Tholins are a class of heteropolymer molecules formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane, often in combination with nitrogen. They usually have a reddish-brown appearance, however, we have recently succeeded, through sequential and non-sequential pyrolysis followed by gas chromatography-mass spectrometry in determining something of the composition of this material. It is clearly not a repetition of the same monomeric unit—and some other term is needed. We propose, as a descriptive term, ‘tholins’, although we were tempted by the phrase ‘star-tar’. Tholins are not one specific compound but rather are descriptive of a spectrum of molecules that give a reddish, 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 this is followed by the formation of ethylene, ethane, acetylene, hydrogen cyanide, and other small simple molecules and small positive ions. These atmospherically-derived substances are distinct from ice tholin, which is formed instead by irradiation of clathrates of water, the surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer solar system are rich in deposits of tholins. Titan tholins are nitrogen-rich organic substances produced by the irradiation of the mixtures of nitrogen and methane found in the atmosphere. Titans atmosphere is 98. 4% nitrogen and the remaining 1. 6% composed of methane, in the case of Titan, the haze and orange-red color of its atmosphere is thought to be caused by the presence of tholins. Neptunes moon Triton is observed to have the color characteristic of tholins. Tritons atmosphere is 99. 9% nitrogen and 0. 1% methane, 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 Plutos atmosphere. Tholins have also detected on the plutino Ixion. The HR4796 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. Tholins do not exist naturally on present-day Earth due to the character of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago. Tholins can act as a screen for protecting planetary surfaces from ultraviolet radiation. A wide variety of bacteria are able to use tholins as their sole source of carbon. Tholins could have been the first microbial food for heterotrophic microorganisms before autotrophy evolved
The trailing hemisphere of Rhea is covered with tholins
Intricate pattern of linear fractures on Europa's surface, likely colored by tholins
Surface of Titan as viewed from the Huygens probe. Tholins are suspected to be the source of the reddish color of both the surface and the atmospheric haze.