Tholins are the organic aerosols thought to cause Titan’s orange haze.
Titan’s atmosphere is dominated by nitrogen, with methane providing the carbon-rich ingredient for complex photochemistry. Ultraviolet sunlight and energetic particles break apart simple molecules high in the atmosphere, triggering reactions that build larger nitrogen-bearing organic compounds. These particles form a dense, orange-brown haze around the moon.
“Tholin” does not name one single chemical. It is a broad term for complex, laboratory-produced organic material resembling the reddish solids and aerosols expected on several cold worlds. Astronomer Carl Sagan and chemist Bishun Khare coined the term after experiments using methane-containing gas mixtures.
The haze is not made primarily of carbonates, sulfates, or silicates. Those minerals may be relevant to planetary surfaces elsewhere, but Titan’s distinctive atmospheric color comes from carbon-and-nitrogen-rich photochemical particles. Some tholin-like material eventually settles onto Titan’s surface, adding to its reddish organic deposits.