Last year’s close-up photos of Pluto from the New Horizons probe were a revelation, but don’t forget the dwarf planet’s proportionally sizable moon Charon. The surface of that world presented its own puzzles of geology and history.

For starters, Charon sported a dark and dusty red cap at its illuminated northern pole. A later image taken looking back at the moon's dark southern pole, dimly lit by “Pluto-shine,” showed that the pole was also darker—perhaps due to a similar reddish cap. The early hypothesis was that, similar to dark regions of Pluto’s surface, this cap was a thin residue that solid organic compounds formed from reactions of gases catalyzed by incoming solar radiation and charged particles.

There's just one problem with this idea: Pluto is the one with the gases, not Charon…

A new study from a large team led by Lowell Observatory researcher Will Grundy did the work of testing the feasibility of this hypothesis. Could Charon be getting methane deliveries from Pluto and turning them into stylish red pole-wear?

The heart of the test is a computer model of Charon’s surface temperatures, since the cap-coloring process requires freezing methane gas onto the surface. Charon is tilted (like Pluto’s orbit), and its 248-year trip around the Sun means that the winter pole stays in frigid darkness for long periods. While the northern pole is currently a summer-y -220 degrees Celsius, for example, it would have stayed below -250 degrees Celsius for the entirety of the (Earth) years 1860 to 1990. Methane gas could freeze out below about -248 degrees Celsius, so the opportunity is certainly there.

Pluto’s atmosphere is constantly leaking methane and a little nitrogen gas, and the researchers estimate that Charon catches about 2.5 percent of the methane that New Horizons measured escaping Pluto. (If you’re keeping track, that’s about 270 billion molecules landing on each square meter of Charon’s surface each second.) That gas should spend enough time floating around Charon that it will visit the dark winter pole where it can freeze out, leaving the lightest dusting of solid methane on the surface.

If that’s where the story ended, you wouldn’t be reading about it right now—without UV light to catalyze reactions, the methane would simply sublimate back to gas when sunshine eventually returned. But even in the darkness of the pole, some amount of ultraviolet radiation strikes the surface after being scattered off course by the odd particle in space. That’s enough to drive the reactions that convert methane into heavier organic molecules that will stay put come spring.

The researchers estimate that a little less than a quarter of the frozen methane undergoes this second transformation. If this were to occur evenly from the pole down to about 45 degrees latitude, you’d build up a layer of dark, reddish crud all of 0.16 millimeters thick, provided you waited a million years.

That might not sound like much (because it isn’t), but it’s probably enough to lead to Charon’s present appearance—partly because this has been going on for far more than a million years, and partly because the dark polar mantling actually looks quite thin. The wavelengths of light present in the images show that the polar surface is mostly water ice despite the color. The dark dust has been mixed in with ice dust kicked up by sporadic impacts.

The mixing with water ice may even be important to the reddish color. When researchers react methane ice with UV radiation in the lab, it eventually just turns black. The water ice might help keep the reddish material “fresh.”

Examining all the physics involved, this hypothesis seems to work out. The cause of Charon’s red caps could very well be the freezing and transformation of methane that escaped from Pluto. A reddish spot was also spied on Nix, another moon orbiting Pluto, so it’s possible that the same process plays out there. But the researchers note that much less of Pluto’s fleeing methane molecules should get delivered to Nix. If you thought the accumulation of dark dust on Charon was weak, they estimate it would probably build up about 20,000 times more slowly on Nix. So perhaps that puzzle demands a separate explanation.

Nature, 2016. DOI: 10.1038/nature19340 (About DOIs).