Tholins (after the ancient Greek word θολός (tholós) meaning "sepia ink") are a class of heteropolymer molecules formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. 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. 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 he obtained in his Miller-Urey-type experiments on the gas mixtures that are found in Titan's atmosphere. 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.
As illustrated to the right, tholins are believed to form through a chain of chemical reactions. This begins with the dissociation and ionization of molecular nitrogen and methane 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 coagulate and deposit 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.
The surfaces of comets, centaurs, and many icy moons in the outer Solar System are rich in deposits of tholins. "Triton tholin" and "Titan tholin" are nitrogen-rich organic substances produced by the irradiation of the gaseous mixtures of nitrogen and methane found in the atmospheres of these moons; Triton's atmosphere is 99.9% nitrogen and 0.1% methane, while Titan's atmosphere is 98.4% nitrogen and the remaining 1.6% composed of methane and trace amounts of other gases. In the case of Titan, the haze and orange-red color of its atmosphere is thought to be caused by the presence of tholins. It is believed that tholins also occur on the dwarf planet Pluto and its moon Charon and are responsible for their red colors as well as the blue tint of Pluto's atmosphere. The plutino Ixion is also high in this substance.
Tholins may have also been detected in the stellar system of an eight-million-year-old star known as 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.
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 issue). Tholins do not exist naturally on present-day Earth due to the oxidizing character of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago.
A wide variety of soil bacteria are able to use tholins as their sole source of carbon. It is thought tholins may have been the first microbial food for heterotrophic microorganisms before autotrophy evolved.
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Experts have long thought that reddish substances are generated as a particular color of ultraviolet light from the sun, called Lyman-alpha, strikes molecules of the gas methane (CH4) in Pluto’s atmosphere, powering chemical reactions that create complex compounds called tholins.
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