View source for Tholin

{{short description|Class of molecules formed by ultraviolet irradiation of organic compounds}}

[[File:Mordor Macula Cropped.png|thumb|[[Mordor Macula]], a dark region on [[Charon (moon)|Charon]]'s north pole. The region is stained a dark brown by deposits of tholins]]
'''Tholins''' (after the Greek  {{lang|grc|θολός}} (''{{lang|grc-Latn|tholós}}'') "hazy" or "muddy";<ref name="sagan1979"/> from the [[ancient Greek]] word meaning "sepia ink") are a wide variety of [[organic compounds]] formed by solar [[ultraviolet]] or [[cosmic rays|cosmic ray]] irradiation of simple carbon-containing compounds such as [[carbon dioxide]] ({{chem|C|O|2}}), [[methane]] ({{chem|CH|4}}) or [[ethane]] ({{chem|C|2|H|6}}), often in combination with [[nitrogen]] ({{chem|N|2}}) or [[water]] ({{chem|H|2|O}}).<ref name="cometary tholins"/><ref name="Hörst">[[Sarah Hörst]] [http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html "What in the world(s) are tholins?"], Planetary Society, July 23, 2015. Retrieved 30 Nov 2016.</ref> Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically [[nitrile]]s and [[hydrocarbon]]s, and their degraded forms such as [[amine]]s and [[Phenyl group|phenyl]]s. Tholins do not form naturally on modern-day [[Earth]], but they are found in great abundance on the surfaces of icy bodies in the outer [[Solar System]], and as reddish [[aerosol]]s in the atmospheres of outer Solar System planets and moons. 

In the presence of water, tholins could be raw materials for prebiotic chemistry (i.e., the non-living chemistry that forms the basic chemicals of which life is made). Their existence has implications for the [[Abiogenesis|origins of life]] on Earth and possibly on other planets. As particles in an atmosphere, tholins scatter light, and can affect [[Planetary habitability|habitability]].

Tholins may be produced in a laboratory, and are usually studied as a heterogeneous mixture of many chemicals with many different structures and properties. Using techniques like [[thermogravimetric analysis]], [[astrochemistry|astrochemists]] analyze the composition of these tholin mixtures, and the exact character of the individual chemicals within them.<ref name="Planetary and Space Science pp. 279–288">{{cite journal |last1=Nna-Mvondo |first1=Delphine |last2=de la Fuente |first2=José L. |last3=Ruiz-Bermejo |first3=Marta |last4=Khare |first4=Bishun |last5=McKay |first5=Christopher P. |title=Thermal characterization of Titan's tholins by simultaneous TG–MS, DTA, DSC analysis |journal=Planetary and Space Science |date=September 2013 |volume=85 |pages=279–288 |doi=10.1016/j.pss.2013.06.025 |bibcode=2013P&SS...85..279N }}</ref>

==Overview==
[[File:Poly(acrylonitrile).png|thumb|[[Polyacrylonitrile]], one hypothesized polymeric component of tholins, mostly in chemically degraded form as polymers containing [[cyanide|nitrile]] and [[amine|amino]] groups. It is used experimentally to create tholin mixtures.<ref name="Planetary and Space Science pp. 279–288"/>]]

The term "tholin" was coined by astronomer [[Carl Sagan]] and his colleague [[Bishun Khare]] to describe the difficult-to-characterize substances they obtained in his [[Miller–Urey experiment|Miller–Urey-type experiments]] on the methane-containing gas mixtures such as those found in [[Titan_(moon)|Titan]]'s atmosphere.<ref name="sagan1979">{{cite journal| title =Tholins: organic chemistry of interstellar grains and gas|first1=Carl |last1=Sagan |author1-link=Carl Sagan |first2=Bishun |last2=Khare |author2-link=Bishun Khare|doi=10.1038/277102a0 |journal=Nature |volume =277 |issue= 5692 |date= 11 January 1979 |pages =102–107 |bibcode = 1979Natur.277..102S |s2cid=4261076 }}</ref> Their paper proposing the name "tholin" said:
<blockquote>For the past decade we have been producing in our laboratory a variety of complex organic solids from mixtures of the cosmically abundant gases {{chem|CH|4}}, {{chem|C|2|H|6}}, {{chem|NH|3}}, {{chem|H|2|O}}, HCHO, and {{chem|H|2|S}}. The product, synthesized by ultraviolet (UV) light or spark discharge, is a brown, sometimes sticky, residue, which has been called, because of its resistance to conventional analytical chemistry, "intractable polymer".  [...] We propose, as a model-free descriptive term, 'tholins' (Greek Θολός, muddy; but also Θόλος, vault or dome), although we were tempted by the phrase 'star-tar'.<ref name="Hörst" /><ref name="sagan1979" /></blockquote>

Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, including [[heteropolymer]]s,<ref>[https://uanews.arizona.edu/story/bit-titan-earth-helps-search-lifes-origins A Bit of Titan on Earth Helps in the Search for Life's Origins]. Lori Stiles, ''University of Arizona''. 19 October  2004.</ref><ref>{{cite journal|doi=10.1016/j.icarus.2014.04.042|title=Amino acids generated from hydrated Titan tholins: Comparison with Miller–Urey electric discharge products|journal=Icarus|volume=237|pages=182–189|year=2014|last1=Cleaves|first1=H. James|last2=Neish|first2=Catherine|last3=Callahan|first3=Michael P.|last4=Parker|first4=Eric|last5=Fernández|first5=Facundo M.|last6=Dworkin|first6=Jason P.|bibcode=2014Icar..237..182C|doi-access=free}}</ref> that give a reddish, organic surface covering on certain planetary surfaces. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups.<ref name='Colors MU69'/> Sagan and Khare note "The properties of tholins will depend on the energy source used and the initial abundances of precursors, but a general physical and chemical similarity among the various tholins is evident."<ref name="sagan1979" />

Some researchers in the field prefer a narrowed definition of tholins, for example S. Hörst wrote: "Personally, I try to use the word 'tholins' only when describing the laboratory-produced samples, in part because we do not really know yet how similar the material we produce in the lab is to the material found on places like Titan or Triton (or Pluto!)."<ref name="Hörst" /> French researchers also use the term tholins only when describing the laboratory-produced samples as analogues.<ref name='Dubois 2018'>{{Cite journal | arxiv=1807.04569| doi=10.1016/j.icarus.2018.07.006| bibcode=2019Icar..317..182D| title=In situ investigation of neutrals involved in the formation of Titan tholins| year=2019| last1=Dubois| first1=David| last2=Carrasco| first2=Nathalie| last3=Petrucciani| first3=Marie| last4=Vettier| first4=Ludovic| last5=Tigrine| first5=Sarah| last6=Pernot| first6=Pascal| journal=Icarus| volume=317| pages=182–196| s2cid=119446074}}</ref> NASA scientists also prefer the word 'tholin' for the products of laboratory simulations, and use the term 'refractory residues' for actual observations on astronomical bodies.<ref name='Colors MU69'/>

==Formation==
[[File:Formation of tholins in Titan's upper atmosphere.svg|thumb|upright=1.4|The formation of tholins in the [[atmosphere of Titan]]]]
===Artificially===
The key elements of tholins are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthesized tholins has confirmed earlier identifications of chemical groups present, including primary [[amine]]s, [[nitrile]]s, and [[alkyl]] portions such as {{chem|CH|2}}/{{chem|CH|3}} forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of {{chem|N|2}}:{{chem|CH|4}} gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous [[Miller–Urey experiment]] conducted in 1952.<ref>{{cite journal |title=New experimental constraints on the composition and structure of tholins |author=Eric Quirico |author2=Gilles Montagnac |author3=Victoria Lees |author4=Paul F. McMillan |author5=Cyril Szopa |author6=Guy Cernogora |author7=Jean-Noël Rouzaud |author8=Patrick Simon |author9=Jean-Michel Bernard |author10=Patrice Coll |author11=Nicolas Fray |author12=Robert D. Minardi |author13=François Raulin |author14=Bruno Reynard |author15=Bernard Schmitt |journal=Icarus |volume=198 |issue=1 |pages=218–231 |date=November 2008 |doi=10.1016/j.icarus.2008.07.012|bibcode=2008Icar..198..218Q }}</ref>

===Naturally===
As illustrated to the right, tholins are thought to form in nature through a chain of chemical reactions known as [[pyrolysis]] and [[radiolysis]]. This begins with the [[dissociation (chemistry)|dissociation]] and [[ionization]] of molecular [[nitrogen]] ({{chem|N|2}}) and [[methane]] ({{chem|CH|4}}) by energetic particles and solar radiation. This is followed by the formation of [[ethylene]], [[ethane]], [[acetylene]], [[hydrogen cyanide]], and other small simple molecules and small positive ions. Further reactions form [[benzene]] and other organic molecules, and their polymerization leads to the formation of an aerosol of heavier molecules, which then condense and precipitate on the planetary surface below.<ref name="Waite2007">{{Cite journal
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Tholins formed at low pressure tend to contain nitrogen atoms in the interior of their molecules, while tholins formed at high pressure are more likely to have nitrogen atoms located in terminal positions.<ref>{{cite web |date=9 March 2004 |url=http://ca.pittcon.org/technical+program/tpabstra04.nsf/Agenda+Time+Slots+Web/233224B2F6F17D3785256D9E00725026?Opendocument |last1=McGuigan |first1=M. |last2=Sacks |first2=R.D. |title=Comprehensive Two Dimensional Gas Chromatography Study of Tholin Samples Using Pyrolysis Inlet and TOF-MS Detection |website=Pittcon Conference & Expo}}</ref><ref>{{cite journal |last1=McGuigan |first1=M.A. |last2=Waite |first2=J.H. |last3=Imanaka |first3=H. |last4=Sacks |first4=R.D. |date=2006 |title=Analysis of Titan tholin pyrolysis products by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry |journal= Journal of Chromatography A|volume=1132 |issue=1–2 |pages=280–288 |doi=10.1016/j.chroma.2006.07.069 |pmid=16934276 }}</ref>

