Evidence mounts for subsurface ocean on Pluto
Three new studies based on data returned by New Horizons add to existing evidence that Pluto harbors a subsurface ocean. The studies focus on the very bright Sputnik Planitia, the left side of Tombaugh Regio and Pluto’s iconic “heart”, which featured prominently in the first photos returned during the July 2015 flyby.
James Keane and Isamu Matsuyama published a study in the journal Nature. They argue that nitrogen buildup in Sputnik Planita changed the planet’s spin, reorienting it toward its current position.
Unlike Earth’s axis, which is tilted 23.5 degrees, Pluto’s axis is tilted to the point that the dwarf planet essentially lies on its side. As such, each of the poles alternates in receiving the most sunlight while the equatorial regions remain cold throughout Pluto’s 248-Earth-year orbit. Without a subsurface ocean, scientists think this orientation would likely not have happened.
A 600-mile (1,000-kilometer) wide basin probably created by an impacting meteorite, Sputnik Planitia is aligned directly with the tidal axis between Pluto and its largest moon Charon, the line along which the gravitational pull of Charon on Pluto is strongest.
The chance of this happening by coincidence is just 5 percent, which is leading scientists to suspect extra mass at Sputnik Planitia interacted with the tidal forces between Pluto and Charon, resulting in Pluto’s reorientation.
“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, the lead author of a second study, also published in Nature.
Sputnik Planitia is filled with frozen nitrogen ice formed via condensation when the region is cold and shadowed. The ice eventually returns to gaseous form when the region is exposed to sunlight and heats up.
“Each time Pluto goes around the Sun, a bit of nitrogen accumulates in the heart,” Keane said. “Once enough ice has piled up, maybe a hundred meters (328 feet) thick, it starts to overwhelm the planet’s shape, which dictates the planet’s orientation. And if you have an excess of mass in one spot on the planet, it wants to go to the equator. Eventually, over millions of years, it will drag the whole planet over.”
The Gravitational Anomaly
Keane and Matsuyama combined New Horizons‘ data with data produced by computer models in which they moved the heart feature around the planet to determine what such movement would do to its spin axis.
Following the meteorite impact that created Sputnik Planitia, much of the icy crust in the region was blasted away. If a subsurface ocean were present, its liquid would have pushed up against the weakened crust in the wake of the explosion.
By itself, the nitrogen ice in the basin is not thick enough to have sufficient mass to reorient Pluto. Without an underground ocean upwelling beneath it, the nitrogen ice would have to be 25 miles (40 kilometers) thick for there to be enough mass to tip the planet over.
In contrast, a nitrogen layer just 4 miles (7 kilometers) thick above a subsurface ocean does produce enough mass to do this, as Nimmo and colleagues determined.
Nimmo said migrating rock, another possible cause of Pluto’s reorientation, is not likely to have caused the additional mass as the rock is too far down and unlikely to have been moved around in such a way that would result in the current scenario.
“We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean,” Nimmo said.
Under the Surface
New Horizons scientists found evidence nitrogen ice beneath Sputnik Planitia is constantly being refreshed by convection. According to co-investigator Richard Binzel of the Massachusetts Institute of Technology, a weak spot at the bottom of the basin drives this convection, allowing heat from Pluto’s interior to rise and cause the ice to “bubble”.
Internal heat produced by the radioactive decay of rock has likely kept the underground ocean from completely freezing over the last 4.5 billion years.
The ocean is likely composed of liquid water along with ammonia or other substances that act as an antifreeze, preventing it from freezing entirely and keeping it in a slushy rather than completely liquid state.
Surface fractures imaged by New Horizons could be caused by the gradual freezing of this ocean. Because water expands when it freezes, the process would cause stress to the rock and ice directly above it, causing it to crack. Pluto’s lack of an equatorial bulge also points to it having a subsurface ocean, as oceans make it impossible for worlds to maintain that bulge.
Keane said if volatile ices continue to accumulate in Sputnik Planitia, the planet could still be reorienting itself.
Binzel and co-author Alissa Earle published a third study in the journal Icarus where they argued moderate temperatures in Pluto’s equatorial region have prevented its ice from completely melting, keeping Sputnik Planitia bright for millions of years and possibly keeping Pluto in its current position.
“This basin has been there a long time and had this bright ice spot for a very long time. And that may have helped to get it rotated to where it is today,” Earle said.
The presence of an underground ocean on Pluto suggests other dwarf planets in the Kuiper Belt may also have subsurface oceans.
Binzel said while some scientists suspected Pluto to harbor a subsurface ocean, obtaining data this compelling from a flyby mission was unexpected.
“Pluto just continues to surprise us,” Binzel said.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.