View source for Mercury (planet)

{{Short description|First planet from the Sun}}
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{{Use mdy dates|date=August 2024}}
{{Use American English|date=August 2024}}
{{Infobox planet
| name = Mercury
| symbol = [[File:Mercury symbol (bold).svg|24px|☿|class=skin-invert]]
| image = Mercury in true color.jpg
| caption = Mercury in [[False color#True color|true color]] (by ''[[MESSENGER]]'' in 2008)
| background = LightSalmon
| orbit_ref = <ref name="horizons" />
| epoch = [[J2000]]
| aphelion = {{convert|0.466697|AU|e6km|2|abbr=unit|lk=in}}
| perihelion = {{convert|0.307499|AU|e6km|2|abbr=unit}}
| semimajor = {{convert|0.387098|AU|e6km|2|abbr=unit}}
| eccentricity = {{val|0.205630}}<ref name="fact" />
| period = {{plainlist |
* {{val|87.9691|u=day}}
* {{val|0.240846|u=[[julian year (astronomy)|yr]]}}
* 0.5 Mercury [[synodic day]]s
  }}
| synodic_period = 115.88&nbsp;d<ref name="fact" />
| avg_speed = 47.36&nbsp;km/s<ref name="fact" />
| inclination = {{plainlist |
* 7.005° to [[ecliptic]]
* 3.38° to [[Sun]]'s [[equator]]
* 6.35° to [[invariable plane]]<ref name=Souami_Souchay_2012/>
  }}
| asc_node = 48.331°
| arg_peri = 29.124°
| mean_anomaly = 174.796°
| satellites = None
| allsatellites = yes
| mean_radius = {{plainlist |
* {{nowrap|{{val|fmt=commas|2439.7|1.0|u=km}}}}<ref name="nasa" /><ref name="Seidelmann2007" />
* {{val|0.3829}} Earths
  }}
| flattening = 0.0009<ref name="fact" />
| surface_area = {{plainlist |
* {{val|7.48|e=7|u=km2}}<ref name="nasa" />
* 0.147 Earths
  }}
| volume = {{plainlist |
* {{val|6.083|e=10|u=km3}}<ref name="nasa" />
* 0.056 Earths
  }}
| mass = {{plainlist |
* {{val|3.3011|e=23|u=kg}}<ref name="Mazarico2014" />
* 0.055 Earths
  }}
| density = {{val|5.427|u=g/cm3}}<ref name="nasa" />
| surface_grav = {{cvt|3.7|m/s2|g0|lk=out}}<ref name="nasa" />
| moment_of_inertia_factor = {{val|0.346|0.014}}<ref name="Margot2012" />
| escape_velocity = 4.25&nbsp;km/s<ref name="nasa" />
| rotation = {{val|176|u=day}}<ref name="ESO">{{cite web | title=ESO | website=ESO | url=https://www.eso.org/public/outreach/eduoff/vt-2004/mt-2003/mt-mercury-rotation.html | access-date=June 3, 2021 | archive-date=December 4, 2008 | archive-url=https://web.archive.org/web/20081204160221/https://www.eso.org/public/outreach/eduoff/vt-2004/mt-2003/mt-mercury-rotation.html | url-status=live }}</ref>
| sidereal_day = {{plainlist |
* {{val|58.646|u=d}}
* {{val|1407.5|u=h}}<ref name="nasa" />
  }}
| rot_velocity = {{cvt|10.892|km/h|m/s|disp=out}}
| axial_tilt = {{nowrap|2.04{{prime}} ± 0.08{{prime}}}} (to orbit)<ref name="Margot2012" /><br />(0.034°)<ref name="fact" />
| right_asc_north_pole = {{plainlist |
* {{RA|18|44|2}}
* 281.01° <ref name="fact" /><ref name="iau2015">{{Cite journal |last1=Archinal |first1=B. A. |last2=Acton |first2=C. H. |last3=A'Hearn |first3=M. F. |last4=Conrad |first4=A. |last5=Consolmagno |first5=G. J. |last6=Duxbury |first6=T. |last7=Hestroffer |first7=D. |last8=Hilton |first8=J. L. |last9=Kirk |first9=R. L. |last10=Klioner |first10=S. A. |last11=McCarthy |first11=D. |last12=Meech |first12=K. |last13=Oberst |first13=J. |last14=Ping |first14=J. |last15=Seidelmann |first15=P. K. |date=2018 |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015 |url=http://link.springer.com/10.1007/s10569-017-9805-5 |journal=Celestial Mechanics and Dynamical Astronomy |language=en |volume=130 |issue=3 |page=22 |doi=10.1007/s10569-017-9805-5 |bibcode=2018CeMDA.130...22A |issn=0923-2958|url-access=subscription }}</ref>
  }}
| declination = 61.41° <ref name="iau2015" />
| albedo = {{plainlist |
* 0.088 ([[Bond albedo|Bond]])<ref name="Mallama" />
* 0.142 ([[Geometric albedo|geom.]])<ref name="MallamaMercury" />
  }}
| magnitude = −2.48 to +7.25<ref name="Mallama_and_Hilton" />
| abs_magnitude = −0.4<ref name="IMCCE">{{cite web | title=Encyclopedia - the brightest bodies | website=IMCCE | url=https://promenade.imcce.fr/en/pages5/572.html | access-date=May 29, 2023 | archive-date=July 24, 2023 | archive-url=https://web.archive.org/web/20230724225002/https://promenade.imcce.fr/en/pages5/572.html | url-status=live }}</ref>
| angular_size = 4.5–13{{pprime}}<ref name="fact" />
| single_temperature = {{cvt|437|K|°C|0}} ([[Effective temperature|blackbody temperature]])<ref name="American Chemical Society 2013">{{cite web | title=Atmospheres and Planetary Temperatures | website=American Chemical Society | date=July 18, 2013 | url=https://www.acs.org/climatescience/energybalance/planetarytemperatures.html | access-date=January 3, 2023| archiveurl=https://web.archive.org/web/20230127144936/https://www.acs.org/climatescience/energybalance/planetarytemperatures.html |archivedate=January 27, 2023}}</ref>
| temp_name1 = 0°N, 0°W <ref name="vasa">{{cite journal |title=Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits |first1=Ashwin R. |last1=Vasavada |first2=David A. |last2=Paige |first3=Stephen E. |last3=Wood |date=February 19, 1999 |url=http://www.gps.caltech.edu/classes/ge151/references/vasavada_et_al_1999.pdf |journal=Icarus |volume=141 |pages=179–193 |id=Figure 3 with the "TWO model"; Figure 5 for pole |bibcode=1999Icar..141..179V |doi=10.1006/icar.1999.6175 |issue=2 |access-date=February 18, 2012 |archive-date=November 13, 2012 |archive-url=https://web.archive.org/web/20121113124427/http://www.gps.caltech.edu/classes/ge151/references/vasavada_et_al_1999.pdf |url-status=live |issn=0019-1035 }}</ref>
| min_temp_1 = −173 °C
| mean_temp_1 = 67 °C
| max_temp_1 = 427 °C
| temp_name2 = 85°N, 0°W<ref name="vasa" />
| min_temp_2 = −193 °C
| mean_temp_2 = −73 °C
| max_temp_2 = 106.85 °C
<!---| temp_name3 = 90°N<ref name="vasa" /> |min_temp3=180K |mean_temp3=180K | max_temp_3 = 180 K (−93 °C) -->| pronounced = {{IPAc-en|audio=en-us-Mercury.ogg|ˈ|m|ɜːr|k|j|ʊr|i}}
| adjectives = Mercurian {{IPAc-en|m|ər|ˈ|k|jʊər|i|ə|n}},<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Mercurian |archive-url=https://web.archive.org/web/20200327132540/https://www.lexico.com/definition/mercurian |url-status=dead |archive-date=March 27, 2020 |title=Mercurian |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref><br/>Mercurial {{IPAc-en|m|ər|ˈ|k|jʊər|i|ə|l}}<ref>{{Cite dictionary |url=http://www.lexico.com/definition/Mercurial |archive-url=https://web.archive.org/web/20191222132107/https://www.lexico.com/definition/mercurial |url-status=dead |archive-date=December 22, 2019 |title=Mercurial |dictionary=[[Lexico]] UK English Dictionary UK English Dictionary |publisher=[[Oxford University Press]]}}</ref>
| atmosphere = yes
| surface_pressure = trace (≲ 0.5&nbsp;nPa)
| atmosphere_ref = <ref name="fact">{{cite web | first=David R. | last=Williams | title=Mercury Fact Sheet | url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html | publisher=NASA | access-date=April 19, 2021 | date=November 25, 2020 | archive-date=April 3, 2019 | archive-url=https://web.archive.org/web/20190403160651/https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html | url-status=live }}</ref><ref name=Milillo_et_al_2005>{{cite journal | title=Surface-Exosphere-Magnetosphere System Of Mercury | last1=Milillo | first1=A. | last2=Wurz | first2=P. | last3=Orsini | first3=S. | last4=Delcourt | first4=D. | last5=Kallio | first5=E. | last6=Killen | first6=R. M. | last7=Lammer | first7=H. | last8=Massetti | first8=S. | last9=Mura | first9=A. | last10=Barabash | first10=S. | last11=Cremonese | first11=G. | last12=Daglis | first12=I. A. | last13=Angelis | first13=E. | last14=Lellis | first14=A. M. | last15=Livi | first15=S. | last16=Mangano | first16=V. | last17=Torkar | first17=K. | journal=Space Science Reviews | volume=117 | issue=3–4 | pages=397–443 | date=April 2005 | doi=10.1007/s11214-005-3593-z | bibcode=2005SSRv..117..397M | s2cid=122285073 }}</ref><ref name=Berezhnoy2018>{{cite journal | title=Chemistry of impact events on Mercury | last=Berezhnoy | first=Alexey A. | journal=Icarus | volume=300 | pages=210–222 | date=January 2018 | doi=10.1016/j.icarus.2017.08.034 | bibcode=2018Icar..300..210B }}</ref>
| atmosphere_composition = {{plainlist |
* atomic [[oxygen]]
* [[sodium]]
* [[magnesium]]
* atomic [[hydrogen]]
* [[potassium]]
* [[calcium]]
* [[helium]]
* Trace amounts of [[iron]], [[aluminium]], [[argon]], [[dinitrogen]], [[dioxygen]], [[carbon dioxide]], [[water vapor]], [[xenon]], [[krypton]], and [[neon]]
  }}
}}

'''Mercury''' is the first [[planet]] from the [[Sun]] and the [[List of Solar System objects by size|smallest in the Solar System]]. It is a [[rocky planet]] with a trace atmosphere and a surface [[gravity]] slightly higher than that of [[Mars]]. The surface of Mercury is similar to Earth's [[Moon]], being heavily [[Impact crater|cratered]], with an expansive [[rupes]] system generated from [[thrust fault]]s, and bright [[ray system]]s, formed by [[ejecta]]. Its largest crater, [[Caloris Planitia]], has a diameter of {{convert|1,550|km|mi|abbr=on}}, which is about one-third the diameter of the planet ({{Convert|4880|km|mi|abbr=on|disp=or}}). 
Being the most [[inferior planet|inferior]] orbiting planet, it always appears close to the sun in [[Earth]]'s sky, either as a "morning star" or an "evening star". It is the planet with the highest [[delta-v]] required for travel from Earth, as well as to and from the [[List of planets in the Solar System|other planets]] in the [[Solar System]].

Mercury's [[sidereal year]] (88.0 Earth days) and [[sidereal day]] (58.65 Earth days) are in a 3:2 ratio, in a [[Tidal locking#Planets|spin–orbit resonance]]. Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days. Above the planet's surface is an extremely tenuous [[exosphere]] and a [[Mercury's magnetic field|faint magnetic field]] that is strong enough to deflect [[solar wind]]s. Combined with its high [[orbital eccentricity]], the planet's surface has widely varying [[sunlight]] intensity and temperature, with the [[equator]]ial regions ranging from {{convert|-170|C|F|sigfig=2}} at night to {{convert|420|C|F|sigfig=2}} during sunlight. Due to its very small [[axial tilt]], the planet's poles are [[Permanently shadowed crater|permanently shadowed]]. This strongly suggests that [[Ice|water ice]] could be present in the craters.

Like the other planets in the Solar System, Mercury formed approximately 4.5 billion years ago. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with [[planetesimal]]s and rock vaporization; as of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Its [[Mantle (geology)|mantle]] is highly homogeneous, which suggests that Mercury had a [[magma ocean]] early in its history, like the Moon. According to current [[Scientific modelling|models]], Mercury may have a solid [[silicate]] crust and mantle overlaying a solid outer core, a deeper liquid core layer, and a solid inner core. Mercury is expected to be destroyed, along with Venus, and possibly the Earth and the Moon, when the Sun becomes a [[Red giant|Red Giant]] in approximately seven or eight billion years.<ref>{{Cite web|url=https://science.nasa.gov/exoplanets/resources/life-and-death/chapter-6/|title=Chapter 6: Aging Into Gianthood - NASA Science|date=December 11, 2018}}</ref>

Mercury is a [[classical planet]] that has been observed and recognized throughout history as a planet (or wandering star). In English, it is named after the [[ancient Roman]] god {{Lang|la|Mercurius}} ([[Mercury (mythology)|Mercury]]), god of commerce and communication, and the messenger of the gods. The first successful flyby of Mercury was conducted by ''[[Mariner 10]]'' in 1974, and it has since been [[Exploration of Mercury|visited and explored]] by the ''[[MESSENGER (spacecraft)|MESSENGER]]'' and ''[[BepiColombo]]'' orbiters.

== Nomenclature ==
Historically, humans knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the [[ancient Greeks]] had realized the two stars were one.<ref name="Dunne"/> They knew the planet as {{Lang|grc|Στίλβων|italic=no}} {{Lang|grc-latn|Stilbōn}}, meaning "twinkling", and {{Lang|grc|Ἑρμής|italic=no}} {{Lang|grc-latn|[[Hermes|Hermēs]]}}, for its fleeting motion,<ref>{{LSJ|sti/lbwn|Στίλβων}}, {{LSJ|*(ermh{{=}}s|Ἑρμῆς|ref}}.</ref> a name that is retained in modern [[Greek language|Greek]] ({{Lang|el|Ερμής|italic=no}} {{Lang|el-latn|Ermis}}).<ref>{{cite web |url=http://www.greek-names.info/greek-names-of-the-planets/ |title=Greek Names of the Planets |access-date=July 14, 2012 |quote=''Ermis'' is the Greek name of the planet Mercury, which is the closest planet to the Sun. It is named after the Greek God of commerce, Ermis or Hermes, who was also the messenger of the Ancient Greek gods. |date=April 25, 2010 |archive-date=May 9, 2010 |archive-url=https://web.archive.org/web/20100509164917/http://www.greek-names.info/greek-names-of-the-planets/ |url-status=live }} See also the [[:el:Ερμής (πλανήτης)|Greek article about the planet]].</ref> The Romans named the planet after the swift-footed Roman messenger god, [[Mercury (mythology)|Mercury]] (Latin {{Lang|la|Mercurius}}), whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet,<ref name="Dunne">{{cite book |title=The Voyage of Mariner&nbsp;10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |chapter-url=https://history.nasa.gov/SP-424/ch1.htm |publisher=NASA History Office |date=1978 |chapter=Chapter One |url=https://history.nasa.gov/SP-424/ |access-date=July 12, 2017 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}</ref><ref>{{cite book |first=Eugène Michel |last=Antoniadi |others=Translated from French by Moore, Patrick |date=1974 |title=The Planet Mercury |publisher=Keith Reid Ltd |location=Shaldon, Devon |pages=9–11 |isbn=978-0-904094-02-2}}</ref> though some associated the planet with [[Apollo]] instead, as detailed by [[Pliny the Elder]].<ref name="DRGA Planetae">[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0063%3Aalphabetic+letter%3DP%3Aentry+group%3D4%3Aentry%3Dplanetae-cn "Planetae"], in ''Dictionary of Greek and Roman Antiquities'', pp. 922, 923.</ref> The [[astronomical symbol]] for Mercury is a stylized version of Hermes' [[caduceus]]; a [[Christian cross]] was added in the 16th century:[[File:Mercury symbol (fixed width).svg|16px|☿]].<ref>{{cite book |title=Astronomy: A Textbook |first=John Charles |last=Duncan |date=1946 |publisher=Harper & Brothers |page=125 |quote=The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods.}}</ref><ref name="jones-1999">{{cite book |last=Jones |first=Alexander |url=https://books.google.com/books?id=8MokzymQ43IC&pg=PA62 |title=Astronomical papyri from Oxyrhynchus |date=1999 |isbn=9780871692337 |pages=62–63 |publisher=American Philosophical Society |quote=It is now possible to trace the medieval symbols for at least four of the five planets to forms that occur in some of the latest papyrus horoscopes ([ [[P.Oxy.]] ] 4272, 4274, 4275 [...]). Mercury's is a stylized caduceus. |access-date=March 19, 2023 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411141114/https://books.google.com/books?id=8MokzymQ43IC&pg=PA62 |url-status=live }}</ref>

== Physical characteristics ==
[[File:Terrestrial planet size comp 2024.png|thumb|upright=1.4|Mercury to scale among the [[Inner Solar System]] [[planetary-mass object]]s. From left: Mercury, [[Venus]],
[[Earth]], the [[Moon]], [[Mars]] and [[Ceres (dwarf planet)|Ceres]]]]

Mercury is one of four [[terrestrial planet]]s in the [[Solar System]], which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an [[equator]]ial [[radius]] of {{convert|2439.7|km}}.<ref name="fact"/> Mercury is also [[list of Solar System objects by radius|smaller]]—albeit more massive—than the largest [[natural satellite]]s in the Solar System, [[Ganymede (moon)|Ganymede]] and [[Titan (moon)|Titan]]. Mercury consists of approximately 70% metallic and 30% [[silicate]] material.<ref name="strom" />