Tholins may be a major constituent of the [[interstellar medium]].<ref name="sagan1979"/> On Titan, their chemistry is initiated at high altitudes and participates in the formation of solid organic particles.<ref name='Dubois 2018'/> 

These atmospherically derived substances are distinct from ''ice tholin II'', which are formed instead by irradiation ([[radiolysis]]) of [[clathrate]]s of [[water]] and organic compounds such as methane ({{chem|CH|4}}) or ethane ({{chem|C|2|H|6}}).<ref name='cometary tholins'>{{cite journal |last1=McDonald |first1=G.D. |last2=Whited |first2=L.J. |last3=DeRuiter |first3=C. |last4=Khare |first4=B.N. |last5=Patnaik |first5=A. |last6=Sagan |first6=C. |title=Production and chemical analysis of cometary ice tholins |journal=Icarus |volume=122 |issue=1 |pages=107–117 |doi=10.1006/icar.1996.0112 |bibcode=1996Icar..122..107M|year=1996 |doi-access=free }}</ref><ref name='Cruikshank 2005'>{{cite journal |doi=10.1016/j.icarus.2004.09.003 |bibcode=2005Icar..175..268C|url=http://astrochemistry.org/docs/Cruikshanketal2005.pdf|title=A spectroscopic study of the surfaces of Saturn's large satellites: HO ice, tholins, and minor constituents|year=2005|last1=Cruikshank|first1=D.|display-authors=etal|journal=Icarus|volume=175|issue=1|pages=268–283}}</ref> The radiation-induced synthesis on ice are independent of temperature.<ref name='cometary tholins'/>

Models show that, even when far from UV radiation of a star, [[cosmic ray]] doses may be fully sufficient to convert carbon-containing ice grains entirely to complex organics in less than the lifetime of the typical [[interstellar cloud]].<ref name='cometary tholins'/>

==Biological significance==
Some researchers have speculated that Earth may have been seeded by organic compounds early in its development by tholin-rich comets, providing the raw material necessary for life to develop.<ref name="sagan1979" /><ref name='cometary tholins'/> (See [[Miller–Urey experiment]] for discussion.) Tholins do not exist naturally on present-day Earth due to the oxidizing properties of the free oxygen component of its atmosphere ever since the [[Great Oxygenation Event]] around 2.4 billion years ago.<ref name='prebiotic chem 2013'>{{Cite journal | doi=10.2174/13852728113179990078| pmid=24143126| pmc=3796891| title=Atmospheric Prebiotic Chemistry and Organic Hazes| year=2013| last1=Trainer| first1=Melissa| journal=Current Organic Chemistry| volume=17| issue=16| pages=1710–1723}}</ref>

Laboratory experiments<ref name='Coll 2010'>{{Cite journal |bibcode = 2010AGUFM.P31C1551C|title = Prebiotic chemistry on Titan ? The nature of Titan's aerosols and their potential evolution at the satellite surface|last1 = Coll|first1 = P. J.|last2 = Poch|first2 = O.|last3 = Ramirez|first3 = S. I.|last4 = Buch|first4 = A.|last5 = Brassé|first5 = C.|last6 = Raulin|first6 = F.|journal = AGU Fall Meeting Abstracts|pages = P31C–1551|year = 2010|volume = 2010}}</ref> suggest that tholins near large pools of liquid water that might persist for thousands of years could facilitate the formation of prebiotic chemistry to take place,<ref name='Tholins Europa'/><ref name="Hörst "/> and has implications for the [[Abiogenesis|origins of life]] on Earth and possibly other planets.<ref name="Hörst "/><ref name='prebiotic chem 2013'/> Also, as particles in the atmosphere of an [[exoplanet]], tholins affect the light scatter and act as a screen for protecting planetary surfaces from [[ultraviolet]] radiation, affecting [[Planetary habitability|habitability]].<ref name="Hörst "/><ref>{{cite web |url=http://www.spectroscopynow.com/details/ezine/sepspec14793ezine/Mooning-over-Titans-atmosphere.html |title=Mooning over Titan's atmosphere |website=SpectroscopyNOW |date=15 October 2006}}</ref> Laboratory simulations found derived residues related to [[amino acid]]s as well as [[urea]], with important [[Astrobiology|astrobiological]] implications.<ref name='prebiotic chem 2013'/><ref name='Coll 2010'/><ref>{{Cite journal |pmid = 21161385|year = 2011|last1 = Ruiz-Bermejo|first1 = M.|last2 = Rivas|first2 = L. A.|last3 = Palacín|first3 = A.|last4 = Menor-Salván|first4 = C.|last5 = Osuna-Esteban|first5 = S.|title = Prebiotic synthesis of protobiopolymers under alkaline ocean conditions|journal = Origins of Life and Evolution of the Biosphere |volume = 41|issue = 4|pages = 331–45|doi = 10.1007/s11084-010-9232-z|bibcode = 2011OLEB...41..331R|s2cid = 19283373}}</ref>

