Back in BAN Issue #704 I talked about consulting a bit on a new limited-run comic book series about sending vampires to Mars. It looks really fun and funny, and I had a great time talking with the writer and co-creator George O'Connor about it. The artist, Fernando Pinto, even drew up a panel with me getting turned. It’s fantastic.

The comic is funded through Kickstarter, and they’re more than 80% to their goal with only a few days left. If you have a few bucks and want to see how vampires fare on a spaceship bound for the (blood) red planet, then chip some in! Thanks!

How did Pluto form its heart?

Tombaugh Regio is the proper name for the huge valentine-shaped region dominating the views we got when the New Horizons space probe shot past Pluto in 2015. The two lobes of the heart are different; the eastern one (on the right in the images) is rough and consists of highlands (hillier higher-elevation territory), while the western one, called Sputnik Planitia, is brighter, white, lower in elevation, and smooth.

It’s clearly some sort of impact feature, but the elongated shape and composition have been difficult to model using the usual cosmic impact software simulators planetary scientists employ.

But new research seems to have cracked this nut (broken the heart?): Scientists simulated the impact at a variety of angles and speeds, and an impactor of various sizes and compositions, but made a critical assumption: The material impacting was so cold that it didn’t vaporize or even really melt after the impact. This is very different than what you expect for impacts on, say, Earth, where the energies are so huge that the rock and even metal in an impacting body can melt or vaporize [link to journal paper].

Why make this assumption? For one thing, it’s colder in the outer solar system, so far from the Sun. But critically, impact speeds are lower. For one thing, Pluto is smaller, so it has less gravity, and doesn’t pull as hard on an incoming impactor, accelerating it. Also, orbital speeds out there are far slower than in the inner solar system, so the impacts tend to be much lower energy. 

The best model fit they got to the shape of Sputnik Planitia yielded some surprises. The impact was at a low angle, just about 30° from horizontal. That’s not a surprise; the elongated shape of the cardio-lobe-shaped region implies a low angle impact. But what they also found is that the icy mantle underneath Pluto’s surface is mostly material from the impactor that plopped down and stayed in place. It may be slightly higher density than the native Pluto mantle material around it. Also, the rocky core of the incoming body sank down to Pluto’s core and just kinda sits there. The inner volume of Pluto is so cold that the lump didn’t get distributed, but is relatively intact, a mass on the side. 

This explains a long-standing mystery. Sputnik Planitia is low elevation, a depression several kilometers lower than the surrounding terrain. This impact happened when Pluto was relatively young, and over time the spin of Pluto should’ve been affected by this redistribution of mass. Less mass in Sputnik Planitia means that Pluto should’ve tipped over and put this region near the pole, but instead it sits at the equator even after several eons. Why? 

The new impact idea explains that; the impactor was denser than Pluto, so all that material in the mantle and the lump on the core are heavier than surrounding material. Over time Pluto’s spin would naturally tip to put those near the equator.

An interesting outcome of this is that their models imply there doesn’t have to be a lot of liquid water under the surface of Pluto. There’s been some speculation of a liquid (well, slushy) ocean under Sputnik Planitia, mostly due to the smooth surface. The lack of craters implies something is repaving the surface there, and one thought is that a thick liquid is convecting (warm material rising and cooler sinking) under it. I wonder about this, though: The surface of the region is segmented into polygons, which is expected for a frozen surface over a liquid (I’ve seen this myself in Colorado on frozen lakes), and there are little black pits in the ice that are small in the center of the plates and get bigger closer to the edges. That implies a plume of material rising underneath at a given plate’s center, which pushes material toward the edges. The pits grow due to sublimation (ice going directly from solid to gas), so they form in the center and move out to the edges, getting bigger as they do (I talk about this a lot more in the Pluto chapter of my book, Under Alien Skies, which is coming out in paperback soon!). If there’s no liquid under Sputnik Planitia this gets a lot harder to explain. They don’t mention the pits in the paper, so I’m curious how this new model might explain them; I suspect it’ll wind up being a hybrid of the two ideas, with some liquid under the surface but not necessarily an ocean.

In general a lot of evolution of planets is due to impacts, but the icy worlds that inhabit the outer solar system are smaller and colder, and impacts there will have much larger and much longer lasting effects. Changing up our assumptions of how they form could explain more of what we see out there, things that were not expected and defy our presumptions.

Pluto itself was just one shocking reveal after another as images came in from New Horizons. It’s been almost a decade now since that incredible encounter, and new science is still coming in. I expect that will be true for many decades to come.

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