Pluto Has a Cold Wandering Heart, and Maybe a Hidden Ocean
Scientists looking deep into the heart of Pluto have discovered more evidence that the petite planet could be hiding a liquid ocean beneath its icy surface.
In two papers published Wednesday in the journal Nature, researchers say Sputnik Planatia, the bright left lobe of Pluto's "heart" is on the move -- slowly drifting toward the dwarf planet's equator.
One way to explain this drift is if an ocean of water lies beneath the planet's surface, the scientists conclude.
James Keane, a graduate student at the University of Arizona and the first author of one of the papers, explained that the phenomenon responsible for making Pluto's heart move has been observed on other terrestrial bodies like the moon, Mars, Venus, Mercury, and our own planet Earth. Scientists call it "true polar wander."
It's not that the physical feature is moving across the surface, he said. Instead, the planet is reorienting itself on its axis. Picture a billiards 8-ball, with the 8 facing you. Now imagine turning it slowly until the 8 is facing the floor.
Keane, who is writing his dissertation on true polar wander, said planets reorient themselves essentially because they are lazy.
"The axis a planet spins on is not random," he said. "They like to spin in such a way that they minimize the amount of energy in the spin. That means they like to spin around the shortest axis."
Polar wander occurs when a 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.
The distinctive Sputnik Planatia is near Pluto's equator, which indicates that it has comparatively more mass than other parts of the planet, Keane said. It's a little counterintuitive however, since Sputnik Planatia is a basin, or a giant hole in the ground, that was probably carved by a large impact earlier in the planet's history.
To determine how the impact basin could still have enough mass to explain how it got to its telltale location on Pluto, both teams turned to computer models to test different scenarios.
"It turns out Pluto is pretty easy to reorient this way," Keane said.
Keane's group found that the depression could gain enough mass to cause true polar wander by 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 of volatile ices like methane nitrogen and carbon wind up in the impact basin due to seasonal snowfall, it's possible for Sputnik Planatia to gain what's called a positive mass anomaly, Keane said.
The other study, led by planetary scientist Francis Nimmo at UC Santa Cruz, comes to a slightly different conclusion. Nimmo and his colleagues suggest that the positive mass anomaly was probably caused by a subsurface ocean that moved closer to the surface in the area of the impact crater.
Because liquid water is more dense than ice, the extra ocean in this part of Pluto would give Sputnik Planatia 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.
He added that his group took a more skeptical approach to the positive mass anomaly problem, but that none of its findings contradict this other scenario.
Previous studies have also indicated that Pluto has a subsurface ocean and Keane said the pattern of fractures on the planet's surface do indeed suggest a liquid sea beneath its surface.
"It will be hard to know for certain if there is an ocean unless we send another spacecraft out there, and that's not going to happen anytime soon," he said.
However, both papers show that it takes some combination of active geology and weather to cause Pluto's heart to wander.
"The outer solar system is turning out to be a lot more dynamic than we thought it could be," Keane said.
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