Water may be the liquid we are most familiar with, but it behaves unlike another other liquid we might name. Water expands as it freezes, making it less dense than the liquid form. It’s why lakes on Earth can freeze over in the winter, encapsulating the surface and the organisms below. The curious way water freezes is why we think Pluto—the dwarf planet quickly becoming more interesting than other, actual planets—may currently be hiding a water ocean underneath kilometers of ice.

According to a study published by Noah Hammond and colleagues in Geophysical Research Letters, the best evidence for a water ocean sloshing beneath the frozen heart of Pluto is the geology of its surface.

“What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion,” Hammond said in a press release. “A subsurface ocean that was slowly freezing over would cause this kind of expansion.”

But not just any ice could cause the cracks and crevices indicating expansion. Under enough pressure, the ice that you’d put into a drink on a hot day turns into the very creatively named “Ice II.” Ice II has more closely packed atoms than typical ice, so it contracts like other liquids when frozen. If Pluto had frozen from its surface to its core, there would be sufficient pressure to create ice II, causing global contraction. New Horizons saw the opposite happening on the surface, implying there’s still some water down there.

Hammond estimates that Pluto’s icy shell is thick enough — over 300 kilometers — to form ice II all the way through. The fact that the “extensional tectonic features” appear despite this makes it more likely that Pluto is not ice all the way down. The dwarf planet simply refuses to stop interesting us, to stop amazing Hammond.

“The possibility that you could have vast liquid water ocean habitats so far from the sun on Pluto — and that the same could also be possible on other Kuiper belt objects as well — is absolutely incredible.”

What do you think about this possible development? Let us know in the comments below!

Images: NASA