On Earth, a wide variety of [[Soil biology|soil bacteria]] are able to use laboratory-produced tholins as their sole source of carbon. Tholins could have been the first microbial food for [[heterotroph]]ic microorganisms before [[autotroph]]y evolved.<ref>{{cite journal |last1=Stoker |first1=C.R. |last2=Boston |first2=P.J. |last3=Mancinelli |first3=R.L. |last4=Segal |first4=W. |last5=Khare |first5=B.N. |last6=Sagan |first6=C. |date=1990 |title=Microbial metabolism of tholin |journal=Icarus |volume=85 |issue=1 |pages=241–256 |doi=10.1016/0019-1035(90)90114-O |pmid=11538367 |bibcode=1990Icar...85..241S|url=https://zenodo.org/record/1253904 }}</ref><ref>{{cite journal | pmid=11538106 | year=1991 | last1=McKay | first1=C. P. | title=Urey Prize Lecture: Planetary Evolution and the Origin of Life | journal=Icarus | volume=91 | issue=1 | pages=93–100 | doi=10.1016/0019-1035(91)90128-g | bibcode=1991Icar...91...93M }}</ref>

==Occurrence==
[[File:Huygens surface color.jpg|thumb|left|upright|The surface of Titan as viewed from the [[Huygens (spacecraft)|''Huygens'' lander]]. Tholins are suspected to be the source of the reddish color of both the surface and the atmospheric haze.]]
Sagan and Khare note the presence of tholins through multiple locations: "as a constituent of the Earth's primitive oceans and therefore relevant to the [[origin of life]]; as a component of red aerosols in the atmospheres of the outer planets and Titan; present in [[comet]]s, carbonaceous chondrites asteroids, and pre-planetary solar nebulae; and as a major constituent of the [[interstellar medium]]."<ref name="sagan1979" /> The surfaces of comets, [[centaur (planetoid)|centaur]]s, and many icy moons and [[Kuiper belt|Kuiper-belt]] objects in the outer Solar System are rich in deposits of tholins.<ref>{{Cite journal | doi=10.1016/j.icarus.2015.12.017| title=Sublimation of water ice mixed with silicates and tholins: Evolution of surface texture and reflectance spectra, with implications for comets| journal=Icarus| volume=267| pages=154–173| year=2016| last1=Poch| first1=Olivier| last2=Pommerol| first2=Antoine| last3=Jost| first3=Bernhard| last4=Carrasco| first4=Nathalie| last5=Szopa| first5=Cyril| last6=Thomas| first6=Nicolas| bibcode=2016Icar..267..154P| s2cid=56028928}}</ref>

===Moons===
====Titan====
[[Titan (moon)|Titan]] tholins are nitrogen-rich<ref name="McDonald1994"/><ref>{{Cite journal | doi=10.1016/j.icarus.2012.03.003| bibcode=2012Icar..221..844D| title=New insights into the structure and chemistry of Titan's tholins via 13C and 15N solid state nuclear magnetic resonance spectroscopy| journal=Icarus| volume=221| issue=2| pages= 844–853 | year=2012| last1=Derenne| first1=S.| last2=Coelho| first2=C.| last3=Anquetil| first3=C.| last4=Szopa| first4=C.| last5=Rahman| first5=A.S.| last6=McMillan| first6= P.F. | last7=Corà| first7=F.| last8=Pickard| first8=C.J.| last9=Quirico| first9=E.| last10=Bonhomme| first10=C.| url=http://discovery.ucl.ac.uk/1377547/2/1377547.pdf| doi-access= free}}</ref> organic substances produced by the irradiation of the gaseous mixtures of nitrogen and methane found in the atmosphere and surface of Titan. Titan's atmosphere is about 97% nitrogen, 2.7±0.1% methane and the remaining trace amounts of other gases.<ref>{{cite book |title=Titan: Exploring an Earthlike World |url= https://archive.org/details/titanexploringea00cous |url-access=limited |first1=Athena |last1=Coustenis |first2=Frederic W. |last2=Taylor |publisher=World Scientific |date=2008 |pages= [https://archive.org/details/titanexploringea00cous/page/n172 154]–155 |isbn=978-981-270-501-3}}</ref> In the case of Titan, the haze and orange-red color of its atmosphere are both thought to be caused by the presence of tholins.<ref name="Waite2007" /><ref>{{cite web | url=https://astrobiology.nasa.gov/nai/annual-reports/2010/jpl-titan/task-34-tholin-chemical-analysis/ | title=Task 3.4 Tholin Chemical Analysis |website= NASA Astrobiology Institute |date = August 2010}}</ref>

