Tholins

Tholins (after the Greek θολός (tholós) "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a class of heteropolymer molecules formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane, often in combination with nitrogen. Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surface of icy bodies in the outer Solar System, and as reddish aerosols in the atmosphere of outer solar system planets and moons. They usually have a reddish-brown appearance.

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-type experiments on the methane-containing gas mixtures such as those found in Titan's atmosphere. Their paper proposing the name "tholin" said:

For the past decade we have been producing in our laboratory a variety of complex organic solids from mixtures of the cosmically abundant gases CH4, C2H6, NH3, H2O, HCHO, and H2S. 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”. However, we have recently succeeded, through sequential and non-sequential pyrolysis followed by gas chromatography-mass spectrometry (GC-MS) in determining something of the composition of this material. It is clearly not a polymer—a repetition of the same monomeric unit—and some other term is needed... We propose, as a model-free descriptive term, ‘tholins’ (Gk ϴὸλος, muddy; but also ϴoλòς, vault or dome), although we were tempted by the phrase ‘star-tar’.

Tholins are not one specific compound but rather are descriptive of a spectrum of molecules that give a reddish, organic surface covering on certain planetary surfaces. 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."

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