If there's one place in our solar system you would expect to be as frozen solid as a box of forgotten ice lollies, it would be Pluto. However, PhD student Noah Hammond at Brown University says that data returned in 2015 by NASA's unmanned New Horizons deep-space probe indicates that the dwarf planet may have a subsurface ocean similar to those suspected to exist on some moons of Jupiter and may be responsible for the unusual surface features.

When New Horizons flew by Pluto on July 14 of last year, it may have been a brief encounter, but it has supplied plenty of surprises. Instead of a bland world of ice punctuated by the odd drift of frozen oxygen snow, the unmanned probe returned images of a complex landscape that was recently very active in geological terms.

The images returned showed mountains capped with methane snow, plains, rifts hundreds of kilometers long, and strange pillowy mounds, but the idea that hundreds of kilometers beneath all this might be a slowly freezing ocean down was not the obvious conclusion to draw.

But according to Hammond, the data returned from New Horizons allowed him to update models of Pluto's thermal evolution. If oceans are to exist anywhere, there has to be enough heat present to keep water in a liquid state. On Earth and ancient Mars, most of that heat came from the Sun. On Jupiter's moons, the suspected oceans may be warmed by the tidal forces that deform the moons as they revolve about the giant planet and are believed to power the volcanoes of Io.

Pluto is another matter entirely. It's far too distant from the Sun to receive any warmth and there's no nearby giant planet to drive tides. Even Pluto's large companion moon Charon can't generate any heat because the two bodies long ago tidally locked with each other, which is essentially like jamming a stick in the spokes of the heat engine.

But Hammond contends heat is coming from somewhere because otherwise Pluto should have shrunk millions of year ago. Instead, the odd surface formations indicate that the planet has been expanding like a huge cake that billows and cracks in the oven. In this case, the expansion is consistent with the presence of an underground ocean trapped beneath the icy crust.

According to some scientists, the most likely cause of the suspected heat is the decay of radioactive elements that keep the interior of the planet warm, which is one of the contributors to the heat of the Earth's core. This warming effect, along with the insulation of the exotic snows and ices on the surface, has slowed down the eventual cooling of Pluto and has allowed the expansion features to occur, since as water cools and freezes, it expands.

Hammond says that by plugging in updated data from the flyby of Pluto's diameter and density, he and other researchers were able to show that if a subsurface ocean existed on Pluto it would be at least 260 km (162 mi) beneath the surface of the dwarf planet. This means that if the ocean was now frozen, the pressures at that depth could convert the ice into an exotic form called ice II. This is a compact, crystalline form of ice that contracts rather than expands, which would have resulted in Pluto shrivelling like a freeze-dried apricot. Since the opposite has happened, Hammond believes there is a case for a Plutonian sea.

"That's amazing to me," says 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."

The research as published in Geophysical Research Letters.

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