Pluto’s frozen heart may hide slushy underground ocean

Pluto would reorient itself to situate Sputnik Planitia at the anti-Charon point if the basin was inordinately heavier than the rest of the dwarf planet – for complicated physics reasons, this orientation is what makes the lopsided planet most stable.

Such a buildup would have created a gravitational anomaly for Sputnik Planitia. Now, however, two studies published Wednesday in Nature are strengthening the case that Pluto’s icy heart contains a warmer, wetter inner world. On the right is a cutaway image of Pluto shows a section through the area of Sputnik Planitia, with dark blue representing a subsurface ocean and light blue for the frozen crust. Together, the two studies tell a remarkable, nearly fantastical origin story involving giant asteroids, buried oceans, and exotic ices that twisted entire worlds upside down in eons past.

But Sputnik Planitia is different.

“Each time Pluto goes around the sun, a bit of nitrogen accumulates in the heart”, Keane said. Fissures and fractures around Tombaugh Regio and other parts of the planet suggested a subsurface layer of watery slush might be slowly solidifying, breaking up the surface as it expands like ice cubes in a freezer-but other, drier possibilities could also explain such cracks. “Pluto may still be moving and wobbling around today due to the transport of volatiles, and this same process may be responsible for continuing geological activity on other worlds”. “So we calculated Pluto’s size with its interior heat flow, and found that underneath Sputnik Planitia, at those temperatures and pressures, you could have a zone of water-ice that could be at least viscous”. The other study is made by James Keane of the University of Arizona. “On a planetary scale, this process breaks the surface around the planet and creates the faults we see today”. The best explanation, they posit, is an icy ocean below the crust.

Simplified illustration of true polar wander. “I think it’s going to keep planetary scientists extremely busy for a long time”. So far, there is evidence that three of Saturn’s moons, three of Jupiter’s moons, Neptune’s moon Triton, and probably the asteroid belt dwarf planet Ceres, have water. 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.

“Once enough ice has piled up, maybe a hundred meters thick, it starts to overwhelm the planet’s shape, which dictates the planet’s orientation”, Keane said in a statement.

Something odd is happening on Pluto.

Over the course of millions of years, the gravitational weight of this dense ocean could have been enough to manipulate the rotation of the planet, and orientate Sputnik Planitia to face Charon. Illustration by James Tuttle Keane.

“If you put an extra weight on the surface of Pluto, Pluto will roll over to place the weight on the equator at a point directly opposite Charon”.

That’s where the new studies comes in. Since this putative impact, the basin has filled with nitrogen and methane ice, and traces of carbon monoxide ice.

Keane models indicate that Sputnik Planitia has accumulated a lot of volatiles, enough to yield a several-mile thick sheet of ice. “So it’s hard to figure out how that rock would get moved around in the way that you would need”. “And an ocean is a natural way to get that”, said first author of a paper on the new findings Francis Nimmo, Professor at University of California, Santa Cruz in the US.

New Horizons is the NASA mission, launched in 2006, that made a close approach to Pluto in July, giving researchers stunning views and unprecedented amounts of information about the dwarf planet.

According to Nimmo, the meteorite impact on Pluto ejected a huge quantity of water ice, leaving a crater with only a thin layer of ice at its bottom. “What happens then is that water underneath pushes up on the remaining ice, and rises up”. “So you have to come up with a way to hide that extra mass”.

Comparison of mantle uplift in the Moon’s Orientale Basin with proposed ocean uplift in Sputnik Planitia. “Pluto just continues to surprise us”. “Volatiles, oceans, and things we haven’t explored in detail”.

Still the idea is exciting to Binzel who says, “We are in a new era where water seems to be more prevalent than we had previously thought”. After its flyby of Pluto, mission scientists identified a second target – an icy Kuiper Belt body called 2014 MU69 – which the probe should reach in 2019.