====Europa====
[[Image:PIA20028 - Europa's varied surface features (rotated).jpg|thumb|right|Linear fractures on Europa's surface, likely colored by tholins.]]
Colored regions on Jupiter's satellite [[Europa (moon)|Europa]] are thought to be tholins.<ref name='Tholins Europa'>{{Cite journal |doi = 10.1029/2002JE001841|bibcode = 2002JGRE..107.5114B|title = A new energy source for organic synthesis in Europa's surface ice|year = 2002|last1 = Borucki|first1 = Jerome G.|last2 = Khare|first2 = Bishun|last3 = Cruikshank|first3 = Dale P.|journal = Journal of Geophysical Research: Planets|volume = 107|issue = E11|pages = 24-1-24-5|doi-access = free}}</ref><ref name='amino acids'>{{Cite journal |bibcode = 2017AAS...22913804W|title = MISE: A Search for Organics on Europa|last1 = Whalen|first1 = Kelly|last2 = Lunine|first2 = Jonathan I.|last3 = Blaney|first3 = Diana L.|author3-link= Diana Blaney |journal = American Astronomical Society Meeting Abstracts |volume = 229|pages = 138.04|year = 2017}}</ref><ref name='JPL2015'>{{cite news |url=http://www.jpl.nasa.gov/news/news.php?feature=4602 |title=Europa Mission to Probe Magnetic Field and Chemistry |work=Jet Propulsion Laboratory |date=27 May 2015 |access-date=2017-10-23}}</ref><ref>{{Cite journal |bibcode = 2005DPS....37.5810K|title = Impact Driven Chemistry on Europa's Surface|last1 = Khare|first1 = B. N.|last2 = Nna Mvondo|first2 = D.|last3 = Borucki|first3 = J. G.|last4 = Cruikshank|first4 = D. P.|last5 = Belisle|first5 = W. A.|last6 = Wilhite|first6 = P.|last7 = McKay|first7 = C. P.|journal = Bulletin of the American Astronomical Society |volume=37|page = 753 |year = 2005}}</ref> The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where [[pyrolysis]] and [[radiolysis]] take place. In order to generate colored tholins on Europa there must be a source of materials (carbon, nitrogen, and water), and a source of energy to drive the reactions. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as [[Cryovolcano|cryovolcanic]] events that resurface the body, and to accumulate from space as interplanetary dust.<ref name='Tholins Europa'/>

====Rhea====
[[File:PIA18438-SaturnMoon-Rhea-20141104-fig2.jpg|thumb|The trailing hemisphere of Saturn's moon [[Rhea (moon)|Rhea]] is covered with tholins.]]
[[File:Pluto’s Heart - Like a Cosmic Lava Lamp.jpg|thumb|Close-up view of Sputnik Planitia on Pluto as viewed by the ''[[New Horizons]]'' spacecraft, showing nitrogen ice glaciers and reddish-colored tholins.]]

The extensive dark areas on the trailing hemisphere of Saturn's moon [[Rhea (moon)|Rhea]] are thought to be deposited tholins.<ref name='Cruikshank 2005'/>

====Triton====
Neptune's moon [[Triton (moon)|Triton]] is observed to have the reddish color characteristic of tholins.<ref name="McDonald1994">{{cite journal|doi=10.1006/icar.1994.1046|pmid=11539478|title=Chemical Investigation of Titan and Triton Tholins|journal=Icarus|volume= 108 |issue=1|pages=137–145|year=1994|last1=McDonald|first1=Gene D.|last2=Thompson|first2=W.Reid|last3=Heinrich|first3=Michael|last4=Khare|first4=Bishun N. |last5= Sagan|first5=Carl|bibcode=1994Icar..108..137M|doi-access=free}}</ref> Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide.<ref>[https://www.universetoday.com/56042/triton/ Neptune's Moon Triton]. Matt Williams, ''Universe Today''. 16 October 2016.</ref><ref>{{cite web|url = https://science.nasa.gov/neptune/moons/triton/ |title=Triton|website= NASA Science|date=21 November 2017 |access-date = 14 November 2023}}</ref>

