Tholins are a class of molecules formed by from ultraviolet irradiation of simple carbon-containing compounds to create organic compounds.
Molecular Nitrogen (N2) and Methane (CH4)
⬇️
☀️ Solar Radiation and Cosmic Rays ☀️
⬇️
Ethylene (C4H4) and other simple molecules
⬇️
Benzene (C6H6) and other complex molecules
⬇️
Nitriles, hydrocarbons, and negatively charged organic ions such as amines and phenyls
⬇️
Tholins (as a complex molecular slush)
Tholins can be found throughout the solar system and are believed to be a major constituent of the interstellar medium. They give a reddish, organic surface covering on certain planetary surfaces. While astronomers have yet to get their paws on a naturally deposited tholins, they have been able to recreate them in the lab using a combination of carbon, nitrogen, and hydrogen.
On Saturn’s moon Rhea, the outer crust is composed of methane ice. Over millions of years of ultraviolet irradiation from the Sun and cosmic rays, the methane ice has broken down some into a complex slush of organic molecules that turns the surface of the moon’s trailing hemisphere a reddish-brown hue. While the entire moon is affected, the trailing hemisphere shows it most prominently as it faces away from the direction of travel, with the leading hemisphere shielding it from dust and ice that Rhea’s gravitational pull vacuums up from around Saturn.
A similar process occurs on other celestial bodies such as Pluto, Charon, Arrokoth, and Neptune’s icy moon Triton. Some of the closest tholin deposits to us are found on Jupiter’s icy moon Europa, where tholins are believed to have stained much of its heavily cracked surface a deep reddish-brown hue.
However, on Saturn’s moon Titan, tholin formation occurs at high altitudes and precipitates down in the formation of organic compounds to tint the surface of the moon a reddish brown beneath its predominantly methane atmosphere. These tholins scatter light, deepening the amber hue of the atmosphere and acting as a screen helping to protect the surface from ultraviolet radiation. These particulates then accumulate on the surface of the moon and become a part of Titan’s “soil”.
Tholin snow, eh? What a cursed snowcone that must make.
Could the formation of tholins in the outer solar system help shed some light on the origins of life on Earth? Does the presence of tholins increase the habitability of certain celestial bodies? Perhaps, and that’s why they’re a subject of great interest to astrobiologists as well.
Recommended reading
- Tholin on Wikipedia
- Dissecting the Dirt on Titan, NASA Science Editorial Team, May 2007
- Cassini Spacecraft Reveals Evidence of Tholin Formation at High Altitudes in Titan’s Atmosphere, NASA Science Editorial Team, May 2007
- Tholins and their relevance for astrophysical issues, Cambridge University Press, Feb 2008