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Pluto's reorientation
Sputnik Planitia, the left lobe of Pluto’s “heart,” is thought to have formed as the result of a giant impact early in the dwarf planet’s history. This graphic shows how the icy world reoriented itself as the impact basin filled with volatile ices. (Credit: James Keane / University of Arizona)

Pluto’s famous heart-shaped feature may well have migrated over the course of millions of years as the dwarf planet spun, and that would add to the evidence for a slushy ocean hidden beneath the ice, two groups of scientists say.

In separate reports published today by the journal Nature, the scientists say a reorientation of the faraway world’s most famous feature would provide the best explanation for phenomena observed during last year’s flyby of NASA’s New Horizons probe – including patterns of cracks in the ice.

Most tellingly, it would explain why the heart-shaped feature, known informally as Tombaugh Regio, lines up almost precisely opposite Pluto’s biggest moon, Charon.

“We asked, ‘What’s the chance of that randomly happening?’ And it’s less than 5 percent that it would be so perfectly opposite,” MIT Professor Richard Binzel, a co-author of one of the reports, said in a news release. “And then the question becomes, what was it that caused the alignment?”

The scientists’ explanation starts with a massive collision between Pluto and another celestial body early in its history. That would have created an impact basin in the area that’s known today as Sputnik Planitia, which is the left-hand lobe of Tombaugh Regio’s heart.

Computer models suggest that, over time, the basin filled in with subsurface slush as well as nitrogen ice that settled out of Pluto’s thin atmosphere.

Such a buildup would have created a gravitational anomaly for Sputnik Planitia. As Pluto and Charon whirled around each other, the anomaly would have forced the planet to reorient itself on its axis. The disruptive effect would be similar to that caused by putting a wad of chewing gum on a spinning top, or throwing a spitball.

Eventually, the reorientation would put Sputnik Planitia’s mass on the equator, opposite to the side of the planet that perpetually faces Charon.

The phenomenon, known as polar wander, is thought to have affected the orientation of Earth’s moon and Mars billions of years ago. But on Pluto, the process may still be active, the scientists said.

“Its entire geology – glaciers, mountains, valleys – seems to be linked to volatile processes. That’s different from most other planets and moons in our solar system,” the University of Arizona’s James Keane said in a news release. Keane is the principal author of the second report published by Nature.

Keane and his colleagues point to characteristic cracks in Pluto’s surface ice, which matches up with what would be expected from computer models for frozen water from a subsurface ocean.

“It’s like freezing ice cubes,” Keane said. “As the water turns to ice, it expands. On a planetary scale, this process breaks the surface around the planet and creates the faults we see today.”

The scenario is consistent with the study published by Binzel and his colleagues.

“We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean,” the study’s principal author, Francis Nimmo of the University of California at Santa Cruz, said in a news release.

The subsequent buildup of frozen nitrogen would add to the gravitational anomaly, creating the thick glaciers that the New Horizons probe saw covering the surface of Sputnik Planitia.

Other studies have suggested that Pluto still possesses a slushy subsurface ocean, and the patterns of cracks observed in the surface ice are consistent with that view, Nimmo and his colleagues say.

Nimmo suspects that Pluto’s ocean contains compounds such as ammonia that act as an antifreeze. He also notes that other large objects in the Kuiper Belt, the broad ring of icy material that lies beyond Neptune’s orbit, are similar to Pluto in size and density.

“When we look at these other objects, they may be equally interesting, not just frozen snowballs,” he said.

In addition to Nimmo and Binzel, the authors of “Reorientation of Sputnik Planitia Implies a Subsurface Ocean on Pluto” include D.P. Hamilton, W.B. McKinnon, P.M. Schenk, C.J. Bierson, R.A. Beyer, J.M. Moore, S.A. Stern, H.A. Weaver, C.B. Olkin, L.A. Young, K.E. Smith and the New Horizons Geology, Geophysics and Imaging Theme Team.

In addition to Keane, the authors of “Reorientation and Faulting of Pluto Due to Volatile Loading Within Sputnik Planitia” include Isamu Matsuyama, Shunichi Kamata and Jordan Steckloff.

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