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Like a giant Thermos, Pluto keeps its underground ocean from freezing

Gas trapped in a lattice of ice might buffer Pluto’s subsurface sea from the cooling effects of its icy crust above.

ByKatherine J. WuNOVA NextNOVA Next
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An image of Sputnik Planitia, the western lobe of Pluto's pale tan "heart," captured by NASA’s New Horizons spacecraft. Sputnik Planitia's surprising "heaviness" is evidence of Pluto's subsurface ocean. Image Credit: NASA/JHUAPL/SwRI

Pluto is no place to party down.

On this frigid outpost, orbiting nearly 4 billion miles from the Sun, light is scarce. Temperatures regularly plunge below -380 degrees Fahrenheit, and only a tenuous, nitrogen-rich atmosphere exists above a mountainous surface wreathed in a shell of ice.

This cold, dark neighborhood of our Solar System might be the last place you’d expect to find liquid water. But puny Pluto has always been full of big surprises: Despite its inhospitable exterior, this dwarf planet is thought to harbor a vast, global ocean just below its frozen crust.

Now, a group of scientists may have an explanation for how this supposed subterranean sea keeps from freezing over. Their research, published today in the journal Nature Geoscience, suggests that Pluto is built like a giant, spherical thermos, using a cushion of air trapped at the base of its icy shell to insulate the warmth of its inner ocean.

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“This is a very cool paper,” says Prabal Saxena, a planetary scientist at NASA’s Goddard Space Flight Center who was not involved in the study. “How do you have something that’s hot underneath, and something cold above, and not have them speaking to each other? That’s what this layer potentially provides a solution to.”

Because no direct observations of Pluto’s underground ocean have been made, there’s no guarantee that the theory holds water. But the study’s results bolster the case for Pluto’s inner warmth—and may hint at how liquid waters still run deep on other icy worlds.

“This study could be paradigm-shifting,” says Anne Verbiscer, a planetary scientist at the University of Virginia who was not involved in the study. “This isn’t just about Pluto...this could be the case on many other ocean worlds, and that’s the number one significance of this work.”

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NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Image Credit: NASA/JHUAPL/SwRI

The paper’s findings build on a growing pile of evidence that Pluto is nothing like the dead, desolate planetary runt it was once made out to be. Though scientists have long wondered if the dwarf planet could be home to liquid water, it wasn’t until 2015, when NASA’s New Horizons spacecraft conducted an historic flyby of Pluto, that strong evidence for its ocean began to surface.

Much of the critical data beamed back from New Horizons involved researchers’ first close-up look at a region called Sputnik Planitia, an ice-filled impact basin that occupies the left lobe of a massive, heart-shaped blemish on Pluto’s surface. Sputnik Planitia is thought to be an age-old battle scar, left behind from an ancient collision that excavated a hole in the dwarf planet’s exterior—a rocky rendezvous that, in theory, should make this region of Pluto’s ice crust lighter than the rest.

But Sputnik Planitia isn’t lighter at all. It seems that it’s actually heavier—so much so that it’s permanently tilted the whole of Pluto over to one side, like a paper clip taped to a balloon. In 2016, a team of researchers led by Francis Nimmo, a planetary scientist at the University of California Santa Cruz, proposed a solution to this Plutonian paradox: The extra weight could come from water, which is denser as a liquid than a solid, welling up beneath Sputnik Planitia. The impactor that created the basin in the first place is believed to have excavated some of the ice from the surrounding crust. With the ice beneath Sputnik Planitia thinned, a subsurface ocean would have the space to flood the void and put Pluto off balance.

This theory, however, left Nimmo and his colleagues with another perplexing inconsistency. For a time, an underground ocean could be sustained by heat emanating from the decay of radioactive elements in Pluto’s rocky core. For this sub-Sputnik oasis to stay put, however, the ice above it would need to remain thinner than the rest of Pluto’s crust. That’s all well and good when ice stays extremely cold and thus resistant to flow. But if exposed to a source of warmth, like liquid water, ice can (very slowly) start to run like heated honey. Eventually, this would even out the crust, stripping Pluto if its slant. But Sputnik Planitia’s weight is stubborn—which meant there had to be a way for the crust to keep its cool.

