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Sputnik Planum
The left side of Pluto’s bright “heart” is known informally as Sputnik Planum. (Credit: NASA / JHUAPL / SwRI)

Scientists have been saying for months that Pluto could have a salty, sloshing ocean beneath its icy surface, but now they’ve worked out just how deep it could go. The answer? As deep as 60 miles, or nine times the depth of Earth’s deepest seas.

The estimate, based on computer modeling of the impact dynamics behind a heart-shaped region of Pluto, was published this month in Geophysical Research Letters.

The bright-colored “heart,” first seen last year by NASA’s New Horizons probe, is arguably Pluto’s best-known surface feature. But it’s actually two features. Scientists say the left lobe of the heart, known informally as Sputnik Planum, was created in the aftermath of an ancient impact. The object that made that impact is thought to have been about 120 miles (200 kilometers) wide.

In June, researchers reported that the geological features mapped on Pluto’s surface would be consistent with the presence of a liquid water ocean far below, perhaps heated by the decay of radioactive materials in Pluto’s rocky core. But they didn’t estimate the size of the ocean.

This month’s findings address that question.

A team led by Brown University geologist Brandon Johnson developed a computer model that charted how Pluto’s mass distribution should affect its orientation relative to Charon, the dwarf planet’s largest moon.

Pluto and Charon show the same face to each other as they spin in their orbits, a relationship known as tidal locking. And it so happens that Sputnik Planum is always front and center, as seen from Charon. That suggests that Sputnik Planum is a positive mass anomaly – that is, it has a higher-than-average concentration of mass.

Typically, an impact region like Sputnik Planum would be a crater with a lower-than-average concentration of mass – in other words, a negative mass anomaly. In order for the region to be on the positive side, something must be pushing up from beneath the ice.

“This scenario requires a liquid ocean,” Johnson said in a news release. “We wanted to run computer models of the impact to see if this is something that would actually happen. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It’s also sensitive to how salty the ocean is, because the salt content affects the density of the water.”

The researchers ran a wide variety of computer simulations to determine which combination of factors would produce an impact region of Sputnik Planum’s size. The best match was a scenario in which Pluto has an subsurface ocean layer that’s more than 60 miles (100 kilometers) thick, with a salinity of around 30 percent.

“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly —and it appears as though it is — this ocean layer of at least 100 kilometers has to be there,” Johnson said.

As additional readings from last year’s New Horizons flyby become available, researchers are likely to get a clearer idea of what lies beneath on Pluto. They may find out that a different scenario provides a better match for the data. But if the current scenario holds true, Pluto’s hidden ocean would go nine times deeper than the Pacific Ocean’s Marianas Trench, and be saltier than the saltiest water in Utah’s Great Salt Lake.

Could such an environment sustain life? Maybe someday we’ll find out.

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