Time for what seems to be the obligatory Monday NASA post, but I’m sure you won’t mind as these seem to be quite popular. The latest pictures back from New Horizons are of Charon, the largest of Pluto’s five moons, the others being Styx, Nix, Hydra and Kerberos (possibly the best set of names out there in my opinion). Before we begin, though, a pedantic point of order. It seems it’s being frequently reported this week that Charon is the largest satellite relative to its planet in the solar system. On the face of it this is fine but Charon is so big compared to Pluto that the centre of mass of the pair of them is not inside the body of Pluto. Instead of Charon orbiting a point within Pluto like every other moon does with its planet, in this case both Pluto and Charon orbit a point in empty space between the two. This means that Charon isn’t a moon of Pluto at all, they’re actually a binary dwarf planet system, which I think is actually a lot more cool. Oh, and whilst we’re at it, I have no idea how to pronounce Charon properly. I’ve heard it KA-ron and SHA-ron (not CHA-ron) both from proper astronomers so, take your pick, I guess.
Anyway, pedantry aside we have a wonderful picture to scour; let’s take a look, it’s a beauty.
Look. At. The. Detail. Really, stop reading this and go back and have another look. Even better, click through to this original image from NASA, download it, set it as your background and look at it every day forever. I’ve got two favourite bits. The reddish part to the north partly because I love that hue but mostly because it has been named Mordor Macula and the Lord of the Rings geek in me just can’t handle it. My other fave is the lower right portion next to the terminator; the terminator being the boundary between day and nighttime as opposed to a homicidal cyborg. This area is especially interesting because it is where the sunlight hits the planet obliquely and therefore gives the most detail allowing us to get a real feel for the mountains and craters.
The part that has got the scientists all fired up, however, is the canyon stretching all the way across the body, it’s as if Charon was almost split in two some time in the past. Currently, the best theory we have as to how this happened on such a small, cold world is cryovolcanism, which must be one of the coolest words ever. It’s theorised that there was a subsurface ocean of water that eventually froze, as ice crystals form the crystal structure forces the water molecules to take up more space than they had when they were liquid. This means that a litre of water takes up a smaller volume than a kilogram of ice. This makes ice less dense than water and is why your ice cubes float in your cuba libre. It’s also why Charon looks like it has been used as a piñada one too many times. As the ocean froze it expanded and cracked the surface allowing a lava-esque flow of water to smooth off some of the surface.
A decade ago even some of the scientists involved in the New Horizons mission were a little worried that the Pluto system wouldn’t reveal too much of interest. Those fears have been well and truly laid to rest.