===Dwarf planets===
====Pluto====
Tholins occur on the [[dwarf planet]] [[Pluto]]<ref>{{cite web |url=http://www.nasa.gov/nh/pluto-the-other-red-planet |title=Pluto: The 'Other' Red Planet |author=<!--Staff writer(s); no by-line.--> |date=3 July 2015 |website=NASA |access-date=2015-07-06 |quote=Experts have long thought that reddish substances are generated as a particular color of ultraviolet light from the sun, called Lyman-alpha, strikes molecules of the gas methane ({{chem|CH|4}}) in Pluto's atmosphere, powering chemical reactions that create complex compounds called tholins.}}</ref> and are responsible for red colors<ref>[http://www.businessinsider.com/nasa-released-an-incredibly-detailed-photo-of-snow-and-something-else-on-pluto-2016-3 "NASA released an incredibly detailed photo of snow - and something else - on Pluto"], ''Business Insider Australia,'' Mar. 6, 2016 (accessed 28 Feb. 2018).</ref> as well as the blue tint of the [[atmosphere of Pluto]].<ref>{{cite news |author=Amos, Jonathan |title=New Horizons: Probe captures Pluto's blue hazes |work= BBC News |date=8 October 2015 |url=https://www.bbc.com/news/science-environment-34481347}}</ref> The reddish-brown cap of the north pole of [[Charon (moon)|Charon]],<ref name= "Hörst" /> the largest of five [[moons of Pluto]], is thought to be composed of tholins, produced from methane, nitrogen and related gases released from the atmosphere of Pluto and transferred over about {{convert|19,000|km|mi|abbr=on}} distance to the orbiting moon.<ref name="NASA-20150909">{{cite web |last=Albert |first=P.T. |title= New Horizons Probes the Mystery of Charon's Red Pole |url=https://blogs.nasa.gov/pluto/2015/09/09/new-horizons-probes-the-mystery-of-charons-red-pole/ |date=9 September 2015 |work=[[NASA]] |access-date= 2015-09-09}}</ref><ref name="NYT-20160914">{{cite news |last1=Bromwich |first1=Jonah Engel |last2=St. Fleur |first2=Nicholas |title=Why Pluto's Moon Charon Wears a Red Cap |url=https://www.nytimes.com/2016/09/15/science/pluto-moon-charon.html |date=14 September 2016 |work=[[New York Times]] |access-date=14 September 2016}}</ref><ref>{{cite conference |url=https://www.hou.usra.edu/meetings/plutosystem2019/pdf/7014.pdf |title=The Long-Term Evolution of Pluto's Atmosphere and Its Effect on Charon's Surface Tholin Formation |author1=H. S. Shi |author2=I. L. Lai |author3=W. H. Ip |conference=Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) |year=2019}}</ref>

====Ceres====
Tholins were detected on the dwarf planet [[Ceres (dwarf planet)#Carbon|Ceres]] by the [[Dawn (spacecraft)|''Dawn'' mission]].<ref name="PO-20170216">{{cite web |title=Dawn discovers evidence for organic material on Ceres (Update) |url=https://phys.org/news/2017-02-scientists-geology-ceres.html |date=16 February 2017 |work=[[Phys.org]] |access-date=17 February 2017}}</ref><ref name='organics 2018'>{{cite journal|doi=10.1016/j.icarus.2017.12.039|title=The surface composition of Ceres' Ezinu quadrangle analyzed by the Dawn mission|journal=Icarus|volume=318|pages=124–146|year=2019|last1=Combe|first1=Jean-Philippe|display-authors=etal|bibcode=2019Icar..318..124C|s2cid=125598869}}</ref> Most of the planet's surface is extremely rich in carbon, with approximately 20% carbon by mass in its near surface.<ref name='PO 12 2018'>[https://phys.org/news/2018-12-team-evidence-carbon-rich-surface-ceres.html Team finds evidence for carbon-rich surface on Ceres]. Southwest Research Institute. Published by ''PhysOrg''. 10 December 2018.</ref><ref name='Nature 12 2018'>{{Cite journal | doi=10.1038/s41550-018-0656-0|title = An aqueously altered carbon-rich Ceres| journal=[[Nature Astronomy]]|volume = 3|issue = 2|pages = 140–145|year = 2019|last1 = Marchi|first1 = S.|display-authors=etal|bibcode = 2019NatAs...3..140M|s2cid = 135013590}}</ref> The carbon content is more than five times higher than in [[carbonaceous chondrite]] meteorites analyzed on Earth.<ref name='Nature 12 2018'/>

