Tholin

Tholins (after the Greek θολός (tholós) 'hazy' or 'muddy'; from the ancient Greek word meaning 'sepia ink') are a wide variety of organic compounds formed by solar ultraviolet irradiation or cosmic rays from simple carbon-containing compounds such as carbon dioxide (CO2), methane (CH4) or ethane (C2H6), often in combination with nitrogen (N2) or water (H2O). Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups. 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 aerosols in the atmosphere of outer Solar System planets and moons.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'. 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 (after the Greek θολός (tholós) 'hazy' or 'muddy'; from the ancient Greek word meaning 'sepia ink') are a wide variety of organic compounds formed by solar ultraviolet irradiation or cosmic rays from simple carbon-containing compounds such as carbon dioxide (CO2), methane (CH4) or ethane (C2H6), often in combination with nitrogen (N2) or water (H2O). Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups. 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 aerosols in the atmosphere of outer Solar System planets and moons. In the presence of water, tholins can be raw materials for prebiotic chemistry, i.e. the non-living chemistry that forms the basic chemicals which form life. Their existence has implications for the origins of life on Earth and possibly on other planets. As particles in an atmosphere, tholins scatter light, and can affect habitability. 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: Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, including heteropolymers, 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. 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.' 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!).' French researchers also use the term tholins only when describing the laboratory-produced samples as analogues. 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. Tholins may be a major constituent of the interstellar medium. On Titan, their chemistry is initiated at high altitudes and participates in the formation of solid organic particles. Their key elements are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthetized 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. 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 and ionization of molecular nitrogen (N2) and methane (CH4) 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. 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. 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). The radiation-induced synthesis on ice are non-dependant on temperature. 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 (see Miller–Urey experiment for discussion related to this). 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.