Pluto's heart suggests that a subsurface ocean likely exists on the planet. New simulations of an asteroid impact that created the heart-shaped feature hint that the dwarf planet could be hiding a deep underground ocean.

In a new study, researchers simulated an asteroid impact that likely produced the Sputnik Planum, a flat icy patch on Pluto's surface that makes up the western half of the dwarf planet's heart-shaped feature.

Because Pluto and its largest moon Charon are both tidally locked, they always face each other with the same side. Sputnik Planum happens to be on the side of Pluto that always faces Charon. The position implies that the area is what is known as positive mass anomaly, which means that the side is on average more massive than the rest of Pluto's surface.

Brown University geologist Brandon Johnson this does not make sense at first because a negative mass anomaly is expected if you blast out a bunch of materials.

The researchers ran simulations of an asteroid striking the dwarf planet's surface and found that during this impact, the 125-mile asteroid produced a huge dent on Pluto. After the Sputnik Planum basin was formed, it was partially filled in by nitrogen ice.

Although it added some mass to the basin, the ice layer was not thick enough to give Sputnik Planum positive mass. The rest of the mass may have been produced by a liquid that lurks beneath the dwarf planet's surface.

After the large impact produced the dent, a rebound that pulls material from the dwarf planet's interior upward occurred. If the upwelled material was denser than what was removed by the impact, the crater would have the same mass as it used to have before the impact, a phenomenon called isostatic compensation.

Because water is denser than ice, if a liquid water is hidden beneath Pluto's icy shell, it may have welled up after the impact and evened out the crater's mass. If the basin has neutral mass to begin with, the nitrogen layer that filled the basin later would be enough to produce a positive mass anomaly.

"This scenario requires a liquid ocean," Johnson said. "It's pretty amazing to me that you have this body so far out in the solar system that still may have liquid water."

Scientists likewise found that liquid water hidden beneath the dwarf planet's icy shell could be as salty as the Dead Sea on Earth and is at least 60 miles deep, which is far deeper than the deepest part of the Earth's ocean at only about 7 miles deep.

"We find that SP can only achieve a large positive mass anomaly if Pluto has a more than 100 km thick salty ocean," the researchers wrote in their study published in Geophysical Research Letters on Sept. 19. "Furthermore, our work supports the hypothesis that SP basin has an impact origin."

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