====Makemake====
[[Makemake]] exhibits [[methane]], large amounts of [[ethane]] and tholins, as well as smaller amounts of [[ethylene]], [[acetylene]] and high-mass [[alkane]]s may be present, most likely created by [[photolysis]] of methane by solar radiation.<ref name="brown">{{cite journal|title=Methane and Ethane on the Bright Kuiper Belt Object 2005 FY<sub>9</sub>|author=Mike Brown|display-authors=4 |author2=K. M. Barksume| author3=G. L. Blake|author4=E. L. Schaller|author5=D. L. Rabinowitz| author6=H. G. Roe |author7=C. A. Trujillo|journal= The Astronomical Journal |date=2007 |volume=133| issue=1|pages= 284–289| doi= 10.1086/509734|bibcode=2007AJ....133..284B|s2cid=12146168 |url=http://authors.library.caltech.edu/7288/1/BROaj07.pdf}}</ref><ref name="brown2015" >{{cite journal| title=Irradiation products on the dwarf planet Makemake |author= M. E. Brown |author2= E. L. Schaller| author3= G. A. Blake| journal=The Astronomical Journal |volume=149 |issue= 3 |page= 105|year=2015| doi= 10.1088/0004-6256/149/3/105| bibcode = 2015AJ....149..105B |s2cid= 39534359 |url= https://authors.library.caltech.edu/57340/1/1538-3881_149_3_105.pdf }}</ref><ref>{{cite journal |doi=10.1086/509734|bibcode=2007AJ....133..284B|url=https://authors.library.caltech.edu/7288/1/BROaj07.pdf|title=Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9|year=2007|last1=Brown|first1=M. E.|last2=Barkume|first2=K. M.|last3=Blake|first3=G. A.|last4=Schaller|first4=E. L.|last5=Rabinowitz|first5=D. L.|last6=Roe|first6=H. G.|last7=Trujillo|first7=C. A.|journal=The Astronomical Journal|volume=133|issue=1|pages=284–289|s2cid=12146168 }}</ref>

===Kuiper belt objects and Centaurs===
The reddish color typical of tholins is characteristic of many [[Trans-Neptunian object]]s, including [[plutino]]s in the outer Solar System such as [[28978 Ixion]].<ref name=Boehnhardt2004>{{cite journal |author=H. Boehnhardt |date=2004 |title=Surface characterization of 28978 Ixion (2001 KX76) |journal=[[Astronomy and Astrophysics Letters]]
 |volume=415 |pages=L21–L25 |doi=10.1051/0004-6361:20040005 |bibcode=2004A&A...415L..21B
 |display-authors=1 |last2=Bagnulo|first2=S. |last3=Muinonen |first3=K. |last4=Barucci
 |first4=M. A. |last5=Kolokolova |first5=L. |last6=Dotto |first6=E. |last7=Tozzi |first7=G. P.
 |issue=2|doi-access=free }}</ref> Spectral reflectances of [[Centaur (minor planet)|Centaurs]] also suggest the presence of tholins on their surfaces.<ref>{{cite journal |doi=10.1023/B:MOON.0000031948.39136.7d |bibcode=2003EM&P...92..315C|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040012770.pdf|title=Spectral Models of Kuiper Belt Objects and Centaurs|year=2003|last1=Cruikshank|first1=Dale P.|last2=Dalle Ore|first2=Cristina M.|author2-link=Cristina Dalle Ore|journal=Earth, Moon, and Planets|volume=92|issue=1–4|pages=315–330|s2cid=189906047|hdl=2060/20040012770|hdl-access=free}}</ref><ref>{{cite journal |url=https://authors.library.caltech.edu/9247/1/BARaj08.pdf |doi=10.1088/0004-6256/135/1/55|bibcode=2008AJ....135...55B|title=Near-Infrared Spectra of Centaurs and Kuiper Belt Objects|year=2008|last1=Barkume|first1=K. M.|last2=Brown|first2=M. E.|last3=Schaller|first3=E. L.|journal=The Astronomical Journal|volume=135|issue=1|pages=55–67|citeseerx=10.1.1.62.5470|s2cid=12245232 }}</ref><ref>{{Cite journal | doi=10.3847/1538-3881/aab14e| bibcode=2018AJ....155..170S| title=Surface Ice and Tholins on the Extreme Centaur 2012 DR30| year=2018| last1=Szabó| first1=Gy. M.| last2=Kiss |display-authors=etal | journal=The Astronomical Journal| volume=155| issue=4| page=170| doi-access=free}}</ref> The ''New Horizons'' exploration of the [[classical Kuiper belt object]] [[486958 Arrokoth]] revealed reddish color at its surface, suggestive of tholins.<ref name='Colors MU69'>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2019/pdf/2051.pdf |title=THE COLORS OF 486958 2014 MU69 ("ULTIMA THULE"): THE ROLE OF SYNTHETIC ORGANIC SOLIDS (THOLINS) |last1=Cruikshank|first1=D.|display-authors=etal |collaboration=New Horizons Composition Team |conference=50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) |date=January 2019}}</ref><ref>[https://www.space.com/42864-new-horizons-ultima-thule-flyby-what-to-expect.html NASA to Make Historic New Year's Day Flyby of Mysterious Ultima Thule. Here's What to Expect.]  Nola Taylor Redd, ''Space.com''. 31 December 2018.</ref>

