Could there be a subsurface ocean on Pluto?

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Dec 10, 2017

With vaporous plumes shooting out from supposed hydrothermal vents deep beneath its frozen surface, which is probably hiding an ocean swimming with strange microbes, Enceladus is currently in the spotlight as the ocean world scientists are impatient to probe. But then there’s Pluto.

Wait. What was that about a hypothetical ocean on Pluto?

Turns out that Enceladus, Europa, Titan, and the other Saturnian and Jovian moons aren’t the only objects in space being eyed for their subsurface oceans. Pluto is now being suspected of hiding one of these subterranean worlds by researchers at NASA’s Goddard Space Flight Center, who recently published a study in Icarus about how trans-Neptunian objects (TNOs) like Pluto and its moon (because it’s still not a planet) could have formed and retained liquid water oceans through billions of years.

“These objects need to be considered as potential reservoirs of water and life,” said team lead Prabal Saxena. “If our study is correct, we now may have more places in our solar system that possess some of the critical elements for extraterrestrial life.”

So how could Pluto, which seems like a dead world to those of us who aren’t studying macro views of it in a lab, have a liquid ocean where extraterrestrial things may or may not be thriving? At -350 Fahrenheit, the un-planet’s surface is so freezing that even the formation of ice is warped into something amorphous. But. Pluto and its moons have densities similar to Enceladus and other objects thought to be hiding subsurface oceans, and there have been traces of crystalline water ice and ammonia hydrates found after an analysis of light reflected from some TNOs. Neither of these should survive on the surface for long. Space radiation breaks down ammonia hydrates and twists the crystalline structure of water ice into that amorphous form.

If you find something on the surface that shouldn’t be there, that probably means it emerged from somewhere else, and that somewhere else is what scientists believe to be a subterranean layer of liquid water which shot to the surface when a cryovolcano erupted. Cryovolcanoes spew a “lava” of frozen volatiles like water and ammonia. If such a thing had erupted on the doomed city of Pompeii, its victims would have been mummified in ice.

Could there really be an ocean–and possibly aliens–under this frozen, forbidding surface? Credit: NASA

If there really are liquid oceans on Pluto and other TNOs, how did they get there? Radioactive elements were incorporated into them as they formed billions of years ago. As anything radioactive decays, it releases heat, and that heat could have been enough to melt a layer of ice from the crust into a subsurface ocean that lasted so long as the heat from this phase decay could maintain it. There is still something problematic here. Radioactive elements will eventually decay into more stable ones, meaning there would have been no more heat to maintain this hypothetical ocean. Again, there’s a “but wait” here. Pluto is orbited by moons. Gravitational interactions with its moons could generate enough heat to keep it an ocean world beneath the surface.

Moons happen when massive collisions between celestial objects send material flying into space. Gravity from the parent object causes this debris to coalesce into a lesser object or objects, which then try to find the most stable orbit possible. The moon will then engage in what NASA calls a “gravitational dance” with its parent to ultimately achieve stability. A satellite reaches stability when it spins at a rate that keeps the same side facing the parent as it travels along a circular orbit aligned with the parent’s equator. As the moon and planet (or sort-of-planet) try to figure that out, gravitational attraction from each of them generates heat-releasing friction by stretching and relaxing their interiors. This is tidal heating—and the reason why there could really be an ocean beneath the frozen wastes of Pluto.

“Crucially, our study also suggests that tidal heating could make deeply buried oceans more accessible to future observations by moving them closer to the surface,” study co-author Joe Renaud explained. “If you have a liquid water layer, the additional heat from tidal heating would cause the next adjacent layer of ice to melt.”

Even liquid water on Pluto doesn’t automatically mean it’s crawling with alien microorganisms, though it could be with the right energy source and chemical balance. Martians are overrated. Plutonians just sound so much cooler.

(via NASA Goddard)