What are tholins on Pluto?

Tholins are complex organic compounds found on the surface of Pluto. These substances are responsible for the dark coloration observed in certain regions of the dwarf planet, most notably in the area known as Cthulhu Regio, which lies to the west of the heart-shaped Tombaugh Regio.1

While much of Pluto's surface is covered in highly reflective ices—such as nitrogen, methane, and carbon monoxide—the presence of tholins creates a stark contrast in reflectivity. Whereas bright regions like Tombaugh Regio have a high albedo, the tholin-rich Cthulhu Regio is significantly darker, with a reflectivity of only 0.1 to 0.2.1