What’s beneath the surface? NASA/JHUAPL/SwRI

We think that Pluto is hiding a liquid ocean, but why it hasn’t frozen is a big mystery. Now it seems that gas trapped inside the bottom layer of its icy outer shell may be keeping it warm.

A number of observations point towards an underground ocean on Pluto, including deep cracks on its surface that seem to come from subsurface water freezing and expanding. But unlike other subsurface ocean worlds in our solar system, such as the icy moons Europa and Enceladus, Pluto is not stretched and warmed by the gravitational pull of a larger nearby object, meaning its ocean should be frozen.

To solve this puzzle, we need to figure out how Pluto is trapping the small amount of heat from the decay of radioactive elements in its rocky core. Francis Nimmo at the University of California, Santa Cruz and his colleagues have proposed that an extra layer between the ocean and the shell would do the trick.


The layer would be made out of a material called a gas hydrate, which occurs when gas molecules get trapped between frozen water molecules. “It’s not bubbles, it’s a little microscopic cage for keeping gas atoms in,” says Nimmo. “It doesn’t look very different from regular ice, but it’s got all that gas in there.”

Gas hydrates are much better insulators than water ice, so the researchers calculated that this extra layer could keep the ocean around and maintain the ice shell as we see it now. This may help explain why Pluto’s tenuous atmosphere has lots of nitrogen but almost no methane – it’s much easier for methane to get caught in a gas hydrate and kept underground.

Maintaining a liquid ocean would be good for any potential life under the shell, but the layer of gas hydrates might not be, because it keeps the ice above it extremely cold, Nimmo says. “If the ice above is really cold and stiff, that’s going to make it much harder to get stuff from the surface down to the ocean, and people often think that’s an important part to keeping life going,” he says.

Journal reference: Nature Geoscience, DOI: 10.1038/s41561-019-0369-8