Editing Tholin (section)

==Overview==
[[File:Poly(acrylonitrile).png|150px|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'/>