Pluto Has Oceans Under Ice?

Frigid Pluto, home to some of our solar system's chilliest real estate, may well harbor an ocean beneath its miles-thick ice shell, new research suggests.

Despite its extreme cold, the dwarf planet still appears to be warm enough to "easily" have a subsurface ocean, according to a new model of the rate at which radioactive heat might still warm Pluto's core.

And that ocean wouldn't be a mere puddle, noted planetary scientist Guillaume Robuchon of the University of California, Santa Cruz.

Rather, the ocean could be 60 to 105 miles (100 to 170 kilometers) thick beneath a 120-mile (200-kilometer) layer of ice, Robuchon said at an annual meeting of the American Geophysical Union in San Francisco earlier this week.

If so, Pluto would join a list of outer solar system bodies—such as Saturn's moons Titan and Enceladus—believed to possibly hold liquid water, a key ingredient for life as we know it. (See "Saturn Moon Has Ice Volcano—And Maybe Life?" and "Saturn Moon Has Water Geysers.")

Liquid-Preserving Heat Under Pluto Ice?

Pluto's heat would come from the decay of radioactive nuclides, particularly potassium-40, in rocks deep in the dwarf planet's interior.

Even though Pluto's surface is probably colder than -380°F (-230°C), there could still be plenty of liquid-preserving heat beneath the ice cap, the new model suggests.

"Ice is a good insulator," said Robuchon's collaborator, Francis Nimmo, also of the University of California, Santa Cruz.

Robuchon added that his model of Pluto's interior readily produces an under-ice ocean, as long as Pluto's core rocks contains at least a hundred parts per billion of radioactive potassium. (Earth's rocks contain approximately 0.01 percent of potassium-40.)

(See "New Pluto Pictures Unveiled; Hubble's Sharpest Yet.")

However, for an ocean to exist, Pluto's rocks must be concentrated in a rocky core, with water and ice layered on the surface.

If the body is instead a stew of water and ice throughout, Nimmo said, "it's back to the drawing board."

Looking for Cracks

When the New Horizons spacecraft reaches Pluto in 2015, it should be easy to test whether Pluto actually has a subsurface ocean.

If there's no ocean, Pluto should be comparatively flattened at its poles, containing a "fossil" equatorial bulge left over from early in its history, when the body was spinning more rapidly.

(Related: "Pluto Has 'Upside Down' Atmosphere.")

Such a bulge exists on Earth's moon, which of course is ocean free, Nimmo said.

Also, if there's an ocean on Pluto, the surface should show cracks created as Pluto gradually lost heat and the ice cap thickened over billions of years.

That's because the freezing ice would have expanded, causing the surface to bulge upward, cracking in the process.

If there was only ice, and never an ocean, the cooling of the planet should have contracted the ice rather than expanded it. Once formed, ice contracts as it cools.

"We're making predictions," Nimmo said, "and will find out whether they're right or wrong when New Horizons gets there."