Pluto’s most iconic feature — its “icy heart” — may have been responsible for tipping the dwarf planet over. Scientists believe the 600-mile-wide region of frozen plains known as Sputnik Planitia gained enough mass over the years, causing Pluto to tilt to its current orientation. And that could mean there’s a subsurface ocean lurking underneath the dwarf planet.
The cracks and faults on Pluto’s surface tell the story of its rollover, according to two new studies published today in Nature. Researchers used computer models to simulate Pluto’s reorientation, which would have put a lot of stress on the crust and created these cracks. Those models match up pretty well with the patterns of canyons and mountains that NASA’s New Horizons spacecraft saw when it flew by Pluto last year. “It’s a second line of evidence proving that this actually happened,” Isamu Matsuyama, an assistant professor at the University of Arizona’s Lunar and Planetary Lab and the author of one of the studies, tells The Verge.
As for how the flip occurred, the two Nature studies offer complementary arguments. Matsuyama’s study says that the low-lying Sputnik Planitia filled up with a bunch of nitrogen ice, gaining mass that pushed Pluto over. But the second study says the nitrogen ice wasn’t enough to completely change Pluto’s orientation. Even more weight was needed, and a dense ocean lurking just underneath Sputnik Planitia would have been enough to do the trick. “That tips you over the edge,” Francis Nimmo, professor of earth and planetary sciences at University of California Santa Cruz and lead author of the second study, tells The Verge.
Nimmo’s study is just further evidence that liquid may be teaming underneath Pluto, making this dwarf planet one of a growing group of objects in our Solar System that harbor oceans. “Subsurface oceans are common,” Amy Barr Mlinar, a senior scientist at the Planetary Science Institute, who was not involved in today’s studies, tells The Verge. “Fifteen or 20 years ago, very few people thought this way.”
Sputnik Planitia is located in a very special place on Pluto, right next to something called a tidal axis — the imaginary line that connects Pluto and its largest moon Charon. This axis dictates how Pluto moves if its mass changes. If you were to add extra weight to a certain point on Pluto, the entire dwarf planet would reorient itself so that the weighted point would end up next to this axis.
Scientists think that Sputnik Planitia made the same move. “It started out somewhere else on the surface but then it tilted Pluto over so that Sputnik Planitia moved to this point directly opposite Charon,” says Nimmo. To do that, Sputnik Planitia must have acquired enough mass at some point to knock Pluto on its side. So how exactly did it pack on the pounds? Matsuyama argues that the position and low altitude of this region is what helped the area gather mass.
Sputnik Planitia is near the equator on Pluto — but unlike Mars or Earth, Pluto’s equator is home to some of the coldest regions on the dwarf planet. That’s because Pluto’s axis is extremely tilted compared to Earth’s, making it easier for regions around Pluto’s equator to be in permanent shadow. As a result, these shaded regions can get extremely cold, causing the nitrogen gas in Pluto’s atmosphere to freeze into surface ice. This ice then gathered into the low lying plains of Sputnik Planitia. “If you have a region around the equator that’s lower, it’s a lot easier to accumulate more material there,” Matsuyama says. “There’s more room to accumulate extra stuff.”
The idea is that all this extra nitrogen ice may have added enough mass to Sputnik Planitia to tip Pluto over. But Nimmo says the region needed even more mass to fully reorient Pluto — and that’s where the subsurface ocean comes into play.
Sputnik Planitia is thought to be the leftover of another object slamming into the surface of Pluto long ago. This impact probably carved away a big chunk of the ice shell on Pluto’s surface, creating the depressed region. If there is an ocean underneath Pluto, it would have reacted to this collision. The water would have pushed ice back up to the surface of the impact site, as a way of balancing things out. “You end up with an ocean that’s closest to the surface underneath Sputnik Planitia than it is in the surroundings,” says Nimmo. And since water is denser than ice, this nearby ocean would have provided the extra weight needed to tilt Pluto.
It’s not the first time someone has suggested Pluto may be harboring an ocean. Experts think it’s possible that radioactive materials inside the dwarf planet are decaying, providing enough heat that could keep an ocean as a liquid. If so, that means Pluto could join the ranks of moons like Enceladus and Europa, which are thought to have oceans as well. And that opens up the possibility of even more subsurface oceans in our Solar System. “We know there are other objects similar in size to Pluto in the Kuiper Belt,” says Nimmo. “If this study is correct, maybe some of these other big objects have subsurface oceans too.”