Looking into Pluto’s ocean using mathematical models and images from the New Horizons spacecraft

An ocean of liquid water deep beneath Pluto’s icy surface is coming into focus thanks to new calculations by Alex Nguyen, a graduate student in earth, environmental and planetary sciences in arts and sciences at Washington University in St. Louis.

In an article published in the magazine IcarusNguyen used mathematical models and images from the New Horizons spacecraft that passed by Pluto in 2015 to take a closer look at the ocean that likely covers the planet under a thick shell of nitrogen, methane and water ice.

Patrick McGovern of the Lunar and Planetary Institute in Houston co-authored the paper.

For decades, planetary scientists assumed that Pluto could not support an ocean. The surface temperature is about -220°C, a temperature so cold that even gases such as nitrogen and methane freeze solid. Water should not be given a chance.

“Pluto is a small body,” says Nguyen, who received his Ph.D. research at Washington University as an Olin Chancellor’s Fellow and a National Science Foundation Graduate Research Fellow. “It should have lost almost all of its heat shortly after it formed, so basic calculations would indicate that it is frozen to the core.”

But in recent years, leading scientists, including William B. McKinnon, professor of earth, environmental and planetary sciences in Arts & Sciences, have gathered evidence suggesting that Pluto likely contains an ocean of liquid water beneath its ice. That inference came from several lines of evidence, including Pluto’s cryovolcanoes that spew ice and water vapor. Although there is still some debate, “it is now generally accepted that Pluto has an ocean,” Nguyen said.

The new study examines the ocean in more detail, even if it’s far too deep beneath the ice for scientists to ever see. Nguyen and McGovern created mathematical models to explain the cracks and bulges in the ice covering Pluto’s Sputnik Platinum basin, the site of a meteor collision billions of years ago. Their calculations suggest that the ocean in this region is under a shell of water ice 40 to 80 km thick, a blanket of protection that likely prevents the inner ocean from freezing solid.

They also calculated the likely density or salinity of the ocean based on the breaks in the ice above it. They estimate that Pluto’s ocean is at most about 8% denser than Earth’s seawater, or about the same as Utah’s Great Salt Lake. If you could somehow reach Pluto’s ocean, you could float effortlessly.

As Nguyen explained, that density level would explain the abundance of fractures on the surface. If the ocean were significantly less dense, the ice shell would collapse, creating many more fractures than actually observed. If the ocean were much denser, there would be fewer fractures. “We estimate a sort of Goldilocks zone where the density and shell thickness are just right,” he said.

Space agencies have no plans to return to Pluto anytime soon, so many of its mysteries will remain for future generations of researchers. Whether it’s called a planet, an asteroid or just one of many objects in the outer reaches of the solar system, it’s worth studying, Nguyen said. “From my perspective, it’s a planet.”