Why are Pluto and Charon so different?
“So what happened to all the volatiles that Charon surely had at one point? You see, if Pluto is 30% ice/70% rock, but Charon is almost 100% ice, that means they couldn’t have formed together! They must have different origins, and therefore must have coalesced to form a binary pair later on. But when you get two worlds that are this close together, the one that’s denser, with more concentrated mass, can potentially strip the most loosely-held material off of the other one. In the case of the less dense, lower mass Charon, that likely means it once had these same volatile materials — like nitrogen and methane — and that Pluto stole them!”
When New Horizons approached the Pluto system last year, it discovered two vastly different worlds in Pluto and Charon. While Pluto had mountains, plains, ridges, and surface regions with vastly different properties, Charon looked more like our Moon: cold, airless, mostly uniform and full of craters. As soon as we learned these two world were so different, the question of “why” was brought to the forefront. Yet the very same images hold clues: beneath Pluto’s volatile, ever-changing surface may lie a world that’s not so different from Charon. It’s only the easily sublimated molecules like nitrogen and methane that make the difference, and the reason Pluto has them exclusively is that it likely stole them from Charon a long time ago.