The creation of Sputnik Planum may have caused Pluto to shift on its axis, accidentally revealing a hidden liquid ocean in the process.
The 1300 km-long, 900-km wide basin, located in the icy heart-shaped region called Tombaugh Regio, is heavy enough that its mass actually caused the dwarf planet to reorient, effectively rolling over, to position the basin on the axis of Pluto’s tidal tug-of-war with Charon. Sputnik Planum may owe its prodigious, planet-flipping mass to the presence of a subsurface ocean beneath Pluto’s icy crust.
Planetary systems tend to settle into the configuration that conserves energy the best, so if a planet has an area of extra mass in one spot, such as the huge Tharsis volcanic rise on Mars, the planet will actually roll on its axis so that mass is closer to the equator, along the axis of tidal forces. If the planet has a lighter area, like the Aitken impact basin on the Moon, the planet will tend to roll to put that negative mass near one of its poles, along its axis of rotation.
Pluto’s surface bears recognizable scars of that reorientation. When a planet rolls over, every point on its surface experience a change in the tidal stresses that act on it. That can stretch, crack, and compress rock and ice, producing a trademark pattern of faults. That’s the global fault pattern that New Horizons researchers spotted on Pluto.
And those cracks hinted that at some point in its history, Pluto probably had an ocean beneath its surface, a mantle of, if not liquid water, then at least partially fluid icy slush. Most of the faults observed on the surface are caused by expansion, or stretching, which Keane and his colleagues say “probably reflects global expansion due to the freezing of a subsurface ocean.” That’s a lot like what happens here on Earth, when water freezes in rock crevices and expands, widening the cracks.
A subsurface ocean may also be just the thing to help explain why Pluto rolled the way it did.
This is How Pluto Rolls
At first glance, you’d expect Sputnik Planum to be a negative mass; after all, it’s basically a big hole in the ground, albeit one partially filled with nitrogen ice. If that were the case, then the imbalance it caused would have made Pluto reorient so that Sputnik Planum ended up near one of the poles, along the rotational axis — but that’s not what happened. Sputnik Planum lies just 248 mi (400 km) off Pluto’s tidal axis, and very close to the equator. Keane and his colleagues say that means that the basin must somehow actually be a bulge of extra mass, cleverly disguised as a depression.
To understand how Pluto may have reoriented, Keane and his colleagues build a computer model of Pluto, with a core rich in silicate, a liquid water ocean layer acting as a mantle, and on top, a weak crust, rich in water. Keane and his colleagues tried modelling what would happen if Sputnik Planum formed at different points on Pluto’s surface, and with different masses. . It turns out that there’s only about a 5% chance that Sputnik Planum ended up this close to the tidal axis purely by happenstance.
“First, Sputnik Planum could not have formed in any random location,” wrote Keane and his colleagues in a paper published in Nature. The only starting points that allowed Sputnik Planum to end up in its current position were in a quadrant of Pluto’s northern hemisphere, opposite Charon. That’s because shifting a whole planet, even a dwarf planet, on its axis requires some energy, so there’s only so far it can shift in one migration.
The researchers also found that Sputnik Planum couldn’t just be a hole, notable only for the mass that was missing. That would have caused Pluto to reorient so Sputnik Planum ended up near the north pole, not the equator.
Sputnik Planum has accumulated a huge layer of nitrogen ice, probably 1.8 to 6 mi (3 to 10 km) thick, since its formation, but even that isn’t enough mass to counteract all the missing mass in the basin. In fact, Nimmo and colleagues wrote in a second paper published in Nature, it would take a nitrogen ice layer about 24.8 mi (40 km) thick to accomplish that. Something else had to be going on beneath the surface, and according to Nimmo, the most plausible answer is is a subsurface ocean which welled upward in the wake of an asteroid impact sometime in Pluto’s past.
Making An Impact
Many scientists believe that Sputnik Planum is a crater carved out of Pluto’s ice by a long-ago asteroid impact, and if that’s the case, then the effects of that impact could have made the basin much more massive than it appears.
When the impact dug tens of kilometers of ice out of the floor of Sputnik Planum, it would also have weakened the rocky crust beneath, which would have allowed the subsurface ocean – if it was there – to well upward, closer to the surface. That’s happened elsewhere, much closer to home.
“On the Moon, a combination of impact-driven uplift of dense mantle material and later surface addition of lavas after the crust has cooled and strengthened results in impact basins showing a positive gravitational anomaly,” Nimmo and colleagues wrote. On Pluto, the “dense mantle material” may actually have been liquid water, and the basin may have gathered nitrogen ice rather than lava after the fact.
When Nimmo and his colleagues modelled that, they found that Sputnik Planum only needed to accumulate about a 4.3 mi (7 km) layer of nitrogen ice to give it enough weight to turn the dwarf planet. That’s right in the middle of the range of previous estimates for the thickness of Sputnik Planum’s ice, based on New Horizons data, so the model seems to fit. “If Sputnik Planum is a positive gravitational anomaly at the present day,” they wrote, “a subsurface ocean with a thinned shell beneath the basin provides a viable explanation.”
There are other possibilities, the researchers acknowledged, but most of them don’t match observations or current models as well as the liquid ocean idea. For instance, solid ice beneath the surface in Sputnik Planum wouldn’t produce the types of faults that New Horizons observed. “Further work will be required to exclude these other alternatives definitively,” Nimmo and colleagues wrote.
Uncharted Waters
If there is a subsurface ocean on Pluto, it’s not clear yet exactly what it might look like. Keane’s team has suggested that it may be slushy, partially frozen water ice, rather than flowing liquid water. Nimmo and his team have suggested that Pluto’s hidden ocean may not be pure water at all, but may actually contain some ammonia, or even ethanol. It may even be a thing of the past, long since frozen back into solid ice, although Nimmo and his team say the ocean must still be in liquid form in order to keep Sputnik Planum in its current position.
Questions remain to be answered, but the possibility is one more sign that Pluto is a much more dynamic place than anyone realized a year ago, and it hints at the potential for even more interesting things waiting beneath the surface of other Kuiper Belt Objects.
“Various Kuiper Belt Objects of somewhat similar sizes and densities to Pluto are known; among these bodies, subsurface oceans are probably a common phenomenon,” Nimmo and his colleagues wrote.