=== Internal structure ===
[[File:Mercury with magnetic field.svg|left|thumb|upright=1.0|Mercury's internal structure and magnetic field]]
Mercury appears to have a solid silicate [[Crust (geology)|crust]] and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core.<ref>{{cite web |url=https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |title=MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities |publisher=NASA |editor-first=Tricia |editor-last=Talbert |date=March 21, 2012 |access-date=April 20, 2018 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112170032/https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |url-status=dead }}</ref><ref>{{Cite web |url=https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |title=Scientists find evidence Mercury has a solid inner core |format=Press release |date=April 17, 2023 |last1=Genova |first1=Antonio |display-authors=et al |website=AGU Newsroom |language=en-US |access-date=April 17, 2019 |archive-date=April 17, 2019 |archive-url=https://web.archive.org/web/20190417162031/https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |url-status=live }}</ref> The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements.<ref>{{cite book | chapter=The Chemical Composition of Mercury | last1=Nittler | first1=Larry R. | last2=Chabot | first2=Nancy L. | last3=Grove | first3=Timothy L. | last4=Peplowski | first4=Patrick N. | title=Mercury: The View after MESSENGER | editor1-first=Sean C. | editor1-last=Solomon | editor2-first=Larry R. | editor2-last=Nittler | editor3-first=Brian J. | editor3-last=Anderson | isbn=9781316650684 | series=Cambridge Planetary Science Book Series | publication-place=Cambridge, UK | publisher=Cambridge University Press | year=2018 | pages=30–51 | doi=10.1017/9781316650684.003 | arxiv=1712.02187 | bibcode=2018mvam.book...30N | s2cid=119021137 }}</ref> The planet's density is the second highest in the Solar System at 5.427&nbsp;g/cm<sup>3</sup>, only slightly less than Earth's density of 5.515&nbsp;g/cm<sup>3</sup>.<ref name="fact" /> If the effect of [[gravitational compression]] were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3&nbsp;g/cm<sup>3</sup> versus Earth's 4.4&nbsp;g/cm<sup>3</sup>.<ref>{{cite web |date=May 8, 2003 |url=https://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |title=Mercury |publisher=US Geological Survey |access-date=November 26, 2006 |archive-url=https://web.archive.org/web/20060929091534/http://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |archive-date=September 29, 2006 |url-status=dead }}</ref> Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the [[planetary core|core]], Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.<ref>{{cite journal |title=On the Internal Structures of Mercury and Venus |last=Lyttleton |first=Raymond A. |author-link=Raymond Lyttleton |journal=Astrophysics and Space Science |volume=5 |issue=1 |pages=18–35 |year=1969 |doi=10.1007/BF00653933 |bibcode=1969Ap&SS...5...18L |s2cid=122572625 }}</ref>

The radius of Mercury's core is estimated to be {{convert|2020|±|30|km|mi|abbr=on}}, based on interior models constrained to be consistent with a [[moment of inertia factor]] of {{val|0.346|0.014}}.<ref name="Margot2012" /><ref name="Hauck_etal_2013" /> Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.<ref name="cornell">{{cite news |last=Gold |first=Lauren |url=https://news.cornell.edu/stories/2007/05/cu-astronomer-finds-mercury-has-molten-core |title=Mercury has molten core, Cornell researcher shows |work=[[Cornell Chronicle]] |publisher=[[Cornell University]] |date=May 3, 2007 |access-date=July 17, 2025 |archive-date=May 8, 2025 |archive-url=https://web.archive.org/web/20250508142359/https://news.cornell.edu/stories/2007/05/cu-astronomer-finds-mercury-has-molten-core |url-status=live}}</ref><ref name="nrao">{{cite news |last=Finley |first=Dave |date=May 3, 2007 |title=Mercury's Core Molten, Radar Study Shows |publisher=National Radio Astronomy Observatory |url=http://www.nrao.edu/pr/2007/mercury/ |access-date=May 12, 2008 |archive-date=May 3, 2012 |archive-url=https://web.archive.org/web/20120503202505/http://www.nrao.edu/pr/2007/mercury/ |url-status=live }}</ref> The mantle-crust layer is in total {{convert|420|km|mi|abbr=on}} thick.<ref>{{cite journal |last1=Hauck |first1=Steven A. |display-authors=etal |title=The curious case of Mercury's internal structure |journal=Journal of Geophysical Research: Planets |date=May 6, 2013 |volume=118 |issue=6 |pages=1204–1220 |doi=10.1002/jgre.20091 |bibcode=2013JGRE..118.1204H |s2cid=17668886 |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |access-date=June 5, 2023 |hdl=1721.1/85633 |hdl-access=free |archive-date=June 5, 2023 |archive-url=https://web.archive.org/web/20230605175115/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |url-status=live }}</ref> Projections differ as to the size of the crust specifically; data from the {{nowrap|''Mariner 10''}} and ''MESSENGER'' probes suggests a thickness of {{convert|35|km|mi|abbr=on}}, whereas an [[Airy isostacy]] model suggests a thickness of {{convert|26|±|11|km|mi|abbr=on}}.<ref name="Padovan2015" /><ref>{{Cite book |last1=Solomon |first1=Sean C. |url=https://books.google.com/books?id=4o92DwAAQBAJ |title=Mercury: The View after MESSENGER |last2=Nittler |first2=Larry R. |last3=Anderson |first3=Brian J. |date=December 20, 2018 |publisher=Cambridge University Press |isbn=978-1-107-15445-2 |pages=534 |language=en |access-date=November 19, 2022 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162217/https://books.google.com/books?id=4o92DwAAQBAJ |url-status=live }}</ref><ref>{{cite journal | title=A thin, dense crust for Mercury | last=Sori | first=Michael M. | journal=Earth and Planetary Science Letters | volume=489 | pages=92–99 | date=May 2018 | doi=10.1016/j.epsl.2018.02.033 | bibcode=2018E&PSL.489...92S | doi-access=free }}</ref> One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.<ref>{{cite journal |title=Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere |last1=Schenk |first1=Paul M. |last2=Melosh |first2=H. Jay |author-link2=H. Jay Melosh |journal=Abstracts of the 25th Lunar and Planetary Science Conference |volume=1994 |pages=1994LPI....25.1203S |bibcode=1994LPI....25.1203S |date=March 1994 }}</ref><ref>{{cite conference | last1=Watters | first1=T. R. | first2=F. | last2=Nimmo | first3=M. S. | last3=Robinson | title=Chronology of Lobate Scarp Thrust Faults and the Mechanical Structure of Mercury's Lithosphere | conference=Lunar and Planetary Science Conference | page=1886 | year=2004 | bibcode= 2004LPI....35.1886W }}</ref><ref>{{cite journal | journal=Geology | date=November 1998 | volume=26 | issue=11 | pages=991–994 | title=Topography of lobate scarps on Mercury; new constraints on the planet's contraction | first1=Thomas R. | last1=Watters | first2=Mark S. | last2=Robinson | first3=Anthony C. | last3=Cook | doi=10.1130/0091-7613(1998)026<0991:TOLSOM>2.3.CO;2 | bibcode=1998Geo....26..991W }}</ref>

Mercury's core has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common [[chondrite]] meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.<ref name="Benz" /> Early in the Solar System's history, Mercury may have been struck by a [[planetesimal]] of approximately {{frac|1|6}} Mercury's mass and several thousand kilometers across.<ref name="Benz" /> The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.<ref name="Benz" /> A similar process, known as the [[giant impact hypothesis]], has been proposed to explain the formation of Earth's Moon.<ref name="Benz" />

Alternatively, Mercury may have formed from the [[solar nebula]] before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the [[protostar|protosun]] contracted, temperatures near Mercury could have been between 2,500 and 3,500&nbsp;K and possibly even as high as 10,000&nbsp;K.<ref name="CameronAGW1">{{cite journal |title=The partial volatilization of Mercury |last=Cameron |first=Alastair G. W. |author-link=Alastair G. W. Cameron |journal=Icarus |volume=64 |issue=2 |pages=285–294 |year=1985 |doi=10.1016/0019-1035(85)90091-0 |bibcode=1985Icar...64..285C}}</ref> Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the [[solar wind]].<ref name="CameronAGW1" /> A third hypothesis proposes that the solar nebula caused [[drag (physics)|drag]] on the particles from which Mercury was [[Accretion (astrophysics)|accreting]], which meant that lighter particles were lost from the accreting material and not gathered by Mercury.<ref>{{cite journal |title=Iron/silicate fractionation and the origin of Mercury |last=Weidenschilling |first=Stuart J. |journal=Icarus |volume=35 |issue=1 |pages=99–111 |year=1987 |doi=10.1016/0019-1035(78)90064-7 |bibcode=1978Icar...35...99W}}</ref>

Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first ''[[MESSENGER]]'', which ended in 2015, found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events.<ref name="csmon20110929">{{cite news |url=https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |title=Messenger's message from Mercury: Time to rewrite the textbooks |work=The Christian Science Monitor |first=Mark |last=Sappenfield |date=September 29, 2011 |access-date=August 21, 2017 |archive-date=August 21, 2017 |archive-url=https://web.archive.org/web/20170821214604/https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |url-status=live }}</ref> ''[[BepiColombo]]'', which will arrive at Mercury in 2025, will make observations to test these hypotheses.<ref name="ESA-Bepi">{{cite web |url=http://sci.esa.int/bepicolombo/ |title=BepiColombo |series=Science & Technology |publisher=European Space Agency |access-date=April 7, 2008 |archive-date=March 6, 2018 |archive-url=https://web.archive.org/web/20180306193632/http://sci.esa.int/bepicolombo/ |url-status=live }}</ref> The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.<ref name="intra">{{cite news |url=https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |title=Messenger sheds light on Mercury's formation |work=Chemistry World |first=Jon |last=Cartwright |date=September 30, 2011 |access-date=August 21, 2017 |archive-date=August 6, 2017 |archive-url=https://web.archive.org/web/20170806063258/https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |url-status=live }}</ref>

=== Surface geology ===
{{Main|Geology of Mercury}}
{{multiple image
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 | image1   = Mercury render with Blender 01.png
 | image2   = Mercury render with Blender 02.png
 | footer   = Mercury rendered with [[Blender (software)|Blender]] with data from [[NASA]] and the [[United States Geological Survey|USGS]]
}}
Mercury's surface is similar in appearance to that of the Moon, showing extensive [[Lunar mare|mare]]-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more [[heterogeneous]] than the surface of [[Mars]] or the Moon, both of which contain significant stretches of similar geology, such as [[Lunar mare|maria]] and plateaus.<ref name="awst169_18_18" /> [[Albedo]] features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and [[ray system]]s. Larger albedo features correspond to higher reflectivity plains.<ref>{{cite conference | title=Albedo Features of Mercury | last1=Hughes | first1=E. T. | last2=Vaughan | first2=W. M. | conference=43rd Lunar and Planetary Science Conference, held March 19–23, 2012 at The Woodlands, Texas | volume=1659 | id=2151 | date=March 2012 | bibcode=2012LPI....43.2151H }}</ref> Mercury has "[[wrinkle-ridge]]s" (dorsa), Moon-like [[highland]]s, mountains (montes), plains (planitiae), escarpments (rupes), and valleys ([[Vallis (planetary geology)|valles]]).<ref>{{cite web |last=Blue |first=Jennifer |date=April 11, 2008 |url=https://planetarynames.wr.usgs.gov/ |title=Gazetteer of Planetary Nomenclature |publisher=US Geological Survey |access-date=April 11, 2008 |archive-date=April 12, 2012 |archive-url=https://web.archive.org/web/20120412082057/http://planetarynames.wr.usgs.gov/ |url-status=live }}</ref><ref name="DunneCh7">{{cite book |title=The Voyage of Mariner&nbsp;10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |author-link2=Eric Burgess |chapter-url=https://history.nasa.gov/SP-424/ch7.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Seven |url=https://history.nasa.gov/SP-424/ |access-date=May 28, 2008 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}</ref>

[[File:Unmasking the Secrets of Mercury.jpg|thumb|left|[[MESSENGER#Scientific instruments|MASCS]] spectrum scan of Mercury's surface by ''MESSENGER'']]
The planet's mantle is chemically heterogeneous, suggesting the planet went through a [[magma ocean]] phase early in its history. Crystallization of minerals and convective overturn resulted in a layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early [[Chemical reduction|chemically reducing]] conditions than is found on other terrestrial planets. The surface is dominated by iron-poor [[pyroxene]] and [[olivine]], as represented by [[enstatite]] and [[forsterite]], respectively, along with sodium-rich [[plagioclase]] and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.<ref>{{cite journal | title=The Surface Composition of Mercury | first1=Larry R. | last1=Nittler | first2=Shoshana Z. | last2=Weider | journal=Elements | year=2019 | volume=15 | issue=1 | pages=33–38 | doi=10.2138/gselements.15.1.33 | bibcode=2019Eleme..15...33N | s2cid=135051680 }}</ref><ref>{{cite journal | title=The Role of Reducing Conditions in Building Mercury | first1=Camille | last1=Cartier | first2=Bernard J. | last2=Wood | journal=Elements | volume=15 | number=1 | pages=39–45 | date=February 2019 | doi=10.2138/gselements.15.1.39 | bibcode=2019Eleme..15...39C | s2cid=135268415 }}</ref>

Names for features on Mercury come from a variety of sources and are set according to the [[IAU]] [[planetary nomenclature]] system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or [[fossa (geology)|fossae]] are named for works of architecture. Montes are named for the word "hot" in a variety of languages. [[Plain]]s or planitiae are named for [[Mercury (god)|Mercury]] in various languages. [[Escarpment]]s or [[rupēs]] are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.<ref>{{cite web |url=https://planetarynames.wr.usgs.gov/Page/Categories |title=Categories for Naming Features on Planets and Satellites |publisher=US Geological Survey |access-date=August 20, 2011 |archive-date=July 8, 2014 |archive-url=https://web.archive.org/web/20140708063522/http://planetarynames.wr.usgs.gov/Page/Categories |url-status=live }}</ref>

==== Impact basins and craters ====
[[File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg|thumb|left|{{anchor|Munch|Sander|Poe}}Enhanced-color image of craters [[Munch (crater)|Munch]] (left), [[Sander (crater)|Sander]] (center), and [[Poe (crater)|Poe]] (right) amid volcanic plains (orange) near [[Caloris Basin]]]]

Mercury was heavily bombarded by comets and [[asteroid]]s during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the [[Late Heavy Bombardment]] that ended 3.8 billion years ago.<ref>{{cite journal |last=Strom |first=Robert G. |year=1979 |volume=24 |issue=1 |title=Mercury: a post-Mariner assessment |journal=Space Science Reviews |pages=3–70 |bibcode=1979SSRv...24....3S |doi=10.1007/BF00221842 |s2cid=122563809 }}</ref> Mercury received impacts over its entire surface during this period of intense crater formation,<ref name="DunneCh7" /> facilitated by the lack of any [[atmosphere]] to slow impactors down.<ref>{{cite journal |last1=Broadfoot |first1=A. Lyle |first2=Shailendra |last2=Kumar |first3=Michael J. S. |last3=Belton |author-link3=Michael J. Belton |first4=Michael B. |last4=McElroy |author-link4=Michael McElroy (scientist) |title=Mercury's Atmosphere from Mariner&nbsp;10: Preliminary Results |journal=Science |volume=185 |issue=4146 |date=July 12, 1974 |pages=166–169 |doi=10.1126/science.185.4146.166 |pmid=17810510 |bibcode=1974Sci...185..166B|s2cid=7790470 }}</ref> During this time Mercury was [[volcano|volcanically]] active; basins were filled by [[magma]], producing smooth plains similar to the maria found on the Moon.<ref>{{cite book | date=1997 | doi=10.3133/i2596 | title=Geology of the solar system | series=IMAP 2596 | publisher=U.S. Geological Survey }}</ref><ref>{{cite journal |last1=Head |first1=James W. |author-link1=James W. Head |last2=Solomon |first2=Sean C. |author-link2=Sean Solomon |title=Tectonic Evolution of the Terrestrial Planets |journal=Science |year=1981 |volume=213 |issue=4503 |pages=62–76 |doi=10.1126/science.213.4503.62 |pmid=17741171 |bibcode=1981Sci...213...62H |hdl=2060/20020090713 |url=https://www.planetary.brown.edu/pdfs/323.pdf |citeseerx=10.1.1.715.4402 |access-date=October 25, 2017 |archive-date=July 21, 2018 |archive-url=https://web.archive.org/web/20180721153426/http://www.planetary.brown.edu/pdfs/323.pdf |url-status=dead }}</ref> One of the most unusual craters is [[Apollodorus (crater)|Apollodorus]], or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site.<ref>{{cite web |title=Scientists see Mercury in a new light |url=https://www.sciencedaily.com/releases/2008/02/080201093149.htm |website=Science Daily |date=February 28, 2008 |access-date=April 7, 2008 |archive-date=December 5, 2020 |archive-url=https://web.archive.org/web/20201205202019/https://www.sciencedaily.com/releases/2008/02/080201093149.htm |url-status=live }}</ref>

[[Craters on Mercury]] range in diameter from small bowl-shaped cavities to [[multi-ringed impact basin]]s hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.<ref name="Spudis01">{{cite journal |first=Paul D. |last=Spudis |author-link=Paul Spudis |title=The Geological History of Mercury |journal=Workshop on Mercury: Space Environment, Surface, and Interior, Chicago |issue=1097 |year=2001 |page=100 |bibcode=2001mses.conf..100S}}</ref> According to [[International Astronomical Union]] rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named.<ref name="Ritzel" />

