View source for Biosignature

{{Short description|Substance providing scientific evidence of past or present life}}
{{other uses|Biomarker (disambiguation)}}
A '''biosignature''' (sometimes called '''chemical fossil''' or '''molecular fossil''') is any substance&nbsp;&ndash; such as an element, [[isotope]], [[molecule]], or [[phenomenon]]&nbsp;&ndash; that provides [[scientific evidence]] of past or present [[life]] on a planet or moon.<ref name=SSG >{{Cite book| last2=Beaty | last1=Steele| contribution=Final report of the MEPAG Astrobiology Field Laboratory Science Steering Group (AFL-SSG)| title=The Astrobiology Field Laboratory | publisher=the Mars Exploration Program Analysis Group (MEPAG) - NASA| place=U.S.| page=72| date=September 26, 2006| contribution-url=http://mepag.jpl.nasa.gov/reports/AFL_SSG_WHITE_PAPER_v3.doc| chapter-format=.doc|display-authors=etal}}</ref><ref>{{cite web | url = http://www.science-dictionary.com/definition/biosignature.html | archive-url = https://web.archive.org/web/20100316062919/http://www.science-dictionary.com/definition/biosignature.html | archive-date = 2010-03-16 | title = Biosignature - definition | access-date = 2011-01-12 | year = 2011 | work = Science Dictionary }}</ref><ref name='Biosignatures 2011'>{{cite journal | vauthors = Summons RE, Amend JP, Bish D, Buick R, Cody GD, Des Marais DJ, Dromart G, Eigenbrode JL, Knoll AH, Sumner DY | display-authors = 6 | title = Preservation of martian organic and environmental records: final report of the Mars biosignature working group | journal = Astrobiology | volume = 11 | issue = 2 | pages = 157–81 | date = March 2011 | pmid = 21417945 | doi = 10.1089/ast.2010.0506 | url = http://eaps.mit.edu/geobiology/recent%20pubs/AST-2010-0506-Summons_Mars%20Taphonomy.pdf | first8 = J. L. | last10 = Sumner | last9 = Knoll | last8 = Eigenbrode | first9 = A. H. | last7 = Dromart | bibcode = 2011AsBio..11..157S | first10 = D. Y. | first7 = G | hdl = 1721.1/66519 | s2cid = 9963677 | hdl-access = free | access-date = 2013-06-22 | archive-date = 2019-11-28 | archive-url = https://web.archive.org/web/20191128212415/http://eaps.mit.edu/geobiology/recent%20pubs/AST-2010-0506-Summons_Mars%20Taphonomy.pdf }}</ref> Measurable attributes of life include its physical or chemical structures, its use of [[Thermodynamic free energy|free energy]], and the production of [[biomass]] and [[Cellular waste product|wastes]].

The field of [[astrobiology]] uses biosignatures as evidence for the search for past or present extraterrestrial life.<ref>{{Cite journal |last1=Malaterre |first1=Christophe |last2=ten Kate |first2=Inge Loes |last3=Baqué |first3=Mickael |last4=Debaille |first4=Vinciane |last5=Grenfell |first5=John Lee |last6=Javaux |first6=Emmanuelle J. |last7=Khawaja |first7=Nozair |last8=Klenner |first8=Fabian |last9=Lara |first9=Yannick J. |last10=McMahon |first10=Sean |last11=Moore |first11=Keavin |last12=Noack |first12=Lena |last13=Patty |first13=C.H. Lucas |last14=Postberg |first14=Frank |date=2023-11-01 |title=Is There Such a Thing as a Biosignature? |url=https://www.liebertpub.com/doi/10.1089/ast.2023.0042 |journal=Astrobiology |language=en |volume=23 |issue=11 |pages=1213–1227 |doi=10.1089/ast.2023.0042 |pmid=37962841 |bibcode=2023AsBio..23.1213M |hdl=2268/308868 |issn=1531-1074|hdl-access=free }}</ref> Candidate biosignatures strongly indicate some of the [[earliest known life forms]], aid studies of the [[origin of life on Earth]] as well as the possibility of [[life on Mars]], [[life on Venus|Venus]] and elsewhere in the universe.

== Types ==
Biosignatures can be grouped into ten broad categories:<ref name="NASA strategy 2015">[https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf NASA Astrobiology Strategy 2015] {{Webarchive|url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf|date=2016-12-22}}.(PDF), NASA.</ref>

#[[Isotope]] patterns: Isotopic evidence or patterns that require biological processes.
#[[Chemistry]]: Chemical features that require biological activity.
#[[Organic matter]]: Organics formed by biological processes.
#[[Mineral]]s: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., [[Magnetite|biomagnetite]]).
#Microscopic structures and textures: Biologically-formed cements, microtextures, [[Micropaleontology|microfossils]], and films.
#Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, [[biofilm]]s (e.g., [[stromatolite]]s), or [[fossil]]s of larger organisms.
#Temporal variability: Variations in time of atmospheric gases, [[Reflectance|reflectivity]], or macroscopic appearance that indicates life's presence.
#Surface reflectance features: Large-scale reflectance features due to biological pigments.
#Atmospheric gases: Gases formed by [[Metabolism|metabolic]] processes, which may be present on a planet-wide scale.
#[[Technosignature]]s: Signatures that indicate a technologically advanced civilization.<ref name="WP-20201231">{{cite news |last=Frank |first=Adam |title=A new frontier is opening in the search for extraterrestrial life - The reason we haven't found life elsewhere in the universe is simple: We haven't looked until now. |url=https://www.washingtonpost.com/outlook/2020/12/31/breakthrough-listen-seti-technosignatures/ |date=31 December 2020 |newspaper=[[The Washington Post]] |access-date=1 January 2021 }}</ref>

== Viability ==
Determining whether an observed feature is a true biosignature is complex. There are three criteria that a potential biosignature must meet to be considered viable for further research: Reliability, survivability, and detectability.<ref>{{cite journal | vauthors = Domagal-Goldman SD, Meadows VS, Claire MW, Kasting JF | title = Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets | journal = Astrobiology | volume = 11 | issue = 5 | pages = 419–41 | date = June 2011 | pmid = 21663401 | pmc = 3133782 | doi = 10.1089/ast.2010.0509 | bibcode = 2011AsBio..11..419D }}</ref><ref>{{cite journal | vauthors = Seager S, Schrenk M, Bains W | title = An astrophysical view of Earth-based metabolic biosignature gases | journal = Astrobiology | volume = 12 | issue = 1 | pages = 61–82 | date = January 2012 | pmid = 22269061 | doi = 10.1089/ast.2010.0489 | bibcode = 2012AsBio..12...61S | hdl = 1721.1/73073 | s2cid = 18142901 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Meadows VS | title = Reflections on O2 as a Biosignature in Exoplanetary Atmospheres | journal = Astrobiology | volume = 17 | issue = 10 | pages = 1022–1052 | date = October 2017 | pmid = 28443722 | pmc = 5655594 | doi = 10.1089/ast.2016.1578 }}</ref><ref name=":3" />[[File:O2 false positives2.png|thumb|False positive mechanisms for oxygen on a variety of planet scenarios. The molecules in each large rectangle represent the main contributors to a spectrum of the planet's atmosphere. The molecules circled in yellow represent the molecules that would help confirm a false positive biosignature if they were detected. Furthermore, the molecules crossed out in red would help confirm a false positive biosignature if they were ''not'' detected. Cartoon adapted from [[Victoria Meadows]]' 2018 oxygen as a biosignature study.<ref name=":3">{{cite journal | vauthors = Meadows VS, Reinhard CT, Arney GN, Parenteau MN, Schwieterman EW, Domagal-Goldman SD, Lincowski AP, Stapelfeldt KR, Rauer H, DasSarma S, Hegde S, Narita N, Deitrick R, Lustig-Yaeger J, Lyons TW, Siegler N, Grenfell JL | display-authors = 6 | title = Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment | journal = Astrobiology | volume = 18 | issue = 6 | pages = 630–662 | date = June 2018 | pmid = 29746149 | pmc = 6014580 | doi = 10.1089/ast.2017.1727 | arxiv = 1705.07560 | bibcode = 2018AsBio..18..630M }}</ref>]]

=== Reliability ===
A biosignature must be able to dominate over all other processes that may produce similar physical, spectral, and chemical features. When investigating a potential biosignature, scientists must carefully consider all other possible origins of the biosignature in question. Many forms of life are known to mimic geochemical reactions. One of the theories on the [[Abiogenesis|origin of life]] involves molecules developing the ability to catalyse geochemical reactions to exploit the energy being released by them. These are some of the earliest known metabolisms (see [[methanogenesis]]).<ref>{{cite journal | vauthors = Ver Eecke HC, Butterfield DA, Huber JA, Lilley MD, Olson EJ, Roe KK, Evans LJ, Merkel AY, Cantin HV, Holden JF | display-authors = 6 | title = Hydrogen-limited growth of hyperthermophilic methanogens at deep-sea hydrothermal vents | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 34 | pages = 13674–9 | date = August 2012 | pmid = 22869718 | pmc = 3427048 | doi = 10.1073/pnas.1206632109 | bibcode = 2012PNAS..10913674V | doi-access = free }}</ref><ref name=":4">{{cite journal | vauthors = Szostak J | title = How Did Life Begin? | journal = Nature | volume = 557 | issue = 7704 | pages = S13–S15 | date = May 2018 | pmid = 29743709 | doi = 10.1038/d41586-018-05098-w | bibcode = 2018Natur.557S..13S | doi-access = free }}</ref> In such case, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should. A disequilibrium such as this could be interpreted as an indication of life.<ref name=":4" />

=== Survivability ===
A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism's use of metabolic reactions for energy is the production of [[metabolic waste]]. In addition, the structure of an organism can be preserved as a [[fossil]] and we know that some fossils on Earth are [[Earliest known life forms|as old as 3.5 billion years]].<ref>{{Cite web|url=https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html|title=Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks|website=Phys.org |author=University of New South Wales |date=May 9, 2017 |language=en-us|access-date=2019-06-12}}</ref><ref>{{Cite journal|last1=Ward|first1=Colin R.|last2=Walter|first2=Malcolm R.|last3=Campbell|first3=Kathleen A.|last4=Kranendonk|first4=Martin J. Van|last5=Djokic|first5=Tara|date=2017-05-09|title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits|journal=Nature Communications|language=en|volume=8|article-number=15263|doi=10.1038/ncomms15263|pmid=28486437|pmc=5436104|issn=2041-1723|bibcode=2017NatCo...815263D}}</ref> These byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it.

=== Detectability ===
A biosignature must be detectable with the most latest technology to be relevant in scientific investigation. This seems to be an obvious statement, however, there are many scenarios in which life may be present on a planet yet remain undetectable because of human-caused limitations.

==== False positives ====
Every possible biosignature is associated with its own set of unique [[False positives and false negatives|false positive]] mechanisms or non-biological processes that can mimic the detectable feature of a biosignature. An important example is using [[oxygen]] as a biosignature. On Earth, the majority of life is centred around oxygen. It is a byproduct of [[photosynthesis]] and is subsequently used by other life forms to breathe. Oxygen is also readily detectable in [[Spectral line|spectra]], with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature. However, finding oxygen alone in a planet's atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. One possibility is that oxygen can build up abiotically via [[Photodissociation|photolysis]] if there is a low inventory of non-condensable gasses or if the planet loses a lot of water.<ref>{{cite journal | vauthors = Luger R, Barnes R | title = Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs | journal = Astrobiology | volume = 15 | issue = 2 | pages = 119–43 | date = February 2015 | pmid = 25629240 | pmc = 4323125 | doi = 10.1089/ast.2014.1231 | bibcode = 2015AsBio..15..119L | arxiv = 1411.7412 }}</ref><ref>{{cite journal |last1=Wordsworth |first1=Robin |last2=Pierrehumbert |first2=Raymond |title=Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets |journal=The Astrophysical Journal |date=1 April 2014 |volume=785 |issue=2 |pages=L20 |doi=10.1088/2041-8205/785/2/L20 |bibcode=2014ApJ...785L..20W |arxiv=1403.2713 |s2cid=17414970 }}</ref><ref name="Lisse 2020">{{cite journal|title=A Geologically Robust Procedure for Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen Is a Modern Earth-like Biosignature|journal=Astrophysical Journal Letters|year=2020|last=Lisse|first=Carey|volume=898|issue=577|page=L17|doi=10.3847/2041-8213/ab9b91|arxiv=2006.07403 |bibcode=2020ApJ...898L..17L|s2cid=219687224 |doi-access=free }}</ref> Finding and distinguishing a biosignature from its potential false-positive mechanisms is one of the most complicated parts of testing for viability because it relies on human ingenuity to break an abiotic-biological degeneracy, if nature allows.

==== False negatives ====
Opposite to false positives, [[False positives and false negatives|false negative]] biosignatures arise in a scenario where life may be present on another planet, but some processes on that planet make potential biosignatures undetectable.<ref>{{cite journal |last1=Reinhard |first1=Christopher T. |last2=Olson |first2=Stephanie L. |last3=Schwieterman |first3=Edward W. |last4=Lyons |first4=Timothy W. |title=False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth |journal=Astrobiology |date=April 2017 |volume=17 |issue=4 |pages=287–297 |doi=10.1089/ast.2016.1598 |pmid=28418704 |pmc=5399744 |bibcode=2017AsBio..17..287R |arxiv=1702.01137 }}</ref> This is an ongoing problem and area of research in preparation for future telescopes that will be capable of observing exoplanetary atmospheres.

==== Human limitations ====
There are many ways in which humans may limit the viability of a potential biosignature. The resolution of a telescope becomes important when vetting certain false-positive mechanisms, and many current telescopes do not have the capabilities to observe at the resolution needed to investigate some of these. In addition, probes and telescopes are worked on by huge collaborations of scientists with varying interests. As a result, new probes and telescopes carry a variety of instruments that are a compromise to everyone's unique inputs. For a different type of scientist to detect something unrelated to biosignatures, a sacrifice may have to be made in the capability of an instrument to search for biosignatures.<ref>{{Cite book|url=https://www.nap.edu/catalog/12951/new-worlds-new-horizons-in-astronomy-and-astrophysics|title=New Worlds, New Horizons in Astronomy and Astrophysics|date=2010-08-13|isbn=978-0-309-15799-5|language=en|last1=Board |first1=Space Studies |publisher=National Academies Press }}</ref>

== General examples ==

=== Geomicrobiology ===
{{Life timeline}}
[[File:Calcidiscus leptoporus 05.jpg|thumb|left|200px|Electron micrograph of microfossils from a sediment core obtained by the [[Deep Sea Drilling Program]] ]]The ancient record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved over geologic time. Some related disciplines such as [[geochemistry]], [[geobiology]], and [[geomicrobiology]] often use biosignatures to determine if living [[organism]]s are or were present in a sample. These possible biosignatures include: (a) [[microfossils]] and [[stromatolites]]; (b) molecular structures ([[biomarkers]]) and [[Isotope|isotopic compositions]] of carbon, nitrogen and hydrogen in [[organic matter]]; (c) multiple sulfur and oxygen isotope ratios of minerals; and (d) abundance relationships and isotopic compositions of redox-sensitive metals (e.g., Fe, Mo, Cr, and rare earth elements).<ref name="PSARC">{{cite web|url=http://php.scripts.psu.edu/dept/psarc/index.php?page=executive-summary|title=SIGNATURES OF LIFE FROM EARTH AND BEYOND|year=2009|work=Penn State Astrobiology Research Center (PSARC)|publisher=Penn State|access-date=2011-01-14|archive-date=2018-10-23|archive-url=https://web.archive.org/web/20181023073522/http://php.scripts.psu.edu/dept/psarc/index.php?page=executive-summary}}</ref><ref>{{cite web|url=http://astrobiology.nasa.gov/articles/2008/7/30/reading-archaean-biosignatures/|title=Reading Archaean Biosignatures|last1=Tenenbaum|first1=David|date=July 30, 2008|publisher=NASA|archive-url=https://web.archive.org/web/20141129094552/http://astrobiology.nasa.gov/articles/2008/7/30/reading-archaean-biosignatures/|archive-date=November 29, 2014|access-date=2014-11-23}}</ref>

For example, the particular [[fatty acids]] measured in a sample can indicate which types of [[bacterium|bacteria]] and [[archaea]] live in that environment. Another example is the long-chain [[fatty alcohol]]s with more than 23 atoms that are produced by [[plankton]]ic [[bacteria]].<ref>{{Cite web |url=http://www.cyberlipid.org/simple/simp0003.htm |title=Fatty alcohols |access-date=2006-04-01 |archive-date=2012-06-25 |archive-url=https://web.archive.org/web/20120625155634/http://www.cyberlipid.org/simple/simp0003.htm }}</ref> When used in this sense, geochemists often prefer the term [[biomarker]]. Another example is the presence of straight-chain [[lipids]] in the form of [[alkanes]], [[alcohols]], and [[fatty acids]] with 20&ndash;36 [[carbon]] atoms in soils or sediments. [[Peat]] deposits are an indication of originating from the [[epicuticular wax]] of higher [[plant]]s.

