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Pluto’s subsurface ocean may date back to planet’s formation

Pluto as imaged by New Horizons. Image Credit: NASA/JHUAPL/SwRI

Scientists who recently studied the detailed images of Pluto’s surface returned by NASA’s New Horizons spacecraft in July 2015 have found evidence that the dwarf planet’s subsurface ocean may have been around since Pluto’s formation over four billion years ago.

Pluto has long been thought to have begun its life as a cold icy rock far from the Sun. Its underground liquid ocean was believed to have been created significantly later through heat generated by the radioactive decay of rock, which melted part of its icy shell.

But now, new evidence in the form of Pluto’s varied surface features suggests it formed as a hot world with a liquid ocean beneath its surface. That heat may have been the product of violent impacts on proto-Pluto, combined with decay of radioactive elements in its rocks. Every impacting object would have generated more heat through the release of gravitational energy.

If Pluto had begun its life cold and subsequently experienced gradual heating from the radioactive decay of internal rock, its surface would show evidence of compression because water contracts as it melts. On the other hand, if Pluto had started out hot, part of its subsurface ocean would have frozen over time, producing visible features created by expansion of its surface.

Analysis of New Horizons images shows only the latter, extensional features such as a system of ridges and troughs, and cracks in its icy shell, noted Carver Bierson, a graduate student at the University of California Santa Cruz (UC), who led the study.

Arrows indicate extensional faults on Pluto’s surface. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

“We see lots of evidence of expansion, but we don’t see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean,” Bierson said.

“For a long time, people have thought about the thermal evolution of Pluto and the ability of an ocean to survive to the present day,” explained Frances Nimmo, also of UC Santa Cruz. “Now that we have images of Pluto’s surface from NASA’s New Horizons mission, we can compare what we see with the predictions of different thermal evolution models.”

Both ancient and recent extensional features are visible in surface images, she added.

Because of the speed at which New Horizons was traveling during the flyby, its instruments could photograph only about half of its northern hemisphere in high resolution. There could be compression features in the areas that weren’t imaged, but that will remain a mystery until a second spacecraft is sent to map the rest of the planet.

A hot start would produce a liquid underground ocean from the very beginning, possibly rendering Pluto habitable for microbial life.

Today, Pluto has a surface temperature of minus 380 degrees Fahrenheit (minus 228 degrees Celsius) or 45 degrees Kelvin, making it one of the solar system’s coldest worlds. Bierson noted Pluto’s geology acts as a record of its history, confirming it formed violently enough to generate sufficient heat to warm its interior and create the subsurface ocean.

Also key to whether Pluto began its life cold or hot is the rate at which it formed. If Pluto formed in a period of less than 30,000 years, it probably started out hot. However, if it took several million years to form through accretion, the only way it could have had a hot start is if the energy of large impacting objects is buried underneath its surface.

“How Pluto was put together in the first place matters a lot for its thermal evolution. If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough, the heat gets trapped inside,” Nimmo stated.

A hot start means Pluto could have been habitable for microbial life for a long time. It also raises the question as to whether other Kuiper Belt dwarf planets might also have subsurface oceans that are just as old.

“Even in this cold environment so far from the Sun, all these worlds might have formed fast and hot, with liquid oceans,” Bierson said.

A paper about the study has been published in the journal Nature Geoscience.

More of Pluto’s extensional faults are visible here. Image Credit: NASA/JHUAPL/SwRI



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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