{{multiple image |direction=horizontal |align=right |total_width=400
|image1=The Mighty Caloris (PIA19213).png | caption1=Overhead view of Caloris Basin
|image2=PIA19450-PlanetMercury-CalorisBasin-20150501.jpg | caption2=Perspective view of Caloris Basin – high (red); low (blue)
}}

The largest known crater is [[Caloris Planitia]], or Caloris Basin, with a diameter of {{convert|1550|km|mi|abbr=on}}.<ref name="newscientist30012008">{{cite news |url=https://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |title=Bizarre spider scar found on Mercury's surface |date=January 30, 2008 |publisher=NewScientist.com news service |first=David |last=Shiga |access-date=September 4, 2017 |archive-date=December 10, 2014 |archive-url=https://web.archive.org/web/20141210213025/http://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |url-status=live }}</ref> The impact that created the Caloris Basin was so powerful that it caused [[lava]] eruptions and left a concentric mountainous ring ~{{convert|2|km|mi|abbr=on}} tall surrounding the [[impact crater]]. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt.<ref name="Spudis01" />

At the [[antipodes|antipode]] of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface.<ref>{{cite journal |last1=Schultz |first1=Peter H. |author-link1=Peter H. Schultz |last2=Gault |first2=Donald E. |year=1975 |title=Seismic effects from major basin formations on the moon and Mercury |journal=Earth, Moon, and Planets |volume=12 |issue=2 |pages=159–175 |doi=10.1007/BF00577875 |bibcode=1975Moon...12..159S|s2cid=121225801 }}</ref> Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.<ref>{{cite journal |last1=Wieczorek |first1=Mark A. |last2=Zuber |first2=Maria T. |author-link2=Maria Zuber |title=A Serenitatis origin for the Imbrian grooves and South Pole-Aitken thorium anomaly |journal=Journal of Geophysical Research |year=2001 |volume=106 |issue=E11 |pages=27853–27864 |url=http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |access-date=May 12, 2008 |doi=10.1029/2000JE001384 |bibcode=2001JGR...10627853W |doi-access=free |archive-date=May 12, 2011 |archive-url=https://web.archive.org/web/20110512152936/http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |url-status=live }}</ref>

[[File:EW1027346412Gnomap.png|thumb|Tolstoj basin is along the bottom of this image of Mercury's limb]]
Overall, 46 impact basins have been identified.<ref>{{cite journal | title=Large impact basins on Mercury: Global distribution, characteristics, and modification history from MESSENGER orbital data | last1=Fassett | first1=Caleb I. | last2=Head | first2=James W. | last3=Baker | first3=David M. H. | last4=Zuber | first4=Maria T. | last5=Smith | first5=David E. | last6=Neumann | first6=Gregory A. | last7=Solomon | first7=Sean C. | last8=Klimczak | first8=Christian | last9=Strom | first9=Robert G. | last10=Chapman | first10=Clark R. | last11=Prockter | first11=Louise M. | last12=Phillips | first12=Roger J. | last13=Oberst | first13=Jürgen | last14=Preusker | first14=Frank | journal=Journal of Geophysical Research | volume=117 | id=E00L08 | date=October 2012 | at=15 pp. | article-number=2012JE004154 | doi=10.1029/2012JE004154 | bibcode=2012JGRE..117.0L08F | doi-access=free }}</ref> A notable basin is the {{convert|400|km|mi|abbr=on|adj=mid}}-wide, multi-ring [[Tolstoj Basin]] that has an ejecta blanket extending up to {{convert|500|km|mi|abbr=on}} from its rim and a floor that has been filled by smooth plains materials. [[Beethoven Basin]] has a similar-sized ejecta blanket and a {{convert|625|km|mi|abbr=on|adj=mid}}-diameter rim.<ref name="Spudis01" /> Like the Moon, the surface of Mercury has likely incurred the effects of [[space weathering]] processes, including solar wind and [[micrometeorite]] impacts.<ref>{{cite journal |title=Albedo of Immature Mercurian Crustal Materials: Evidence for the Presence of Ferrous Iron |journal=Lunar and Planetary Science |volume=39 |issue=1391 |year=2008 |page=1750 |last1=Denevi |first1=Brett W. |author-link1=Brett Denevi |last2=Robinson |first2=Mark S. |bibcode=2008LPI....39.1750D}}</ref>

==== Plains ====
There are two geologically distinct plains regions on Mercury.<ref name="Spudis01" /><ref name="WagWolIva01" /> Gently rolling, hilly [[Inter-crater plains on Mercury|plains in the regions between craters]] are Mercury's oldest visible surfaces,<ref name="Spudis01" /> predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about {{convert|30|km|mi|abbr=on}} in diameter.<ref name="WagWolIva01" />

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins.<ref name="Spudis01" /> All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.<ref name="Spudis01" />

==== Compressional features ====
An unusual feature of Mercury's surface is the numerous compression folds, or [[rupes]], that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury.<ref>{{cite journal | title=Wrinkle ridges on Mercury and the Moon within and outside of mascons | last1=Schleicher | first1=Lisa S. | last2=Watters | first2=Thomas R. | last3=Martin | first3=Aaron J. | last4=Banks | first4=Maria E. | journal=Icarus | volume=331 | pages=226–237 | date=October 2019 | doi=10.1016/j.icarus.2019.04.013 | bibcode=2019Icar..331..226S | s2cid=150072193 }}</ref> As Mercury's interior cooled, it contracted and its surface began to deform, creating [[wrinkle ridge]]s and [[lobate scarp]]s associated with [[thrust fault]]s. The scarps can reach lengths of {{convert|1000|km|mi|abbr=on}} and heights of {{convert|3|km|mi|abbr=on}}.<ref name = "Choi2016.09">{{cite web |url=http://www.space.com/34199-earthquakes-rock-mercury-today.html |title=Mercuryquakes May Currently Shake Up the Tiny Planet |last=Choi |first=Charles Q. |date=September 26, 2016 |website=[[Space.com]] |access-date=September 28, 2016 |archive-date=September 28, 2016 |archive-url=https://web.archive.org/web/20160928040406/http://www.space.com/34199-earthquakes-rock-mercury-today.html |url-status=live }}</ref> These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent.<ref name="Dzurisin1978">{{cite journal |last=Dzurisin |first=Daniel |date=October 10, 1978 |title=The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments |journal=Journal of Geophysical Research |volume=83 |issue=B10 |pages=4883–4906 |bibcode=1978JGR....83.4883D |doi=10.1029/JB083iB10p04883}}</ref> Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~{{convert|1–7|km|mi|abbr=on}}.<ref name="Watters2016">{{cite journal |last1=Watters |first1=Thomas R. |last2=Daud |first2=Katie |last3=Banks |first3=Maria E. |last4=Selvans |first4=Michelle M. |last5=Chapman |first5=Clark R. |last6=Ernst |first6=Carolyn M. |title=Recent tectonic activity on Mercury revealed by small thrust fault scarps |journal=Nature Geoscience |date=September 26, 2016 |doi=10.1038/ngeo2814 |volume=9 |issue=10 |pages=743–747 |bibcode=2016NatGe...9..743W}}</ref> Most activity along the major thrust systems probably ended about 3.6–3.7&nbsp;billion years ago.<ref>{{cite journal | title=Dating long thrust systems on Mercury: New clues on the thermal evolution of the planet | first1=L. | last1=Giacomini | first2=M. | last2=Massironi | first3=V. | last3=Galluzzi | first4=S. | last4=Ferrari | first5=P. | last5=Palumbo | journal=Geoscience Frontiers | volume=11 | issue=3 | date=May 2020 | pages=855–870 | doi=10.1016/j.gsf.2019.09.005 | bibcode=2020GeoFr..11..855G | s2cid=210298205 | doi-access=free | hdl=11577/3314386 | hdl-access=free }}</ref> Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present.<ref name = "Choi2016.09" /><ref name= "Watters2016" />

====Volcanism====
[[File:Picasso crater.png|thumb|The arc-shaped pit on the eastern side of [[Picasso (crater)|Picasso crater]] may have formed from the collapse of a magma tube]]
There is evidence for [[pyroclastic flow]]s on Mercury from low-profile [[shield volcano]]es.<ref name="Kerber 2009">{{cite journal |title=Explosive volcanic eruptions on Mercury: Eruption conditions, magma volatile content, and implications for interior volatile abundances |journal=Earth and Planetary Science Letters |date=August 15, 2009 |last1=Kerber |first1=Laura |last2=Head |first2=James W. |last3=Solomon |first3=Sean C. |last4=Murchie |first4=Scott L. |last5=Blewett |first5=David T. | volume=285 | issue=3–4 | pages=263–271 | doi=10.1016/j.epsl.2009.04.037 | bibcode=2009E&PSL.285..263K |doi-access=free }}</ref><ref name="Volcanism 2011">{{cite journal |title=Flood Volcanism in the Northern High Latitudes of Mercury Revealed by ''MESSENGER'' |journal=Science |date=September 30, 2011 |last1=Head |first1=James W. |last2=Chapman |first2=Clark R. |last3=Strom |first3=Robert G. |last4=Fassett |first4=Caleb I. |last5=Denevi |first5=Brett W. |volume=333 |issue=6051 |pages=1853–1856 |doi=10.1126/science.1211997 |bibcode=2011Sci...333.1853H |pmid=21960625 |s2cid=7651992 |url=https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |access-date=August 20, 2019 |archive-date=July 19, 2018 |archive-url=https://web.archive.org/web/20180719031049/https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |url-status=live }}</ref><ref name="becca">{{cite journal |last1=Thomas |first1=Rebecca J. |last2=Rothery |first2=David A. |last3=Conway |first3=Susan J. |last4=Anand |first4=Mahesh |title=Long-lived explosive volcanism on Mercury |journal=Geophysical Research Letters |date=September 16, 2014 |volume=41 |issue=17 |pages=6084–6092 |doi=10.1002/2014GL061224 |bibcode=2014GeoRL..41.6084T |s2cid=54683272 |url=http://oro.open.ac.uk/40782/ |access-date=July 19, 2017 |archive-date=August 22, 2017 |archive-url=https://web.archive.org/web/20170822012357/http://oro.open.ac.uk/40782/ |url-status=live }}</ref> Fifty-one pyroclastic deposits have been identified,<ref name="Groudge 2014">{{cite journal |title=Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data |journal=Journal of Geophysical Research |date=March 2014 |last1=Groudge |first1=Timothy A. |last2=Head |first2=James W. |doi=10.1002/2013JE004480 |volume=119 |issue=3 |pages=635–658 |bibcode=2014JGRE..119..635G |s2cid=14393394 |url=https://www.planetary.brown.edu/pdfs/4334.pdf |access-date=August 25, 2019 |archive-date=July 18, 2019 |archive-url=https://web.archive.org/web/20190718161242/http://www.planetary.brown.edu/pdfs/4334.pdf |url-status=dead }}</ref> where 90% of them are found within impact craters.<ref name="Groudge 2014"/> A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.<ref name="Groudge 2014"/>

A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to {{convert|8|km|mi|abbr=on}} in diameter. It is thus a "[[compound volcano]]".<ref name="Rothery 2014">{{cite journal |title=Prolonged eruptive history of a compound volcano on Mercury: Volcanic and tectonic implications |journal=Earth and Planetary Science Letters |date=January 1, 2014 |last1=Rothery |first1=David A. |last2=Thomas |first2=Rebeca J. |last3=Kerber |first3=Laura |volume=385 |pages=59–67 |bibcode=2014E&PSL.385...59R |doi=10.1016/j.epsl.2013.10.023 |url=http://oro.open.ac.uk/38842/1/Rothery2.pdf |access-date=August 20, 2019 |archive-date=March 6, 2020 |archive-url=https://web.archive.org/web/20200306081432/http://oro.open.ac.uk/38842/1/Rothery2.pdf |url-status=live }}</ref> The vent floors are at least {{convert|1|km|mi|abbr=on}} below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit.<ref name="Rothery 2014"/> Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.<ref name="Rothery 2014"/>

=== Surface conditions and exosphere ===
{{Main|Atmosphere of Mercury}}
[[File:North pole of Mercury -- NASA.jpg|thumb|Composite of the north pole of Mercury, where a large volume of water ice lies in permanently dark craters.<ref name="NYTimes2012-11-28" />]]
The surface temperature of Mercury ranges from {{Convert|100 to 700|K|C F}}.<ref name=":0">{{cite book |last=Prockter |first=Louise |title=Ice in the Solar System |publisher=Johns Hopkins APL Technical Digest |volume=26 |issue=2 |date=2005 |url=https://www.jhuapl.edu/content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |access-date=July 27, 2009 |archive-date=September 24, 2021 |archive-url=https://web.archive.org/web/20210924085243/https://www.jhuapl.edu/Content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |url-status=live }}</ref> It never rises above 180&nbsp;K at the poles,<ref name="vasa" /> due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At [[perihelion]], the equatorial [[subsolar point]] is located at longitude 0°W or 180°W, and it climbs to a temperature of about {{val|700|u=K}}. During [[aphelion]], this occurs at 90° or 270°W and reaches only {{val|550|u=K}}.<ref>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=463 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> On the dark side of the planet, temperatures average {{val|110|u=K}}.<ref name="vasa" /><ref>{{cite journal |last1=Murdock |first1=Thomas L. |last2=Ney |first2=Edward P. |title=Mercury: The Dark-Side Temperature |journal=[[Science (journal)|Science]] |year=1970 |volume=170 |issue=3957 |pages=535–537 |doi=10.1126/science.170.3957.535 |pmid=17799708 |bibcode=1970Sci...170..535M|s2cid=38824994 }}</ref> The intensity of [[sunlight]] on Mercury's surface ranges between 4.59 and 10.61 times the [[solar constant]] (1,370 W·m<sup>−2</sup>).<ref>{{cite book |title=Physics and Chemistry of the Solar System |last=Lewis |first=John S. |publisher=Academic Press |date=2004 |url=https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |access-date=June 3, 2008 |isbn=978-0-12-446744-6 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162200/https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |url-status=live }}</ref>

Although daylight temperatures at the surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102&nbsp;K, far lower than the global average.<ref>{{cite journal |last1=Ingersoll |first1=Andrew P. |last2=Svitek |first2=Tomas |last3=Murray |first3=Bruce C. |title=Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars |journal=Icarus |volume=100 |issue=1 |pages=40–47 |year=1992 |bibcode=1992Icar..100...40I |doi=10.1016/0019-1035(92)90016-Z}}</ref> This creates a [[Cold trap (astronomy)|cold trap]] where ice can accumulate. Water ice strongly reflects [[radar]], and observations by the 70-meter [[Goldstone Solar System Radar]] and the [[Very Large Array|VLA]] in the early 1990s revealed that there are patches of high radar [[Reflection (physics)|reflection]] near the poles.<ref name=Slade_et_al_1992>{{cite journal |last1=Slade |first1=Martin A. |last2=Butler |first2=Bryan J. |last3=Muhleman |first3=Duane O. |year=1992 |title=Mercury radar imaging – Evidence for polar ice |journal=[[Science (journal)|Science]] |volume=258 |issue=5082 |pages=635–640 |doi=10.1126/science.258.5082.635 |pmid=17748898 |bibcode=1992Sci...258..635S|s2cid=34009087 }}</ref> Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation.<ref>{{cite web |last=Williams |first=David R. |date=June 2, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |title=Ice on Mercury |publisher=NASA Goddard Space Flight Center |access-date=May 23, 2008 |archive-date=January 31, 2011 |archive-url=https://web.archive.org/web/20110131225129/http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |url-status=live }}</ref> The presence of [[ice|water ice]] was confirmed using ''MESSENGER'' images of craters at the north pole.<ref name="NYTimes2012-11-28">{{cite news |url=https://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |title=On Closest Planet to the Sun, NASA Finds Lots of Ice |work=[[The New York Times]] |first=Kenneth |last=Chang |date=November 29, 2012 |page=A3 |archive-date=November 29, 2012 |archive-url=https://web.archive.org/web/20121129194012/http://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |url-status=live |quote=Sean C. Solomon, the principal investigator for MESSENGER, said there was enough ice there to encase [[Washington, D.C.]], in a frozen block two and a half miles deep.}}</ref>

The icy crater regions are estimated to contain about 10<sup>14</sup>–10<sup>15</sup>&nbsp;kg of ice,<ref name="Zahnle1">{{cite journal |last1=Rawlins |first1=Katherine |last2=Moses |first2=Julianne I. |last3=Zahnle |first3=Kevin J. |author-link3=Kevin J. Zahnle |title=Exogenic Sources of Water for Mercury's Polar Ice |journal=Bulletin of the American Astronomical Society |year=1995 |volume=27 |bibcode=1995DPS....27.2112R |page=1117}}</ref> and may be covered by a layer of [[regolith]] that inhibits [[Sublimation (phase transition)|sublimation]].<ref>{{cite journal |last1=Harmon |first1=John K. |last2=Perillat |first2=Phil J. |last3=Slade |first3=Martin A. |title=High-Resolution Radar Imaging of Mercury's North Pole |journal=Icarus |volume=149 |issue=1 |pages=1–15 |year=2001 |doi=10.1006/icar.2000.6544 |bibcode=2001Icar..149....1H}}</ref> By comparison, the [[Antarctica|Antarctic]] ice sheet on Earth has a mass of about 4{{e|18}}&nbsp;kg, and Mars's south polar cap contains about 10<sup>16</sup>&nbsp;kg of water.<ref name="Zahnle1" /> The origin of the ice on Mercury is not yet known, but the two most likely sources are from [[outgassing]] of water from the planet's interior and deposition by impacts of comets.<ref name="Zahnle1" />

