A class of molecules that form by the action of ultraviolet radiation or spark discharge on gases of simple compounds, including CH₄, C₂H₆, H₂O, HCHO, and H₂S. They are usually brown, sometimes sticky. Tholins (from the Greek tholos, meaning muddy) do not form on Earth but are found on the surface of Titan and other bodies in the outer solar system. The term was coined by the astronomer Carl Sagan and the chemist Bishun Khare.