<<Tholins (after the Greek θολός (tholós) "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") is an informal name given to a wide variety of organic compounds
formed by solar ultraviolet irradiation or cosmic rays from simple carbon-containing compounds such as carbon dioxide, 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.
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, CHO, 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 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. 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.
The reddish color typical of tholins is characteristic of many Trans-Neptunian Objects, including plutinos in the outer Solar System such as 28978 Ixion. Spectral reflectances of Centaurs also suggest the presence of tholins on their surfaces. New Horizons flyby of (486958) 2014 MU69 (Ultima Thule) revealed red color at the surface, suggestive of hydrocarbons and other organic compounds.
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." The surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer Solar System are rich in deposits of tholins.
Neptune's moon Triton is observed to have the reddish color characteristic of tholins. Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide.
Tholins occur on the dwarf planet Pluto and are responsible for red colors as well as the blue tint of the atmosphere of Pluto. The reddish-brown cap of the north pole of Charon, 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 19,000 km distance to the orbiting moon.
In February 2017, organic compounds were detected on the dwarf planet Ceres, later identified as tholins.
Makemake exhibits methane, large amounts of ethane and tholins, as well as smaller amounts of ethylene, acetylene and high-mass alkanes may be present, most likely created by photolysis of methane by solar radiation.
Tholins were also detected in situ by the Rosetta mission to comet 67P/Churyumov–Gerasimenko. Tholins are not typically characteristic of main-belt asteroids, but have been detected on the asteroid 24 Themis.
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. The HR 4796 system is approximately 220 light years from Earth.
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.>>