One option may involve an extremely cold (but not frozen) liquid ocean pumped full of a kind of antifreeze, like ammonia. That’s not unheard of in our Solar System, but the amount of ammonia required to set Pluto askew—close to 30 percent of the ocean’s weight—is a little outside the realm of probability, Nimmo says.

But the newest theory—the brainchild of Hokkaido University planetary scientist Shunichi Kamata—may offer the most promising alternative yet: a gassy buffer at the base of Pluto’s ice shell that minimizes contact between the dwarf planet’s chilly crust and the warmer waters beneath.

To test this idea, a team of researchers led by Kamata and Nimmo conducted a series of computer simulations that charted Pluto’s evolution with and without this insulating lattice of gas and ice. Their model showed that, in the absence of this layer, Pluto’s ocean would have frozen long ago. But because gas isn’t very good at transferring heat, the simple addition of this aerated cushion was enough to make Pluto’s cool exterior compatible with its sloshy interior.

Where these gases originated is still up for debate, but they might trace all the way back to Pluto’s conception some 4.6 billion years ago, when ice latched onto gas present in the dwarf planet’s starting material. Reactions at Pluto’s toasty core, which is still active today, have probably also exuded gas that gets detained by ice before it reaches the surface.

The process won’t go on forever, though. Pluto’s subsurface sea is still slowly freezing—and as water turns to bulky ice, the dwarf planet’s girth is gradually expanding. Stretch marks of Pluto’s burgeoning bloat are even visible on the surface, in the form of geologic faults.

Ironically, as the ocean freezes, the ice shell and its base layer of gas both thicken, upping the capacity for insulation. “Most likely, it’s getting progressively harder for Pluto to lose its heat,” Nimmo says. “It’s actually a good way of keeping this ocean alive for a very long time.”

What’s more, the new theory might have the potential to solve another Plutonian mystery: the unusually high levels of nitrogen in the dwarf planet’s atmosphere. Unlike methane and carbon monoxide, nitrogen is difficult to lock up in cages of ice, Kamata says. As this trio of gases migrates from Pluto’s interior to its surface, carbon-bearing gases end up selectively captured, leaving nitrogen to dominate the skies above.

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The proposed interior structure of Pluto. A thin layer of gas trapped in ice, labeled here as "clathrate hydrate," works as a thermal insulator between Pluto's icy crust and the waters below, keeping the ocean from freezing and the ice from melting. Image Credit: Kamata et al., Nature Geosciences, 2019

The gas shield theory also isn’t mutually exclusive with other explanations for Pluto’s oceans, Saxena says. For instance, ammonia could still play a role in maintaining the water’s liquid state (and it could do so at a relatively low concentration). Either way, he says, it’s becoming increasingly apparent that “liquid water is pretty resilient in the Solar System...and that’s exciting.”

However, Catherine Walker, a planetary scientist at NASA who was not involved in the study, points out that, while this is a “very cool way to think about this,” the existence of a gas-infused ice layer isn’t exactly easy to test. New Horizons is now deep in the Kuiper Belt, and there are no imminent plans for humankind to drill deep into Pluto.

But if a spacecraft were to be sent back to Pluto, Nimmo thinks it would be interesting to go back to the place where it all began: Sputnik Planitia. If this spot is indeed heavy with liquid, Nimmo says, it will probably also have higher gravity than surrounding areas.

In the meantime, Walker says, what we’re left with is a new way to look at oceans in our universe. If scientists’ suspicions are right, Pluto’s ocean is far from alone: Ice-laden moons like Europa and Enceladus may sport subterranean seas as well.

“Sometimes, we have a very Earth-centric view,” Walker says. “But we’re really just scratching the surface of what’s [possible]. The more you look at places that seem simple, the more you realize nothing is.”

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