New Horizons never got as close to Charon as it did to Pluto, so we didn't learn as much about it. One feature stood out from other ice worlds we have seen orbiting actual planets, however: a red "cap" near the north pole nicknamed Mordor Macula. Tempting as it is to imagine "Make Pluto A Planet Again" scrawled by aliens against the ruddy background, astronomers have looked for more likely explanations, and now think they have one.
Almost immediately, astronomers suspected the cap was made up of tholins, sticky hydrocarbon residues formed by the breakdown of methane on exposure to ultraviolet light. The methane is thought to have escaped from Pluto, with any that encounters Charon being cooled to a point where it does not immediately escape even this low gravity.
In Science Advances, a team from the SouthWest Research Institute attribute the color to the seasonal changes in Charon's thin atmosphere.
Catchily titled “Charon's Refractory Factory”, the paper proposes a multi-stage process beginning in Charon's winter, when methane freezes in the presence of Lyman-alpha radiation – a form of UV light scattered by interplanetary hydrogen. Although it will be a long time before we can go to check directly, the authors created a replica of Charonian conditions in the new Center for Laboratory Astrophysics and Space Science Experiments.
“Our experiment condensed methane in an ultra-high vacuum chamber under exposure to Lyman-alpha photons to replicate with high fidelity the conditions at Charon's poles," said Dr Ujjwal Raut in a statement. The product is primarily colorless ethane, which stays frozen in the spring when Charon has become warm enough for the methane to become gaseous again and can experience further transformations.
"We think ionizing radiation from the solar wind decomposes the Lyman-alpha-cooked polar frost to synthesize increasingly complex, redder materials responsible for the unique albedo on this enigmatic moon," Raut said.
The team have yet to confirm this part of the process in the lab, something they plan to try next, but Raut described the work as “one of the most illustrative and stark examples of surface-atmospheric interactions so far observed at a planetary body.”
A second paper by the same team, published in Geophysical Research Letters, backed up the conclusion using a computer model. "The model points to 'explosive' seasonal pulsations in Charon's atmosphere due to extreme shifts in conditions over Pluto's long journey around the Sun," said lead author Dr Ben Teolis.
These pulses are thought to last one to four Earth years, which sounds like a long time, until you realize Charon's year is 248 of ours.
Charon doesn't get much sunlight, but it is so tilted that almost all of what does reach it falls on the summer hemisphere, creating a 50° C (90° F) difference between the winter and summer poles.
Around the equinox, frozen methane turns to gas, increasing atmospheric pressure almost a thousand-fold, before refreezing in the other hemisphere. The authors think around 10 percent of the methane Charon captures is eventually converted to reddish hydrocarbons, while the other 90 percent eventually escapes the light gravity, only to be replaced by more material from Pluto.
Our view of Charon's south pole was hidden by night, but there were hints the moon has red shoes to match its cap.
It just goes to show you don't need to be a planet to have an exceptionally cool moon.