Mercury is too small and hot for its [[gravity]] to retain any significant [[atmosphere]] over long periods of time; it does have a tenuous surface-bounded [[exosphere]]<ref>{{cite journal |last1=Domingue |first1=Deborah L. |last2=Koehn |first2=Patrick L. |display-authors=2 |last3=Killen |first3=Rosemary M. |last4=Sprague |first4=Ann L. |last5=Sarantos |first5=Menelaos |last6=Cheng |first6=Andrew F. |last7=Bradley |first7=Eric T. |last8=McClintock |first8=William E. |title=Mercury's Atmosphere: A Surface-Bounded Exosphere |journal=Space Science Reviews |volume=131 |issue=1–4 |pages=161–186 |year=2009 |doi=10.1007/s11214-007-9260-9 |bibcode=2007SSRv..131..161D |s2cid=121301247 |name-list-style=vanc}}</ref> at a surface pressure of less than approximately 0.5&nbsp;nPa (0.005 picobars).<ref name="fact" /> It includes [[hydrogen]], [[helium]], [[oxygen]], [[sodium]], [[calcium]], [[potassium]], [[magnesium]], [[silicon]], and [[hydroxide]], among others.<ref name=Milillo_et_al_2005/><ref name=Berezhnoy2018/> This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. [[Hydrogen atom]]s and [[helium atom]]s probably come from the solar wind, [[diffusion|diffusing]] into Mercury's [[magnetosphere]] before later escaping back into space. The [[radioactive decay]] of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, [[sputtering]] creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O<sup>+</sup>, OH<sup><span style="color:black;">−</span></sup>, and [[hydronium|H<sub>3</sub>O<sup>+</sup>]] was a surprise.<ref>{{cite book |editor-first=Faith |editor-last=Vilas |editor-first2=Clark R. |editor-last2=Chapman |editor-first3=Mildred |editor-last3=Shapley Matthews |last1=Hunten |first1=Donald M. |last2=Shemansky |first2=Donald Eugene |last3=Morgan |first3=Thomas Hunt |date=1988 |publisher=University of Arizona Press |isbn=978-0-8165-1085-6 |chapter=The Mercury atmosphere |title=Mercury |chapter-url=https://www.researchgate.net/publication/23869200_The_Mercury_atmosphere <!-- broken: http://www.uapress.arizona.edu/onlinebks/Mercury/MercuryCh17.pdf --> |url=https://uapress.arizona.edu/book/mercury |access-date=February 19, 2020 |archive-date=February 19, 2020 |archive-url=https://web.archive.org/web/20200219213720/https://uapress.arizona.edu/book/mercury |url-status=live }}</ref><ref>{{cite news |first=Emily |last=Lakdawalla |date=July 3, 2008 |title=MESSENGER Scientists "Astonished" to Find Water in Mercury's Thin Atmosphere |publisher=The Planetary Society |url=https://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |access-date=May 18, 2009 |archive-date=April 4, 2017 |archive-url=https://web.archive.org/web/20170404043436/http://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |url-status=live }}</ref> Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.<ref>{{cite journal |last1=Zurbuchen |first1=Thomas H. |last2=Raines |first2=Jim M. |display-authors=2 |last3=Gloeckler |first3=George |last4=Krimigis |first4=Stamatios M. |last5=Slavin |first5=James A. |last6=Koehn |first6=Patrick L. |last7=Killen |first7=Rosemary M. |last8=Sprague |first8=Ann L. |last9=McNutt Jr. |first9=Ralph L. |last10=Solomon |first10=Sean C. |title=MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment |journal=Science |volume=321 |issue=5885 |pages=90–92 |year=2008 |doi=10.1126/science.1159314 |pmid=18599777 |bibcode=2008Sci...321...90Z |s2cid=206513512 |name-list-style=vanc}}</ref><ref>{{cite news |publisher=University of Michigan |date=June 30, 2008 |title=Instrument Shows What Planet Mercury Is Made Of |url=http://newswise.com/articles/view/542209/ |access-date=May 18, 2009 |archive-date=March 22, 2012 |archive-url=https://web.archive.org/web/20120322021728/http://newswise.com/articles/view/542209/ |url-status=live }}</ref>

Sodium, potassium, and calcium were discovered in the atmosphere during the 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts<ref name="Killen2007">{{cite journal |last1=Killen |first1=Rosemary |title=Processes that Promote and Deplete the Exosphere of Mercury |year=2007 |journal=[[Space Science Reviews]] |volume=132 |issue=2–4 |pages=433–509 |doi=10.1007/s11214-007-9232-0 |ref=Killen2007 |bibcode=2007SSRv..132..433K |last2=Cremonese |first2=Gabrielle |display-authors=2 |last3=Lammer |first3=Helmut |last4=Orsini |first4=Stefano |last5=Potter |first5=Andrew E. |last6=Sprague |first6=Ann L. |last7=Wurz |first7=Peter |last8=Khodachenko |first8=Maxim L. |last9=Lichtenegger |first9=Herbert I. M. |s2cid=121944553 |url=https://boris.unibe.ch/25351/ |access-date=October 16, 2022 |archive-date=October 9, 2022 |archive-url=https://web.archive.org/web/20221009053004/https://boris.unibe.ch/25351/ |url-status=live }}</ref> including presently from [[Comet Encke]].<ref>{{cite journal |first1=Rosemary M. |last1=Killen |first2=Joseph M. |last2=Hahn |title=Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere |journal=Icarus |date=December 10, 2014 |doi=10.1016/j.icarus.2014.11.035 |bibcode=2015Icar..250..230K |volume=250 |pages=230–237 |hdl=2060/20150010116}}</ref> In 2008, magnesium was discovered by ''MESSENGER''.<ref name="McClintock2009">{{cite journal |last1=McClintock |first1=William E. |last2=Vervack |first2=Ronald J. |last3=Bradley |first3=E. Todd |last4=Killen |first4=Rosemary M. |last5=Mouawad |first5=Nelly |last6=Sprague |first6=Ann L. |last7=Burger |first7=Matthew H. |last8=Solomon |first8=Sean C. |last9=Izenberg |first9=Noam R. |title=MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents |journal=Science |year=2009 |volume=324 |doi=10.1126/science.1172525 |pages=610–613 |bibcode=2009Sci...324..610M |pmid=19407195 |issue=5927 |s2cid=5578520 |display-authors=2 }}</ref> Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.<ref name="chaikin1" />

According to NASA, Mercury is not a suitable planet for Earth-like life. It has a [[exosphere|surface boundary exosphere]] instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions.<ref>{{cite web|url= https://solarsystem.nasa.gov/planets/mercury/in-depth/|title= Mercury|date= October 19, 2021|publisher= NASA|accessdate= July 4, 2022|archive-date= July 5, 2022|archive-url= https://web.archive.org/web/20220705191357/https://solarsystem.nasa.gov/planets/mercury/in-depth/|url-status= live}}</ref> Some parts of the subsurface of Mercury may have been [[Planetary habitability|habitable]], and perhaps [[life form]]s, albeit likely primitive [[microorganism]]s, may have existed on the planet.<ref name="NYT-20200324">{{cite news |last=Hall |first=Shannon |title=Life on the Planet Mercury? 'It's Not Completely Nuts' – A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability. |url=https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-url=https://web.archive.org/web/20200324150021/https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-date=March 24, 2020 |url-access=subscription |url-status=live |date=March 24, 2020 |work=[[The New York Times]] |access-date=March 26, 2020 }}</ref><ref name="SR-20200316">{{cite journal | last1=Rodriguez | first1=J. Alexis P. | last2=Leonard | first2=Gregory J. | last3=Kargel | first3=Jeffrey S. | last4=Domingue | first4=Deborah | last5=Berman | first5=Daniel C. | last6=Banks | first6=Maria | last7=Zarroca | first7=Mario | last8=Linares | first8=Rogelio | last9=Marchi | first9=Simone | last10=Baker | first10=Victor R. | last11=Webster | first11=Kevin D. | last12=Sykes | first12=Mark |title=The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System |date=March 16, 2020 |journal=[[Scientific Reports]] |volume=10 |issue=4737 |page=4737 |doi=10.1038/s41598-020-59885-5 |pmid=32179758 |pmc=7075900 |bibcode=2020NatSR..10.4737R }}</ref><ref>{{cite news | title=Vast Collapsed Terrains on Mercury Might be Windows Into Ancient – Possibly Habitable – Volatile-Rich Materials | work=Planetary Science Institute | date=March 16, 2020 | url=https://www.psi.edu/news/mercurychaos | access-date=August 27, 2022 | archive-date=August 28, 2022 | archive-url=https://web.archive.org/web/20220828041010/https://www.psi.edu/news/mercurychaos | url-status=live }}</ref>

=== Magnetic field and magnetosphere ===
{{Main|Mercury's magnetic field}}
[[File:Mercury Magnetic Field NASA.jpg|thumb|Graph showing relative strength of Mercury's magnetic field]]

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, [[magnetic field]]. According to measurements taken by {{nowrap|''Mariner 10''}}, it is about 1.1% the strength of [[Earth's magnetic field|Earth's]]. The magnetic-field strength at Mercury's equator is about {{nowrap|300 [[Tesla (unit)|nT]]}}.<ref>{{cite book |title=Astronomy: The Solar System and Beyond |first=Michael A. |last=Seeds |date=2004 |isbn=978-0-534-42111-3 |publisher=Brooks Cole |edition=4th}}</ref><ref>{{cite web |last=Williams |first=David R. |date=January 6, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |title=Planetary Fact Sheets |publisher=NASA National Space Science Data Center |access-date=August 10, 2006 |archive-date=September 25, 2008 |archive-url=https://web.archive.org/web/20080925071832/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |url-status=live }}</ref> Like that of Earth, Mercury's magnetic field is [[dipolar]]<ref name="chaikin1">{{cite book |first1=J. Kelly |last1=Beatty |last2=Petersen |first2=Carolyn Collins |last3=Chaikin |first3=Andrew |title=The New Solar System |date=1999 |publisher=Cambridge University Press |isbn=978-0-521-64587-4}}</ref> and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth).<ref name="qq">{{cite web |date=January 30, 2008 |url=https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |title=Mercury's Internal Magnetic Field |publisher=NASA |access-date=April 21, 2021 |archive-date=April 21, 2021 |archive-url=https://web.archive.org/web/20210421163118/https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |url-status=live }}</ref> Measurements from both the {{nowrap|''Mariner 10''}} and ''MESSENGER'' space probes have indicated that the strength and shape of the magnetic field are stable.<ref name="qq" />

It is likely that this magnetic field is generated by a [[Dynamo theory|dynamo]] effect, in a manner similar to the magnetic field of Earth.<ref name="cornell" /><ref>{{cite journal |last=Christensen |first=Ulrich R. |title=A deep dynamo generating Mercury's magnetic field |journal=Nature |year=2006 |volume=444 |pages=1056–1058 |doi=10.1038/nature05342 |pmid=17183319 |issue=7122 |bibcode=2006Natur.444.1056C |s2cid=4342216 |url=https://resolver.sub.uni-goettingen.de/purl?gro-2/65564 |access-date=October 29, 2023 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162201/https://publications.goettingen-research-online.de/handle/2/65564 |url-status=live }}</ref> This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong [[tidal heating]] effects caused by the planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect.<ref name="Spohn2001">{{cite journal |last1=Spohn |first1=Tilman |last2=Sohl |first2=Frank |last3=Wieczerkowski |first3=Karin |last4=Conzelmann |first4=Vera |title=The interior structure of Mercury: what we know, what we expect from BepiColombo |journal=Planetary and Space Science |volume=49 |issue=14–15 |pages=1561–1570 |doi=10.1016/S0032-0633(01)00093-9 |bibcode=2001P&SS...49.1561S |year=2001 }}</ref><ref>{{cite journal | title=The tides of Mercury and possible implications for its interior structure | last1=Padovan | first1=Sebastiano | last2=Margot | first2=Jean-Luc | last3=Hauck | first3=Steven A. | last4=Moore | first4=William B. | last5=Solomon | first5=Sean C. | journal=Journal of Geophysical Research: Planets | volume=119 | issue=4 | pages=850–866 | date=April 2014 | doi=10.1002/2013JE004459 | bibcode=2014JGRE..119..850P | s2cid=56282397 }}</ref>

Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth,<ref name="chaikin1" /> is strong enough to trap solar wind [[plasma (physics)|plasma]]. This contributes to the space weathering of the planet's surface.<ref name="qq" /> Observations taken by the {{nowrap|''Mariner 10''}} spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.<ref name="chaikin1" />

During its second flyby of the planet on October 6, 2008, ''MESSENGER'' discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to {{nowrap|800 km}} wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as [[flux transfer event]]s, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via [[magnetic reconnection]].<ref name="NASA060209" /> This also occurs in Earth's magnetic field. The ''MESSENGER'' observations showed the reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by ''MESSENGER''.<ref name="NASA060209">{{cite web |first=Bill |last=Steigerwald |date=June 2, 2009 |title=Magnetic Tornadoes Could Liberate Mercury's Tenuous Atmosphere |publisher=NASA Goddard Space Flight Center |url=http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |access-date=July 18, 2009 |archive-date=May 18, 2012 |archive-url=https://web.archive.org/web/20120518035510/http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |url-status=dead }}</ref>

== Orbit, rotation, and longitude ==
{{multiple image |direction=horizontal |align=right |total_width=400
|image1=ThePlanets Orbits Mercury PolarView.svg |caption1=Orbit of Mercury (2006)
|image2=Mercuryorbitsolarsystem.gif |caption2=Animation of Mercury's and Earth's revolution around the Sun
}}

Mercury has the most [[Orbital eccentricity|eccentric]] orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from {{convert|46000000|to|70000000|km|mi|abbr=on}}. It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same [[semi-major axis]]. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun.

This varying distance to the Sun leads to Mercury's surface being flexed by [[tidal bulge]]s raised by the [[Sun]] that are about 17 times stronger than the Moon's on Earth.<ref>{{cite journal |last1=Van Hoolst |first1=Tim |last2=Jacobs |first2=Carla |year=2003 |title=Mercury's tides and interior structure |journal=Journal of Geophysical Research |volume=108 |issue=E11 |page=7 |doi=10.1029/2003JE002126 |bibcode=2003JGRE..108.5121V|doi-access=free }}</ref> Combined with a 3:2 [[#Spin-orbit resonance|spin–orbit resonance]] of the planet's rotation around its axis, it also results in complex variations of the surface temperature.<ref name="strom">{{cite book |first1=Robert G. |last1=Strom |last2=Sprague |first2=Ann L. |date=2003 |title=Exploring Mercury: the iron planet |publisher=Springer |isbn=978-1-85233-731-5 |url=https://archive.org/details/exploringmercury00stro }}</ref> The resonance makes a single [[solar day]] (the length between two [[meridian (astronomy)|meridian]] transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.<ref name="compare">{{cite web |title=Space Topics: Compare the Planets: Mercury, Venus, Earth, The Moon, and Mars |publisher=Planetary Society |url=http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |access-date=April 12, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20110728044444/http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html |archive-date=July 28, 2011}}</ref>

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the [[ecliptic]]), the largest of all eight known solar planets.<ref name=Williams2019/> As a result, [[transits of Mercury]] across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average.<ref>{{cite web |last=Espenak |first=Fred |date=April 21, 2005 |url=http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |title=Transits of Mercury |publisher=NASA/Goddard Space Flight Center |access-date=May 20, 2008 |archive-date=August 29, 2015 |archive-url=https://archive.today/20150829155710/http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html |url-status=live }}</ref>

Mercury's [[axial tilt]] is almost zero,<ref name="Cosmic1" /> with the best measured value as low as 0.027 degrees.<ref name="Margot2007">{{cite journal |last1=Margot |first1=J. L. |display-authors=4 |last2=Peale |first2=S. J. |last3=Jurgens |first3=R. F. |last4=Slade |first4=M. A. |last5=Holin |first5=I. V. |title=Large Longitude Libration of Mercury Reveals a Molten Core |journal=Science |year=2007 |volume=316 |pages=710–714 |doi=10.1126/science.1140514 |bibcode=2007Sci...316..710M |pmid=17478713 |issue=5825|s2cid=8863681 }}</ref> This is significantly smaller than that of [[Jupiter]], which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 [[arcminutes]] above the horizon.<ref name="Margot2007" /> By comparison, the [[angular size]] of the Sun as seen from Mercury ranges from {{fraction|1|1|4}} to 2 degrees across.<ref>{{cite book | title=From The Sun To The Stars | first=James B. | last=Kaler | author-link=James B. Kaler | year=2016 | page=56 | isbn=9789813143265 | publisher=World Scientific Publishing Company | url=https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | access-date=October 25, 2023 | archive-date=October 31, 2023 | archive-url=https://web.archive.org/web/20231031140355/https://books.google.com/books?id=ZYv4DAAAQBAJ&pg=PA56 | url-status=live }}</ref>

At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same [[Extraterrestrial skies#Mercury|Mercurian day]].{{efn|name=angular}} This is because approximately four Earth days before perihelion, Mercury's angular [[orbital speed|orbital velocity]] equals its angular [[rotational velocity]] so that the Sun's [[improper motion|apparent motion]] ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a [[apparent retrograde motion|retrograde]] direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes.<ref name="strom" /> A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.<ref>{{cite book |title=Popular Astronomy: A Review of Astronomy and Allied Sciences |url=https://books.google.com/books?id=ePc-AQAAIAAJ |year=1896 |publisher=Goodsell Observatory of Carleton College |quote=although in the case of [[Venus]] the libration in longitude due to the eccentricity of the orbit amounts to only 47' on either side of the mean position, in the case of Mercury it amounts to 23° 39' |access-date=December 24, 2016 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162153/https://books.google.com/books?id=ePc-AQAAIAAJ |url-status=live }}</ref>

