The 2015 flyby of the New Horizons spacecraft revealed Pluto to be a far more complex and dynamic dwarf planet than previously imagined. Its surface presented a stunning variation of terrain, including vast plains of nitrogen ice and towering water-ice mountains. Among the most striking discoveries were the prominent reddish-brown patches observed across the surface, giving Pluto a warm, ruddy hue in certain regions. Scientific analysis points to a class of complex organic molecules, known as tholins, that settle across the landscape. This material gives the dwarf planet its distinctive look and provides the answer to what makes up the red stuff on Pluto.
What Are Tholins
The reddish substance coating parts of Pluto is generically known as tholins, a term first coined by astronomer Carl Sagan and his colleague Bishun Khare. Tholins are not a single chemical compound but a broad category of complex, high-molecular-weight organic solids. They are formed when simple carbon-containing compounds are subjected to intense radiation.
These refractory organic solids represent a kind of cosmic tar, often appearing yellow, orange, or dark red, and are abundant on icy bodies across the outer solar system. On Pluto, tholins are formed from the simple volatile ices and gases in the atmosphere, primarily nitrogen and methane. While tholins are organic (containing carbon), they are abiotic, created purely through chemical reactions driven by energy, not by living processes. Their presence contrasts sharply with the vast, bright regions of the dwarf planet covered in simple, colorless ices like frozen nitrogen and methane.
The Atmospheric Creation Process
The process that creates tholins begins high in Pluto’s thin atmosphere, where simple gases are exposed to powerful energy sources. The primary reactants are molecular nitrogen (\(\text{N}_{2}\)) and methane (\(\text{CH}_{4}\)). Solar ultraviolet (UV) radiation, particularly the Lyman-alpha wavelength, acts as the main catalyst, striking these gas molecules and breaking their chemical bonds.
This energy input, along with charged particles from the solar wind and cosmic rays, fragments the stable molecules into highly reactive ions and free radicals. These fragments recombine in a chain reaction, leading to the formation of progressively more complex, heavier hydrocarbon molecules. Initial products include simple, colorless compounds like acetylene (\(\text{C}_{2}\text{H}_{2}\)) and ethylene (\(\text{C}_{2}\text{H}_{4}\)).
As the reactions continue, the molecules undergo polymerization, linking together to form large, solid particles that accumulate and make up the atmospheric haze layers. This haze is composed of “soot-like” particles visible as a distinct blue layer when backlit by the Sun. The continuous production of these complex, reddish, organic macromolecules causes them to slowly precipitate, or rain down, onto the surface, forming a reddish deposit. This process can occur even on the night side of Pluto, driven by a diffuse glow of Lyman-alpha radiation present throughout interplanetary space.
Mapping the Red Hues on Pluto
The tholin particles that settle onto the surface create the dark, reddish regions prominent in New Horizons imagery. The largest deposit is the vast, elongated equatorial region known informally as Cthulhu Macula, officially named Belton Regio. This dark expanse stretches for nearly 3,000 kilometers and is one of the darkest surface features observed.
The concentration of tholins suggests these are either geologically old regions where the material has accumulated over eons or areas where the deposition rate is particularly high. Unlike the bright, volatile ices of Sputnik Planitia, the tholin-rich areas are non-volatile and stable.
Why Tholins Appear Red
This material appears red because of its specific light-scattering properties. The complex organic structure absorbs shorter, higher-energy wavelengths of light (blue and green) while reflecting the longer, lower-energy red wavelengths.
The color of the atmospheric haze presents an apparent paradox, as it appears distinctly blue when viewed against the blackness of space. This blue color is also caused by the tholin particles, which are small enough in the upper atmosphere to scatter blue light much more effectively than red light, similar to how Earth’s sky works. As these particles clump together and grow larger while descending, they eventually form the reddish material that settles on the surface, creating the sharp color contrast between Pluto’s blue atmosphere and its ruddy plains.