New Horizons has long left Pluto behind, but scientists are still unraveling the mystery of this dwarf planet and its satellites. The latest piece of research focuses on the curious red spot of Charon, the dwarf planet’s largest moon, which has been confirmed to be caused by methane stolen from Pluto itself.

Will Grundy from the Lowell Observatory and colleagues analyzed detailed images of the red spot and made some important discoveries. The north pole is covered in a chemical called tholin, which gives both the spot and Pluto its characteristic reddish color.

"This is the first example of a planet's escaping atmosphere affecting its moon's surface," Dr Grundy told IFLScience. "It's like Pluto is a graffiti artist, spray painting Charon's poles with its escaping atmosphere, leaving planet-scale colored spots. This sort of thing might be fairly ordinary among double planets, but we haven't had a close look at such a system before now."

In a paper, published in Nature, the team suggests that the core mechanism for this red spot is its ability to experience extremely cold temperatures. The inclination and orbit of the system with respect to the Sun allows for long, cold winters that last for more than 100 years. The methane is trapped during the cold winter and when spring comes, it is converted into the red-colored chemicals.

Tholins are formed by organic compounds like methane when they are exposed to ultraviolet radiation. The methane is too volatile to stick to the moon and be converted by itself, but the seasonal conditions at the poles are what makes the difference. The team had previously suggested that the methane from Pluto’s atmosphere is trapped in the moon’s north pole and is slowly converted into tholins. This study provides an accurate model for its conversion. 

This is why the presence of tholins is such an interesting revelation; it tells us a lot about the condition on Charon and in the Plutonian system. 

"Every time we look somewhere new, we find surprises," added Grundy. "Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes."

The research suggests that this particular spot is not unique, though. There are indications that Charon's south pole (which was not in view of New Horizons) has a similar feature, and the other smaller moons of Pluto, like Nix, seem to have red spots as well. Obviously the smaller satellites are less massive so, the researchers conclude, the process there is likely to be less efficient.