Life processes may produce a range of biosignatures such as [[nucleic acids]], [[lipid]]s, [[protein]]s, [[amino acid]]s, [[kerogen]]-like material and various morphological features that are detectable in rocks and sediments.<ref name="Beegle">{{cite journal | vauthors = Beegle LW, Wilson MG, Abilleira F, Jordan JF, Wilson GR | s2cid = 7127896 | title = A concept for NASA's Mars 2016 astrobiology field laboratory | journal = Astrobiology | volume = 7 | issue = 4 | pages = 545–77 | date = August 2007 | pmid = 17723090 | doi = 10.1089/ast.2007.0153 | bibcode = 2007AsBio...7..545B }}</ref> [[Microbes]] often interact with geochemical processes, leaving features in the rock record indicative of biosignatures. For example, bacterial micrometer-sized pores in [[carbonate rock]]s resemble inclusions under transmitted light, but have distinct sizes, shapes, and patterns (swirling or dendritic) and are distributed differently from common fluid inclusions.<ref>{{cite journal |last1=Bosak |first1=Tanja |last2=Souza-Egipsy |first2=Virginia |last3=Corsetti |first3=Frank A. |last4=Newman |first4=Dianne K. |title=Micrometer-scale porosity as a biosignature in carbonate crusts |journal=Geology |date=2004 |volume=32 |issue=9 |page=781 |doi=10.1130/G20681.1 |bibcode=2004Geo....32..781B }}</ref> A potential biosignature is a phenomenon that ''may'' have been produced by life, but for which alternate [[Abiotic component|abiotic]] origins may also be possible.

===Morphology===
Another possible biosignature might be [[Morphology (biology)|morphology]] since the shape and size of certain objects may potentially indicate the presence of past or present life. Morphology has sparked debate as it is inconclusive and has resulted in disputed claims of early life on Earth. 

Stromatolites are difficult to identify chemically and are sometimes claimed based on morphology alone. However geological processes may produce false positive candidates. One case is a 3.7 Ga structure in West Greenland which could be explained by tectonic processes.<ref>{{Cite journal |last1=Zawaski |first1=Mike J. |last2=Kelly |first2=Nigel M. |last3=Orlandini |first3=Omero Felipe |last4=Nichols |first4=Claire I. O. |last5=Allwood |first5=Abigail C. |last6=Mojzsis |first6=Stephen J. |date=2020-09-01 |title=Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis |url=https://www.sciencedirect.com/science/article/pii/S0012821X20303538 |journal=Earth and Planetary Science Letters |volume=545 |article-number=116409 |doi=10.1016/j.epsl.2020.116409 |bibcode=2020E&PSL.54516409Z |s2cid=225256458 |issn=0012-821X|url-access=subscription }}</ref><ref name="NG-20181017">{{cite web |last=Wei-Haas |first=Maya |date=17 October 2018 |title='World's oldest fossils' may just be pretty rocks — Analysis of 3.7-billion-year-old outcrops has reignited controversy over when life on Earth began. |url=https://www.nationalgeographic.com/science/2018/10/news-oldest-stromatolite-fossilized-life-rocks-greenland/ |access-date=19 October 2018 |work=[[National Geographic]]}}</ref>

====Martian rocks and meteorites====
[[File:ALH84001 structures.jpg|thumb|right|Some researchers suggested that these microscopic structures on the Martian [[ALH84001]] meteorite could be fossilized bacteria.<ref name="disbelief">{{cite web|url=https://www.usatoday.com/tech/science/space/2006-08-06-mars-life_x.htm|title=After 10 years, few believe life on Mars|last=Crenson|first=Matt | name-list-style = vanc |date=2006-08-06|publisher=[[Associated Press]] (on usatoday.com)|access-date=2009-12-06}}</ref><ref name="life">{{cite journal | vauthors = McKay DS, Gibson EK, Thomas-Keprta KL, Vali H, Romanek CS, Clemett SJ, Chillier XD, Maechling CR, Zare RN | s2cid = 40690489 | display-authors = 6 | title = Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001 | journal = Science | volume = 273 | issue = 5277 | pages = 924–30 | date = August 1996 | pmid = 8688069 | doi = 10.1126/science.273.5277.924 | bibcode = 1996Sci...273..924M }}</ref>]]
Microscopic [[magnetite]] crystals in the Martian [[meteorite]] [[ALH84001]]<ref name="life"/><ref>{{cite journal | vauthors = Friedmann EI, Wierzchos J, Ascaso C, Winklhofer M | title = Chains of magnetite crystals in the meteorite ALH84001: evidence of biological origin | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 5 | pages = 2176–81 | date = February 2001 | pmid = 11226212 | pmc = 30112 | doi = 10.1073/pnas.051514698 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Thomas-Keprta KL, Clemett SJ, Bazylinski DA, Kirschvink JL, McKay DS, Wentworth SJ, Vali H, Gibson EK, McKay MF, Romanek CS | display-authors = 6 | title = Truncated hexa-octahedral magnetite crystals in ALH84001: presumptive biosignatures | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 5 | pages = 2164–9 | date = February 2001 | pmid = 11226210 | pmc = 30110 | doi = 10.1073/pnas.051500898 | doi-access = free }}</ref> are one of the longest-debated of several potential biosignatures in that specimen.<ref>{{Cite web|url=https://www.space.com/33690-allen-hills-mars-meteorite-alien-life-20-years.html|title=Mars Life? 20 Years Later, Debate Over Meteorite Continues |first=Charles Q. | last =Choi | name-list-style = vanc | date = August 2016 |website=Space.com|access-date=2019-06-07}}</ref> The possible [[Biomineralisation|biomineral]] studied in ALH84001 includes putative microbial fossils, tiny rock-like structures whose shape was a potential biosignature because it resembled known bacteria. Most scientists ultimately concluded that these were far too small to be fossilized [[cell (biology)|cell]]s.<ref>{{Citation|last=McSween|first=Harry Y.| name-list-style = vanc |chapter=The Search for Biosignatures in Martian Meteorite Allan Hills 84001|date=2019|pages=167–182|editor-last=Cavalazzi|editor-first=Barbara|series=Advances in Astrobiology and Biogeophysics|publisher=Springer International Publishing|doi=10.1007/978-3-319-96175-0_8|isbn=978-3-319-96175-0|editor2-last=Westall|editor2-first=Frances |title=Biosignatures for Astrobiology|s2cid=186696892 }}</ref> A consensus that has emerged from these discussions, and is now seen as a critical requirement, is the demand for further lines of evidence in addition to any morphological data that supports such extraordinary claims.<ref name="SSG" /> Currently, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection".<ref name="morphology">{{cite book |last1=Garcia-Ruiz |first1=Juan-Manuel Garcia-Ruiz |editor-first1=Richard B. |editor-last1=Hoover |chapter=Morphological behavior of inorganic precipitation systems |name-list-style=vanc |date=December 30, 1999 |title=Instruments, Methods, and Missions for Astrobiology II |volume=SPIE Proceedings 3755 |page=74 |doi=10.1117/12.375088 |s2cid=84764520 |quote=It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection".}}</ref><ref>{{cite news|url=http://astrobiology.ucla.edu/pages/res3e.html|title=Detection and geochemical characterization of Earth's earliest life|author=Agresti|date=3 December 2008|work=NASA Astrobiology Institute|access-date=2013-01-15|archive-url=https://web.archive.org/web/20130123132429/http://astrobiology.ucla.edu/pages/res3e.html|archive-date=23 January 2013|publisher=NASA|author2=House|author3=Jögi|author4=Kudryavstev|author5=McKeegan|author6=Runnegar|author7=Schopf|author8=Wdowiak}}</ref><ref>{{cite journal|vauthors=Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB|date=28 April 2007|title=Evidence of Archean life: Stromatolites and microfossils|url=http://www.cornellcollege.edu/geology/courses/greenstein/paleo/schopf_07.pdf|journal=Precambrian Research|volume=158|issue=3–4|pages=141–155|bibcode=2007PreR..158..141S|doi=10.1016/j.precamres.2007.04.009|access-date=2013-01-15|archive-date=2012-12-24|archive-url=https://web.archive.org/web/20121224202951/http://www.cornellcollege.edu/geology/courses/greenstein/paleo/schopf_07.pdf}}</ref> Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.<ref name="morphology" />

===Chemistry===
No single compound will prove life once existed. Rather, it will be distinctive patterns present in any organic compounds showing a process of selection.<ref name="PhysOrg Cousins 2018">{{cite news |url=https://phys.org/news/2018-01-rover-life-mars-proveit.html |title=Rover could discover life on Mars – here's what it would take to prove it |first=Claire |last=Cousins |work=PhysOrg |date=5 January 2018}}</ref> For example, [[membrane lipid]]s left behind by degraded cells will be concentrated, have a limited size range, and comprise an even number of carbons. Similarly, life only uses left-handed amino acids.<ref name="PhysOrg Cousins 2018" /> Biosignatures need not be chemical, however, and can also be suggested by a distinctive [[magnetic]] biosignature.<ref name="Wall-20111213">{{cite news|url=http://www.space.com/13911-mars-life-search-magnetic-signatures.html|title=Mars Life Hunt Could Look for Magnetic Clues |last=Wall|first=Mike |date=13 December 2011|work=[[Space.com]]|access-date=2011-12-15}}</ref>

[[File:Biomarkers (petroleum).png|thumb|upright|Structures of prime examples of biomarkers (petroleum), from top to bottom: Pristane, Triterpane, Sterane, Phytane and Porphyrin]]

Chemical biosignatures include any suite of complex [[organic compounds]] composed of carbon, hydrogen, and other elements or heteroatoms such as oxygen, nitrogen, and sulfur, which are found in [[Petroleum|crude oils]], [[bitumen]], [[petroleum]] [[source rock]] and eventually show simplification in molecular structure from the parent organic molecules found in all living organisms. They are complex carbon-based molecules derived from formerly [[living organisms]].<ref name="biomarker">Wang, Z.; Stout, S.; Fingas, M. ''Environmental Forensics'', 2006 7, 105-146.</ref> Each biomarker is quite distinctive when compared to its counterparts, as the time required for [[organic matter]] to convert to crude oil is characteristic.<ref>{{cite journal |last1=Stevens |first1=Douglas |last2=Hsu |first2=Chang Samuel |last3=Shi |first3=Quan |title=Petroleum biomarkers analyzed by atmospheric gas chromatography-tandem mass spectroscopy |journal=Waters |url=https://www.waters.com/webassets/cms/library/docs/2013asms_stevens_apgc_petrobiomarkers.pdf |date=2013}}</ref> Most biomarkers also usually have high [[molecular mass]].<ref>{{cite journal |last1=Osadetz |first1=K.G |last2=Pasadakis |first2=N. |last3=Obermajer |first3=M. |title=Definition and characterization of petroleum compositional families using principal component analysis of gasoline and saturate fraction composition ratios |journal=Energy and Resources |date=2002 |volume=1 |pages=3–14 |url=http://publications.gov.sk.ca/documents/310/88704-osadetz.pdf}}</ref>

Some examples of biomarkers found in petroleum are [[pristane]], [[Oleanane|triterpanes]], [[sterane]]s, [[phytane]] and [[porphyrin]]. Such petroleum biomarkers are produced via [[chemical synthesis]] using biochemical compounds as their main constituents. For instance, triterpenes are derived from biochemical compounds found on land [[angiosperm]] plants.<ref>{{cite book |last1=Hsu |first1=Chang S. |last2=Walters |first2=Clifford |last3=Peters |first3=Kenneth E. |title=Analytical advances for hydrocarbon research |date=2003 |pages=223–245}}</ref> The abundance of petroleum biomarkers in small amounts in its reservoir or source rock make it necessary to use sensitive and differential approaches to analyze the presence of those compounds. The techniques typically used include [[gas chromatography]] and [[mass spectrometry]].<ref>{{cite book |last1=Niessen |first1=Wilfried M.A. |title=Current Practice of gas chromatography-mass spectrometry |date=2001 |pages=55–94 |edition=1 |url=https://www.crcpress.com/Current-Practice-of-Gas-Chromatography-Mass-Spectrometry/Niessen/p/book/9780367397425}}</ref>

Petroleum biomarkers are highly important in petroleum inspection as they help indicate the depositional territories and determine the geological properties of oils. For instance, they provide more details concerning their maturity and the source material.<ref>{{cite journal |last1=Chosson |first1=P |last2=Lanau |first2=C |last3=Connan |first3=J |last4=Dessort |first4=D |title=Biodegradation of refractory hydrocarbon biomarkers from petroleum under laboratory conditions |journal=Nature |date=1991 |volume=351 |issue=6328 |pages=640–642 |doi=10.1038/351640a0 |pmid=2052089 |bibcode=1991Natur.351..640C |s2cid=4305795 }}</ref> In addition to that they can also be good parameters of age, hence they are technically referred to as "chemical fossils".<ref>{{cite book |last1=Wang |first1=Zhendi |last2=Stout |first2=Scott A. |title=Oil spill environmental forensics: fingerprinting and source identification |date=2007 |pages=1–53}}</ref> The ratio of pristane to phytane (pr:ph) is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.<ref>{{cite journal |last1=Roushdy |first1=M.I. |last2=El Nady |first2=M.M. |last3=Mostafa |first3=Y.M. |last4=El Gendy |first4=N.Sh. |last5=Ali |first5=H.R. |s2cid=55952894 |title=Biomarkers characteristics of crude oils from some oilfields in the gulf of suez, egypt. |journal=Journal of American Science |date=2010 |volume=6 |issue=11 }}</ref>

