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Tholin
Tholins (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. 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.
==Formation==
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.〔(Abstract )〕
These atmospherically-derived substances are distinct from "ice tholin", which is formed instead by irradiation of clathrates of water and organic compounds such as methane or ethane.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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