A slushy ocean beneath Pluto’s heart?
Since earlier this year (and perhaps before), scientists have been speculating about an underground ocean on Pluto. On November 16, 2016, scientists led by Francis Nimmo at UC Santa Cruz said that a liquid ocean lying deep beneath Pluto’s frozen surface is the “best explanation” for features revealed by the New Horizons spacecraft, in its revelatory sweep past Pluto in July, 2015.
Nimmo’s team has now published a comprehensive study on a Pluto ocean in the peer-reviewed journal Nature.
The idea is that Pluto’s “bulging, viscous, liquid” ocean lies beneath its most prominent surface feature, a heart-shaped region named Tombaugh Regio. These scientists say the existence of a subsurface ocean on Pluto might solve a longstanding puzzle about this region.
That is, for decades, since long before New Horizon’s visit, astronomers have observed that a bright region within Tombaugh Regio – one of the lobes of Pluto’s “heart,” known as Sputnik Planitia – aligns almost exactly opposite the Pluto’s large moon, Charon. This strange orientation of a bright region on Pluto locked in place with this world’s moon has “lacked a convincing explanation,” these scientists say. Astronomer Richard Binzel of MIT, who was part of the study, said in statement:
The New Horizons data say [Sputnik Planitia] is not only opposite Charon, but it’s really close to being almost exactly opposite. So we asked, what’s the chance of that randomly happening? And it’s less than 5 percent that it would be so perfectly opposite.
And then the question becomes, what was it that caused this alignment?
The MIT statement explained:
A thick, heavy ocean, the new data suggest, may have served as a ‘gravitational anomaly,’ or weight, which would factor heavily in Pluto and Charon’s gravitational tug-of-war. Over millions of years, the planet would have spun around, aligning its subsurface ocean and the heart-shaped region above it, almost exactly opposite along the line connecting Pluto and Charon.
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.
Another paper in the same issue of Nature, led by James Keane at the University of Arizona, also argues for reorientation and points to fractures on Pluto as evidence that this happened.
In the subsurface ocean idea, the response to an early impact would have been an upwelling of water pushing up against Pluto’s thinned and weakened crust of ice. At equilibrium, because water is denser than ice, that would still leave a fairly deep basin with a thin crust of ice over the upwelled mass of water. Kimmo explained:
At that point, there is no extra mass at Sputnik Planitia. What happens then is the ice shell gets cold and strong, and the basin fills with nitrogen ice. That nitrogen represents the excess mass.
Nimmo and his colleagues also considered whether the extra mass could be provided by just a deep crater filled with nitrogen ice, with no upwelling of a subsurface ocean. But their calculations showed that this would require an implausibly deep layer of nitrogen, more than 25 miles (40 kilometers) thick. They found that a nitrogen layer about 4 miles (7 km) thick above a subsurface ocean provides enough mass to create a “positive gravity anomaly” consistent with the observations. He said:
We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean.
Bottom line: On November 16, 2016, space scientists using data from the recent New Horizons mission released yet more evidence in two papers in the journal Nature for a subsurface ocean on Pluto. It may lie beneath one lobe of Tombaugh Regio, Pluto’s large and bright heart-shaped region.