[[Geologists]] and [[geochemist]]s use biomarker traces found in crude oils and their related [[source rock]] to unravel the stratigraphic origin and migration patterns of presently existing [[petroleum]] deposits.<ref>{{cite journal |last1=Head |first1=Ian M. |last2=Jones |first2=Martin |last3=Larter |first3=Steve R. |title=Biological activity in the deep subsurface and the origin of heavy oil |journal=Nature |date=2003 |volume=426 |issue=6964 |pages=344–352 |url=http://uig.permedia.ca/pdf/nature_biodegradation.pdf|doi=10.1038/nature02134 |pmid=14628064 |bibcode=2003Natur.426..344H |s2cid=4372154 }}</ref> The dispersion of biomarker molecules is also quite distinctive for each type of oil and its source; hence, they display unique fingerprints. Another factor that makes petroleum biomarkers more preferable than their counterparts is that they have a high tolerance to environmental weathering and corrosion.<ref>{{cite journal |last1=Ashton |first1=Buffy M. |last2=East |first2=Rebecca S. |last3=Walsh |first3=Maud M. |last4=Miles |first4=Scott |last5=Obeton |first5=Edward B. |s2cid=201925529 |title=Studying and Verifying the Use of Chemical Biomarkers for Identifying and Quantitating Oil Residues in the Environment |journal=Journal of Ocean and Climate Systems |date=2000 |pages=1–54}}</ref> Such biomarkers are very advantageous and often used in the detection of [[oil spill]]age in the major waterways.<ref name="biomarker"/> The same biomarkers can also be used to identify contamination in [[lubricant]] oils.<ref>{{cite journal |last1=Bieger |first1=Tilman |last2=Hellou |first2=Jocelyne |last3=Abrajano Jr. |first3=Teofilou A. |title=Petroleum biomarkers as tracers of lubricating oil contamination |journal=Marine Pollution Bulletin |date=1996 |volume=32 |issue=2 |pages=270–274 |doi=10.1016/0025-326X(95)00151-C |bibcode=1996MarPB..32..270B }}</ref> However, biomarker analysis of untreated rock cuttings can be expected to produce misleading results. This is due to potential hydrocarbon contamination and [[biodegradation]] in the rock samples.<ref>{{Cite journal|last1=Ratnayake|first1=Amila Sandaruwan|last2=Sampei|first2=Yoshikazu|date=2019-06-01|title=Organic geochemical evaluation of contamination tracers in deepwater well rock cuttings from the Mannar Basin, Sri Lanka|journal=Journal of Petroleum Exploration and Production Technology|language=en|volume=9|issue=2|pages=989–996|doi=10.1007/s13202-018-0575-8|issn=2190-0566|doi-access=free|bibcode=2019JPEPT...9..989R }}</ref>

====Possible Bioorganic chemistry on Mars====
[[File:1-PIA26368-Perseverance Finds a Rock with Leopard Spots-annotated.png|thumb|[[Cheyava Falls]] discovered at [[Jezero (crater)|Jezero crater]] in 2024 displays elements often associated with microbial life.]]
[[File:Largest Organics Yet Discovered on Mars (SVS14808 - NASA-Mars-Hydrocarbons-2025-Small).jpg|thumb|Discovery of long-chain hydrocarbons by [[Sample Analysis at Mars|SAM]] at [[Gale Crater]] in 2025. These [[fatty acids]] are the key components of [[cell membrane]]s.]]
Organic chemistry has been discovered on Mars, some of which can be explained by geochemical processes. [[Chlorobenzene]] ({{chem|C|6|H|5|Cl}}), for example, was detected as early as the [[Viking lander biological experiments]]<ref name="t513">{{cite journal | last1=Guzman | first1=Melissa | last2=McKay | first2=Christopher P. | last3=Quinn | first3=Richard C. | last4=Szopa | first4=Cyril | last5=Davila | first5=Alfonso F. | last6=Navarro-González | first6=Rafael | last7=Freissinet | first7=Caroline | title=Identification of Chlorobenzene in the Viking Gas Chromatograph-Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars | journal=Journal of Geophysical Research: Planets | volume=123 | issue=7 | date=2018 | issn=2169-9097 | doi=10.1029/2018JE005544 | doi-access=free | pages=1674–1683 | bibcode=2018JGRE..123.1674G | url=https://insu.hal.science/insu-01820363/document | access-date=2025-04-13}}</ref> and later in sedimentary rocks by Curiosity.<ref>{{Cite journal |last1=Freissinet |first1=C. |last2=Glavin |first2=D. P. |last3=Mahaffy |first3=P. R. |last4=Miller |first4=K. E. |last5=Eigenbrode |first5=J. L. |last6=Summons |first6=R. E. |last7=Brunner |first7=A. E. |last8=Buch |first8=A. |last9=Szopa |first9=C. |last10=Archer |first10=P. D. |last11=Franz |first11=H. B. |last12=Atreya |first12=S. K. |last13=Brinckerhoff |first13=W. B. |last14=Cabane |first14=M. |last15=Coll |first15=P. |display-authors=1 |year=2015 |title=Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars |journal=Journal of Geophysical Research: Planets |volume=120 |issue=3 |pages=495–514 |bibcode=2015JGRE..120..495F |doi=10.1002/2014JE004737 |issn=2169-9097 |pmc=4672966 |pmid=26690960 |doi-access=free |first16=P. G. |last16=Conrad |first17=D. J. |last17=Des Marais |first18=J. P. |last18=Dworkin |first19=A. G. |last19=Fairén |first20=P. |last20=François |first21=J. P. |last21=Grotzinger |first22=S. |last22=Kashyap |first23=I. L. |last23=ten Kate |first24=L. A. |last24=Leshin |first25=C. A. |last25=Malespin |first26=M. G. |last26=Martin |first27=F. J. |last27=Martin-Torres |first28=A. C. |last28=McAdam |first29=D. W. |last29=Ming |first30=R. |last30=Navarro-González |first31=A. A. |last31=Pavlov |first32=B. D. |last32=Prats |first33=S. W. |last33=Squyres |first34=A. |last34=Steele |first35=J. C. |last35=Stern |first36=D. Y. |last36=Sumner |first37=B. |last37=Sutter |first38=M.-P. |last38=Zorzano |author39=the MSL Science Team}}</ref> However some abiotic source, such as a [[Fischer–Tropsch process]], could also have produced it.<ref name="k414">{{cite journal | last1=Freissinet | first1=Caroline | last2=Glavin | first2=Daniel P. | last3=Archer | first3=P. Douglas | last4=Teinturier | first4=Samuel | last5=Buch | first5=Arnaud | last6=Szopa | first6=Cyril | last7=Lewis | first7=James M. T. | last8=Williams | first8=Amy J. | last9=Navarro-Gonzalez | first9=Rafael | last10=Dworkin | first10=Jason P. | last11=Franz | first11=Heather. B. | last12=Millan | first12=Maëva | last13=Eigenbrode | first13=Jennifer L. | last14=Summons | first14=R. E. | last15=House | first15=Christopher H. | last16=Williams | first16=Ross H. | last17=Steele | first17=Andrew | last18=McIntosh | first18=Ophélie | last19=Gómez | first19=Felipe | last20=Prats | first20=Benito | last21=Malespin | first21=Charles A. | last22=Mahaffy | first22=Paul R. | title=Long-chain alkanes preserved in a Martian mudstone | journal=Proceedings of the National Academy of Sciences | volume=122 | issue=13 | date=2025 | article-number=e2420580122 | issn=0027-8424 | doi=10.1073/pnas.2420580122 | doi-access=free | pmid=40127274 | pmc=12002291 | bibcode=2025PNAS..12220580F }}</ref> 

Some discoveries have been found in areas confirmed previously be wet, adding weight to their significance. In 2018 at Gale Crater, Curiosity discovered [[Thiophene]] ({{chem|C|4|H|4|S}}) and polymers ([[Polythiophene]]).<ref name="k422">{{cite journal | last1=Westall | first1=Frances | last2=Foucher | first2=Frédéric | last3=Bost | first3=Nicolas | last4=Bertrand | first4=Marylène | last5=Loizeau | first5=Damien | last6=Vago | first6=Jorge L. | last7=Kminek | first7=Gerhard | last8=Gaboyer | first8=Frédéric | last9=Campbell | first9=Kathleen A. | last10=Bréhéret | first10=Jean-Gabriel | last11=Gautret | first11=Pascale | last12=Cockell | first12=Charles S. | title=Biosignatures on Mars: What, Where, and How? Implications for the Search for Martian Life | journal=Astrobiology | volume=15 | issue=11 | date=2015 | issn=1531-1074 | pmid=26575218 | pmc=4653824 | doi=10.1089/ast.2015.1374 | doi-access=free | pages=998–1029 | bibcode=2015AsBio..15..998W }}</ref> Natural sulfur reduction has been proposed as a possible abiotic source.<ref name="n120">{{cite journal | last1=Heinz | first1=Jacob | last2=Schulze-Makuch | first2=Dirk | title=Thiophenes on Mars: Biotic or Abiotic Origin? | journal=Astrobiology | volume=20 | issue=4 | date=2020-04-01 | issn=1531-1074 | doi=10.1089/ast.2019.2139 | doi-access=free | pages=552–561 | pmid=32091933 | bibcode=2020AsBio..20..552H }}</ref> [[Dimethyl sulfide]] ({{chem|CH|2|S}}), another organic molecule chemical associated with microbial life, was also detected.<ref name="EigenbrodeSummons2018">{{cite journal|last1=Eigenbrode|first1=Jennifer L.|last2=Summons|first2=Roger E.|last3=Steele|first3=Andrew|last4=Freissinet|first4=Caroline|last5=Millan|first5=Maëva|last6=Navarro-González|first6=Rafael|last7=Sutter|first7=Brad|last8=McAdam|first8=Amy C.|last9=Franz|first9=Heather B.|last10=Glavin|first10=Daniel P.|last11=Archer|first11=Paul D.|last12=Mahaffy|first12=Paul R.|last13=Conrad|first13=Pamela G.|last14=Hurowitz|first14=Joel A.|last15=Grotzinger|first15=John P.|last16=Gupta|first16=Sanjeev|last17=Ming|first17=Doug W.|last18=Sumner|first18=Dawn Y.|last19=Szopa|first19=Cyril|last20=Malespin|first20=Charles|last21=Buch|first21=Arnaud|last22=Coll|first22=Patrice|title=Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars|journal=Science|volume=360|issue=6393|year=2018|pages=1096–1101|issn=0036-8075|doi=10.1126/science.aas9185|pmid=29880683|bibcode=2018Sci...360.1096E|s2cid=46983230|url=http://spiral.imperial.ac.uk/bitstream/10044/1/60810/2/aas9185_CombinedPDF_v2.pdf|doi-access=free}}</ref> However photochemistry has been proposed to explain its presence elsewhere, such as on comets.<ref name="h224">{{cite journal | arxiv=2501.08892 | doi=10.3847/2041-8213/adafa7 | doi-access=free | title=On the Abiotic Origin of Dimethyl Sulfide: Discovery of Dimethyl Sulfide in the Interstellar Medium | date=2025 | last1=Sanz-Novo | first1=Miguel | last2=Rivilla | first2=Víctor M. | last3=Endres | first3=Christian P. | last4=Lattanzi | first4=Valerio | last5=Jiménez-Serra | first5=Izaskun | last6=Colzi | first6=Laura | last7=Zeng | first7=Shaoshan | last8=Megías | first8=Andrés | last9=López-Gallifa | first9=Álvaro | last10=Martínez-Henares | first10=Antonio | last11=San Andrés | first11=David | last12=Tercero | first12=Belén | last13=De Vicente | first13=Pablo | last14=Martín | first14=Sergio | last15=Requena-Torres | first15=Miguel A. | last16=Caselli | first16=Paola | last17=Martín-Pintado | first17=Jesús | journal=The Astrophysical Journal Letters | volume=980 | issue=2 | pages=L37 | bibcode=2025ApJ...980L..37S }}</ref> In [[Cheyava Falls]] discovered by [[Perseverance (rover)|Perseverance]] in July 2024, [[kerogen]]-like compounds were found.<ref name="n167">{{cite journal | last1=Shen | first1=JianXun | last2=Chen | first2=Yan | last3=Sun | first3=Yu | last4=Liu | first4=Li | last5=Pan | first5=YongXin | last6=Lin | first6=Wei | title=Detection of biosignatures in terrestrial Mars analogs: Strategical and technical assessments | journal=Earth and Planetary Physics | volume=6 | date=2022 | issn=2096-3955 | doi=10.26464/epp2022042 | doi-access=free | page=0 | doi-broken-date=3 September 2025 | url=http://www.eppcgs.org/en/article/pdf/preview/10.26464/epp2022042 | access-date=2025-03-15}}</ref> Formed in the presence of water, it features millimeter-sized splotches resembling "leopard spots" containing [[iron]] and [[phosphate]], elements often associated with microbial life.<ref name="n167" /> In 2025, analysis of rocks from Gale Crater by [[Sample Analysis at Mars|SAM]] found [[decane]] ({{chem|C|10|H|22}}), [[dodecane]] (C<sub>12</sub>H<sub>26</sub>) and [[undecane]] (CH<sub>3</sub>(CH<sub>2</sub>)<sub>9</sub>CH<sub>3</sub>), collectively known as [[fatty acids]], which terrestrial cell membranes are made of.<ref name="k414"/> However these have formed on meteorites, which may have delivered them to Mars.<ref name="k414" />

=== Atmospheric ===

The atmospheric properties of exoplanets are of particular importance, as atmospheres provide the most likely observables for the near future, including habitability indicators and biosignatures.<ref name="NYT-20220915">{{cite news |last=Gertner |first=Jon |title=The Search for Intelligent Life Is About to Get a Lot More Interesting - There are an estimated 100 billion galaxies in the universe, home to an unimaginable abundance of planets. And now, there are new ways to spot signs of life on them. |url=https://www.nytimes.com/2022/09/15/magazine/extraterrestrials-technosignatures.html |date=15 September 2022 |work=[[The New York Times]] |access-date=15 September 2022 }}</ref> Over billions of years, the processes of life on a planet would result in a mixture of chemicals unlike anything that could form in an ordinary chemical equilibrium.<ref name="Lisse 2020" /><ref>{{cite web|url=https://www.technologyreview.com/blog/arxiv/26247/|title=Artificial Life Shares Biosignature With Terrestrial Cousins|date=10 January 2011|work=The Physics arXiv Blog|publisher=MIT|access-date=2011-01-14|archive-date=2018-10-23|archive-url=https://web.archive.org/web/20181023073445/https://www.technologyreview.com/s/422342/artificial-life-shares-biosignature-with-terrestrial-cousins/}}</ref><ref name="Seager 2017">{{cite journal | vauthors = Seager S, Bains W, Petkowski JJ | title = Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry | journal = Astrobiology | volume = 16 | issue = 6 | pages = 465–85 | date = June 2016 | pmid = 27096351 | doi = 10.1089/ast.2015.1404 | bibcode = 2016AsBio..16..465S | url = https://dspace.mit.edu/bitstream/1721.1/109943/1/Seager_Toward%20a%20List%20of%20Molecules%20as.pdf | hdl = 1721.1/109943 | s2cid = 4350250 | hdl-access = free }}</ref> For example, large amounts of [[oxygen]] and small amounts of [[methane]] are generated by life on Earth.

