(CN) – Astronomers have found the extra mass of a liquid ocean deep beneath Pluto’s surface caused the planet to roll over, moving its iconic heart directly opposite the side facing its moon Charon.
Two studies published Wednesday in the journal Nature show how the surface of Pluto might have been scarred when a meteorite slammed into it. The resulting crater then filled with some combination of nitrogen ice and liquid water from an underground ocean – and the additional mass caused Pluto to roll over as much as 60 degrees to its present alignment.
“It’s a big, elliptical hole in the ground, so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that,” said Francis Nimmo, author of one of the Nature studies.
Nimmo’s team concludes that Sputnik Planitia – the low-lying plain on Pluto’s surface that forms one-half of the dwarf planet’s famous heart-shaped feature – formed after a large meteorite blasted away a huge amount of Pluto’s icy crust. According to the authors, an underground ocean would respond with an upwelling of water pushing against the weakened crust of ice. Since water is denser than ice, Sputnik Planitia remained a fairly deep plain with a thin crust of ice over the ocean.
“At that point, there is no extra mass at Sputnik Planitia,” Nimmo said. “What happens then is the ice shell gets cold and strong, and the basin fills with nitrogen ice. That nitrogen represents the excess mass.”
The authors found that a subsurface ocean beneath a nitrogen layer about four miles thick provides enough mass to create a “positive gravity anomaly” consistent with their observations. They determined that just a deep crater filled with nitrogen ice – and no upwelling of an underground ocean – would require an implausibly thick layer of nitrogen.
Several other large objects in the Kuiper belt are similar to the dwarf planet in density and size, and Nimmo thinks they also have subsurface oceans.
“When we look at these other objects, they may be equally interesting, not just frozen snowballs,” he said.
James Keane, a graduate student at the University of Arizona, led the other study which mapped networks of cracks on Pluto’s surface that they say ruptured due to tectonic forces as the dwarf planet rolled over.
“Before New Horizons, people usually only thought of volatiles in terms of a thin frost veneer, a surface effect that might change the color or affect local or regional geology,” Keane said. “That the movement of volatiles and shifting ice around a planet could have a dramatic, planet-moving effect is not something anyone would have predicted.”
Pluto’s heart-shaped feature just north of the equator was revealed in images captured during the flyby of NASA’s New Horizons spacecraft last year.