===Comets and asteroids===
Tholins were detected ''in situ'' by the [[Rosetta (spacecraft)|''Rosetta'']] mission to comet [[67P/Churyumov–Gerasimenko]].<ref>{{Cite journal |doi = 10.1051/0004-6361/201525977|bibcode = 2015A&A...583A..25P|title = OSIRIS observations of meter-sized exposures of H2O ice at the surface of 67P/Churyumov-Gerasimenko and interpretation using laboratory experiments|year = 2015|last1 = Pommerol|first1 = A. |display-authors=etal |journal = Astronomy & Astrophysics|volume = 583|pages = A25|doi-access = free|hdl = 11577/3182682|hdl-access = free}}</ref><ref>{{Cite journal |doi = 10.1126/science.aab0673|pmid = 26228155|bibcode = 2015Sci...349b0673W|title = CHO-bearing organic compounds at the surface of 67P/Churyumov-Gerasimenko revealed by Ptolemy|year = 2015|last1 = Wright|first1 = I. P.|last2 = Sheridan|first2 = S.|last3 = Barber|first3 = S. J.|last4 = Morgan|first4 = G. H.|last5 = Andrews|first5 = D. J.|last6 = Morse|first6 = A. D.|journal = Science|volume = 349|issue = 6247|article-number = aab0673|s2cid = 206637053}}</ref>  Tholins are not typically characteristic of main-belt asteroids, but have been detected on the asteroid [[24 Themis]].<ref name="Campins">{{ Cite journal |last1=Campins |first1=Humberto |last2=Hargrove |first2=K |last3=Pinilla-Alonso |first3=N |last4=Howell |first4=ES|author4-link=Ellen Howell |last5=Kelley |first5=MS |last6=Licandro |first6=J |last7=Mothé-Diniz |first7=T |last8=Fernández |first8=Y |last9=Ziffer |first9=J |title=Water ice and organics on the surface of the asteroid 24 Themis |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |pages=1320–1 |date=2010 |doi=10.1038/nature09029 |pmid=20428164|bibcode = 2010Natur.464.1320C |s2cid=4334032 }}</ref><ref name= "Rivkin2010">{{Cite journal |last1=Rivkin |first1=Andrew S. |last2=Emery |first2=Joshua P. |title=Detection of ice and organics on an asteroidal surface |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |pages=1322–1323 |date=2010 |doi=10.1038/nature09028 |pmid=20428165 |bibcode = 2010Natur.464.1322R |s2cid=4368093 |url=http://somosbacteriasyvirus.com/surface.pdf}}</ref>

===Tholins beyond the Solar System===

Tholins might have also been detected in the stellar system of the young star [[HR 4796|HR 4796A]] using the [[NICMOS|Near-Infrared Camera and Multi-Object Spectrometer (NICMOS)]] aboard the Hubble Space Telescope.<ref>{{cite journal |last1=Kohler |first1=M. |last2=Mann |first2=I. |last3=Li |first3=A. |date=2008 |title=Complex organic materials in the HR 4796A disk? |journal= The Astrophysical Journal|volume=686 |issue=2 |pages=L95–L98 |doi=10.1086/592961 |bibcode=2008ApJ...686L..95K |arxiv=0808.4113|s2cid=13204352 }}</ref> The [[HR 4796]] system is approximately 220 light years from Earth.<ref>{{cite web |url=http://www.spaceflightnow.com/news/n0801/05disk/ |website=Spaceflight Now |date=5 January 2008 |title=Red dust in disk may harbor precursors to life}}</ref>

==See also==
* {{annotated link|Abiogenesis}}
* {{annotated link|Asphaltene}}
* {{annotated link|Hemolithin}}
* {{annotated link|Kerogen}}
* {{annotated link|PAH world hypothesis}}
* {{annotated link|Pseudo-panspermia}}

==References==
{{reflist|36em}}

{{Molecules detected in outer space}}
{{Astrobiology}}
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[[Category:Organic polymers]]
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[[Category:Prebiotic chemistry]]