An exoplanet's color—or reflectance spectrum—can also be used as a biosignature due to the effect of pigments that are uniquely biologic in origin such as the pigments of [[phototroph]]ic and photosynthetic life forms.<ref>{{Cite journal|last1=DasSarma|first1=Shiladitya|last2=Schwieterman|first2=Edward W.|title=Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures|url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/early-evolution-of-purple-retinal-pigments-on-earth-and-implications-for-exoplanet-biosignatures/63A1AD8AF544BEEF4C6D4A2D53130327|journal=International Journal of Astrobiology|language=en|pages=1–10|doi=10.1017/S1473550418000423|arxiv=1810.05150|bibcode=2018arXiv181005150D|issn=1473-5504|year=2018|volume=20|issue=3|s2cid=119341330}}</ref><ref name="Berdyugina 2016">{{cite journal|last1=Berdyugina|first1=Svetlana V.|last2=Kuhn|first2=Jeff|last3=Harrington|first3=David|last4=Santl-Temkiv|first4=Tina|last5=Messersmith|first5=E. John|date=January 2016|title=Remote sensing of life: polarimetric signatures of photosynthetic pigments as sensitive biomarkers|journal=International Journal of Astrobiology|volume=15|issue=1|pages=45–56|bibcode=2016IJAsB..15...45B|doi=10.1017/S1473550415000129|name-list-style=vanc|doi-access=free}}<!--|access-date=2016-05-07 --></ref><ref>{{cite journal | vauthors = Hegde S, Paulino-Lima IG, Kent R, Kaltenegger L, [[Lynn J. Rothschild|Rothschild L]] | title = Surface biosignatures of exo-earths: remote detection of extraterrestrial life | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 13 | pages = 3886–91 | date = March 2015 | pmid = 25775594 | pmc = 4386386 | doi = 10.1073/pnas.1421237112 | bibcode = 2015PNAS..112.3886H | doi-access = free }}</ref><ref name="ColorCatalog">{{cite news|url=http://www.space.com/28906-alien-life-earth-microbe-catalog.html|title=Catalog of Earth Microbes Could Help Find Alien Life|last=Cofield|first=Calla| name-list-style = vanc |date=30 March 2015|work=Space.com|access-date=2015-05-11}}</ref><ref>{{cite journal |last1=Claudi |first1=R. |last2=Erculiani |first2=M. S. |last3=Galletta |first3=G. |last4=Billi |first4=D. |last5=Pace |first5=E. |last6=Schierano |first6=D. |last7=Giro |first7=E. |last8=D'Alessandro |first8=M. |title=Simulating super earth atmospheres in the laboratory |journal=International Journal of Astrobiology |date=20 May 2015 |volume=15 |issue=1 |pages=35–44 |doi=10.1017/S1473550415000117 |s2cid=125008098 }}</ref> Scientists use the Earth as an example of this when looked at from far away (see [[Pale Blue Dot]]) as a comparison to worlds observed outside of the [[Solar System]].<ref>{{cite journal | vauthors = Krissansen-Totton J, Schwieterman EW, Charnay B, Arney G, Robinson TD, Meadows V, Catling DC | title = Is the Pale Blue Dot unique? Optimized photometric bands for identifying Earth-like exoplanets. | journal = The Astrophysical Journal | date = January 2016 | volume = 817 | issue = 1 | page = 31 |doi=10.3847/0004-637X/817/1/31|bibcode=2016ApJ...817...31K|arxiv=1512.00502 | s2cid = 119211858 | doi-access = free }}</ref> Ultraviolet radiation on life forms could also induce [[biofluorescence]] in visible wavelengths that may be detected by the new generation of space observatories under development.<ref name="EA-20190813">{{cite news |author=Cornell University |title=Fluorescent glow may reveal hidden life in the cosmos |url=https://www.eurekalert.org/pub_releases/2019-08/cu-fgm081319.php |date=13 August 2019 |work=[[EurekAlert!]] |access-date=13 August 2019 |author-link=Cornell University |archive-date=13 August 2019 |archive-url=https://web.archive.org/web/20190813234535/https://www.eurekalert.org/pub_releases/2019-08/cu-fgm081319.php }}</ref><ref>{{cite journal | doi = 10.1093/mnras/stz1842 | arxiv = 1608.06930 | volume=488 | title=Biofluorescent Worlds – II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature | year=2019 | journal=Monthly Notices of the Royal Astronomical Society | pages=4530–4545 | last1 = O'Malley-James | first1 = Jack T | last2 = Kaltenegger | first2 = Lisa| issue = 4 | doi-access = free | bibcode = 2019MNRAS.488.4530O | s2cid = 118394043 }}</ref>

Some scientists have reported methods of detecting hydrogen and methane in [[extraterrestrial atmospheres]].<ref name="Nature-20120627">{{cite journal|vauthors=Brogi M, Snellen IA, de Kok RJ, Albrecht S, Birkby J, de Mooij EJ|date=June 2012|title=The signature of orbital motion from the dayside of the planet τ Boötis b|journal=Nature|volume=486|issue=7404|pages=502–4|arxiv=1206.6109|bibcode=2012Natur.486..502B|doi=10.1038/nature11161|pmid=22739313|s2cid=4368217}}</ref><ref name="Wired-20120627">{{cite magazine|last=Mann|first=Adam|date=June 27, 2012|title=New View of Exoplanets Will Aid Search for E.T.|url=https://www.wired.com/wiredscience/2012/06/tau-bootis-b/|magazine=[[Wired (magazine)|Wired]]|access-date=June 28, 2012}}</ref> Habitability indicators and biosignatures must be interpreted within a planetary and environmental context.<ref name="NASA strategy 2015" /> For example, the presence of oxygen and methane together could indicate the kind of extreme thermochemical disequilibrium generated by life.<ref>[https://www.physics.umd.edu/courses/Phys371/AnlageSpring17/Where%20are%20they.pdf Where are they?] (PDF) Mario Livio and Joseph Silk. ''Physics Today'', March 2017.</ref> Two of the top 14,000 proposed atmospheric biosignatures are [[dimethyl sulfide]] and [[chloromethane]] ({{chem|CH|3|Cl}}).<ref name="Seager 2017" /> An alternative biosignature is the combination of methane and carbon dioxide.<ref name="SPC-20180124">{{cite web|url=https://www.space.com/39476-alien-life-biosignature-gases-oxygen.html|title=Alien Life Hunt: Oxygen Isn't the Only Possible Sign of Life|last=Wall|first=Mike|date=24 January 2018|work=[[Space.com]]|access-date=24 January 2018}}</ref><ref name="SA-20180124">{{cite journal|last1=Krissansen-Totton|first1=Joshua|last2=Olson|first2=Stephanie|last3=Catlig|first3=David C.|date=24 January 2018|title=Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life|journal=Science Advances|volume=4|article-number=eaao5747|doi=10.1126/sciadv.aao5747|pmid=29387792|pmc=5787383|arxiv=1801.08211|bibcode=2018SciA....4.5747K|name-list-style=vanc|number=1, eaao5747}}</ref>

[[File:CH4 disequilibrium.png|thumb|Biogenic methane production is the main contributor to the methane flux coming from the surface of Earth. Methane has a photochemical sink in the atmosphere but will build up if the flux is high enough. If there is detectable methane in the atmosphere of another planet, especially with a host star of G or K type, this may be interpreted as a viable biosignature.<ref>{{Cite journal|last=Arney|first=Giada N.|date= March 2019 |title=The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets|journal=The Astrophysical Journal|volume=873|issue=1|pages=L7|doi=10.3847/2041-8213/ab0651|arxiv=2001.10458|issn=2041-8205|bibcode=2019ApJ...873L...7A|s2cid=127742050 |doi-access=free }}</ref>]]
A disequilibrium in the abundance of gas species in an atmosphere can be interpreted as a biosignature. Life has greatly altered the atmosphere on Earth in a way that would be unlikely for any other processes to replicate. Therefore, a departure from equilibrium is evidence for a biosignature.<ref name=":0" /><ref name=":1" /><ref name=":2">{{cite journal | vauthors = Krissansen-Totton J, Bergsman DS, Catling DC | title = On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres | journal = Astrobiology | volume = 16 | issue = 1 | pages = 39–67 | date = January 2016 | pmid = 26789355 | doi = 10.1089/ast.2015.1327 | arxiv = 1503.08249 | bibcode = 2016AsBio..16...39K | s2cid = 26959254 }}</ref><ref>{{Cite journal|last1=Lovelock James Ephraim|last2=Kaplan I. R.|last3=Pirie Norman Wingate|date=1975-05-06|title=Thermodynamics and the recognition of alien biospheres|journal=Proceedings of the Royal Society of London. Series B. Biological Sciences|volume=189|issue=1095|pages=167–181|doi=10.1098/rspb.1975.0051|bibcode=1975RSPSB.189..167L|s2cid=129105448}}</ref> For example, the abundance of methane in the Earth's atmosphere is orders of magnitude above the equilibrium value due to the constant methane flux that life on the surface emits.<ref name=":2" /><ref>{{cite journal | vauthors = Krissansen-Totton J, Arney GN, Catling DC | title = Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 16 | pages = 4105–4110 | date = April 2018 | pmid = 29610313 | pmc = 5910859 | doi = 10.1073/pnas.1721296115 | arxiv = 1804.00763 | bibcode = 2018PNAS..115.4105K | doi-access = free }}</ref> Depending on the host star, a disequilibrium in the methane abundance on another planet may indicate a biosignature.<ref>{{Cite journal|last=Arney|first=Giada N.|date=March 2019 |title=The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets|journal=The Astrophysical Journal|volume=873|issue=1|pages=L7|doi=10.3847/2041-8213/ab0651|arxiv=2001.10458|issn=2041-8205|bibcode=2019ApJ...873L...7A|doi-access=free}}</ref>

==== Atmosphere of Mars ==== 
{{main|Methane on Mars}}
[[File:PIA19088-MarsCuriosityRover-MethaneSource-20141216.png|thumb|250px|[[Atmosphere of Mars#Methane|Methane]] (CH<sub>4</sub>) on Mars - potential sources and sinks.]]

The [[atmosphere of Mars]] contains some gases which have been studied as potential biosignatures, most notably methane, but also ozone and oxygen. 

Mars has traces of ozone (including a seasonal ozone layer over the south pole in winter) and oxygen in its atmosphere both byproducts of life on Earth. [[Mariner 7]] detected Ozone in 1971<ref name="a485">{{cite journal | last1=Montmessin | first1=Franck | last2=Lefèvre | first2=Franck | title=Transport-driven formation of a polar ozone layer on Mars | journal=Nature Geoscience | volume=6 | issue=11 | date=2013 | issn=1752-0894 | doi=10.1038/ngeo1957 | pages=930–933 | bibcode=2013NatGe...6..930M | url=https://www.nature.com/articles/ngeo1957 | access-date=2025-08-01| url-access=subscription }}</ref><ref name="p548">{{cite journal | last=Lindner | first=Bernhard Lee | title=Ozone on Mars: The effects of clouds and airborne dust | journal=Planetary and Space Science | volume=36 | issue=2 | date=1988 | doi=10.1016/0032-0633(88)90049-9 | pages=125–144 | bibcode=1988P&SS...36..125L | url=https://linkinghub.elsevier.com/retrieve/pii/0032063388900499 | access-date=2025-08-01| url-access=subscription }}</ref> and in 1976 by the Viking biological experiments. Significant levels of oxygen were detected in Gale Crater by Curiosity Rover in 2019 with seasonal variability that has not fully been explained.<ref name="m083">{{cite journal | last1=Trainer | first1=Melissa G. | last2=Wong | first2=Michael H. | last3=McConnochie | first3=Timothy H. | last4=Franz | first4=Heather B. | last5=Atreya | first5=Sushil K. | last6=Conrad | first6=Pamela G. | last7=Lefèvre | first7=Franck | last8=Mahaffy | first8=Paul R. | last9=Malespin | first9=Charles A. | last10=Manning | first10=Heidi L.K. | last11=Martín-Torres | first11=Javier | last12=Martínez | first12=Germán M. | last13=McKay | first13=Christopher P. | last14=Navarro-González | first14=Rafael | last15=Vicente-Retortillo | first15=Álvaro | last16=Webster | first16=Christopher R. | last17=Zorzano | first17=María-Paz | title=Seasonal Variations in Atmospheric Composition as Measured in Gale Crater, Mars | journal=Journal of Geophysical Research: Planets | volume=124 | issue=11 | date=2019 | issn=2169-9097 | doi=10.1029/2019JE006175 | doi-access=free | pages=3000–3024 | bibcode=2019JGRE..124.3000T | hdl=10261/206428 | hdl-access=free }}</ref> Studies indicate that the Martian atmosphere was once oxygen-rich.<ref name="y762">{{cite journal | last1=Lanza | first1=Nina L. | last2=Wiens | first2=Roger C. | last3=Arvidson | first3=Raymond E. | last4=Clark | first4=Benton C. | last5=Fischer | first5=Woodward W. | last6=Gellert | first6=Ralf | last7=Grotzinger | first7=John P. | last8=Hurowitz | first8=Joel A. | last9=McLennan | first9=Scott M. | last10=Morris | first10=Richard V. | last11=Rice | first11=Melissa S. | last12=Bell | first12=James F. | last13=Berger | first13=Jeffrey A. | last14=Blaney | first14=Diana L. | last15=Bridges | first15=Nathan T. | last16=Calef | first16=Fred | last17=Campbell | first17=John L. | last18=Clegg | first18=Samuel M. | last19=Cousin | first19=Agnes | last20=Edgett | first20=Kenneth S. | last21=Fabre | first21=Cécile | last22=Fisk | first22=Martin R. | last23=Forni | first23=Olivier | last24=Frydenvang | first24=Jens | last25=Hardy | first25=Keian R. | last26=Hardgrove | first26=Craig | last27=Johnson | first27=Jeffrey R. | last28=Lasue | first28=Jeremie | last29=Le Mouélic | first29=Stéphane | last30=Malin | first30=Michael C. | last31=Mangold | first31=Nicolas | last32=Martìn-Torres | first32=Javier | last33=Maurice | first33=Sylvestre | last34=McBride | first34=Marie J. | last35=Ming | first35=Douglas W. | last36=Newsom | first36=Horton E. | last37=Ollila | first37=Ann M. | last38=Sautter | first38=Violaine | last39=Schröder | first39=Susanne | last40=Thompson | first40=Lucy M. | last41=Treiman | first41=Allan H. | last42=VanBommel | first42=Scott | last43=Vaniman | first43=David T. | last44=Zorzano | first44=Marìa-Paz | title=Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars | journal=Geophysical Research Letters | volume=43 | issue=14 | date=2016-07-28 | issn=0094-8276 | doi=10.1002/2016GL069109 | doi-access=free | pages=7398–7407 | bibcode=2016GeoRL..43.7398L | url=https://hal.science/hal-02373201/document | access-date=2025-08-01}}</ref> Today they are no longer considered valid biosignatures and are proposed to be the result of photodissociation of carbon dioxide.

