Formation of Pluto’s Sputnik Planitia probably pushed the dwarf planet onto its side

At some point in Pluto’s past, the planet tipped over. That’s a weird thing to say about a sphere with no real top or bottom to reference, much less a planet that doesn’t have a sense of up or down outside its own core, or maybe the Sun. However, geological scarring plus multiple simulations centered around the large, heart-shaped feature called Sputnik Planitia indicate that the dwarf planet has likely be doing a fair amount of wobbling on its axis ever since the Sputnik Planitia was filled up with ice and water.

Pluto becomes bottom-heavy

This story starts to make a bit more sense when you consider that Pluto’s moon, Charon, is nearly the same size as the dwarf planet itself. As such, it exerts a significant amount of gravitational influence on its partner, complete with a strong tidal axis— the area where Charon pulls most directly on Pluto. This gravitational relationship then became the sort of fulcrum point for Pluto to wobble around and “tip” towards after the creation of the Sputnik Planitia, an event that basically made the dwarf planet an bottom-heavy sphere.

We’re not sure how Sputnik Planitia was created, but odds are a large object struck Pluto long ago, creating a large depression. Assuming there is some kind of liquid water under the icy exterior of the planet, that water would have pushed more ice up and to the impact site, leaving some of the heavier liquid closer to the surface there than anywhere else. Combined with a build-up of frozen nitrogen pulled out of the atmosphere, this would have not only filled in the impact site, but left it with more mass on the surface than before, triggering the aforementioned tipping in relation to Charon.

What’s warming the water?

This hypothesis is dependent on icy Pluto somehow having liquid water under its surface, but that may not be so far-fetched. Other cold locales in the solar system, like the moons Europa and Enceladus, are likely home to liquid oceans under icy exteriors. In Pluto’s case, the water may be staying warm thanks to radioactive decay in the planet’s core. This internal heat source would then keep hidden water warm enough to be liquid, even if it’s covered by a layer of frozen nitrogen less than -380° Fahrenheit.

Source: Pluto’s ‘icy heart’ may have tilted the dwarf planet over by Loren Grush, The Verge