For the same reason, there are two points on Mercury's equator, 180 degrees apart in [[longitude]], at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to [[Diurnal temperature variation#Temperature lag|diurnal temperature lag]], at 0.4 Mercury days and 0.8 Mercury years past sunrise.<ref>{{cite web |last=Seligman |first=C. |title=The Rotation of Mercury |url=https://cseligman.com/text/planets/mercuryrot.htm |publisher=cseligman.com |at=NASA Flash animation |access-date=July 31, 2019 |archive-date=August 6, 2019 |archive-url=https://web.archive.org/web/20190806213722/http://cseligman.com/text/planets/mercuryrot.htm |url-status=live }}</ref> Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above.<ref>{{cite journal | title=On the Variations in the Insolation at Mercury Resulting from Oscillations of the Orbital Eccentricity | last=van Hemerlrijck | first=E. | journal=The Moon and the Planets | volume=29 | issue=1 | pages=83–93 | date=August 1983 | doi=10.1007/BF00928377 | bibcode=1983M&P....29...83V | s2cid=122761699 }}</ref>

Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average,<ref name="fact" /> but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as {{Convert|82200000|km|AU e6mi|abbr=in}} to Earth, and that is slowly declining: The next approach to within {{Convert|82100000|km|e6mi|abbr=unit|sigfig=2}} is in 2679, and to within {{Convert|82000000|km|e6mi|abbr=unit}} in 4487, but it will not be closer to Earth than {{Convert|80000000|km|e6mi|abbr=unit}} until 28,622.<ref>Mercury Closest Approaches to Earth generated with: <br /> 1. [http://chemistry.unina.it/~alvitagl/solex/ Solex 10]&nbsp; {{Webarchive|url=https://web.archive.org/web/20081220235836/http://chemistry.unina.it/~alvitagl/solex/ |date=December 20, 2008 }} ([http://home.surewest.net/kheider/astro/SolexMerc.txt Text Output file] {{webarchive|url=https://web.archive.org/web/20120309120624/http://home.surewest.net/kheider/astro/SolexMerc.txt |date=March 9, 2012 }}) <br /> 2. [http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 Gravity Simulator charts] {{Webarchive|url=https://web.archive.org/web/20140912091426/http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1235936812 |date=September 12, 2014 }} <br /> 3. [http://home.surewest.net/kheider/astro/Mercury.txt JPL Horizons 1950–2200]&nbsp; {{webarchive|url=https://web.archive.org/web/20151106172707/http://home.surewest.net/kheider/astro/Mercury.txt |date=November 6, 2015 }} {{noprint|(3 sources are provided to address [[WP:OR|original research]] concerns and to support general long-term trends) }}</ref> Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from the planet's high orbital eccentricity.<ref name="strom" /> Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth,<ref name="AIP Publishing 2019" /><ref name="MoreOrLess">{{cite web |last1=Harford |first1=Tim |title=BBC Radio 4 – More or Less, Sugar, Outdoors Play and Planets |url=https://www.bbc.co.uk/programmes/m0001y9p |publisher=BBC |date=January 11, 2019 |quote=Oliver Hawkins, more or less alumnus and statistical legend, wrote some code for us, which calculated which planet was closest to the Earth on each day for the past 50 years, and then sent the results to [[David A. Rothery]], professor of planetary geosciences at the Open University. |access-date=January 12, 2019 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112044935/https://www.bbc.co.uk/programmes/m0001y9p |url-status=live }}</ref> and—in that measure—it is the closest planet to each of the other planets in the Solar System.<ref name="AIP Publishing 2019">{{cite journal |last1=Stockman |first1=Tom |last2=Monroe |first2=Gabriel |last3=Cordner |first3=Samuel |title=Venus is not Earth's closest neighbor |journal=[[Physics Today]] |publisher=[[AIP Publishing]] |date=March 12, 2019 |issue=3 |article-number=30593 |doi=10.1063/PT.6.3.20190312a |bibcode=2019PhT..2019c0593. |issn=1945-0699 |s2cid=241077611}}</ref><ref>{{cite AV media |last1=Stockman |first1=Tom |date=March 7, 2019 |title=Mercury is the closest planet to all seven other planets |medium=video |url=https://www.youtube.com/watch?v=GDgbVIqGADQ | archive-url=https://ghostarchive.org/varchive/youtube/20211028/GDgbVIqGADQ| archive-date=October 28, 2021|access-date=May 29, 2019 |via=YouTube }}{{cbignore}}</ref><ref>{{Citation|title=🌍 Which Planet is the Closest?| date=October 30, 2019 |url=https://www.youtube.com/watch?v=SumDHcnCRuU| archive-url=https://ghostarchive.org/varchive/youtube/20211028/SumDHcnCRuU| archive-date=October 28, 2021|language=en|access-date=July 22, 2021}}{{cbignore}}</ref>{{efn|In astronomical literature, the term "closest planets" often means "the two planets that approach each other most closely". In other words, the orbits of the two planets approach each other most closely. However, this does ''not'' mean that the two planets are closest over a long period of time. For example, essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is "closest to Earth when averaged over time". However, it turns out that using this time-average definition of 'closeness', Mercury can be the "closest planet" to ''all'' other planets in the solar system.}}

=== Longitude convention ===
The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by {{nowrap|''Mariner 10''}}, this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called [[Hun Kal]], which provides the exact reference point for measuring longitude.<ref>{{cite journal | last=Davies | first=M. E. | title=Surface Coordinates and Cartography of Mercury | journal=Journal of Geophysical Research | volume=80 | issue=B17 | pages=2417–2430 | date=June 10, 1975 | doi=10.1029/JB080i017p02417 | bibcode=1975JGR....80.2417D }}</ref><ref>{{cite book | last1=Davies | first1=M. E. | first2=S. E. | last2=Dwornik | first3=D. E. | last3=Gault | first4=R. G. | last4=Strom | title=NASA Atlas of Mercury | publisher=NASA Scientific and Technical Information Office | date=1978 }}</ref> The center of Hun Kal defines the 20°&nbsp;west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury.<ref name="usgs">{{cite web |url=https://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |access-date=October 22, 2009 |title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE) |archive-url=https://web.archive.org/web/20111024101856/http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html |archive-date=October 24, 2011 |url-status=dead}}</ref> The two hottest places on the equator are therefore at longitudes 0°&nbsp;W and 180°&nbsp;W, and the coolest points on the equator are at longitudes 90°&nbsp;W and 270°&nbsp;W. However, the ''MESSENGER'' project uses an east-positive convention.<ref name="ArchinalA'Hearn2010">{{cite journal |last1=Archinal |first1=Brent A. |display-authors=4 |last2=A'Hearn |first2=Michael F. |last3=Bowell |first3=Edward L. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Courtin |first6=Régis |last7=Fukushima |first7=Toshio |last8=Hestroffer |first8=Daniel |last9=Hilton |first9=James L. |last10=Krasinsky |first10=George A. |last11=Neumann |first11=Gregory A. |last12=Oberst |first12=Jürgen |last13=Seidelmann |first13=P. Kenneth |last14=Stooke |first14=Philip J. |last15=Tholen |first15=David J. |last16=Thomas |first16=Peter C. |last17=Williams |first17=Iwan P. |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009 |journal=Celestial Mechanics and Dynamical Astronomy |volume=109 |issue=2 |year=2010 |pages=101–135 |issn=0923-2958 |doi=10.1007/s10569-010-9320-4 |bibcode=2011CeMDA.109..101A|s2cid=189842666 }}</ref>

=== Spin-orbit resonance ===
[[File:Mercury's orbital resonance.svg|thumb|After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.]]

For many years it was thought that Mercury was synchronously [[tidally locked]] with the Sun, [[rotating]] once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky.<ref>{{cite journal |last1=Liu |first1=Han-Shou |last2=O'Keefe |first2=John A. |title=Theory of Rotation for the Planet Mercury |journal=Science |year=1965 |volume=150 |issue=3704 |page=1717 |doi=10.1126/science.150.3704.1717 |pmid=17768871 |bibcode=1965Sci...150.1717L|s2cid=45608770 }}</ref>

The 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution.<ref name="Colombo" /> In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.<ref name="Colombo" />

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days.<ref name="strom" /> A [[sidereal day]] (the period of rotation) lasts about 58.7 Earth days.<ref name="strom" />

Simulations indicate that the orbital eccentricity of Mercury varies [[chaos theory|chaotically]] from nearly zero (circular) to more than 0.45 over millions of years due to [[Perturbation (astronomy)|perturbations]] from the other planets.<ref name="strom" /><ref name="Correia2009">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |title=Mercury's capture into the 3/2 spin-orbit resonance including the effect of core–mantle friction |journal=Icarus |year=2009 |doi=10.1016/j.icarus.2008.12.034 |arxiv=0901.1843 |volume=201 |issue=1 |pages=1–11 |bibcode=2009Icar..201....1C|s2cid=14778204 }}</ref> This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity.<ref name="Correia">{{cite journal |last1=Correia |first1=Alexandre C. M. |last2=Laskar |first2=Jacques |year=2004 |title=Mercury's capture into the 3/2 spin-orbit resonance as a result of its chaotic dynamics |journal=[[Nature (journal)|Nature]] |volume=429 |pages=848–850 |doi=10.1038/nature02609 |pmid=15215857 |issue=6994 |bibcode=2004Natur.429..848C|s2cid=9289925 }}</ref> However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.<ref>{{Cite journal |bibcode=2014Icar..241...26N |last1=Noyelles |first1=B. |last2=Frouard |first2=J. |last3=Makarov |first3=V. V. |last4=Efroimsky |first4=M. |name-list-style=amp |title=Spin-orbit evolution of Mercury revisited |journal=Icarus |pages=26–44 |year=2014 |volume=241 |issue=2014 |doi=10.1016/j.icarus.2014.05.045 |arxiv=1307.0136|s2cid=53690707 }}</ref>

Numerical simulations show that a future [[Secular resonance|secular]] [[Orbital resonance|orbital resonant]] interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.<ref name="Laskar2008">{{cite journal |last=Laskar |first=Jacques |date=March 18, 2008 |title=Chaotic diffusion in the Solar System |journal=[[Icarus (journal)|Icarus]] |volume=196 |issue=1 |pages=1–15 |bibcode=2008Icar..196....1L |doi=10.1016/j.icarus.2008.02.017 |arxiv=0802.3371|s2cid=11586168 }}</ref><ref name="Laskar2009">{{cite journal |last1=Laskar |first1=Jacques |last2=Gastineau |first2=Mickaël |date=June 11, 2009 |title=Existence of collisional trajectories of Mercury, Mars and Venus with the Earth |journal=[[Nature (journal)|Nature]] |volume=459 |issue=7248 |pages=817–819 |doi=10.1038/nature08096 |bibcode=2009Natur.459..817L |pmid=19516336|s2cid=4416436 }}</ref>

=== Advance of perihelion ===
{{Main|Perihelion precession of Mercury}}
[[File:Drehung der Apsidenlinie light.svg|right|thumb|[[Apsidal precession]] of Mercury's orbit]]
In 1859, the French mathematician and astronomer [[Urbain Le Verrier]] reported that the slow [[precession]] of Mercury's orbit around the Sun could not be completely explained by [[Newtonian mechanics]] and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.<ref>{{cite journal | last=Le Verrier | first=Urbain | year=1859 | language=French | url=https://archive.org/stream/comptesrendusheb49acad#page/378/mode/2up | title=Lettre de M. Le Verrier à M. Faye sur la théorie de Mercure et sur le mouvement du périhélie de cette planète | journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences | publication-place=Paris | volume=49 | pages=379–383 }} (At p. 383 in the same volume Le Verrier's report is followed by another, from Faye, enthusiastically recommending to astronomers to search for a previously undetected intra-mercurial object.)</ref> Other explanations considered included a slight oblateness of the Sun. The success of the search for [[Neptune]] based on its perturbations of the orbit of [[Uranus]] led astronomers to place faith in this possible explanation, and the hypothetical planet was named [[Vulcan (hypothetical planet)|Vulcan]], but no such planet was ever found.<ref>{{cite book |first1=Richard |last1=Baum |last2=Sheehan |first2=William |title=In Search of Planet Vulcan, The Ghost in Newton's Clockwork Machine |date=1997 |isbn=978-0-306-45567-4 |publisher=Plenum Press |location=New York |url-access=registration |url=https://archive.org/details/insearchofplanet0000baum }}</ref>

The observed [[perihelion precession]] of Mercury is 5,600 [[arcseconds]] (1.5556°) per century relative to Earth, or {{val|574.10|0.65|u=arcseconds}} per century<ref name="Clemence">{{cite journal |first=Gerald M. |last=Clemence |title=The Relativity Effect in Planetary Motions |journal=Reviews of Modern Physics |volume=19 |issue=4 |pages=361–364 |year=1947 |doi=10.1103/RevModPhys.19.361 |bibcode=1947RvMP...19..361C}}</ref> relative to the inertial [[International Celestial Reference Frame|ICRF]]. Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or {{val|531.63|0.69|u=arcseconds}} per century relative to ICRF.<ref name="Clemence" /> In the early 20th century, [[Albert Einstein]]'s [[general theory of relativity]] provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just {{val|42.980|0.001|u=arcseconds}} per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for [[1566 Icarus]].<ref>{{cite journal |last=Gilvarry |first=John J. |title=Relativity Precession of the Asteroid Icarus |journal=Physical Review |year=1953 |volume=89 |issue=5 |page=1046 |doi=10.1103/PhysRev.89.1046 |bibcode=1953PhRv...89.1046G}}</ref><ref>{{cite web |first=Kevin |last=Brown |url=http://www.mathpages.com/rr/s6-02/6-02.htm |title=6.2 Anomalous Precession |website=Reflections on Relativity |publisher=MathPages |access-date=May 22, 2008 |archive-date=August 3, 2019 |archive-url=https://web.archive.org/web/20190803235349/https://www.mathpages.com/rr/s6-02/6-02.htm |url-status=live }}</ref>

== Observation ==
[[File:Mercury.jpg|thumb|Image mosaic by {{nowrap|''Mariner 10''}}, 1974]]
Mercury's [[apparent magnitude]] is calculated to vary between −2.48 (brighter than [[Sirius]]) around [[superior conjunction]] and +7.25 (below the limit of naked-eye visibility) around [[inferior conjunction]].<ref name="Mallama_and_Hilton" /> The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93.<ref name="Mallama_and_Hilton" /> Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.<ref name=Menzel1964P292-293>{{cite book |last=Menzel |first=Donald H. |author-link=Donald Howard Menzel |title=A Field Guide to the Stars and Planets |date=1964 |series=[[Peterson Field Guides|The Peterson Field Guide Series]] |location=Boston |publisher=[[Houghton Mifflin Co.]] |pages=292–293}}</ref>

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The [[Hubble Space Telescope]] cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun.<ref>{{cite journal |last1=Baumgardner |first1=Jeffrey |last2=Mendillo |first2=Michael |last3=Wilson |first3=Jody K. |title=A Digital High-Definition Imaging System for Spectral Studies of Extended Planetary Atmospheres. I. Initial Results in White Light Showing Features on the Hemisphere of Mercury Unimaged by Mariner&nbsp;10 |journal=The Astronomical Journal |year=2000 |volume=119 |issue=5 |pages=2458–2464 |doi=10.1086/301323 |bibcode=2000AJ....119.2458B|s2cid=17361371 |doi-access=free }}</ref> Because the shift of 0.15 revolutions of Earth in a Mercurian year makes up a seven-Mercurian-year cycle (0.15 × 7 ≈ 1.0), in the seventh Mercurian year, Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven Mercurian years before.<ref name="elongation" />

Like the Moon and Venus, Mercury exhibits [[Lunar phase|phases]] as seen from Earth. It is "new" at [[inferior conjunction]] and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun,<ref name=Menzel1964P292-293/> except at its new phase during a transit. Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the [[opposition effect|opposition brightness surge]] more than compensates for the distance.<ref name="MallamaSky" /> The opposite is true for Venus, which appears brightest when it is a [[crescent]], because it is much closer to Earth than when [[gibbous]].<ref name="MallamaSky" /><ref>{{cite web | last=Espenak | first=Fred | year=1996 | url=http://eclipse.gsfc.nasa.gov/TYPE/venus2.html | title=NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995–2006 | website=Twelve Year Planetary Ephemeris Directory | publisher=NASA | access-date=May 24, 2008 | archive-date=August 17, 2000 | archive-url=https://web.archive.org/web/20000817181616/http://sunearth.gsfc.nasa.gov/eclipse/TYPE/venus2.html | url-status=live }}</ref>
[[File:PIA19247-Mercury-NPolarRegion-Messenger20150316.jpg|thumb|left|False-color map showing the maximum temperatures of the north polar region]]

[[File:Mercury Venus Moon over San Jose 08 Jan 2024.png|thumb|right|Mercury (lower left) as seen from [[San Jose, California]] with Venus and the Moon.]]

Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest [[elongation (astronomy)|elongation]] east and west of the Sun, respectively. At both of these times, Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.<ref name="elongation">{{cite web |title=Mercury Chaser's Calculator |publisher=Fourmilab Switzerland |first=John |last=Walker |url=http://www.fourmilab.ch/images/3planets/elongation.html |access-date=May 29, 2008 |archive-date=August 2, 2009 |archive-url=https://web.archive.org/web/20090802110601/http://www.fourmilab.ch/images/3planets/elongation.html |url-status=live }} (look at 1964 and 2013)</ref><ref name="MercHorizons">{{cite web |title=Mercury Elongation and Distance |url=http://home.surewest.net/kheider/astro/Mercury.txt |access-date=May 30, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130511073623/http://home.surewest.net/kheider/astro/Mercury.txt |archive-date=May 11, 2013 }} – Numbers generated using the Solar System Dynamics Group, [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 Horizons On-Line Ephemeris System] {{Webarchive|url=https://web.archive.org/web/20150707172324/http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=1 |date=July 7, 2015 }}</ref> At greatest ''western'' elongation, Mercury rises at its earliest before sunrise, and at greatest ''eastern'' elongation, it sets at its latest after sunset.<ref name="RASC2007">{{cite book |title=Observer's Handbook 2007 |editor-first=Patrick |editor-last=Kelly |publisher=[[Royal Astronomical Society of Canada]] |date=2007 |isbn=978-0-9738109-3-6 |url=https://archive.org/details/observershandboo00raji_0 }}</ref>

[[File:PIA19422-Mercury-CarnegieRupes-MDIS-MLA-20150416.jpg|thumb|right|False-color image of [[Carnegie Rupes]], a tectonic landform—high terrain (red); low (blue).]]

Mercury is more often and easily visible from the [[Southern Hemisphere]] than from the [[Northern Hemisphere|Northern]]. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere.<ref name="RASC2007" /> In both of these cases, the angle at which the planet's orbit intersects the horizon is maximized, allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid-latitudes, such as Argentina and South Africa.<ref name="RASC2007" />

An alternate method for viewing Mercury involves observing the planet with a [[telescope]] during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This allows the planet to be found easily, even when using telescopes with {{Convert|8|cm|abbr = on}} apertures. However, great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage.<ref>{{cite journal | title=Finding Venus or Mercury in daylight | last=Curtis | first=A. C. | journal=Journal of the British Astronomical Association | volume=82 | pages=438–439 | date=October 1972 | bibcode=1972JBAA...82..438C }}</ref> This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings). The planet is higher in the sky and less atmospheric effects affect the view of the planet. Mercury can be viewed as close as 4° to the Sun near superior conjunction when it is almost at its brightest.

Mercury can, like several other planets and the brightest stars, be seen during a total [[solar eclipse]].<ref name="eclipse">{{cite web |date=January 22, 2003 |title=Total Solar Eclipse of 2006 March 29 |publisher=Department of Physics at Fizik Bolumu in Turkey |first=Tunç |last=Tezel |url=http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |access-date=May 24, 2008 |archive-date=September 12, 2016 |archive-url=https://web.archive.org/web/20160912153809/http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html |url-status=live }}</ref>

== Observation history ==

=== Ancient astronomers ===
[[File:Mercury-bonatti.png|thumb|Mercury, from ''Liber astronomiae'', 1550]]
The earliest known recorded observations of Mercury are from the [[MUL.APIN]] tablets. These observations were most likely made by an [[Assyria]]n astronomer around the 14th century BC.<ref>{{cite journal |title=The Latitude and Epoch for the Origin of the Astronomical Lore in MUL.APIN |first=Bradley E. |last=Schaefer |journal=American Astronomical Society Meeting 210, #42.05 |year=2007 |volume=38 |bibcode=2007AAS...210.4205S |page=157}}</ref> The [[cuneiform]] name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U<sub>4</sub>.UD ("the jumping planet").{{efn|name=Cuneiform MUL}}<ref>{{cite journal |first1=Hermann |last1=Hunger |last2=Pingree |first2=David |title=MUL.APIN: An Astronomical Compendium in Cuneiform |journal=Archiv für Orientforschung |volume=24 |year=1989 |page=146}}</ref> [[Babylonian astronomy|Babylonian records]] of Mercury date back to the 1st millennium BC. The [[Babylonia]]ns called the planet [[Nabu]] after the messenger to the gods in [[Babylonian mythology|their mythology]].<ref name="JHU history">{{cite web |year=2008 |url=http://btc.montana.edu/messenger/elusive_planet/ancient_cultures_2.php |title=MESSENGER: Mercury and Ancient Cultures |publisher=NASA JPL |access-date=April 7, 2008 |archive-date=July 23, 2012 |archive-url=https://web.archive.org/web/20120723001225/http://www.messenger-education.org/elusive_planet/ancient_cultures_2.php |url-status=live }}</ref>

The [[Greeks in Egypt|Greco]]-[[Ancient Egypt|Egyptian]]<ref name="Heath1921">{{Cite book|title=A History of Greek Mathematics|last=Heath|first=Sir Thomas|publisher=Clarendon Press |year=1921|location=Oxford|pages=vii, 273|volume=II|url=https://archive.org/stream/historyofgreekma029268mbp/historyofgreekma029268mbp_djvu.txt}}</ref> astronomer [[Ptolemy]] wrote about the possibility of planetary transits across the face of the Sun in his work ''Planetary Hypotheses''. He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because transits were too infrequent.<ref>{{cite journal |last=Goldstein |first=Bernard R. |title=The Pre-telescopic Treatment of the Phases and Apparent Size of Venus |journal=Journal for the History of Astronomy |page=1 |year=1996 |bibcode=1996JHA....27....1G |volume=27|doi=10.1177/002182869602700101 |s2cid=117218196 }}</ref>

[[File:Shatir500.jpg|thumb|left|[[Ibn al-Shatir]]'s model for the appearances of Mercury, showing the multiplication of [[epicycles]] using the [[Tusi couple]], thus eliminating the Ptolemaic eccentrics and [[equant]].]]

In [[ancient China]], Mercury was known as "the Hour Star" (''Chen-xing'' {{lang|zh|辰星}}). It was associated with the direction north and the phase of water in the [[Five Phases]] system of metaphysics.<ref>{{cite book |first1=David H. |last1=Kelley |last2=Milone |first2=E. F. |last3=Aveni |first3=Anthony F. |date=2004 |title=Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy |publisher=Birkhäuser |isbn=978-0-387-95310-6}}</ref> Modern [[Chinese culture|Chinese]], [[Korean culture|Korean]], [[Japanese culture|Japanese]] and [[Vietnamese culture|Vietnamese]] cultures refer to the planet literally as the "water star" ({{lang|zh|水星}}), based on the [[Five elements (Chinese philosophy)|Five elements]].<ref>{{cite book |first=Jan Jakob Maria |last=De Groot |year=1912 |chapter=Religion in China: universism. a key to the study of Taoism and Confucianism |title=American lectures on the history of religions |volume=10 |page=300 |publisher=G. P. Putnam's Sons |chapter-url=https://books.google.com/books?id=ZAaP7dyjCrAC&pg=PA300 |access-date=January 8, 2010 |archive-date=February 26, 2024 |archive-url=https://web.archive.org/web/20240226150305/https://books.google.com/books?id=ZAaP7dyjCrAC&pg=PA300#v=onepage&q&f=false |url-status=live }}</ref><ref>{{cite book |first=Thomas |last=Crump |year=1992 |title=The Japanese numbers game: the use and understanding of numbers in modern Japan |pages=39–40 |publisher=Routledge |isbn=978-0-415-05609-0}}</ref><ref>{{cite book |first=Homer Bezaleel |last=Hulbert |year=1909 |title=The passing of Korea |page=[https://archive.org/details/passingkorea01hulbgoog/page/n538 426] |publisher=Doubleday, Page & company |url=https://archive.org/details/passingkorea01hulbgoog |access-date=January 8, 2010}}</ref> [[Hindu mythology]] used the name [[Budha]] for Mercury, and this god was thought to preside over Wednesday.<ref>{{cite book |first1=R. M. |last1=Pujari |last2=Kolhe |first2=Pradeep |last3=Kumar |first3=N. R. |date=2006 |title=Pride of India: A Glimpse Into India's Scientific Heritage |publisher=Samskrita Bharati |isbn=978-81-87276-27-2}}</ref> The god [[Odin]] (or Woden) of [[Germanic paganism]] was associated with the planet Mercury and Wednesday.<ref>{{cite book |first=Michael E. |last=Bakich |date=2000 |title=The Cambridge Planetary Handbook |publisher=Cambridge University Press |isbn=978-0-521-63280-5 |url=https://archive.org/details/cambridgeplaneta00baki }}</ref> The [[Maya civilization|Maya]] may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the [[underworld]].<ref>{{cite book |first=Susan |last=Milbrath |date=1999 |title=Star Gods of the Maya: Astronomy in Art, Folklore and Calendars |publisher=University of Texas Press |isbn=978-0-292-75226-9}}</ref> Mercury was sometimes known as '''[[Stilbon (mythology)|Stilbon]]''' ([[Greek language|Greek]]: Στίλβων) meaning 'the shining, glittering'.<ref>{{Cite web |title=Cicero: De Natura Deorum II |url=https://www.thelatinlibrary.com/cicero/nd2.shtml#51 |access-date=2025-02-13 |website=www.thelatinlibrary.com |language=Latin |quote=Huic autem proximum inferiorem orbem tenet PuroeiV, quae stella Martis appellatur, eaque quattuor et viginti mensibus sex, ut opinor, diebus minus eundem lustrat orbem quem duae superiores, infra hanc autem stella Mercuri est (ea Stilbwn appellatur a Graecis), quae anno fere vertenti signiferum lustrat orbem neque a sole longius umquam unius signi intervallo discedit tum antevertens tum subsequens. |trans-quote=But next to this is the lower planet, Puroei, which is called the star of Mars, and it surveys the same planet in twenty-four months and six days, as I believe, less than the two upper planets. Below this is the star of Mercury (it is called Stilbon by the Greeks), which surveys the planet in about a year, and never departs from the sun further than one sign, both in its preceding and following directions.}}</ref>

In [[medieval Islamic astronomy]], the [[Al-Andalus|Andalusian]] astronomer [[Abū Ishāq Ibrāhīm al-Zarqālī]] in the 11th century described the [[deferent]] of Mercury's [[geocentric orbit]] as being oval, like an egg or a [[Pine nut|pignon]], although this insight did not influence his astronomical theory or his astronomical calculations.<ref>{{cite journal |last1=Samsó |first1=Julio |last2=Mielgo |first2=Honorino |bibcode=1994JHA....25..289S |title=Ibn al-Zarqālluh on Mercury |journal=Journal for the History of Astronomy |volume=25 |issue=4 |year=1994 |pages=289–96 [292]|doi=10.1177/002182869402500403 |s2cid=118108131 }}</ref><ref>{{cite journal |first=Willy |last=Hartner |title=The Mercury Horoscope of Marcantonio Michiel of Venice |journal=Vistas in Astronomy |volume=1 |issue=1 |year=1955 |pages=84–138 |bibcode=1955VA......1...84H |doi=10.1016/0083-6656(55)90016-7}} at pp. 118–122.</ref> In the 12th century, [[Ibn Bajjah]] observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the [[Maragheh observatory|Maragha]] astronomer [[Qotb al-Din Shirazi]] in the 13th century.<ref>{{cite conference |title=History of oriental astronomy: proceedings of the joint discussion-17 at the 23rd General Assembly of the International Astronomical Union, organised by the Commission 41 (History of Astronomy), held in Kyoto, August 25–26, 1997 |first=S. M. Razaullah |last=Ansari |publisher=[[Springer Science+Business Media]] |year=2002 |isbn=1-4020-0657-8 |page=137}}</ref> Most such medieval reports of transits were later taken as observations of [[sunspot]]s.<ref>{{cite journal |last=Goldstein |first=Bernard R. |year=1969 |title=Some Medieval Reports of Venus and Mercury Transits |journal=Centaurus |volume=14 |issue=1 |pages=49–59 |doi=10.1111/j.1600-0498.1969.tb00135.x |bibcode=1969Cent...14...49G}}</ref>

In India, the [[Kerala school of astronomy and mathematics|Kerala school]] astronomer [[Nilakantha Somayaji]] in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the [[Tychonic system]] later proposed by [[Tycho Brahe]] in the late 16th century.<ref>{{cite journal |last1=Ramasubramanian |first1=K. |last2=Srinivas |first2=M. S. |last3=Sriram |first3=M. S. |title=Modification of the Earlier Indian Planetary Theory by the Kerala Astronomers (c. 1500 AD) and the Implied Heliocentric Picture of Planetary Motion |journal=Current Science |volume=66 |year=1994 |pages=784–790 |url=http://www.physics.iitm.ac.in/~labs/amp/kerala-astronomy.pdf |access-date=April 23, 2010 |archive-url=https://web.archive.org/web/20101223145939/http://www.physics.iitm.ac.in/~labs/amp/kerala-astronomy.pdf |archive-date=December 23, 2010 |url-status=dead }}</ref>

=== Ground-based telescopic research ===
The first telescopic observations of Mercury were made by [[Thomas Harriot]] and [[Galileo]] from 1610. In 1612, [[Simon Marius]] observed the brightness of Mercury varied with the planet's orbital position and concluded it had phases "in the same way as Venus and the Moon".<ref>{{cite book |last=Gaab |first=Hans |date=2018 |title=Simon Marius and His Research |url=https://books.google.com/books?id=miaeDwAAQBAJ |publisher=Springer |page=256 |isbn=978-3-319-92620-9 |quote=Marius noted in the dedication from June 30, 1612, in the Prognosticon auf 1613 "that Mercury is illuminated by the Sun in the same way as the Venus and the Moon" and reports his observations of the brightness. |access-date=March 22, 2023 |archive-date=April 11, 2023 |archive-url=https://web.archive.org/web/20230411141118/https://books.google.com/books?id=miaeDwAAQBAJ |url-status=live }}</ref> In 1631, [[Pierre Gassendi]] made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by [[Johannes Kepler]]. In 1639, [[Giovanni Zupi]] used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited the Sun.<ref name="strom" />

A rare event in astronomy is the passage of one planet in front of another ([[occultation]]), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737, is the only one historically observed, having been seen by [[John Bevis]] at the [[Royal Greenwich Observatory]].<ref>{{cite journal |last1=Sinnott |first1=Roger W. |author-link=Roger W. Sinnott |last2=Meeus |first2=Jean |author-link2=Jean Meeus |year=1986 |title=John Bevis and a Rare Occultation |journal=Sky and Telescope |volume=72 |page=220 |bibcode=1986S&T....72..220S}}</ref> The next occultation of Mercury by Venus will be on December 3, 2133.<ref>{{cite book |first=Timothy |last=Ferris |date=2003 |title=Seeing in the Dark: How Amateur Astronomers |publisher=Simon and Schuster |isbn=978-0-684-86580-5}}</ref>

The difficulties inherent in observing Mercury meant that it was far less studied than the other planets. In 1800, [[Johann Schröter]] made observations of surface features, claiming to have observed {{Convert|20|km|4 = -high|adj = mid}} mountains. [[Friedrich Bessel]] used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.<ref name="sao188r">{{cite journal |last1=Colombo |first1=Giuseppe |author-link1=Giuseppe Colombo |last2=Shapiro |first2=Irwin I. |author-link2=Irwin I. Shapiro |title=The Rotation of the Planet Mercury |journal=SAO Special Report #188R |bibcode=1965SAOSR.188.....C |volume=188 |pages=188 |date=November 1965}}</ref> In the 1880s, [[Giovanni Schiaparelli]] mapped the planet more accurately, and suggested that Mercury's rotational period was 88 days, the same as its orbital period due to tidal locking.<ref>{{cite journal |last=Holden |first=Edward S. |year=1890 |title=Announcement of the Discovery of the Rotation Period of Mercury [by Professor Schiaparelli] |journal=Publications of the Astronomical Society of the Pacific |volume=2 |issue=7 |page=79 |bibcode=1890PASP....2...79H |doi=10.1086/120099|s2cid=122095054 |doi-access=free }}</ref> This phenomenon is known as [[synchronous rotation]]. The effort to map the surface of Mercury was continued by [[Eugenios Antoniadi]], who published a book in 1934 that included both maps and his own observations.<ref name="chaikin1" /> Many of the planet's surface features, particularly the [[List of albedo features on Mercury|albedo features]], take their names from Antoniadi's map.<ref>{{cite book | url=https://history.nasa.gov/SP-423/sp423.htm | title=Atlas of Mercury | publisher=[[NASA]] Office of Space Sciences | first1=Merton E. | last1=Davies | first2=Stephen E. | last2=Dwornik | first3=Donald E. | last3=Gault | first4=Robert G. | last4=Strom | date=1978 | chapter=Surface Mapping | chapter-url=https://history.nasa.gov/SP-423/surface.htm | access-date=May 28, 2008 | archive-date=October 9, 2019 | archive-url=https://web.archive.org/web/20191009033507/https://history.nasa.gov/SP-423/sp423.htm | url-status=live }}</ref>