Martian methane is an area of ongoing research. With life being the strongest source of methane on Earth, continued observation of such a disequilibrium could be a viable biosignature.<ref name=":0">{{cite journal | vauthors = Lovelock JE | title = A physical basis for life detection experiments | journal = Nature | volume = 207 | issue = 997 | pages = 568–70 | date = August 1965 | pmid = 5883628 | doi = 10.1038/207568a0 | bibcode = 1965Natur.207..568L | s2cid = 33821197 }}</ref><ref name=":1">{{Cite journal|last1=Hitchcock|first1=Dian R.|last2=Lovelock|first2=James E. | name-list-style = vanc |date=1967-01-01|title=Life detection by atmospheric analysis |journal=Icarus|volume=7|issue=1|pages=149–159|doi=10.1016/0019-1035(67)90059-0|issn=0019-1035|bibcode=1967Icar....7..149H}}</ref> Current photochemical models cannot explain the reported rapid variations in space and time.<ref name = "Zahnle_2011">{{cite journal|last1=Zahnle|first1=Kevin|last2=Freedman|first2=Richard S.|last3=Catling|first3=David C.| name-list-style = vanc |date=2011-04-01|title=Is there methane on Mars?|journal=Icarus|volume=212|issue=2|pages=493–503|doi=10.1016/j.icarus.2010.11.027|issn=0019-1035|bibcode=2011Icar..212..493Z|url=https://zenodo.org/record/1259041}}</ref> Neither its fast appearance nor disappearance have been explained.<ref>[http://mepag.jpl.nasa.gov/decadal/TGM_Mars_Panel-cleared-9-4-09.ppt Mars Trace Gas Mission] {{webarchive|url=https://web.archive.org/web/20110721052957/http://mepag.jpl.nasa.gov/decadal/TGM_Mars_Panel-cleared-9-4-09.ppt|date=2011-07-21}} (September 10, 2009).</ref> Because of its tendency to be destroyed in the atmosphere by [[photochemistry]], excess methane could indicate that there must be an active source.<ref name="t435">{{cite journal | last1=Yung | first1=Yuk L. | last2=Chen | first2=Pin | last3=Nealson | first3=Kenneth | last4=Atreya | first4=Sushil | last5=Beckett | first5=Patrick | last6=Blank | first6=Jennifer G. | last7=Ehlmann | first7=Bethany | last8=Eiler | first8=John | last9=Etiope | first9=Giuseppe | last10=Ferry | first10=James G. | last11=Forget | first11=Francois | last12=Gao | first12=Peter | last13=Hu | first13=Renyu | last14=Kleinböhl | first14=Armin | last15=Klusman | first15=Ronald | last16=Lefèvre | first16=Franck | last17=Miller | first17=Charles | last18=Mischna | first18=Michael | last19=Mumma | first19=Michael | last20=Newman | first20=Sally | last21=Oehler | first21=Dorothy | last22=Okumura | first22=Mitchio | last23=Oremland | first23=Ronald | last24=Orphan | first24=Victoria | last25=Popa | first25=Radu | last26=Russell | first26=Michael | last27=Shen | first27=Linhan | last28=Sherwood Lollar | first28=Barbara | last29=Staehle | first29=Robert | last30=Stamenković | first30=Vlada | last31=Stolper | first31=Daniel | last32=Templeton | first32=Alexis | last33=Vandaele | first33=Ann C. | last34=Viscardy | first34=Sébastien | last35=Webster | first35=Christopher R. | last36=Wennberg | first36=Paul O. | last37=Wong | first37=Michael L. | last38=Worden | first38=John | title=Methane on Mars and Habitability: Challenges and Responses | journal=Astrobiology | volume=18 | issue=10 | date=2018 | issn=1531-1074 | pmid=30234380 | pmc=6205098 | doi=10.1089/ast.2018.1917 | doi-access=free | pages=1221–1242 | bibcode=2018AsBio..18.1221Y }}</ref>

Since 2004 there have been several detections of methane in the Mars atmosphere by a variety of instruments onboard orbiters and ground-based landers on the Martian surface as well as Earth-based telescopes.<ref>{{Cite journal|last1=Krasnopolsky|first1=Vladimir A.|last2=Maillard|first2=Jean Pierre|last3=Owen|first3=Tobias C. | name-list-style = vanc |date=2004-12-01|title=Detection of methane in the martian atmosphere: evidence for life?|journal=Icarus|volume=172|issue=2|pages=537–547|doi=10.1016/j.icarus.2004.07.004|issn=0019-1035|bibcode=2004Icar..172..537K}}</ref><ref>{{cite journal | vauthors = Formisano V, Atreya S, [[Thérèse Encrenaz|Encrenaz T]], Ignatiev N, Giuranna M | s2cid = 13533388 | title = Detection of methane in the atmosphere of Mars | journal = Science | volume = 306 | issue = 5702 | pages = 1758–61 | date = December 2004 | pmid = 15514118 | doi = 10.1126/science.1101732 | bibcode = 2004Sci...306.1758F | doi-access = free }}</ref><ref name="auto">{{cite journal | vauthors = Mumma MJ, Villanueva GL, Novak RE, Hewagama T, Bonev BP, Disanti MA, Mandell AM, Smith MD | s2cid = 25083438 | display-authors = 6 | title = Strong release of methane on Mars in northern summer 2003 | journal = Science | volume = 323 | issue = 5917 | pages = 1041–5 | date = February 2009 | pmid = 19150811 | doi = 10.1126/science.1165243 | bibcode = 2009Sci...323.1041M | doi-access = free }}</ref> However 2019 measurements put an upper bound on the overall methane abundance at 0.05 p.p.b.v<ref>{{cite journal | vauthors = Korablev O, Vandaele AC, Montmessin F, Fedorova AA, Trokhimovskiy A, Forget F, Lefèvre F, Daerden F, Thomas IR, Trompet L, Erwin JT, Aoki S, Robert S, Neary L, Viscardy S, Grigoriev AV, Ignatiev NI, Shakun A, Patrakeev A, Belyaev DA, Bertaux JL, Olsen KS, Baggio L, Alday J, Ivanov YS, Ristic B, Mason J, Willame Y, Depiesse C, Hetey L, Berkenbosch S, Clairquin R, Queirolo C, Beeckman B, Neefs E, Patel MR, Bellucci G, López-Moreno JJ, Wilson CF, Etiope G, Zelenyi L, Svedhem H, Vago JL | display-authors = 6 | title = No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations | journal = Nature | volume = 568 | issue = 7753 | pages = 517–520 | date = April 2019 | pmid = 30971829 | doi = 10.1038/s41586-019-1096-4 | bibcode = 2019Natur.568..517K | hdl = 10261/190885 | s2cid = 106411228 | url = http://oro.open.ac.uk/60547/2/2019%20Korablev%20TGO%20methane%20Nature_accepted.pdf }}</ref> contradicting previous observations.

To rule out a biogenic origin for the methane, a future probe or lander hosting a [[mass spectrometer]] will be needed, as the isotopic proportions of [[carbon-12]] to [[carbon-14]] in methane could distinguish between a biogenic and non-biogenic origin, similarly to the use of the [[δ13C]] standard for recognizing biogenic methane on Earth.<ref>[http://rst.gsfc.nasa.gov/Sect19/Sect19_13a.html Remote Sensing Tutorial, Section 19-13a] {{webarchive|url=https://web.archive.org/web/20111021072805/http://rst.gsfc.nasa.gov/Sect19/Sect19_13a.html|date=2011-10-21}} - Missions to Mars during the Third Millennium, Nicholas M. Short Sr., et al., NASA.</ref>

==== Atmosphere of Venus ====
[[File:Venus - December 23 2016.png|alt=The atmosphere of Venus appears darker and lined with shadows. The shadows trace the prevailing wind direction.|thumb|upright=0.9|Cloud structure of the Venusian atmosphere (made visible through [[ultraviolet]] imaging) which contains phosphine, ammonia and ozone considered candidate biosignatures.]]

The [[atmosphere of Venus]] continues to be investigated for [[Life on Venus#Potential biomarkers|potential biomarkers]] though abiotic processes have been put forward as explanations.

[[Ammonia]] ({{chem|NH|3|}}) was first detected in the atmosphere by the [[bromophenol blue]] chemical sensor of [[Venera 8]] in 1972.<ref name="v399">{{cite journal | last1=Goettel | first1=Kenneth A. | last2=Lewis | first2=John S. | title=Ammonia in the Atmosphere of Venus | journal=Journal of the Atmospheric Sciences | volume=31 | issue=3 | date=1974 | issn=0022-4928 | doi=10.1175/1520-0469(1974)031<0828:AITAOV>2.0.CO;2 | doi-access=free | pages=828–830 | bibcode=1974JAtS...31..828G | url=https://journals.ametsoc.org/downloadpdf/journals/atsc/31/3/1520-0469_1974_031_0828_aitaov_2_0_co_2.pdf | access-date=2025-08-05}}</ref> Ammonia is essential to life and is both a metabolic input and output, as such it has been explored as having strong potential as a biosignature.<ref name="w874">{{cite journal | last1=Huang | first1=Jingcheng | last2=Seager | first2=Sara | last3=Petkowski | first3=Janusz J. | last4=Ranjan | first4=Sukrit | last5=Zhan | first5=Zhuchang | title=Assessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres | journal=Astrobiology | volume=22 | issue=2 | date=2022-02-01 | issn=1531-1074 | doi=10.1089/ast.2020.2358 | doi-access=free | pages=171–191 | pmid=35099265 | arxiv=2107.12424 | bibcode=2022AsBio..22..171H }}</ref> [[Pioneer Venus]] also detected substantial quantities of the gas.<ref name="f912">{{cite journal | last1=Limaye | first1=Sanjay S. | last2=Mogul | first2=Rakesh | last3=Smith | first3=David J. | last4=Ansari | first4=Arif H. | last5=Słowik | first5=Grzegorz P. | last6=Vaishampayan | first6=Parag | title=Venus' Spectral Signatures and the Potential for Life in the Clouds | journal=Astrobiology | volume=18 | issue=9 | date=2018 | issn=1531-1074 | pmid=29600875 | pmc=6150942 | doi=10.1089/ast.2017.1783 | doi-access=free | pages=1181–1198 | bibcode=2018AsBio..18.1181L }}</ref> Of particular interest is that unlike the Martian atmosphere where conditions would suit ammonia's presence only transient trace amounts have been detected, on Venus with conditions less conducive to its presence it appears to somehow be replenished.<ref name="o696">{{cite journal | last1=Bains | first1=William | last2=Petkowski | first2=Janusz J. | last3=Rimmer | first3=Paul B. | last4=Seager | first4=Sara | title=Production of ammonia makes Venusian clouds habitable and explains observed cloud-level chemical anomalies | journal=Proceedings of the National Academy of Sciences | volume=118 | issue=52 | date=2021-12-28 | issn=0027-8424 | pmid=34930842 | pmc=8719887 | doi=10.1073/pnas.2110889118 | doi-access=free | article-number=e2110889118 | arxiv=2112.10850 | bibcode=2021PNAS..11810889B }}</ref> A 2021 paper claimed that it could be a byproduct of life that is in turn providing a stable habitable environment for life to continue in the upper atmosphere.<ref name="o696"/> At least one paper puts forward a possible abiotic explanation, proposing that similar processes as [[nitrogen fixation]] in early Earth's atmosphere though caused by mantle oxidation due to the planet's water loss.<ref name="r688">{{cite journal | last=Wordsworth | first=R.D. | title=Atmospheric nitrogen evolution on Earth and Venus | journal=Earth and Planetary Science Letters | volume=447 | date=2016 | doi=10.1016/j.epsl.2016.04.002 | doi-access=free | pages=103–111 | arxiv=1605.07718 | bibcode=2016E&PSL.447..103W }}</ref> Another has proposed that [[lightning]] could be producing it<ref name="k520">{{cite journal | last1=Delitsky | first1=M.L. | last2=Baines | first2=K.H. | title=Storms on Venus: Lightning-induced chemistry and predicted products | journal=Planetary and Space Science | volume=113-114 | date=2015 | doi=10.1016/j.pss.2014.12.005 | pages=184–192 | bibcode=2015P&SS..113..184D | url=https://linkinghub.elsevier.com/retrieve/pii/S0032063314003973 | access-date=2025-08-05| url-access=subscription }}</ref> though whether Venus has lightning at all has been extensively debated.<ref name="e114">{{cite journal | last1=Blaske | first1=C. H. | last2=O'Rourke | first2=J. G. | last3=Desch | first3=S. J. | last4=Borrelli | first4=M. E. | title=Meteors May Masquerade as Lightning in the Atmosphere of Venus | journal=Journal of Geophysical Research: Planets | volume=128 | issue=9 | date=2023 | issn=2169-9097 | doi=10.1029/2023JE007914 | doi-access=free | article-number=e2023JE007914 | bibcode=2023JGRE..12807914B }}</ref>

[[Ozone]] ({{chem|O|3|}}) was first detected at concentrations of up to 1 ppm in the night side upper atmosphere by [[Venus Express]] in 2011.<ref name="m829">{{cite journal | last1=Montmessin | first1=F. | last2=Bertaux | first2=J.-L. | last3=Lefèvre | first3=F. | last4=Marcq | first4=E. | last5=Belyaev | first5=D. | last6=Gérard | first6=J.-C. | last7=Korablev | first7=O. | last8=Fedorova | first8=A. | last9=Sarago | first9=V. | last10=Vandaele | first10=A.C. | title=A layer of ozone detected in the nightside upper atmosphere of Venus | journal=Icarus | volume=216 | issue=1 | date=2011 | doi=10.1016/j.icarus.2011.08.010 | doi-access=free | pages=82–85 | bibcode=2011Icar..216...82M | url=https://orbi.uliege.be/bitstream/2268/100136/1/Montmessin_O3.pdf | access-date=2025-08-01}}</ref> As a byproduct of living organisms this was once regarded as a candidate biosignature.<ref name="p677">{{cite journal | last1=Grenfell | first1=J.L. | last2=Gebauer | first2=S. | last3=Godolt | first3=M. | last4=Palczynski | first4=K. | last5=Rauer | first5=H. | last6=Stock | first6=J. | last7=von Paris | first7=P. | last8=Lehmann | first8=R. | last9=Selsis | first9=F. | title=Potential Biosignatures in Super-Earth Atmospheres II. Photochemical Responses | journal=Astrobiology | volume=13 | issue=5 | date=2013 | issn=1531-1074 | pmid=23683046 | pmc=3941937 | doi=10.1089/ast.2012.0926 | doi-access=free | pages=415–438 | arxiv=1303.6804 | bibcode=2013AsBio..13..415G }}</ref> Known since the 1970s to exists in trace amounts in the Martian atmosphere, Venus in comparison possesses a significant layer similar to but substantially less concentrated than Earth's. Photochemical processes, specifically dissociation of carbon dioxide (CO2) by sunlight, is now offered an explanation for its presence.<ref name="q087">{{cite journal | last1=Calder | first1=Robb | last2=Shorttle | first2=Oliver | last3=Jordan | first3=Sean | last4=Rimmer | first4=Paul | last5=Constantinou | first5=Tereza | title=Abiotic ozone in the observable atmospheres of Venus and Venus-like exoplanets | journal=Monthly Notices of the Royal Astronomical Society | volume=540 | issue=3 | date=2025-06-05 | issn=0035-8711 | doi=10.1093/mnras/staf851 | doi-access=free | pages=2432–2450}}</ref>

