Tholin
Tholins (after the Greek θολός (tholós) "hazy" or "muddy";[1] from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO2), methane (CH4) or ethane (C2H6), often in combination with nitrogen (N2) or water (H2O).
[2][3] Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically nitriles and hydrocarbons, and their degraded forms such as amines and phenyls.
[4] 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.
[1] 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".
Laboratory infrared spectroscopy analysis of experimentally synthesized tholins has confirmed earlier identifications of chemical groups present, including primary amines, nitriles, and alkyl portions such as CH2/CH3 forming complex disordered macromolecular solids.
Laboratory tests generated complex solids formed from exposure of N2:CH4 gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952.
[8] These atmospherically-derived substances are distinct from ice tholin II, which are formed instead by irradiation (radiolysis) of clathrates of water and organic compounds such as methane (CH4) or ethane (C2H6).
[2] 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.
[14] Laboratory experiments[15] 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,[16][3] and has implications for the origins of life on Earth and possibly other planets.
[19][20] 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 comets, carbonaceous chondrites asteroids, and pre-planetary solar nebulae; and as a major constituent of the interstellar medium.
Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust.
[51][52] Tholins might have also been detected in the stellar system of the young star HR 4796A using the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope.
