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The New Horizons spacecraft spied extensional faults on Pluto, a sign that the dwarf planet has undergone a global expansion possibly due to the slow freezing of a subsurface ocean. (Credit: NASA / JHUAPL / SwRI)

Pluto is one of the coldest places in the solar system, but scientists say geological activity could still keep an ocean’s worth of water liquid beneath the dwarf planet’s surface.

Brown University’s Noah Hammond and his colleagues lay out their argument in a paper published in Geophysical Research Letters.

Readings from NASA’s New Horizons probe have shown that Pluto possesses mountains of water ice that rise as high as 11,000 feet, and there’s also evidence of tectonic activity associated with that ice. The newly published paper looks into the implications of those geological forces.

“Our model shows that recent geological activity on Pluto can be driven just from phase changes in the ice – no tides or exotic materials or unusual processes are required,” Hammond said in a news release. Depending on the depth of Pluto’s ice, the pressures far beneath the surface could keep water in a liquid state.

Study co-author Amy Barr, a researcher at the Planetary Science Institute, said the analysis lends support to the idea that oceans may be common among large objects in the Kuiper Belt, the broad ring of icy material that lies beyond the orbit of Neptune.

The key to the case for Pluto’s subsurface ocean is a form of frozen water known as ice II, which is 25 percent denser than the kind of ice we all know and love. Ice II forms only under high pressure and low temperatures, like the conditions that are thought to exist beneath Pluto’s surface.

The research team’s modeling shows that if Pluto’s water completely froze, its icy shell would be dominated by ice II to such an extent that the shell should have contracted. But New Horizons’ readings suggest that it’s expanding instead.

“We don’t see the things on the surface we’d expect if there had been a global contraction,” Hammond said. “So we conclude that ice II has not formed, and therefore that the ocean hasn’t completely frozen.”

Scientists have suggested that the decay of radioactive minerals in Pluto’s rocky core could still be generating enough heat to keep the water in Pluto’s deep interior from freezing.

But the case for Pluto’s ocean is not yet rock-solid: The model shows that the pressure would be high enough to create ice II only if Pluto’s layer of ice goes deeper than 160 miles. If the ice doesn’t go that deep, Pluto’s water could have frozen merely into regular water ice. That would produce the kind of extensional faults seen by New Horizons.

Pluto’s icy shell is thought to have a depth of at least 185 miles, which gives the researchers confidence that they’re right. The presence of other types of ice, such as frozen nitrogen and methane, also bolsters their case.

“Those exotic ices are actually good insulators,” Hammond said. “They may be helping Pluto from losing more of its heat to space.”

In addition to Hammond and Barr, Brown University’s Edgar Parmentier is a co-author of the paper in Geophysical Research Letters, “Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell.”

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