[[Phosphine]] ({{chem|PH|3|}}) was first detected in 2020 by the [[James Clerk Maxwell Telescope]] and the [[Atacama Large Millimeter/submillimeter Array]] in trace amounts in the upper cloud deck. There was no known abiotic source for the quantities detected.<ref name="Greaves et al">{{cite journal |last1=Greaves |first1=Jane S. |last2=Richards |first2=A.M.S. |last3=Bains |first3=W |title=Phosphine gas in the cloud decks of Venus |journal=Nature Astronomy |date=14 September 2020 |volume=5 |issue=7 |pages=655–664 |doi=10.1038/s41550-020-1174-4 |arxiv=2009.06593 |bibcode=2021NatAs...5..655G |s2cid=221655755 |url=https://www.nature.com/articles/s41550-020-1174-4#citeas |access-date=16 September 2020}}</ref><ref name="sample1">{{cite news |last1=Sample |first1=Ian |title=Scientists find gas linked to life in atmosphere of Venus |url=https://www.theguardian.com/science/2020/sep/14/scientists-find-gas-linked-to-life-in-atmosphere-of-venus |access-date=16 September 2020 |work=The Guardian |date=14 September 2020}}</ref> Subsequent analysis and investigation between 2020 and 2015 indicated possible false detection,<ref>{{citation|last1=Snellen|first1=I. A. G.|title=Re-analysis of the 267-GHz ALMA observations of Venus No statistically significant detection of phosphine|journal=Astronomy and Astrophysics|volume=644|pages=L2|year=2020|arxiv=2010.09761|bibcode=2020A&A...644L...2S|doi=10.1051/0004-6361/202039717|last2=Guzman-Ramirez|first2=L.|last3=Hogerheijde|first3=M. R.|last4=Hygate|first4=A. P. S.|last5=van der Tak|first5=F. F. S.|s2cid=224803085}}</ref><ref name="Thompson2020">{{citation|last1=Thompson|first1=M. A.|title=The statistical reliability of 267 GHz JCMT observations of Venus: No significant evidence for phosphine absorption|journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=501|issue=1|pages=L18–L22|year=2021|arxiv=2010.15188|bibcode=2021MNRAS.501L..18T|doi=10.1093/mnrasl/slaa187|doi-access=free |s2cid=225103303}}</ref> or a much lower concentration of 1&nbsp;ppb.<ref name=Greaves202011>{{citation|arxiv=2011.08176|title=Reply to: No evidence of phosphine in the atmosphere of Venus from independent analyses|year=2021|last1=Greaves|first1=Jane S.|last2=Richards|first2=Anita M. S.|last3=Bains|first3=William|last4=Rimmer|first4=Paul B.|last5=Clements|first5=David L.|last6=Seager|first6=Sara|last7=Petkowski|first7=Janusz J.|last8=Sousa-Silva|first8=Clara|last9=Ranjan|first9=Sukrit|last10=Fraser|first10=Helen J.|journal=Nature Astronomy|volume=5|issue=7|pages=636–639|doi=10.1038/s41550-021-01424-x|bibcode=2021NatAs...5..636G|s2cid=233296859}}</ref><ref name="Mogul2020">{{citation|arxiv=2009.12758|title=Is Phosphine in the Mass Spectra from Venus' Clouds?|year=2020|last1=Mogul|first1=Rakesh|last2=Limaye|first2=Sanjay S.|last3=Way|first3=M. J.|last4=Cordova |first4=Jamie A. Jr|doi=10.1002/essoar.10504552.4|s2cid=231854943}}</ref><ref>{{citation|arxiv=2010.07817|title=A stringent upper limit of the PH3 abundance at the cloud top of Venus|year=2020|doi=10.1051/0004-6361/202039559|last1=Encrenaz|first1=T.|author1-link=Thérèse Encrenaz|last2=Greathouse|first2=T. K.|last3=Marcq|first3=E.|last4=Widemann|first4=T.|last5=Bézard|first5=B.|last6=Fouchet|first6=T.|last7=Giles|first7=R.|last8=Sagawa|first8=H.|last9=Greaves|first9=J.|last10=Sousa-Silva|first10=C.|journal=Astronomy & Astrophysics|volume=643|pages=L5|bibcode=2020A&A...643L...5E|s2cid=222377688}}</ref><ref>{{citation|arxiv=2210.13519|year=2022|title=Phosphine in the Venusian Atmosphere: A Strict Upper Limit from SOFIA GREAT Observations|doi=10.1029/2022GL101055 |last1=Cordiner |first1=M. A. |last2=Villanueva |first2=G. L. |last3=Wiesemeyer |first3=H. |last4=Milam |first4=S. N. |last5=De Pater |first5=I. |last6=Moullet |first6=A. |last7=Aladro |first7=R. |last8=Nixon |first8=C. A. |last9=Thelen |first9=A. E. |last10=Charnley |first10=S. B. |last11=Stutzki |first11=J. |last12=Kofman |first12=V. |last13=Faggi |first13=S. |last14=Liuzzi |first14=G. |last15=Cosentino |first15=R. |last16=McGuire |first16=B. A. |journal=Geophysical Research Letters |volume=49 |issue=22 |article-number=e2022GL101055 |bibcode=2022GeoRL..4901055C |s2cid=253086965 }}</ref> However in  September 2024, the preliminary analysis of the [[James Clerk Maxwell Telescope|JCMT-Venus]] data confirmed a concentration of 300 ppb at altitude 55&nbsp;km. Further data processing is still needed to measure phosphine concentration deeper in the Venusian cloud deck.<ref>{{citation|arxiv=2409.13438|title=Venus Phosphine: Updates and lessons learned|year=2024 |last1=Clements |first1=David L. }}</ref>

==== Other planets ====
On September 12, 2023, [[NASA]] announced that their investigation into [[K2-18b|exoplanet K2-18b]] revealed the possible presence of dimethyl sulfide (DMS) and dimethyl disulfide (CH₃SSCH₃), noting "On Earth, this is only produced by life."<ref>{{cite web|url=https://www.nasa.gov/goddard/2023/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18b|title=Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b|date=12 September 2023|access-date=12 September 2023}}</ref> However a follow up study question the James Webb telescope's instrumentation's ability to differentiate the signature of DMS from methane.<ref name="l546">{{cite journal | last1=Tsai | first1=Shang-Min | last2=Innes | first2=Hamish | last3=Wogan | first3=Nicholas F. | last4=Schwieterman | first4=Edward W. | title=Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds | journal=The Astrophysical Journal Letters | volume=966 | issue=2 | date=2024-05-01 | issn=2041-8205 | doi=10.3847/2041-8213/ad3801 | doi-access=free | page=L24| arxiv=2403.14805 | bibcode=2024ApJ...966L..24T }}</ref> Additionally, follow up studies have identified potential abiotic sources.<ref name="k828">{{cite journal | last1=Reed | first1=Nathan W. | last2=Shearer | first2=Randall L. | last3=McGlynn | first3=Shawn Erin | last4=Wing | first4=Boswell A. | last5=Tolbert | first5=Margaret A. | last6=Browne | first6=Eleanor C. | title=Abiotic Production of Dimethyl Sulfide, Carbonyl Sulfide, and Other Organosulfur Gases via Photochemistry: Implications for Biosignatures and Metabolic Potential | journal=The Astrophysical Journal Letters | volume=973 | issue=2 | date=2024-10-01 | issn=2041-8205 | doi=10.3847/2041-8213/ad74da | doi-access=free | page=L38| bibcode=2024ApJ...973L..38R }}</ref>

=== Agnostic biosignatures ===
Because the only form of known life is that on Earth, the search for biosignatures is heavily influenced by the products that life produces on Earth. However, life that is different from life on Earth may still produce biosignatures that are detectable by humans, even though nothing is known about their specific biology. This form of biosignature is called an "agnostic biosignature" because it is independent of the form of life that produces it. It is widely agreed that all life–no matter how different it is from life on Earth–needs a source of energy to thrive.<ref>{{cite journal | vauthors = Benner SA | title = Defining life | journal = Astrobiology | volume = 10 | issue = 10 | pages = 1021–30 | date = December 2010 | pmid = 21162682 | pmc = 3005285 | doi = 10.1089/ast.2010.0524 | bibcode = 2010AsBio..10.1021B }}</ref> This must involve some sort of chemical disequilibrium, which can be exploited for metabolism.<ref>{{Cite book|url=https://www.nap.edu/read/25252/chapter/6|title=Read "An Astrobiology Strategy for the Search for Life in the Universe" at NAP.edu|doi=10.17226/25252|pmid=30986006|year=2019|isbn=978-0-309-48416-9| author1 = National Academies Of Sciences Engineering | author2 = Division on Engineering Physical Sciences |author3 = Space Studies Board | author4 = Committee on Astrobiology Science Strategy for the Search for Life in the Universe|s2cid=243600456 }}</ref><ref name=":0" /><ref name=":1" /> Geological processes are independent of life, and if scientists can constrain the geology well enough on another planet, then they know what the particular geologic equilibrium for that planet should be. A deviation from geological equilibrium can be interpreted as an atmospheric disequilibrium and agnostic biosignature.

=== Antibiosignatures ===
In the same way that detecting a biosignature would be a significant discovery about a planet, finding evidence that life is ''not'' present can also be an important discovery about a planet. Life relies on [[redox]] imbalances to metabolize the resources available into energy. The evidence that nothing on an earth is taking advantage of the "free lunch" available due to an observed redox imbalance is called antibiosignatures.<ref name = "Zahnle_2011" />

==== CO and H2 in Martian atmosphere ====
The Martian atmosphere contains high abundances of [[Photochemistry|photochemically]] produced CO and H<sub>2</sub>, which are reducing molecules. Mars' atmosphere is otherwise mostly oxidizing, leading to a source of untapped energy that life could exploit if it used by a metabolism compatible with one or both of these reducing molecules. Because these molecules can be observed, scientists use this as evidence for an antibiosignature.<ref>{{cite journal | vauthors = Catling DC, Krissansen-Totton J, Kiang NY, Crisp D, Robinson TD, DasSarma S, Rushby AJ, Del Genio A, Bains W, Domagal-Goldman S | display-authors = 6 | title = Exoplanet Biosignatures: A Framework for Their Assessment | journal = Astrobiology | volume = 18 | issue = 6 | pages = 709–738 | date = June 2018 | pmid = 29676932 | pmc = 6049621 | doi = 10.1089/ast.2017.1737 | arxiv = 1705.06381 | bibcode = 2018AsBio..18..709C }}</ref><ref>{{Cite journal|last1=Wang|first1=Yuwei|last2=Tian|first2=Feng|last3=Li|first3=Tong|last4=Hu|first4=Yongyun| name-list-style = vanc |date=2016-03-01|title=On the detection of carbon monoxide as an anti-biosignature in exoplanetary atmospheres|journal=Icarus|volume=266|pages=15–23|doi=10.1016/j.icarus.2015.11.010|issn=0019-1035|bibcode=2016Icar..266...15W}}</ref> Scientists have used this concept as an argument against life on Mars.<ref>{{cite journal | vauthors = Sholes SF, Krissansen-Totton J, Catling DC | title = 2 as Potential Antibiosignatures | journal = Astrobiology | volume = 19 | issue = 5 | pages = 655–668 | date = May 2019 | pmid = 30950631 | doi = 10.1089/ast.2018.1835 | bibcode = 2019AsBio..19..655S | arxiv = 1811.08501 | s2cid = 96435170 }}</ref>

=== Polyelectrolytes ===
{{Main|Polyelectrolyte theory of the gene}}

The [[Polyelectrolyte theory of the gene]] is a proposed generic biosignature. In 2002, Steven A. Benner and Daniel Hutter proposed that for a linear genetic [[biopolymer]] dissolved in water, such as [[DNA]], to undergo [[Evolution|Darwinian evolution]] anywhere in the universe, it must be a [[polyelectrolyte]], a [[polymer]] containing repeating [[Ionic charge|ionic]] charges.<ref>{{Cite journal |last1=Benner |first1=Steven A. |last2=Hutter |first2=Daniel |date=2002-02-01 |title=Phosphates, DNA, and the Search for Nonterrean Life: A Second Generation Model for Genetic Molecules |url=https://linkinghub.elsevier.com/retrieve/pii/S0045206801912325 |journal=Bioorganic Chemistry |language=en |volume=30 |issue=1 |pages=62–80 |doi=10.1006/bioo.2001.1232|pmid=11955003 |url-access=subscription }}</ref> Benner and others proposed methods for concentrating and analyzing these polyelectrolyte genetic biopolymers on Mars,<ref>{{Cite journal |last1=Špaček |first1=Jan |last2=Benner |first2=Steven A. |date=2022-10-01 |title=Agnostic Life Finder (ALF) for Large-Scale Screening of Martian Life During In Situ Refueling |url=https://www.liebertpub.com/doi/10.1089/ast.2021.0070 |journal=Astrobiology |language=en |volume=22 |issue=10 |pages=1255–1263 |doi=10.1089/ast.2021.0070 |pmid=35796703 |bibcode=2022AsBio..22.1255S |issn=1531-1074|url-access=subscription }}</ref> Enceladus,<ref>{{Cite journal |last=Benner |first=Steven A. |date=2017 |title=Detecting Darwinism from Molecules in the Enceladus Plumes, Jupiter's Moons, and Other Planetary Water Lagoons |journal=Astrobiology |volume=17 |issue=9 |pages=840–851 |doi=10.1089/ast.2016.1611 |issn=1531-1074|doi-access=free |pmid=28665680 |pmc=5610385 |bibcode=2017AsBio..17..840B }}</ref> and Europa.<ref>{{Cite journal |last1=Sutton |first1=Mark A. |last2=Burton |first2=Aaron S. |last3=Zaikova |first3=Elena |last4=Sutton |first4=Ryan E. |last5=Brinckerhoff |first5=William B. |last6=Bevilacqua |first6=Julie G. |last7=Weng |first7=Margaret M. |last8=Mumma |first8=Michael J. |last9=Johnson |first9=Sarah Stewart |date=2019-03-29 |title=Radiation Tolerance of Nanopore Sequencing Technology for Life Detection on Mars and Europa |journal=Scientific Reports |volume=9 |issue=1 |page=5370 |doi=10.1038/s41598-019-41488-4 |issn=2045-2322|doi-access=free |pmid=30926841 |pmc=6441015 |bibcode=2019NatSR...9.5370S }}</ref>

== Missions inside the Solar System ==

[[Astrobiology|Astrobiological exploration]] is founded upon the premise that biosignatures encountered in space will be recognizable as [[extraterrestrial life]]. The usefulness of a biosignature is determined not only by the probability of life creating it but also by the improbability of non-biological (abiotic) processes producing it.<ref name="astrobiology">{{cite web|url=http://astrobiology.arc.nasa.gov/roadmap/g5.html|title=Understand the evolutionary mechanisms and environmental limits of life|last=Rothschild|first=Lynn|date=September 2003|publisher=NASA|archive-url=https://web.archive.org/web/20110126083203/http://astrobiology.arc.nasa.gov/roadmap/g5.html|archive-date=2011-01-26|access-date=2009-07-13}}</ref> Concluding that evidence of an extraterrestrial life form (past or present) has been discovered requires proving that a possible biosignature was produced by the activities or remains of life.<ref name="SSG" /> As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists.

