Back in 1979, before the Voyager 1 probe flew by Jupiter and its moon Io, everyone expected to see a dull, dead moon with lots and lots of impact craters and no real personality. And then the images came in, showing the most volcanically active body in the Solar System; heck, Voyager even got to see a volcano erupting! Thanks to that new information, everything that we thought we knew about Io went out the window and the planetologists spent the next few years happily deciphering the why of Io’s volcanism.
Back in 2014, before New Horizons visited Pluto, we expected to see a cold, dead world much like Mercury: covered with impact craters and with nothing younger than a billion years or so on its surface. Instead, the New Horizons images have shown us a world almost as young as Earth, with resurfaced plains and mountains of ice. (Of course, not everything that we saw was unexpected; for example, the smaller moons are irregular chunks of rock, tumbling around each other in space.) And it isn’t only Pluto that is suprising us. Charon, too, is younger than expeected and has a wider variety of features than we thought to see.
So what is driving this? Why do Pluto and Charon look so different? One hint comes from the mountains of Norgay Montes (Norgay‘s Mountains); another comes from Sputnik Planum (the Plains of Sputnik). Those mountains tower 1,000 ft or more above the surrounding plains. They are made up of ice (which, to a planetologist means water ice, methane ice, CO2 ice, nitrogen ice, and just about any other ice) and strongly resemble ice bergs caught in a frozen sea. And then there is Sputnik Planum, which looks like a kettle of boiling soup that has been flash-frozen. In both cases, the geology argues that heat was applied to the bottom of Pluto’s crust. For Norgay Montes, enough heat was applied to make the mountains break off and slump into the underlying ices. And for Sputnik Planum, even more heat was applied, melting everything into a uniform swamp.
But wwhere would that heat come from? We’re not sure yet. Suggestions range from fossil heat left over from when Pluto and Charon formed to a recent massive impact to (my personal favorite) tidal friction just like on Io. Pluto and Charon are more like co-orbiting planets than like a planet and a moon. Charon is nearly 1/8th as massive as Pluto (for comparison, our Moon is just 1.2% of Earth’s mass) and orbits so closely that the two planets are tidally locked; they always show the same face to each other. That situation creates enormous tidal stresses in the crust, which cna create large amounts of heat. Of course, that should mean that the two planets are slowly moving toward each other and should have been destroyed aeons ago. So what keeps them apart? Neptune. Just as Io is kept in place by the gravitational influence of Jupiter’s other moons, Pluto and Chron might get a “spin up” every time that the pair passes by Neptune.
Of course, this is all speculation at this point. We won’t know for sure until we’ve sspent more time studying the data. And we won’t know for really sure until we send an orbiter to Pluto (so expect some final results in three or four decades). What we do know for sure right now is that we more than got our money’s worth. The New Horizons probe has gathered enough data to help us redefine how planets form and what a planet is. Now we just have to figure out what it means!