In June 1962, Soviet scientists at the [[Institute of Radio-engineering and Electronics]] of the [[USSR Academy of Sciences]], led by [[Vladimir Kotelnikov]], became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet.<ref>{{cite journal |first1=John V. |last1=Evans |author-link=John V. Evans (astronomer) |last2=Brockelman |first2=Richard A. |last3=Henry |first3=John C. |last4=Hyde |first4=Gerald M. |last5=Kraft |first5=Leon G. |last6=Reid |first6=Wyatt A. |last7=Smith |first7=W. W. |title=Radio Echo Observations of Venus and Mercury at 23&nbsp;cm Wavelength |year=1965 |journal=Astronomical Journal |volume=70 |bibcode=1965AJ.....70..486E |pages=487–500 |doi=10.1086/109772}}</ref><ref>{{cite book |last=Moore |first=Patrick |title=The Data Book of Astronomy |page=483 |date=2000 |publisher=CRC Press |location=New York |url=https://books.google.com/books?id=fDDpBwAAQBAJ |isbn=978-0-7503-0620-1 |access-date=February 27, 2018 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162216/https://books.google.com/books?id=fDDpBwAAQBAJ |url-status=live }}</ref><ref>{{cite book |title=To See the Unseen: A History of Planetary Radar Astronomy |url=https://archive.org/details/toseeunseenhis00butr |first=Andrew J. |last=Butrica |publisher=[[NASA]] History Office, Washington D.C. |date=1996 |chapter=Chapter 5 |chapter-url=https://history.nasa.gov/SP-4218/ch5.htm |isbn=978-0-16-048578-7 }}</ref> Three years later, radar observations by Americans [[Gordon H. Pettengill]] and Rolf B. Dyce, using the {{Convert|300|m|yd|4=-wide|adj=mid}} [[Arecibo radio telescope]] in [[Puerto Rico]], showed conclusively that the planet's rotational period was about 59 days.<ref>{{cite journal |last1=Pettengill |first1=Gordon H. |last2=Dyce |first2=Rolf B. |title=A Radar Determination of the Rotation of the Planet Mercury |journal=[[Nature (journal)|Nature]] |volume=206 |issue=1240 |pages=451–2 |year=1965 |doi=10.1038/2061240a0 |bibcode=1965Natur.206Q1240P|s2cid=31525579 }}</ref><ref>{{cite web | url=http://scienceworld.wolfram.com/astronomy/Mercury.html | title=Mercury | website=Eric Weisstein's World of Astronomy | publisher=Wolfram Research | access-date=April 18, 2021 | archive-date=November 6, 2015 | archive-url=https://web.archive.org/web/20151106173823/http://scienceworld.wolfram.com/astronomy/Mercury.html | url-status=live }}</ref> The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.<ref>{{cite book |first1=Bruce C. |last1=Murray |last2=Burgess |first2=Eric |date=1977 |title=Flight to Mercury |publisher=Columbia University Press |isbn=978-0-231-03996-3 |url=https://archive.org/details/flighttomercury00bruc }}</ref>

In 1965, Italian astronomer [[Giuseppe Colombo]] noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.<ref>{{cite journal |last=Colombo |first=Giuseppe |title=Rotational Period of the Planet Mercury |journal=Nature |volume=208 |issue=5010 |page=575 |year=1965 |doi=10.1038/208575a0 |bibcode=1965Natur.208..575C|s2cid=4213296 |doi-access=free }}</ref> Data from {{nowrap|''Mariner 10''}} subsequently confirmed this view.<ref name="AtlasM10" /> This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every ''second'' orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.<ref name="sao188r" />

Ground-based optical observations did not shed much further light on Mercury, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters.<ref>{{cite thesis | last=Golden | first=Leslie M. | title=A Microwave Interferometric Study of the Subsurface of the Planet Mercury |year=1977 | publisher=University of California, Berkeley | bibcode=1977PhDT.........9G }}</ref><ref>{{cite journal |last1=Mitchell |first1=David L. |last2=De Pater |first2=Imke |title=Microwave Imaging of Mercury's Thermal Emission at Wavelengths from 0.3 to 20.5&nbsp;cm (1994) |journal=Icarus |volume=110 |issue=1 |pages=2–32 |doi=10.1006/icar.1994.1105 |bibcode=1994Icar..110....2M |year=1994}}</ref> Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, technological advances have led to improved ground-based observations. In 2000, high-resolution [[lucky imaging]] observations were conducted by the [[Mount Wilson Observatory]] {{Convert|1.5|m|ft|abbr=out|adj=mid}} Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the {{nowrap|''Mariner 10''}} mission.<ref>{{cite journal |last1=Dantowitz |first1=Ronald F. |last2=Teare |first2=Scott W. |last3=Kozubal |first3=Marek J. |title=Ground-based High-Resolution Imaging of Mercury |journal=Astronomical Journal |volume=119 |issue=4 |pages=2455–2457 |year=2000 |bibcode=2000AJ....119.2455D |doi=10.1086/301328|s2cid=121483006 |doi-access=free }}</ref> Most of the planet has been mapped by the Arecibo radar telescope, with {{Convert|5|km|abbr = on|adj = on}} resolution, including polar deposits in shadowed craters of what may be water ice.<ref name="Harm06">{{cite journal |title=Mercury: Radar images of the equatorial and midlatitude zones |journal=Icarus |volume=187 |issue=2 |pages=374–405 |year=2007 |bibcode=2007Icar..187..374H |doi=10.1016/j.icarus.2006.09.026 |last1=Harmon |first1=John K. |first2=Martin A. |last2=Slade |first3=Bryan J. |last3=Butler |first4=James W. |last4=Head III |first5=Melissa S. |last5=Rice |first6=Donald B. |last6=Campbell}}</ref>

<gallery widths="200px" heights="200px">
File:Transit Of Mercury, May 9th, 2016.png|Transit of Mercury. Mercury is visible as a black dot below and to the left of center. The dark area above the center of the solar disk is a [[sunspot]].
File:Planet Elongation.jpg|[[Elongation (astronomy)|Elongation]] is the angle between the Sun and the planet, with Earth as the reference point. Mercury appears close to the Sun.
File:PIA19411-Mercury-WaterIce-Radar-MDIS-Messenger-20150416.jpg|Water ice (yellow) at Mercury's north polar region
</gallery>

=== Research with space probes ===
{{Main|Exploration of Mercury}}
[[File:MESSENGER Assembly.jpg|thumb|left|''MESSENGER'' being prepared for launch]]
[[File:PIA18389-MarsCuriosityRover-MercuryTransitsSun-20140603.gif|thumb|Mercury transiting the [[Sun]] as viewed by the Mars rover [[Curiosity (rover)|''Curiosity'']] (June 3, 2014).<ref name="NASA-20140610">{{cite web |last=Webster |first=Guy |title=Mercury Passes in Front of the Sun, as Seen From Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-183 |date=June 10, 2014 |website=[[NASA]] |access-date=June 10, 2014 |archive-date=February 15, 2020 |archive-url=https://web.archive.org/web/20200215091810/https://www.jpl.nasa.gov/news/news.php?release=2014-183 |url-status=live }}</ref>]]

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over {{Convert|91|e6km|e6mi|abbr = off}} into the Sun's gravitational [[potential well]]. Mercury has an [[orbital speed]] of {{Convert|47.4|km/s|abbr = on}}, whereas Earth's orbital speed is {{Convert|29.8|km/s|abbr = on}}.<ref name=Williams2019>{{cite web | first=David R. | last=Williams | date=October 21, 2019 | title=Planetary Fact Sheet – Metric | publisher=NASA | url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/ | access-date=April 20, 2021 | archive-date=July 19, 2012 | archive-url=https://archive.today/20120719082605/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ | url-status=live }}</ref> Therefore, the spacecraft must make a larger change in [[velocity]] ([[delta-v]]) to get to Mercury and then enter orbit,<ref>{{cite book | title=Inner Solar System: Prospective Energy and Material Resources | first=Kris | last=Zacny | isbn=9783319195698 | date=July 2, 2015 | publisher=Springer International Publishing | page=154 | url=https://books.google.com/books?id=ZrAYCgAAQBAJ&pg=PA154 | access-date=March 19, 2023 | archive-date=April 11, 2023 | archive-url=https://web.archive.org/web/20230411141121/https://books.google.com/books?id=ZrAYCgAAQBAJ&pg=PA154 | url-status=live }}</ref> as compared to the delta-v required for, say, [[Exploration of Mars|Mars planetary missions]].

The [[potential energy]] liberated by moving down the Sun's potential well becomes [[kinetic energy]], requiring a delta-v change to do anything other than pass by Mercury. Some portion of this [[delta-v budget]] can be provided from a [[gravity assist]] during one or more fly-bys of Venus.<ref>{{cite journal | title=Hybrid Algorithm for Multiple Gravity-Assist and Impulsive Delta-V Maneuvers | first1=Sam | last1=Wagner | first2=Bong | last2=Wie | journal=Journal of Guidance, Control, and Dynamics | volume=38 | issue=11 | pages=2096–2107 | date=November 2015 | bibcode=2015JGCD...38.2096W | doi=10.2514/1.G000874 }}</ref> To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. [[Aerobraking]] is ruled out because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that required to [[escape velocity|escape]] the Solar System completely. As a result, only three space probes have visited it so far.<ref name="JPLprofile1">{{cite web | url=https://www.jpl.nasa.gov/education/images/pdf/ss-mercury.pdf | title=Mercury | publisher=[[NASA]] Jet Propulsion Laboratory | date=May 5, 2008 | access-date=April 26, 2021 | archive-date=February 9, 2017 | archive-url=https://web.archive.org/web/20170209135719/http://www.jpl.nasa.gov/education/images/pdf/ss-mercury.pdf | url-status=live }}</ref> A proposed alternative approach would use a [[solar sail]] to attain a Mercury-synchronous orbit around the Sun.<ref>{{cite journal |last1=Leipold |first1=Manfred E. |last2=Seboldt |first2=W. |last3=Lingner |first3=Stephan |last4=Borg |first4=Erik |last5=Herrmann |first5=Axel Siegfried |last6=Pabsch |first6=Arno |last7=Wagner |first7=O. |last8=Brückner |first8=Johannes |title=Mercury sun-synchronous polar orbiter with a solar sail |year=1996 |journal=Acta Astronautica |volume=39 |issue=1 |pages=143–151 |doi=10.1016/S0094-5765(96)00131-2 |bibcode=1996AcAau..39..143L}}</ref>
{{clear}}

==== ''Mariner 10'' ====
{{Main|Mariner 10}}
[[File:Mariner 10.jpg|thumb|''Mariner 10'', the first probe to visit Mercury]]

The first spacecraft to visit Mercury was NASA's {{nowrap|''Mariner 10''}} (1974–1975).<ref name="Dunne" /> The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets.<ref name="DunneCh4">{{cite book |title=The Voyage of Mariner&nbsp;10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |name-list-style=amp |chapter-url=https://history.nasa.gov/SP-424/ch4.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Four |url=https://history.nasa.gov/SP-424/ |access-date=May 28, 2008 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}</ref> {{nowrap|''Mariner 10''}} provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools.<ref>{{cite web |date=October 1976 |first=Tony |last=Phillips |url=http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html |title=NASA 2006 Transit of Mercury |website=SP-423 Atlas of Mercury |publisher=NASA |access-date=April 7, 2008 |archive-date=March 25, 2008 |archive-url=https://web.archive.org/web/20080325090336/http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html |url-status=dead }}</ref> Unfortunately, the same face of the planet was lit at each of {{nowrap|''Mariner 10''{{'s}}}} close approaches. This made close observation of both sides of the planet impossible,<ref>{{cite web |url=http://sci.esa.int/bepicolombo/56015-missions-to-mercury/ |title=BepiColumbo – Background Science |publisher=European Space Agency |access-date=June 18, 2017 |archive-date=July 1, 2017 |archive-url=https://web.archive.org/web/20170701093446/http://sci.esa.int/bepicolombo/56015-missions-to-mercury/ |url-status=live }}</ref> and resulted in the mapping of less than 45% of the planet's surface.<ref name="USATMessenger">{{cite news |url=https://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm |title=MESSENGER to test theory of shrinking Mercury |newspaper=USA Today |first=Tariq |last=Malik |date=August 16, 2004 |access-date=May 23, 2008 |archive-date=July 26, 2011 |archive-url=https://web.archive.org/web/20110726231423/http://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm |url-status=live }}</ref>

The spacecraft made three close approaches to Mercury, the closest of which took it to within {{Convert|327|km|abbr=on}} of the surface.<ref name="AtlasM10">{{cite book |last=Davies |first=Merton E. |display-authors=etal |url=https://history.nasa.gov/SP-423/sp423.htm |title=SP-423 Atlas of Mercury |publisher=[[NASA]] Office of Space Sciences |chapter=Mariner&nbsp;10 Mission and Spacecraft |chapter-url=https://history.nasa.gov/SP-423/mariner.htm |date=1978 |access-date=May 30, 2008 |name-list-style=vanc |archive-date=October 9, 2019 |archive-url=https://web.archive.org/web/20191009033507/https://history.nasa.gov/SP-423/sp423.htm |url-status=dead}}</ref> At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant [[dynamo]] effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet. For many years after the {{nowrap|''Mariner 10''}} encounters, the origin of Mercury's magnetic field remained the subject of several competing theories.<ref name="Ness1">{{cite journal |last=Ness |first=Norman F. |year=1978 |title=Mercury – Magnetic field and interior |journal=Space Science Reviews |volume=21 |issue=5 |pages=527–553 |bibcode=1978SSRv...21..527N |doi=10.1007/BF00240907|s2cid=120025983 }}</ref><ref>{{cite journal |last1=Aharonson |first1=Oded |last2=Zuber |first2=Maria T |last3=Solomon |first3=Sean C |title=Crustal remanence in an internally magnetized non-uniform shell: a possible source for Mercury's magnetic field? |journal=Earth and Planetary Science Letters |year=2004 |volume=218 |issue=3–4 |pages=261–268 |doi=10.1016/S0012-821X(03)00682-4 |bibcode=2004E&PSL.218..261A}}</ref>

On March 24, 1975, just eight days after its final close approach, {{nowrap|''Mariner 10''}} ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down.<ref name="DunneCh8">{{cite book |title=The Voyage of Mariner&nbsp;10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |name-list-style=amp |chapter-url=https://history.nasa.gov/SP-424/ch8.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Eight |url=https://history.nasa.gov/SP-424/ |access-date=July 12, 2017 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}</ref> {{nowrap|''Mariner 10''}} is thought to be still orbiting the Sun, passing close to Mercury every few months.<ref>{{cite web |date=April 2, 2008 |first=Ed |last=Grayzeck |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-085A |title=Mariner&nbsp;10 |website=NSSDC Master Catalog |publisher=NASA |access-date=April 7, 2008 |archive-date=September 8, 2018 |archive-url=https://web.archive.org/web/20180908063158/https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A |url-status=live }}</ref>

==== ''MESSENGER'' ====
{{Main|MESSENGER}}
[[File:Details of MESSENGER's Impact Location.jpg|thumb|Estimated details of the impact of ''MESSENGER'' on April 30, 2015]]
A second NASA mission to Mercury, named ''MESSENGER'' (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury.<ref>{{cite web |year=2005 |url=http://www.spaceref.com/news/viewsr.html?pid=18956 |title=MESSENGER Engine Burn Puts Spacecraft on Track for Venus |publisher=SpaceRef.com |access-date=March 2, 2006}}</ref> A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008,<ref name="MessCountdown">{{cite web |url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115 |title=Countdown to MESSENGER's Closest Approach with Mercury |date=January 14, 2008 |publisher=Johns Hopkins University Applied Physics Laboratory |access-date=May 30, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130513080731/http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115 |archive-date=May 13, 2013}}</ref> and a third on September 29, 2009.<ref>{{cite web |title=MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations |url=http://messenger.jhuapl.edu/news_room/details.php?id=136 |date=September 30, 2009 |publisher=MESSENGER Mission News |access-date=September 30, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20130510175510/http://messenger.jhuapl.edu/news_room/details.php?id=136 |archive-date=May 10, 2013}}</ref> Most of the hemisphere not imaged by {{nowrap|''Mariner 10''}} was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission,<ref name="MessCountdown" /> and then entered a one-year extended mission into 2013. In addition to continued observations and mapping of Mercury, ''MESSENGER'' observed the 2012 [[solar maximum]].<ref name="Extended2013">{{cite news | url=http://www.upi.com/Top_News/US/2011/11/15/NASA-extends-spacecrafts-Mercury-mission/UPI-55131321408343/ | title=NASA extends spacecraft's Mercury mission | work=United Press International | date=November 15, 2011 | access-date=November 16, 2011 | archive-date=May 31, 2013 | archive-url=https://web.archive.org/web/20130531103344/http://www.upi.com/Top_News/US/2011/11/15/NASA-extends-spacecrafts-Mercury-mission/UPI-55131321408343/ | url-status=live }}</ref>

[[File:MercuryTopo.png|thumb|right|Topography of Mercury based on MDIS (Mercury Dual Imaging System) data]] The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than {{nowrap|''Mariner 10''}}, assorted [[spectrometer]]s to determine the abundances of elements in the crust, and [[magnetometer]]s and devices to measure velocities of charged particles. Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure.<ref name="messenger_faq">{{cite web |url=http://messenger.jhuapl.edu/Resources/Resources/MESSENGER_FS22811_PRESS.PDF |title=MESSENGER: Fact Sheet |publisher=[[Applied Physics Laboratory]] |date=February 2011 |access-date=August 21, 2017 |archive-date=August 22, 2017 |archive-url=https://web.archive.org/web/20170822012453/http://messenger.jhuapl.edu/Resources/Resources/MESSENGER_FS22811_PRESS.PDF |url-status=live }}</ref> ''MESSENGER''{{'s}} final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015.<ref name="results 2015">{{cite news |last=Wall |first=Mike |url=http://www.space.com/28948-messenger-mercury-probe-final-days.html |title=NASA Mercury Probe Trying to Survive for Another Month |work=Space.com |date=March 29, 2015 |access-date=April 4, 2015 |archive-date=April 3, 2019 |archive-url=https://web.archive.org/web/20190403102620/https://www.space.com/28948-messenger-mercury-probe-final-days.html |url-status=live }}</ref><ref name="NYT-20150427">{{cite news |last=Chang |first=Kenneth |title=NASA's Messenger Mission Is Set to Crash Into Mercury |url=https://www.nytimes.com/2015/04/28/science/nasas-messenger-mission-is-set-to-crash-into-mercury.html |archive-url=https://web.archive.org/web/20150429030725/http://www.nytimes.com/2015/04/28/science/nasas-messenger-mission-is-set-to-crash-into-mercury.html |archive-date=April 29, 2015 |url-access=subscription |url-status=live |date=April 27, 2015 |work=[[The New York Times]] |access-date=April 27, 2015}}</ref><ref name="NYT-20150430">{{cite news |last=Corum |first=Jonathan |title=Messenger's Collision Course With Mercury |url=https://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |date=April 30, 2015 |work=[[The New York Times]] |access-date=April 30, 2015 |archive-date=March 31, 2019 |archive-url=https://web.archive.org/web/20190331205609/https://www.nytimes.com/interactive/2015/04/30/science/space/messenger-collides-with-mercury.html |url-status=live }}</ref> The spacecraft's impact with Mercury occurred at 3:26:01&nbsp;p.m. [[Eastern Time Zone|EDT]] on April 30, 2015, leaving a crater estimated to be {{convert|16|m|ft|abbr=on}} in diameter.<ref>{{cite web |date=June 3, 2015 |title=Best Determination of MESSENGER's Impact Location |url=https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-1605.html |archive-url=https://web.archive.org/web/20230411141108/https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-1605.html |archive-date=April 11, 2023 |access-date=October 6, 2023 |website=messenger.jhuapl.edu |publisher=[[Johns Hopkins Applied Physics Laboratory]]}}</ref>