Possible examples of a biosignature include complex [[Organic compound|organic molecules]] or structures whose formation is virtually unachievable in the absence of life:<ref name="astrobiology" />

# Cellular and extracellular morphologies
# [[Biomolecule]]s in rocks
# Bio-organic molecular structures
# [[Chirality]]
# [[Biogenic silica|Biogenic minerals]]
# Biogenic isotope patterns in minerals and organic compounds
# Atmospheric gases
# [[Photosynthetic pigment]]s

=== The ''Viking'' missions to Mars ===
{{Main|Viking biological experiments}}
The [[Viking Lander|''Viking'' missions]] to Mars in the 1970s conducted the first experiments which were explicitly designed to look for biosignatures on another planet. Each of the two [[Viking program|''Viking'' landers]] carried three [[Viking Biological Experiments|life-detection experiments]] which looked for signs of [[metabolism]]; however, the results were declared inconclusive.<ref name="Beegle" /><ref>Levin, G and P. Straaf. 1976. Viking Labeled Release Biology Experiment: Interim Results. Science: vol: 194. pp: 1322-1329.</ref><ref name="Chambers">{{Cite book|title=Life on Mars; The Complete Story|last=Chambers|first=Paul|publisher=Blandford|year=1999|isbn=0-7137-2747-0|place=London|url-access=registration|url=https://archive.org/details/lifeonmarscomple00cham}}</ref><ref>{{cite journal | vauthors = Klein HP, Horowitz NH, Levin GV, Oyama VI, Lederberg J, Rich A, Hubbard JS, Hobby GL, Straat PA, Berdahl BJ, Carle GC, Brown FS, Johnson RD | s2cid = 24957458 | display-authors = 6 | title = The viking biological investigation: preliminary results | journal = Science | volume = 194 | issue = 4260 | pages = 99–105 | date = October 1976 | pmid = 17793090 | doi = 10.1126/science.194.4260.99 | bibcode = 1976Sci...194...99K }}</ref><ref name="ExoMars">{{Cite web|url=https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/ExoMars|title=ExoMars|website=www.esa.int}}</ref>{{Excessive citations inline|date=March 2025}}

=== Mars Science Laboratory ===
{{Main|Timeline of Mars Science Laboratory}}
The ''Curiosity'' rover from the [[Mars Science Laboratory]] mission, with its [[Curiosity rover|''Curiosity'' rover]] is currently assessing the potential past and present [[planetary habitability|habitability]] of the Martian environment and is attempting to detect biosignatures on the surface of Mars.<ref name="Biosignatures 2011" /> Considering the MSL instrument payload package, the following classes of biosignatures are within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases.<ref name="Biosignatures 2011" /> The ''Curiosity'' rover targets [[outcrop]]s to maximize the probability of detecting 'fossilized' [[organic matter]] preserved in sedimentary deposits.

=== ExoMars Orbiter ===
The 2016 ExoMars [[Trace Gas Orbiter]] (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission. It delivered the [[Schiaparelli EDM lander|''Schiaparelli'' EDM lander]] and then began to settle into its science orbit to map the sources of [[Atmosphere of Mars#Methane|methane on Mars]] and other gases, and in doing so, will help select the landing site for the [[Rosalind Franklin (rover)|''Rosalind Franklin'' rover]] to be launched in 2028.<ref>{{cite news|url=http://english.ruvr.ru/2012_07_15/ExoMars-program-gathers-strength/|title=ExoMars program gathers strength|last=Pavlishchev|first=Boris|date=Jul 15, 2012|work=The Voice of Russia|access-date=2012-07-15|archive-date=2012-08-06|archive-url=https://web.archive.org/web/20120806205001/http://english.ruvr.ru/2012_07_15/ExoMars-program-gathers-strength/}}</ref> The primary objective of the ''Rosalind Franklin'' rover mission is the search for biosignatures on the surface and subsurface by using a drill able to collect samples down to a depth of {{convert|2|m|ft}}, away from the destructive radiation that bathes the surface.<ref name="ExoMars" /><ref name="MSL-main_page">{{cite web|url=http://marsprogram.jpl.nasa.gov/msl/mission/|archive-url=https://web.archive.org/web/20060305231951/http://marsprogram.jpl.nasa.gov/msl/mission/|archive-date=2006-03-05|title=Mars Science Laboratory: Mission|publisher=NASA/JPL|access-date=2010-03-12}}</ref>

=== Mars 2020 Rover ===
[[File:1-PIA26368-Perseverance Finds a Rock with Leopard Spots-annotated.png|thumb|Cheyava Falls rock]]

The [[Mars 2020]] rover, which launched in 2020, is intended to investigate an [[astrobiology|astrobiologically]] relevant ancient environment on Mars, investigate its surface [[Geology of Mars|geological processes]] and history, including the assessment of its past [[Planetary habitability|habitability]], the possibility of past [[life on Mars]], and potential for preservation of biosignatures within accessible geological materials.<ref name="AP-20130709">{{cite news|url=http://apnews.excite.com/article/20130709/DA7EA0K83.html|title=Panel: Next Mars rover should gather rocks, soil|last=Chang|first=Alicia|date=July 9, 2013|access-date=July 12, 2013|agency=[[Associated Press]]|archive-date=November 4, 2014|archive-url=https://web.archive.org/web/20141104235435/http://apnews.excite.com/article/20130709/DA7EA0K83.html}}</ref><ref name="nasa-letapp20121220">{{cite web|url=https://mepag.jpl.nasa.gov/announcements/Call_for_2020_Mars_Science_Rover-G.pdf|title=Call for ''Letters of Application'' for Membership on the Science Definition Team for the 2020 Mars Science Rover|last=Schulte|first=Mitch|date=December 20, 2012|publisher=NASA|id=NNH13ZDA003L}}</ref> In addition, it will cache the most interesting samples for possible future transport to Earth.

In 2024, Perseverance found a rock, called [[Cheyava Falls]], during its exploration of the [[Jezero Crater]]. The rover's instruments detected organic compounds within the rock.<ref name=nasa>{{cite web |title=NASA's Perseverance Rover Scientists Find Intriguing Mars Rock |url=https://www.jpl.nasa.gov/news/nasas-perseverance-rover-scientists-find-intriguing-mars-rock |website=NASA Jet Propulsion Laboratory (JPL) |access-date=31 July 2024}}  {{PD-notice}}</ref><ref>{{cite web |title=Perseverance Finds a Rock With 'Leopard Spots' |url=https://www.jpl.nasa.gov/images/pia26368-perseverance-finds-a-rock-with-leopard-spots |website=NASA Jet Propulsion Laboratory (JPL) |access-date=31 July 2024}}  {{PD-notice}}</ref> According to NASA, Cheyava Falls "possesses qualities that fit the definition of a possible indicator of ancient life".<ref name=atlantic>{{cite web |last1=Koren |first1=Marina |title=We Might Get Thrilling News About Aliens … in 2040 |url=https://www.theatlantic.com/science/archive/2024/07/mars-rock-cheyava-falls-alien-life/679261/ |website=The Atlantic |access-date=31 July 2024 |language=en |date=26 July 2024}}</ref><ref name=nasa/>

On 10 September 2025, NASA reported a "potential biosignature" finding in Cheyava Falls: organic-carbon–bearing [[mudstone]]s hosting sub-millimetre nodules and millimetre-scale reaction fronts enriched in ferrous [[iron phosphate]] and [[iron sulfide]], consistent with [[vivianite]] and [[greigite]] imply low-temperature, post-depositional [[redox reaction]]s between organics and Fe–S–P minerals; these textures and chemistries qualify as potential biosignatures but requiring further study and sample return for confirmation.<ref name=nasa2025>{{cite web |last= |first= |date=10 September 2025 |title=NASA Says Mars Rover Discovered Potential Biosignature Last Year |url=https://www.nasa.gov/news-release/nasa-says-mars-rover-discovered-potential-biosignature-last-year/ |website=NASA |access-date=10 September 2025}} {{PD-notice}}</ref><ref name=Hurowitz>{{Cite journal|title=Redox-driven mineral and organic associations in Jezero Crater, Mars|first1=Joel A.|last1=Hurowitz|first2=M. M.|last2=Tice|first3=A. C.|last3=Allwood|first4=M. L.|last4=Cable|date=September 10, 2025|journal=Nature|volume=645|issue=8080|pages=332–340|doi=10.1038/s41586-025-09413-0 |display-authors=1 |pmid=40931152 |pmc=12422973 |bibcode=2025Natur.645..332H }}</ref> On Earth, vivianite is frequently found in sediments, peat bogs, and around decaying organic matter. Similarly, certain forms of microbial life on Earth can produce greigite.<ref name=nasa2025/> The same organic materials can be produced by non-biological processes which require "hot conditions" like volcanic activity; the rock location suggests that it was underwater, and there is no detected past volcanic activity in that region.<ref name=natgeo/>

If confirmed, this biosignature would mean that there were a microbial life on Mars around 3.5 billion years ago. According to geologist [[Michael Tice]]:<ref name=natgeo>{{Cite web|url=https://www.nationalgeographic.com/science/article/mars-rock-alien-life-microbes-chemistry|title=This is the best evidence yet for ancient life on Mars|date=September 12, 2025|website=Science}}</ref>
{{Quote|text=If the Cheyava Falls results ultimately do lead to the proof of ancient life on Mars ... that means two different planets hosted microbes getting their energy through the same means at about the same time in the distant past. That could suggest that early life learns how to survive in this way regardless of where it originated.}}

=== Titan ''Dragonfly'' ===
NASA's ''[[Dragonfly (Titan space probe)|Dragonfly]]''<ref>{{Cite web|url=http://dragonfly.jhuapl.edu/index.php|title=Dragonfly|website=dragonfly.jhuapl.edu|access-date=2019-06-07}}</ref> lander/aircraft concept is proposed to launch in 2028 and would seek evidence of biosignatures on the organic-rich surface and atmosphere of [[Titan (moon)|Titan]], as well as study its possible prebiotic [[primordial soup]].<ref>[[bibcode:2017DPS....4921902B|Dragonfly: Exploring Titan's Surface with a New Frontiers Relocatable Lander]]. American Astronomical Society, DPS meeting #49, id.219.02. October 2017.</ref><ref name="LPSC 2017">{{cite conference | vauthors = Turtle P, Barnes JW, Trainer MG, Lorenz RD, MacKenzie SM, Hibbard KE, Adams D, Bedini P, Langelaan JW, Zacny K | title = Dragonfly: Exploring titan's prebiotic organic chemistry and habitability. |conference= Lunar and Planetary Science Conference | year = 2017 | url = https://www.hou.usra.edu/meetings/lpsc2017/eposter/1958.pdf }}</ref> Titan is the largest moon of [[Saturn]] and is widely believed to have a large subsurface ocean consisting of a salty brine.<ref>{{Cite journal| vauthors = Fortes AD |date=2000-08-01|title=Exobiological Implications of a Possible Ammonia–Water Ocean inside Titan|journal=Icarus|volume=146|issue=2|pages=444–452|doi=10.1006/icar.2000.6400|issn=0019-1035|bibcode=2000Icar..146..444F}}</ref><ref>{{Cite journal| vauthors = Grasset O, Sotin C, Deschamps F |date=2000-06-01|title=On the internal structure and dynamics of Titan|journal=Planetary and Space Science|volume=48|issue=7|pages=617–636|doi=10.1016/S0032-0633(00)00039-8|issn=0032-0633|bibcode=2000P&SS...48..617G}}</ref> In addition, scientists believe that Titan may have the conditions necessary to promote [[Prebiotic (chemistry)|prebiotic]] chemistry, making it a prime candidate for biosignature discovery.<ref>{{Cite web|url=https://phys.org/news/2013-04-nasa-team-complex-chemistry-titan.html|title=NASA team investigates complex chemistry at Titan|work=Phys.org |date=April 3, 2013 |author=JPL/NASA|access-date=2019-06-07}}</ref><ref>{{Cite web|url=http://theconversation.com/saturns-moon-titan-may-harbour-simple-life-forms-and-reveal-how-organisms-first-formed-on-earth-81527|title=Saturn's moon Titan may harbour simple life forms – and reveal how organisms first formed on Earth|last=Desai|first=Ravi|website=The Conversation|date=July 27, 2017|access-date=2019-06-07}}</ref><ref>{{cite journal | vauthors = Gudipati MS, Jacovi R, Couturier-Tamburelli I, Lignell A, Allen M | title = Photochemical activity of Titan's low-altitude condensed haze | journal = Nature Communications | volume = 4 | article-number = 1648 | date = 2013-04-03 | pmid = 23552063 | doi = 10.1038/ncomms2649 | bibcode = 2013NatCo...4.1648G | doi-access = free }}</ref>

{{See also|Life on Titan}}

=== ''Europa Clipper'' ===

[[File:Europa Clipper spacecraft model.png|thumb|''Europa Clipper'']]
NASA's ''[[Europa Clipper]]'' probe is designed as a flyby mission to Jupiter's smallest [[Galilean moons|Galilean moon]], [[Europa (moon)|Europa]].<ref>{{Cite web|url=https://www.jpl.nasa.gov/missions/europa-clipper/|title=Europa Clipper|website=www.jpl.nasa.gov|access-date=2019-06-07}}</ref> The mission launched in October 2024 and is set to reach Europa in April 2030, where it will investigate the potential for habitability on Europa. Europa is one of the best candidates for biosignature discovery in the [[Solar System]] because of the scientific consensus that it retains a subsurface ocean, with two to three times the volume of water on Earth. Evidence for this subsurface ocean includes:

*''[[Voyager 1]]'' (1979): The first close-up photos of Europa are taken. Scientists propose that a subsurface ocean could cause the tectonic-like marks on the surface.<ref>{{cite journal | vauthors = Smith BA, Soderblom LA, Johnson TV, Ingersoll AP, Collins SA, Shoemaker EM, Hunt GE, Masursky H, Carr MH, Davies ME, Cook AF, Boyce J, Danielson GE, Owen T, Sagan C, Beebe RF, Veverka J, Strom RG, McCauley JF, Morrison D, Briggs GA, Suomi VE | display-authors = 6 | title = The jupiter system through the eyes of voyager 1 | journal = Science | volume = 204 | issue = 4396 | pages = 951–72 | date = June 1979 | pmid = 17800430 | doi = 10.1126/science.204.4396.951 | bibcode = 1979Sci...204..951S | s2cid = 33147728 }}</ref>
*''[[Galileo (spacecraft)|Galileo]]'' (1997): The [[magnetometer]] aboard this probe detected a subtle change in the magnetic field near Europa. This was later interpreted as a disruption in the expected magnetic field due to the current induction in a conducting layer on Europa. The composition of this conducting layer is consistent with a salty subsurface ocean.<ref>{{cite journal | vauthors = Kivelson MG, Khurana KK, Russell CT, Volwerk M, Walker RJ, Zimmer C | s2cid = 44381312 | title = Galileo magnetometer measurements: a stronger case for a subsurface ocean at Europa | journal = Science | volume = 289 | issue = 5483 | pages = 1340–3 | date = August 2000 | pmid = 10958778 | doi = 10.1126/science.289.5483.1340 | bibcode = 2000Sci...289.1340K }}</ref>
*[[Hubble Space Telescope]] (2012): An image was taken of Europa which showed evidence for a plume of water vapor coming off the surface.<ref>{{Cite web|url=https://www.spacetelescope.org/news/heic1322/|title=Hubble discovers water vapour venting from Jupiter's moon Europa|website=www.spacetelescope.org|access-date=2019-06-07}}</ref><ref>{{Cite web|url=https://www.spacetelescope.org/images/opo1633a/|title=Photo composite of suspected water plumes on Europa|website=www.spacetelescope.org|access-date=2019-06-07}}</ref>

The ''Europa Clipper'' probe includes instruments to help confirm the existence and composition of a subsurface ocean and thick icy layer. In addition, the instruments will be used to map and study surface features that may indicate tectonic activity due to a subsurface ocean.<ref>{{Cite journal|last1=Phillips|first1=Cynthia B.|last2=Pappalardo|first2=Robert T.| name-list-style = vanc |date=2014-05-20|title=Europa Clipper Mission Concept: Exploring Jupiter's Ocean Moon|journal=Eos, Transactions American Geophysical Union|volume=95|issue=20|pages=165–167|doi=10.1002/2014EO200002|bibcode=2014EOSTr..95..165P|doi-access=free}}</ref>

