Pluto’s moon Charon (enhanced colors).
Last year’s close-up photos of Pluto from the New Horizons probe were a revelation, but don’t forget the dwarf planet’s comparatively large moon Charon. The surface of that world presented its own puzzles of geology and history.
For starters, Charon wore a dark and dusty red cap at his illuminated north pole. A later image, looking back at the moon’s dark south pole dimly lit by “Pluto gleam,” showed that the pole was also darker — perhaps because of a similar reddish cap. The early hypothesis was that, similar to dark regions of Pluto’s surface, this hood was a thin residue that formed solid organic compounds 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 job of testing the feasibility of this hypothesis. Could Charon get methane deliveries from Pluto and turn them into stylish red pole-wear?
At the heart of the test is a computer model of Charon’s surface temperatures, as the cap coloring process requires methane gas to be frozen on the surface. Charon is tilted (like Pluto’s orbit), and its 248-year journey around the sun means that the winter pole remains in icy darkness for long periods. While the North Pole is currently a summer temperature of -220 degrees Celsius, it would have remained below -250 degrees Celsius for the whole from the (Earth) 1860s to 1990s. Methane gas could freeze below about -248 degrees Celsius, so the chance is definitely there.
Pluto’s atmosphere is constantly leaking methane and some nitrogen gas, and the researchers estimate that Charon captures about 2.5 percent of the methane New Horizons measured escaping Pluto. (If you keep track, that’s about 270 billion molecules landing on every square meter of Charon’s surface every second.) That gas should float around Charon enough time to visit the dark winter pole where it can freeze, causing the lightest dusting of solid methane on the surface.
If the story ended there, you wouldn’t be reading about it now – without UV light to catalyze reactions, the methane would simply sublimate back into gas when the sunshine finally returned. But even in the darkness of the pole, some amount of ultraviolet radiation hits the surface after being scattered off course by a strange particle in space. That’s enough to drive the reactions that convert methane into heavier organic molecules that stay put in the spring.
The researchers estimate that just under a quarter of frozen methane undergoes this second transformation. If this happened evenly from the pole to about 45 degrees latitude, you’d build up a layer of dark, reddish crude oil 0.16 millimeters thick if you just waited a million years.
That may not sound like much (because it isn’t), but it’s probably enough to lead to Charon’s current appearance – partly because this has been going on for over a million years, and partly because the dark polar mantle is there actually looks pretty good. thin. The wavelengths of light in the images show that the polar surface is mostly water ice despite its color. The dark dust is mixed with ice dust blown up by sporadic impacts.
Mixing it with water ice may even be important for the reddish color. When researchers in the lab let methane ice react with UV radiation, it eventually just turns black. The water ice can help keep the reddish material “fresh.”
If we examine all the physics involved, this hypothesis seems to work. 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, that’s how it is possible that the same process takes place there. But the researchers note that far fewer of Pluto’s fleeing methane molecules should be delivered to Nix. If you thought the accumulation of dark dust on Charon was weak, they estimated it would probably accumulate about 20,000 times slower on Nix. So maybe that puzzle calls for a separate explanation.
Nature2016. DOI: 10.1038/nature19340 (About DOIs).