- For the Swedish politician, see Claes Tholin.
Tholin is a heteropolymer formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in the outer solar system. They usually have reddish-brown appearance.
"Triton tholin" and "Titan tholin" are nitrogen-rich organic substances produced by the irradiation of gaseous mixtures of nitrogen and methane such as that found in those moons' atmospheres; Triton's atmosphere is 99.9% nitrogen and 0.1% methane and Titan's atmosphere is 98.4% nitrogen and the remaining 1.6% composed of methane and trace amounts of other gases. These atmospherically derived substances are distinct from "ice tholin", which is formed by irradiation of clathrates of water and organic compounds such as methane or ethane. The plutino Ixion is also high in this compound.
The surfaces of comets, centaurs, and many icy moons in the outer solar system are rich in deposits of Triton, Titan and ice tholins. 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 Urey-Miller experiment for discussion related to this issue. Tholins do not exist naturally on Earth due to oxidizing character of its atmosphere. The haze and orange-red color of Titan's atmosphere and Centaur-class planetoids is thought to be caused by presence of tholins.
A theoretical model explains formation of tholins by the dissociation and ionization of molecular nitrogen and methane by energetic particles and solar radiation, formation of ethylene, ethane, acetylene, hydrogen cyanide, and other small simple molecules and small positive ions, further formation of benzene and other organic molecules, their polymerization and formation of aerosol of heavier molecules, which then coagulate and deposit on the planetary surface. 
The term "tholin" (from the Greek word meaning "muddy") was coined by famed astronomer Carl Sagan to describe the difficult-to-characterize substances he obtained in his Urey-Miller-type experiments on the gas mixtures that are found in Titan's atmosphere. It is not a specific compound but is a term generally used to describe the reddish, organic component of planetary surfaces.
Tholins formed at low pressure tend to contain nitrogen atoms inside of their molecules while tholins formed at high pressure display nitrogen atoms more likely located in terminal positions. 
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.