=== Enceladus ===
[[File:Enceladus geysers June 2009.jpg|thumb|An image of the plumes of water and ice coming from the surface of Enceladus. Future missions will investigate these geysers to determine the composition and look for signs of life.]]
Although there are no set plans to search for biosignatures on [[Saturn]]'s sixth-largest moon, [[Enceladus]], the prospects of biosignature discovery there are exciting enough to warrant several mission concepts that may be funded in the future. Similar to Jupiter's moon Europa, there is much evidence for a subsurface ocean to also exist on Enceladus. Plumes of water vapor were first observed in 2005 by the [[Cassini–Huygens|Cassini]] mission<ref>{{cite journal | vauthors = Porco CC, Helfenstein P, Thomas PC, Ingersoll AP, Wisdom J, West R, Neukum G, Denk T, Wagner R, Roatsch T, Kieffer S, Turtle E, McEwen A, Johnson TV, Rathbun J, Veverka J, Wilson D, Perry J, Spitale J, Brahic A, Burns JA, Delgenio AD, Dones L, Murray CD, Squyres S | s2cid = 6976648 | display-authors = 6 | title = Cassini observes the active south pole of Enceladus | journal = Science | volume = 311 | issue = 5766 | pages = 1393–401 | date = March 2006 | pmid = 16527964 | doi = 10.1126/science.1123013 | bibcode = 2006Sci...311.1393P | url = https://authors.library.caltech.edu/36593/7/PorcoCC.SOM.pdf }}</ref><ref>{{Cite web|url=http://www.esa.int/Our_Activities/Space_Science/Herschel/Enceladus_rains_water_onto_Saturn|title=Enceladus rains water onto Saturn|website=European Space Agency|date=26 July 2011|access-date=2019-06-07}}</ref> and were later determined to contain salt as well as organic compounds.<ref>{{cite journal | vauthors = Postberg F, Schmidt J, Hillier J, Kempf S, Srama R | title = A salt-water reservoir as the source of a compositionally stratified plume on Enceladus | journal = Nature | volume = 474 | issue = 7353 | pages = 620–2 | date = June 2011 | pmid = 21697830 | doi = 10.1038/nature10175 | bibcode = 2011Natur.474..620P | s2cid = 4400807 }}</ref><ref>{{Cite web|url=http://www.esa.int/Our_Activities/Space_Science/Cassini_samples_the_icy_spray_of_Enceladus_water_plumes|title=Cassini samples the icy spray of Enceladus' water plumes|website=European Space Agency |date=22 June 2011 |access-date=2019-06-07}}</ref> In 2014, more evidence was presented using gravimetric measurements on Enceladus to conclude that there is in fact a large reservoir of water underneath an icy surface.<ref>{{Cite journal|last=Witze|first=Alexandra|title=Icy Enceladus hides a watery ocean|url=http://www.nature.com/news/icy-enceladus-hides-a-watery-ocean-1.14985|journal=Nature News|doi=10.1038/nature.2014.14985|year=2014|s2cid=131145017|url-access=subscription}}</ref><ref>{{cite journal |last1=Iess |first1=L. |last2=Stevenson |first2=D. J. |last3=Parisi |first3=M. |last4=Hemingway |first4=D. |last5=Jacobson |first5=R.A. |last6=Lunine |first6=Jonathan I. |last7=Nimmo |first7=F. |last8=Armstrong |first8=J. W. |last9=Asmar |first9=S. W. |last10=Ducci |first10=M. |last11=Tortora |first11=P. |title=The Gravity Field and Interior Structure of Enceladus |date=April 4, 2014 |journal=[[Science (journal)|Science]] |volume=344 |number=6179 |pages=78–80 |doi=10.1126/science.1250551 |bibcode=2014Sci...344...78I |pmid=24700854|s2cid=28990283 |url=https://authors.library.caltech.edu/45462/7/Iess-SM.pdf }}</ref><ref>{{Cite news |url=https://www.bbc.com/news/science-environment-26872184|title=Saturn moon hides 'great lake' |last=Amos |first=Jonathan |date=2014-04-03 |access-date=2019-06-07 }}</ref> Mission design concepts include:

*[[Enceladus Life Finder]] (ELF)<ref>{{Cite book|author1=Reh, K. |author2=Spilker, L. |author3=Lunine, Jonathan I. |author4=Waite Jr., Jack Hunter |author5=Cable, M. L. |author6=Postberg, Frank |author7=Clark, K. |title=2016 IEEE Aerospace Conference |chapter=Enceladus Life Finder: The search for life in a habitable Moon |date=March 2016 |pages=1–8 |doi=10.1109/AERO.2016.7500813 |isbn=978-1-4673-7676-1 |s2cid=22950150 }}</ref><ref>{{Cite web|url=https://spaceflightnow.com/2015/04/06/diverse-destinations-considered-for-new-interplanetary-probe/|title=Diverse destinations considered for new interplanetary probe |date=2015-04-06 |work=Spaceflight Now|last=Clark|first=Stephen|access-date=2019-06-07}}</ref>
*[[Enceladus Life Signatures and Habitability]]<ref>{{Cite web|url=http://futureplanets.blogspot.com/2017/08/proposed-new-frontiers-missions.html|title=Future Planetary Exploration: Proposed New Frontiers Missions|date=2017-08-04|website=Future Planetary Exploration|access-date=2019-06-07|archive-url=https://web.archive.org/web/20170920093515/http://futureplanets.blogspot.com/2017/08/proposed-new-frontiers-missions.html|archive-date=2017-09-20}}</ref>
* Enceladus Organic Analyzer<ref>{{Cite web|url=http://eoa.ssl.berkeley.edu/|title=EOA – Enceladus Organic Analyzer|access-date=2019-06-07}}</ref>
* [[Enceladus Explorer]] (En-Ex)<ref>{{cite journal |last1=Konstantinidis |first1=Konstantinos |last2=Flores Martinez |first2=Claudio L. |last3=Dachwald |first3=Bernd |last4=Ohndorf |first4=Andreas |last5=Dykta |first5=Paul |last6=Bowitz |first6=Pascal |last7=Rudolph |first7=Martin |last8=Digel |first8=Ilya |last9=Kowalski |first9=Julia |last10=Voigt |first10=Konstantin |last11=Förstner |first11=Roger |title=A lander mission to probe subglacial water on Saturn׳s moon Enceladus for life |journal=Acta Astronautica |date=January 2015 |volume=106 |pages=63–89 |doi=10.1016/j.actaastro.2014.09.012 |bibcode=2015AcAau.106...63K }}</ref>
* [[Explorer of Enceladus and Titan]] (E<sup>2</sup>T)<ref>{{Cite web|url=https://e2tmission.wordpress.com/|title=E2T - Explorer of Enceladus and Titan|website=E2T - Explorer of Enceladus and Titan|access-date=2019-06-07}}</ref>
* [[Journey to Enceladus and Titan]] (JET)<ref>{{Cite web|url=https://www.science.org/content/article/updated-nasa-taps-missions-tiny-metal-world-and-jupiter-trojans|title=Updated: NASA taps missions to tiny metal world and Jupiter Trojans|last1=Voosen|first1=Paul|date=2017-01-04|website=Science {{!}} AAAS|access-date=2019-06-07}}</ref><ref>{{Cite conference |title=JET: Journey to Enceladus and Titan |conference=42nd Lunar and Planetary Science Conference |page = 1326|bibcode = 2011LPI....42.1326S |vauthors=Sotin C, [[Kathrin Altwegg|Altwegg K]], Brown RH, Hand K, Lunine JI, Soderblom J, Spencer J, Tortora P, ((JET Team)) |year=2011}}</ref>
* [[Life Investigation For Enceladus]] (LIFE)<ref>{{cite journal | vauthors = Tsou P, Brownlee DE, McKay CP, Anbar AD, Yano H, [[Kathrin Altwegg|Altwegg K]], Beegle LW, Dissly R, Strange NJ, Kanik I | s2cid = 34375065 | display-authors = 6 | title = LIFE: Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life | journal = Astrobiology | volume = 12 | issue = 8 | pages = 730–42 | date = August 2012 | pmid = 22970863 | doi = 10.1089/ast.2011.0813 | bibcode = 2012AsBio..12..730T }}</ref>
* Testing the Habitability of Enceladus's Ocean ([[Testing the Habitability of Enceladus's Ocean|THEO]])<ref>{{Cite journal| vauthors = MacKenzie SM, Caswell TE, Phillips-Lander CM, Stavros EN, Hofgartner JD, Sun VZ, Powell KE, Steuer CJ, O'Rourke JG, Dhaliwal JK, Leung CW |date=2016-09-15|title=THEO concept mission: Testing the Habitability of Enceladus's Ocean|journal=Advances in Space Research|volume=58|issue=6|pages=1117–1137|doi=10.1016/j.asr.2016.05.037|issn=0273-1177|bibcode=2016AdSpR..58.1117M|arxiv=1605.00579|s2cid=119112894}}</ref>

All of these concept missions have similar science goals: To assess the habitability of Enceladus and search for biosignatures, in line with the strategic map for exploring the ocean-world Enceladus.<ref>{{Cite journal|last=Sherwood|first=Brent | name-list-style = vanc |date=2016-09-01|title=Strategic map for exploring the ocean-world Enceladus|journal=Acta Astronautica|series=Space Flight Safety|volume=126|pages=52–58|doi=10.1016/j.actaastro.2016.04.013|issn=0094-5765|bibcode=2016AcAau.126...52S|s2cid=112827329 }}</ref>

== Searching outside of the Solar System ==

At 4.2 [[light-year]]s (1.3 [[parsec]]s, 40&nbsp;trillion [[km]], or 25&nbsp;trillion miles) away from Earth, the closest potentially habitable [[exoplanet]] is [[Proxima Centauri b]], which was discovered in 2016.<ref name="proxima b discovery paper">{{cite journal
| bibcode         = 2016Natur.536..437A
| title           = A terrestrial planet candidate in a temperate orbit around Proxima Centauri
| journal         = Nature
| volume          = 536
| issue           = 7617
| pages           = 437–440
| last1           = Anglada-Escudé
| first1          = Guillem
| last2           = Amado
| first2          = Pedro J.
| last3           = Barnes
| first3          = John
| last4           = Berdiñas
| first4          = Zaira M.
| last5           = Butler
| first5          = R. Paul
| last6           = Coleman
| first6          = Gavin A. L.
| last7           = de la Cueva
| first7          = Ignacio
| last8           = Dreizler
| first8          = Stefan
| last9           = Endl
| first9          = Michael
| last10          = Giesers
| first10         = Benjamin
| last11          = Jeffers
| first11         = Sandra V.
| last12          = Jenkins
| first12         = James S.
| last13          = Jones
| first13         = Hugh R. A.
| last14          = Kiraga
| first14         = Marcin
| last15          = Kürster
| first15         = Martin
| last16          = López-González
| first16         = María J.
| last17          = Marvin
| first17         = Christopher J.
| last18          = Morales
| first18         = Nicolás
| last19          = Morin
| first19         = Julien
| last20          = Nelson
| first20         = Richard P.
| last21          = Ortiz
| first21         = José L.
| last22          = Ofir
| first22         = Aviv
| last23          = Paardekooper
| first23         = Sijme-Jan
| last24          = Reiners
| first24         = Ansgar
| last25          = Rodríguez
| first25         = Eloy
| last26          = Rodríguez-López
| first26         = Cristina
| last27          = Sarmiento
| first27         = Luis F.
| last28          = Strachan
| first28         = John P.
| last29          = Tsapras
| first29         = Yiannis
| last30          = Tuomi
| first30         = Mikko
| first31=Mathias |last31=Zechmeister
| display-authors  = 3
| year            = 2016
| arxiv           = 1609.03449
| doi             = 10.1038/nature19106
| pmid            = 27558064
| s2cid = 4451513
| url=https://www.nature.com/articles/nature19106 
}}</ref><ref>{{cite journal | vauthors = Meadows VS, Arney GN, Schwieterman EW, Lustig-Yaeger J, Lincowski AP, Robinson T, Domagal-Goldman SD, Deitrick R, Barnes RK, Fleming DP, Luger R, Driscoll PE, Quinn TR, Crisp D | display-authors = 6 | title = The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants | journal = Astrobiology | volume = 18 | issue = 2 | pages = 133–189 | date = February 2018 | pmid = 29431479 | pmc = 5820795 | doi = 10.1089/ast.2016.1589 | arxiv = 1608.08620 | bibcode = 2018AsBio..18..133M }}</ref> This means it would take more than 18,100 years to get there if a vessel could consistently travel as fast as the ''[[Juno (spacecraft)|Juno]]'' spacecraft (250,000 kilometers per hour or 150,000 miles per hour).<ref>{{Cite web|url=https://www.missionjuno.swri.edu/earth-flyby/story/how-fast-can-juno-go|title=How Fast Can Juno Go?|website=Mission Juno|access-date=2019-06-08}}</ref> It is currently not feasible to send humans or even probes to search for biosignatures outside of the Solar System. The only way to search for biosignatures outside of the [[Solar System]] is by observing exoplanets with telescopes.

There have been no plausible or confirmed biosignature detections outside of the Solar System. Despite this, it is a rapidly growing field of research due to the prospects of the next generation of telescopes. The [[James Webb Space Telescope]], which launched in December 2021, will be a promising next step in the search for biosignatures. Although its wavelength range and resolution will not be compatible with some of the more important atmospheric biosignature gas bands like oxygen, it will still be able to detect some evidence for oxygen false positive mechanisms.<ref>{{Cite journal|last1=Lincowski|first1=Andrew P.|last2=Meadows|first2=Victoria S.|last3=Lustig-Yaeger|first3=Jacob | name-list-style = vanc |date=2019-05-17|title=The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST|journal=The Astronomical Journal|volume=158|issue=1|page=27|arxiv=1905.07070v1|doi=10.3847/1538-3881/ab21e0|bibcode=2019AJ....158...27L|s2cid=158046684 |doi-access=free }}</ref>

The new generation of ground-based 30-meter class telescopes ([[Thirty Meter Telescope]] and [[Extremely Large Telescope]]) will have the ability to take high-resolution spectra of exoplanet atmospheres at a variety of wavelengths.<ref>{{Cite arXiv |vauthors = Crossfield IJ |date=2016-04-21|title=Exoplanet Atmospheres and Giant Ground-Based Telescopes|class=astro-ph.IM |eprint=1604.06458v1}}</ref> These telescopes will be capable of distinguishing some of the more difficult false positive mechanisms such as the abiotic buildup of oxygen via photolysis. In addition, their large collecting area will enable high angular resolution, making [[direct imaging]] studies more feasible.

== Bibliography ==
* Gaines, Susan M.; Eglinton, Geoffrey; Rullkotter, Jurgen (2008). [[Echoes of Life: What Fossil Molecules Reveal about Earth History]]. Oxford University Press. ISBN 978-0-19-517619-3.

== See also ==
* [[Bioindicator]]
* [[Taphonomy]]
* [[Technosignature]]

== References ==
{{reflist}}

{{Astrobiology}}
{{Extraterrestrial life}}
{{Portal bar|Astronomy|Biology}}

[[Category:Astrobiology]]
[[Category:Astrochemistry]]
[[Category:Bioindicators]]
[[Category:Biology terminology]]
[[Category:Search for extraterrestrial intelligence]]
[[Category:Petroleum geology]]