New Horizons Finds Clues of an Ocean on Pluto

Simulations of Pluto's geological evolution suggest more evidence that a liquid ocean may lie beneath the dwarf planet's surprisingly young surface.

There's long been speculation that there may be an ocean hiding below Pluto's frozen surface, but until now observational evidence has been inconclusive. However, as we continue to stare in awe at the ever-sharpening dwarf planet's complex landscape, some tantalizing clues as to what lies beneath are beginning to present themselves.

Probably the most profound discovery by NASA's New Horizons mission, which buzzed Pluto on July 14, 2015, is that the world has an active geology. This is a surprise; Pluto orbits the sun around 40 times times further away than the Earth orbits the sun. Shouldn't it be a barren, frozen wasteland? Apparently not. Pluto has complex geology, sporting a surprisingly young surface composed of a rich assortment of chemicals, including water, ammonia and methane ices.

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The planet's apparent youth is caused by a complex interplay between the surface ice sublimating into the world's thin atmosphere (which, by the way, may even support clouds) and internal motions of ice that act like a lava lamp, slowly cycling chemicals from below.

This dynamic behavior seems odd and it could be a strong indicator that Pluto has an ocean -- albeit a very alien one. Now, in a paper published by the journal Geophysical Research Letters, planetary scientists have carried out numerical simulations of Pluto's geological evolution and their conclusions are fascinating.

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"Thanks to the incredible data returned by New Horizons, we were able to observe tectonic features on Pluto's surface, update our thermal evolution model with new data and infer that Pluto most likely has a subsurface ocean today," said lead author and graduate student Noah Hammond, of Brown University, in a statement. "What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion.

"A subsurface ocean that was slowly freezing over would cause this kind of expansion."

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As water freezes, it expands -- a reason why you should never put a glass bottle of water in the freezer; it would shatter as the water freezes. As water ice expands, it also becomes buoyant (re: icebergs or the ice cubes floating in your G&T).

OK, so there's features that show Pluto has expanded, but who's to say that the dwarf planet did have a subsurface ocean, which has since frozen long ago? Well, after freezing, the frozen subsurface ocean would experience extreme pressure and increasingly cold conditions as Pluto's internal heat is radiated into space. What would then form is a phase of ice that we don't experience in everyday life here on Earth. Known as "ice II," this phase of water ice can only be produced in the laboratory, but is thought to exist in the icy moons of the outer solar system such as Jupiter's Ganymede. In this phase, water molecules form a crystalline structure that cause ice II's volume to contract and its density to increase. It is no longer buoyant.

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Therefore, if Pluto's subsurface ocean froze long ago in the solar system's history, Hammond's simulations show that New Horizons should have seen ancient compression features during its flyby, such as the wrinkles and mare ridges found on the surface of Earth's moon -- features created by contraction after the moon cooled. In fact, the opposite is true; Pluto's landscape is filled with geological features that suggest expansion. This only adds to the hypothesis that Pluto's strikingly dynamic surface is a sign that there's a liquid water ocean below covered with a crust of ice.

"In our paper, we look at tectonic features on the surface of Pluto to understand the interior and we run thermal evolution models to help us understand how Pluto's interior may have evolved over time," said Hammond. "Our study further supports that hypothesis by showing that if the ocean froze, ice II would likely form, causing compressional tectonic features which are absent from the surface."

"Many people thought that Pluto would be geologically 'dead,' that it would be covered in craters and have an ancient surface," added coauthor Amy C. Barr, of the Planetary Science Institute (PSI). "Our work shows how even Pluto, at the edge of the solar system, with very little energy, can have tectonics."

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These findings are incredible and New Horizons is showing us that even without the constant heating from solar radiation that the small planetary bodies in the once-considered frozen badlands of the solar system can actually support their own internally-heated oceans of liquid water and other volatiles. And Pluto is showcasing just how diverse the worlds beyond Neptune can be.

Source: Planetary Science Institute, Brown University