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.
"Triton tholin" and "Titan tholin" are nitrogen-rich organic substances produced by the irradiation of gaseous mixtures of N2 and CH4 such as that found in those moons' atmospheres; Triton's atmosphere is 99.9% N2 and 0.1% CH4 and Titan's atmosphere is 94% N2 and 6% CH4. These substances are distinct from "ice tholin", which is formed by irradiation of clathrates of water and organic compounds such as methane or ethane.
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.