Planetary Sciences News

Ancient Impact’s Seismic Waves Reveal Pluto’s Ocean, Core

By modeling the waves produced by a massive, ancient impact, scientists have begun to unlock the secrets of Pluto’s interior.

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How do you study the interior of a dwarf planet 5 billion kilometers away? You use an ancient impact.

Millions of years ago, a huge asteroid struck Pluto, creating the landscape we know as Sputnik Planitia, a feature that makes up half of Pluto’s “heart.” A team of scientists now have re-created those ancient waves to study Pluto’s internal structure. They found that Pluto might have a substantially thick internal ocean and a core that hints at an environment potentially habitable to life.

“Once you go even a little bit beneath the surface of Pluto, there’s just a lot of question marks,” Adeene Denton, a planetary scientist at Purdue University in West Lafayette, Ind., told Eos. “And the work that I did was trying to remove a few of those question marks.” Denton was scheduled to present the research at the canceled Lunar and Planetary Science Conference on 17 March.

In July 2015, NASA’s New Horizons spacecraft unveiled Pluto to us in unprecedented detail as it zoomed by the dwarf planet at 14 kilometers per second (that’s about 31,000 miles per hour). Scientists discovered a geologically active surface covered in ice mountains, smooth plains, and oozing, nitrogen ice glaciers. There’s even a thin, hazy atmosphere made up of nitrogen, carbon dioxide, and methane.

Satellite image of Pluto and its telltale heart
NASA’s New Horizons spacecraft captured this image of Pluto as it whizzed by on 14 July 2015. Sputnik Planitia, a vast region of ice that makes up part of Pluto’s heart, is thought to be an ancient impact basin. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Scientists are also fairly certain there’s an ocean sloshing around beneath Pluto’s icy surface, but the details of that ocean remain a mystery. How deep is the ocean? What kind of rock makes up its floor? Could that ocean be interacting with its rocky floor in a way that produces chemicals necessary to sustain life? These are the questions Denton set out to answer.

Pluto’s farside, which New Horizons saw from a distance as it approached the dwarf planet, inspired Denton to investigate further. At almost exactly opposite Sputnik Planitia (at Sputnik’s “antipode”), other scientists had spotted geological features that resemble grabens on Earth. Grabens are extensional rifts that form as a continental plate is split and pulled apart by faults. They look like a series of high plateaus and valleys, striping the surface.

“Curiously, a lot of the weirdest stuff on Pluto’s far side is close to the antipode of Sputnik Planitia,” James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory who has studied these antipodal features, said in an email. “When these seismic waves collide on the opposite side of the body, they can amplify and result in intense surface deformation.”

Denton wondered whether Pluto’s internal structure (namely, its core and subsurface ocean) acted as a lens to focus the impact waves to deform the opposite surface. On Earth, scientists use instruments that measure seismic waves—waves of energy produced by large movements or breaking of rock, either on Earth’s surface or below—to study our planet’s interior. These waves travel faster or slower through different kinds of material, and studying them helped us understand the structure of Earth.

Black-and-white satellite image of Pluto’s far side
This image of Pluto’s farside was captured by NASA’s New Horizons spacecraft on 11 July 2015, when New Horizons was still 4 million kilometers (2.5 million miles) away from Pluto. Credit: NASA/JHUAPL/SWRI

But we can’t send seismometers to Pluto, so Denton decided to study the waves of the ancient impact that created Sputnik Planitia. Using a computer program that models impacts, Denton tweaked the interior properties of Pluto to see how different combinations of core composition and ocean thickness affected seismic wave propagation.

What Denton found was that a core made primarily of serpentine and a 150-kilometer-thick ocean could act to focus those waves strongly enough to rupture Pluto’s surface antipodal to the impact. Other compositions, like a dunnite core or a thicker or thinner ocean, didn’t focus the waves in the right way, Denton said.

Finding a serpentine core could lead to some exciting new questions. Serpentine is a mineral that forms when seawater reacts with rock. On Earth, these reactions occur deep in the ocean, between seawater and exposed mantle rock, which is primarily made of olivine and pyroxene. When the rock and seawater react, serpentine forms, and those reactions release heat and molecules useful to life. In our oceans, bacteria colonize places where serpentinization occurs, such as in the Lost City hydrothermal field in the northern Atlantic Ocean. Some scientists propose that serpentinization could have occurred—or could be an active process—on other ocean worlds, like Saturn’s moon Enceladus or Jupiter’s moon Europa.

If Pluto does have a serpentine core and a substantial subsurface ocean, then “theoretically, you have an environment that might be conducive to life in some capacity,” Denton said.

The new research “sets up a lot of new testable hypotheses for a future Pluto mission, which NASA is actively talking about,” Keane said.

And the research has implications that stretch farther than to just Pluto. “There are a lot more Kuiper Belt objects out there than just Pluto, and if Pluto is geologically active and hosts a potentially habitable location in its interior, that really opens up what’s possible for Kuiper Belt objects in general,” Denton said.

—JoAnna Wendel (@JoAnnaScience), Freelance Writer

Citation: Wendel, J. (2020), Ancient impact’s seismic waves reveal Pluto’s ocean, core, Eos, 101, https://doi.org/10.1029/2020EO142337. Published on 06 April 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
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