Pluto’s icy heart may hide underground ocean

Before the New Horizons flyby, planetary scientist James Keane saw himself as a “Pluto skeptic.”

Boy was he wrong. The distant dwarf planet is home to mysterious mountains that could be ice volcanoes and icebergs of water ice floating atop a plain of frozen nitrogen. Chief among Pluto’s fascinating — and endearing — features is its bright, white heart full of ice and nitrogen snow.

In one of two new papers on Pluto published Wednesday in the journal Nature, Keane and his colleagues write that the western lobe of this heart — an ice-filled impact basin called Sputnik Planitia — may have acted like an extra weight on one side of a bowling ball, causing the planet to wobble away from its original spin axis. A second study led by University of California, Santa Cruz planetary scientist Francis Nimmo suggests that Sputnik Planitia may be so heavy because an underground ocean is hiding beneath the icy crust.

Keane, a Ph.D. student at the University of Arizona, and Nimmo were immediately drawn to Sputnik Planitia (pronounced plaNI-sha) because it is located at what’s called the antiCharon point — the spot on Pluto’s surface that is directly opposite the side that faces Pluto’s largest moon.

“We were struck that the brightest thing, the most anomalous thing, was at a special location on Pluto,” Keane said. “If you were to draw a line connecting the center of Charon through the center of Pluto it would come out on the far side of Pluto and it would go right throw Sputnik Planitia.”

This is suggestive of a phenomenon called true polar wandering: The planet’s spin axis remains the same relative to the rest of the solar system, but the planet reorients so that its poles are at two new locations.

Data from New Horizons suggests that Pluto could have sloshing subsurface ocean of liquid water. The planet has cracks on its surface characteristic of the planet expanding and contracting as parts of that ocean melt and freeze.

If there is water in Pluto’s interior, then it would likely upwell beneath Sputnik Planitia, where the crust is thinner. That would make this spot heavier than the rest of the dwarf-planet, since liquid water is denser than ice.

It’s possible that neither of these theories explains Pluto’s polar wandering. For one thing, scientists haven’t conclusively proved that Pluto even has an interior ocean. For another, not all of the volatile ices that Keane discusses in his paper are heavier than water ice, so it’s difficult to know exactly how much heavier than the rest of Pluto Sputnik Planitia might really be.