Scientists looking deep into the "heart" of Pluto have discovered more evidence that the dwarf planet could be hiding a liquid ocean beneath its icy surface.
In two papers published this week in the journal Nature, researchers say Sputnik Planitia, the bright left lobe of a giant heart-shaped plain on Pluto's surface, is on the move — slowly drifting toward the petite planet's equator.
One possible explanation for this drift would be an underground ocean, the scientists said.
Sputnik Planitia is a smooth basin about 600 miles wide, and it's covered in bright nitrogen ice, according to measurements taken by NASA's New Horizons spacecraft. Previous studies suggest that the basin was created in a long-ago collision with another body at least 125 miles across.
As it happens, Sputnik Planitia is located in a particularly interesting spot on Pluto's surface.
"We call it the tidal axis," said James Keane, a graduate student at the University of Arizona and the first author of the other paper. "If you drew a line from the center of Pluto's moon Charon through the center of Pluto and out the other side, you would come very close to the location where Sputnik Planitia is today."
Both research teams agree that the location of Sputnik Planitia is no accident: They say the mini-planet reoriented itself to make sure the formation ended up where it did in a process called "true polar wander."
True polar wander occurs when a planet or dwarf planet forms a new feature that changes its balance of mass. If one area suddenly has more mass — perhaps because an enormous volcano has erupted on that spot — the planet will reorient itself so the more massive feature is closer to the equator. If a part of the planet suddenly loses a lot of mass, that feature will drift toward the poles.
It's not that the physical feature is moving across the surface, Keane said. Instead, the entire planet is reorienting itself. Picture an 8-ball in billiards, with the 8 facing you. Now imagine turning it slowly until the 8 is facing the floor. That's essentially what happened with Pluto after its fateful collision.
The fact that Sputnik Planitia is near Pluto's equator indicates that it has comparatively more mass than other parts of the dwarf planet, Keane said.
It's a little counterintuitive, however, since Sputnik Planitia is essentially a giant hole in the ground.
To determine how the impact basin could still have enough mass to cause it to migrate to its current location, both teams turned to computer models to test different scenarios.
Keane's group found that the depression could gain enough mass to cause true polar wander through a combination of two factors.
If most of the material ejected from the crater during the impact ultimately landed along the sides of the crater, and if a disproportionate amount of volatile ices such as methane, nitrogen and carbon wound up in the impact basin due to seasonal snowfall, it's possible for Sputnik Planitia to gain what's called a positive mass anomaly, Keane said.
The other study came to a slightly different conclusion.
In that paper, Nimmo and his colleagues suggest that the positive mass anomaly was probably caused by an underground ocean that moved closer to the surface in the area of the impact crater.
Because liquid water is more dense than ice, the ocean would give Sputnik Planitia enough mass to explain its distinct location.
"If they are correct, that means the ocean still has to be there, because if it had turned to ice, it would be the same density as the rest of the crust," Keane said.
Despite Pluto's great distance from the sun, Nimmo said, it is entirely possible for it to have a liquid water ocean that endures to this day.
"Pluto has quite a lot of rock on its inside, and the radioactive elements in that rock could provide the heat" that would keep the water from freezing, he said.
In addition, the thick layer of ice on the dwarf planet's surface would prevent that heat from radiating out into the outer solar system.
"The radioactivity of the rock and the insulating blanket of ice could give Pluto a liquid ocean that can last for billions of years," he said.
Brandon Johnson, a planetary scientist at Brown University who was not involved in either of the new studies, said the case for the reorientation of Pluto is a lot stronger, thanks to the work of both teams.
He noted that Keane's group used computer models to show that the pattern of cracks on Pluto's surface could only occur if true polar wander had indeed taken place.
"That's really a very convincing line of evidence that this true polar wander actually happened," he said.
Johnson is inclined to agree with Nimmo's group that Pluto has a subsurface ocean, but he said more work would need to be done before anyone can say that for certain.
"Somewhere down the pipeline there may be an analysis of another geological feature that either supports the subsurface ocean or argues against it," he said. "But the current body of work supports an ocean more and more."
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7:15 p.m.: This article was updated with quotes from the authors of the Nature papers and with other details.