==== ''BepiColombo'' ====
{{Main|BepiColombo}}
The [[European Space Agency]] and the [[Japanese Space Agency]] developed and launched a joint mission called ''BepiColombo'', which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere.<ref name="ESAColumboGoAhead">{{cite web |title=ESA gives go-ahead to build BepiColombo |date=February 26, 2007 |publisher=[[European Space Agency]] |url=http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html |access-date=May 29, 2008 |archive-date=March 31, 2008 |archive-url=https://web.archive.org/web/20080331042423/http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html |url-status=live }}</ref>  ''BepiColombo'' was launched on October 20, 2018.<ref>{{cite web |title=BepiColombo Fact Sheet |url=http://sci.esa.int/bepicolombo/47346-fact-sheet/ |publisher=[[European Space Agency]] |access-date=December 19, 2016 |date=December 1, 2016 |archive-date=May 20, 2016 |archive-url=http://arquivo.pt/wayback/20160520023104/http://sci.esa.int/bepicolombo/47346%2Dfact%2Dsheet/ |url-status=live }}</ref> It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year.<ref name="ESAColumboGoAhead" /> The mapper probe carries an array of spectrometers similar to those on ''MESSENGER'', and will study the planet at many different wavelengths including [[infrared]], [[ultraviolet]], [[X-ray]] and [[gamma ray]].<ref>{{cite web |title=Objectives |publisher=European Space Agency |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350 |date=February 21, 2006 |access-date=May 29, 2008 |archive-date=September 28, 2006 |archive-url=https://web.archive.org/web/20060928055507/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350 |url-status=live }}</ref> ''BepiColombo'' conducted the first of its six planned Mercury flybys on October 1, 2021,<ref>{{cite web|url=https://www.nasaspaceflight.com/2021/10/bepicolombo-first-mercury-flyby/|title=BepiColombo completes first Mercury flyby, science provides insight into planet's unique environment|website=[[NASA|NASA Spaceflight]]|date=October 24, 2021|accessdate=October 8, 2022|last1=Warren|first1=Haygen|archive-date=October 8, 2022|archive-url=https://web.archive.org/web/20221008121521/https://www.nasaspaceflight.com/2021/10/bepicolombo-first-mercury-flyby/|url-status=live}}</ref> and the sixth was completed on January 9, 2025. The spacecraft will enter the planet's orbit in 2026.<ref name="BBCBepi">{{cite web |last=Fullbrook |first=Danny |date=January 9, 2025 |title=New Images of Mercury Captured by UK Spacecraft |website=[[BBC]] |url=https://www.bbc.com/news/articles/cx2v2r1jm7go |access-date=January 10, 2025 }}</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Astronomy on Mercury]]
* [[Colonization of Mercury]]
* [[Mercury (astrology)|Mercury in astrology]]
* [[Mercury in fiction]]
* [[Outline of Mercury (planet)]]
{{div col end}}

== Notes ==
{{reflist |group=lower-alpha |refs=

{{efn |name=Cuneiform MUL
| Some sources precede the cuneiform transcription with "MUL". "MUL" is a cuneiform sign that was used in the Sumerian language to designate a star or planet, but it is not considered part of the actual name. The "4" is a reference number in the Sumero–Akkadian transliteration system to designate which of several syllables a certain cuneiform sign is most likely designating.
}}

{{efn |name=angular
| The Sun's total angular displacement during its apparent retrograde motion as seen from the surface of Mercury is ~1.23°, while the Sun's angular diameter when the apparent retrograde motion begins and ends is ~1.71°, increasing to ~1.73° at perihelion (midway through the retrograde motion).
}}

}}
{{clear}}

== References ==
{{reflist |colwidth=30em |refs=
<ref name=Ritzel>{{cite news |last=Ritzel |first=Rebecca |title=Ballet isn't rocket science, but the two aren't mutually exclusive, either |newspaper=The Washington Post |location=Washington, D.C., United States |date=December 20, 2012 |url=https://www.washingtonpost.com/entertainment/theater_dance/ballet-isnt-rocket-science-but-the-twoarentmutually-exclusive-either/2012/12/20/83fae9d6-3e2b-11e2-ae43-cf491b837f7b_story.html |access-date=December 22, 2012 |archive-date=December 23, 2012 |archive-url=https://web.archive.org/web/20121223125341/http://www.washingtonpost.com/entertainment/theater_dance/ballet-isnt-rocket-science-but-the-twoarentmutually-exclusive-either/2012/12/20/83fae9d6-3e2b-11e2-ae43-cf491b837f7b_story.html |url-status=live }}</ref>

<ref name="horizons">{{cite web |date=April 7, 2008 |first=Donald K. |last=Yeomans |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27@Sun%27&OUT_UNITS=%27AU-D%27 |title=HORIZONS Web-Interface for Mercury Major Body |publisher=[[JPL Horizons On-Line Ephemeris System]] |access-date=April 7, 2008 |archive-date=August 18, 2023 |archive-url=https://web.archive.org/web/20230818120425/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%271%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27%40Sun%27&OUT_UNITS=%27AU-D%27 |url-status=live }} – Select "Ephemeris Type: Orbital Elements", "Time Span: 2000-01-01 12:00 to 2000-01-02". ("Target Body: Mercury" and "Center: Sun" should be defaulted to.) Results are instantaneous [[osculating orbit|osculating]] values at the precise [[J2000]] epoch.</ref>

<ref name=Souami_Souchay_2012>{{cite journal
 | title=The solar system's invariable plane
 | last1=Souami | first1=D. | last2=Souchay | first2=J.
 | journal=Astronomy & Astrophysics
 | volume=543 | id=A133 | pages=11 | date=July 2012
 | doi=10.1051/0004-6361/201219011 | bibcode=2012A&A...543A.133S | doi-access=free }}</ref>

<ref name="nasa">{{cite web |date=February 15, 2021 |first1=Phillips |last1=Davis |last2=Barnett |first2=Amanda |url=https://solarsystem.nasa.gov/planets/mercury/overview/ |title=Mercury |website=Solar System Exploration |publisher=NASA Jet Propulsion Laboratory |access-date=April 21, 2021 |archive-date=April 18, 2021 |archive-url=https://web.archive.org/web/20210418111218/https://solarsystem.nasa.gov/planets/mercury/overview/ |url-status=live }}</ref>

<ref name="Seidelmann2007">{{cite journal| doi = 10.1007/s10569-007-9072-y| last1 = Seidelmann| first1 = P. Kenneth| last2 = Archinal| first2 = Brent A.| last3 = A'Hearn<!-- written A'hearn here, mostly A'Hearn elsewhere -->| first3 = Michael F.| display-authors = 3| last4 = Conrad| first4 = Albert R.| last5 = Consolmagno| first5 = Guy J.| last6 = Hestroffer| first6 = Daniel| last7 = Hilton| first7 = James L.| last8 = Krasinsky| first8 = Georgij A.| last9 = Neumann| first9 = Gregory A.| last10=Oberst | first10=Jürgen | last11=Stooke | first11=Philip J. | last12=Tedesco | first12=Edward F. | last13=Tholen | first13=David J. | last14=Thomas | first14=Peter C. | last15=Williams | first15=Iwan P. | year = 2007| title = Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006| journal = Celestial Mechanics and Dynamical Astronomy| volume = 98| issue = 3| pages = 155–180| bibcode = 2007CeMDA..98..155S| s2cid = 122772353| ref = {{sfnRef|Seidelmann Archinal A'hearn et al.|2007}}| doi-access = free}}</ref>

<ref name="Benz">{{cite journal |title=Collisional stripping of Mercury's mantle |last1=Benz |first1=W. |last2=Slattery |first2=W. L. |last3=Cameron |first3=Alastair G. W. |journal=Icarus |volume=74 |issue=3 |pages=516–528 |year=1988 |doi=10.1016/0019-1035(88)90118-2 |bibcode=1988Icar...74..516B |url=https://zenodo.org/record/1253898 |access-date=August 25, 2019 |archive-date=September 5, 2019 |archive-url=https://web.archive.org/web/20190905034903/https://zenodo.org/record/1253898 |url-status=live }}</ref>

<ref name="awst169_18_18">{{cite journal |first=Jefferson |last=Morris |date=November 10, 2008 |title=Laser Altimetry |journal=[[Aviation Week & Space Technology]] |volume=169 |issue=18 |page=18 |quote=Mercury's crust is more analogous to a marbled cake than a layered cake.}}</ref>

<ref name="WagWolIva01">{{cite conference |last1=Wagner |first1=Roland J. |last2=Wolf |first2=Ursula |last3=Ivanov |first3=Boris A. |last4=Neukum |first4=Gerhard |title=Application of an Updated Impact Cratering Chronology Model to Mercury' s Time-Stratigraphic System |work=Workshop on Mercury: Space Environment, Surface, and Interior. Proceedings of a workshop held at The Field Museum. |date=October 4–5, 2001 |location=Chicago, IL |publisher=Lunar and Planetary Science Institute |page=106 |bibcode=2001mses.conf..106W}}</ref>

<ref name="Cosmic1">{{cite book |last=Biswas |first=Sukumar |date=2000 |title=Cosmic Perspectives in Space Physics |publisher=Springer |series=Astrophysics and Space Science Library |page=176 |isbn=978-0-7923-5813-8}}</ref>

<ref name="Mallama">{{cite arXiv|title=The spherical bolometric albedo for planet Mercury |first1=Anthony |last1=Mallama |year=2017 |eprint=1703.02670 <!--|bibcode=2017arXiv170302670M-->|class=astro-ph.EP }}</ref>

<ref Name="MallamaMercury">{{cite journal |last1=Mallama |first1=Anthony |last2=Wang |first2=Dennis |last3=Howard |first3=Russell A. |title=Photometry of Mercury from SOHO/LASCO and Earth |journal=Icarus |volume=155 |issue=2 |pages=253–264 |year=2002 |doi=10.1006/icar.2001.6723 |bibcode=2002Icar..155..253M}}</ref>

<ref Name="MallamaSky">{{cite journal |last=Mallama |first=Anthony |title=Planetary magnitudes |journal=Sky and Telescope |volume=121 |issue=1 |pages=51–56 |year=2011|bibcode=2011S&T...121a..51M }}</ref>

<ref name="Margot2012">{{cite journal |last1=Margot |first1=Jean-Luc |last2=Peale |first2=Stanton J. |last3=Solomon |first3=Sean C. |last4=Hauck |first4=Steven A. |last5=Ghigo |first5=Frank D. |last6=Jurgens |first6=Raymond F. |last7=Yseboodt |first7=Marie |last8=Giorgini |first8=Jon D. |last9=Padovan |first9=Sebastiano|last10=Campbell|first10=Donald B. |title=Mercury's moment of inertia from spin and gravity data |journal=Journal of Geophysical Research: Planets |volume=117 |issue=E12 |article-number=2012JE004161 |year=2012 |pages=n/a |issn=0148-0227 |doi=10.1029/2012JE004161 |bibcode=2012JGRE..117.0L09M|citeseerx=10.1.1.676.5383 |s2cid=22408219 }}</ref>

<ref name="Mazarico2014">{{cite journal |last1=Mazarico |first1=Erwan |last2=Genova |first2=Antonio |last3=Goossens |first3=Sander |last4=Lemoine |first4=Frank G. |last5=Neumann |first5=Gregory A. |last6=Zuber |first6=Maria T. |last7=Smith |first7=David E. |last8=Solomon |first8=Sean C. |title=The gravity field, orientation, and ephemeris of Mercury from MESSENGER observations after three years in orbit |journal=Journal of Geophysical Research: Planets |volume=119 |issue=12 |year=2014 |pages=2417–2436 |issn=2169-9097 |doi=10.1002/2014JE004675 |hdl=1721.1/97927 |bibcode=2014JGRE..119.2417M |s2cid=42430050 |url=https://iris.uniroma1.it/bitstream/11573/1242426/2/Mazarico_the-gravity_2014.pdf |access-date=August 25, 2019 |archive-date=September 29, 2021 |archive-url=https://web.archive.org/web/20210929073931/https://iris.uniroma1.it/retrieve/handle/11573/1242426/1160748/Mazarico_the-gravity_2014.pdf |url-status=live }}</ref>

<ref name="Padovan2015">{{cite journal |doi=10.1002/2014GL062487 |title=Thickness of the crust of Mercury from geoid-to-topography ratios |journal=Geophysical Research Letters |volume=42 |issue=4 |pages=1029 |year=2015 |last1=Padovan |first1=Sebastiano |last2=Wieczorek |first2=Mark A. |last3=Margot |first3=Jean-Luc |last4=Tosi |first4=Nicola |last5=Solomon |first5=Sean C. |bibcode=2015GeoRL..42.1029P |s2cid=31442257 |url=http://www.escholarship.org/uc/item/403071pz |doi-access=free |access-date=December 15, 2018 |archive-date=February 12, 2019 |archive-url=https://web.archive.org/web/20190212190428/https://escholarship.org/uc/item/403071pz |url-status=live }}</ref>

<ref name="Colombo">{{cite journal |last1=Colombo |first1=Giuseppe |author-link1=Giuseppe Colombo|last2=Shapiro |first2=Irwin I. |author-link2=Irwin I. Shapiro|title=The rotation of the planet Mercury |journal=Astrophysical Journal |year=1966 |volume=145 |page=296 |doi=10.1086/148762 |bibcode=1966ApJ...145..296C|doi-access=free |hdl=2060/19660004284 |hdl-access=free }}</ref>

<ref name="Mallama_and_Hilton">{{cite journal |title=Computing apparent planetary magnitudes for The Astronomical Almanac |journal=Astronomy and Computing |first1=Anthony |last1=Mallama |first2=James L. |last2=Hilton |volume=25 |pages=10–24 |date=October 2018 |doi=10.1016/j.ascom.2018.08.002 |bibcode=2018A&C....25...10M |arxiv=1808.01973|s2cid=69912809 }}</ref>

<ref name="Hauck_etal_2013">{{cite journal |last1=Hauck |first1=Steven A. |last2=Margot |first2=Jean-Luc |last3=Solomon |first3=Sean C. |last4=Phillips |first4=Roger J. |last5=Johnson |first5=Catherine L. |last6=Lemoine |first6=Frank G. |last7=Mazarico |first7=Erwan |last8=McCoy |first8=Timothy J. |last9=Padovan |first9=Sebastiano |last10=Peale |first10=Stanton J. |last11=Perry |first11=Mark E. |last12=Smith |first12=David E. |last13=Zuber |first13=Maria T. |title=The curious case of Mercury's internal structure|journal=Journal of Geophysical Research: Planets |year=2013 |volume=118 |issue=6 |pages=1204–1220 |doi=10.1002/jgre.20091|bibcode=2013JGRE..118.1204H |hdl=1721.1/85633 |s2cid=17668886 |hdl-access=free }}</ref>

}}

== External links ==
{{Spoken Wikipedia|En-Mercury(Planet).ogg|date=January 16, 2008}}
{{Sister project links|display=Mercury|voy=no|commonscat=yes|d=y|wikt=Mercury|m=no|mw=no|species=no|n=no|v=Solar System, technical/Mercury|s=Category:Planetary_astronomy}}
* {{cite book |url=https://history.nasa.gov/SP-423/sp423.htm |title=Atlas of Mercury |publisher=NASA |date=1978 |id=SP-423}}
* [https://planetarynames.wr.usgs.gov/Page/MERCURY/target Mercury nomenclature] and [https://web.archive.org/web/20130217190331/http://planetarynames.wr.usgs.gov/Page/mercuryQuadMap map with feature names] from the USGS/IAU ''Gazetteer of Planetary Nomenclature''
* [http://messenger-act.actgate.com/msgr_public_released/react_quickmap.html Equirectangular map of Mercury] {{Webarchive|url=https://web.archive.org/web/20160520025929/http://messenger-act.actgate.com/msgr_public_released/react_quickmap.html |date=May 20, 2016 }} by Applied Coherent Technology Corp
* [https://www.google.com/maps/space/mercury/ 3D globe of Mercury] by Google
* [http://solarviews.com/eng/mercury.htm Mercury] at Solarviews.com
* [http://www.astronomycast.com/2007/08/episode-49-mercury/ Mercury] by Astronomy Cast
* [http://messenger.jhuapl.edu/ ''MESSENGER'' mission web site]
* [http://www.esa.int/bepicolombo ''BepiColombo'' mission web site]

{{Mercury (planet)|state=expanded}}
{{Solar System}}
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