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Under Pluto’s icy heart, a slushy sea may stir

Why scientists suspect there’s an underground ocean on the dwarf planet.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

The dwarf planet Pluto orbits at the edge of our solar system, 3.67 billion miles from the sun, where temperatures on its surface can plummet to minus 400 degrees Fahrenheit. Yet even in that cold, dark region of space, scientists speculate the planet may have an ocean of liquid water, or slushy ice, just beneath its surface.

“We expected that Pluto would be full of surprises, but this one knocked our socks off,” says Richard Binzel, an MIT professor who has been studying the dwarf planet since the 1980s.

On Tuesday, he and a team of scientists from NASA, MIT, and universities across the United States published a study in Nature based on observations from the New Horizons spacecraft. It finds evidence for such an ocean, specifically located in the left “lobe” of Pluto’s heart-shaped basin, which you can see as the bright area in this image of Pluto below.

Why scientists think there’s an underground ocean on Pluto

An ocean under the heart of Pluto is the leading theory to explain a mystery about why Pluto’s heart-shaped basin never faces Pluto’s moon, called Charon.

Pluto and Charon are tidally locked in orbit. That means the planet and its moon always have the same sides facing each other.

Here’s a GIF from NASA of Pluto and Charon orbiting one another. Imagine Pluto is a man holding a rope attached to a ball. As Pluto spins around in a circle, holding the rope, he’s always facing the ball. And the ball is always facing him.

When the New Horizon spacecraft began sending back images of Pluto’s spectacular heart-shaped basin, scientists thought it was peculiar that the basin was almost perfectly aligned with the orbit of Charon.

Scientists are skeptical of coincidences. So what could cause Pluto’s “heart” to line up in this way? One explanation: There’s something there throwing Pluto off balance.

“There’s a little extra mass there [in the heart],” Binzel says. “We’re trying to understand what it is that could contribute that mass. And the answer we come to is maybe there’s this dense subsurface layer of liquid water, or a slushy layer ... pushing up in the region.”

When the ocean formed in Pluto’s heart, it made the dwarf planet reorient itself so that the heavy ocean was on the opposite side of Charon. “That extra mass is going to line up either exactly toward Charon or opposite Charon,” Binzel says. It’s what the laws of physics dictate.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

So how did the ocean get there in the first place?

Binzel suspects that Pluto’s “heart,” particularly its left ventricle region, called Sputnik Planitia, was created by an impact with another object in the Kuiper belt, an asteroid belt near the edge of the solar system.

Illustration by James Tuttle Keane.

That impact gouged out a piece of the surface of Pluto, making the crust very thin at the point of impact. Underground water, kept warm by Pluto’s radioactivity, then flooded this area of thin crust like water in a blister. This formed the extra mass that caused Pluto to reorient itself so the impact zone faced away from Charon.

Binzel doubts scientists will be able to absolutely confirm the existence of an ocean without sending a spacecraft to orbit Pluto. And he admits it is possible that the source of the mass underneath Pluto’s heart is not water. A second paper in Nature today outlines a competing hypothesis: that it’s ice that accounts for the mass, not liquid water. (You can read The Verge’s report on it here.)

Nevertheless, both papers agree: There’s something massive in the heart of Pluto that causes the planet to line up with Charon in its peculiar way. And there will be more discoveries to come.

By studying Pluto, and other faraway objects in the solar system, we can get a sense of features that could conceivably exist on the billions of other planets in our galaxy.

In the case of Pluto, Binzel says, “Nature is so much more creative than we can imagine.” And what about life in this subterranean ocean? Could it exist there? It’s very, very unlikely, but as Binzel says, wherever there’s water, it opens the possibility.