View of 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 view thanks to new calculations by Alex Nguyen, a doctoral student in earth, environmental and planetary 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 flew by Pluto in 2015 to take a closer look at the ocean that likely covers the planet under a thick blanket 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 like nitrogen and methane freeze solid. Water shouldn’t have a chance.

“Pluto is a small body,” said Nguyen, who is conducting his doctoral research at Washington University as an Olin Chancellor’s Fellow and National Science Foundation Graduate Research Fellow. “It may have lost almost all of its heat shortly after its formation, so simple calculations suggest that it was frozen to its core.”

But in recent years, prominent scientists, including William B. McKinnon, professor of earth, environmental and planetary sciences in the Faculty of Arts and Sciences, have gathered evidence suggesting that Pluto likely has an ocean of liquid water beneath the ice contains. This conclusion was based on several lines of evidence, including Pluto’s cryovolcanoes, which 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 takes a closer look at the ocean, even though it lies far too deep beneath the ice for scientists to ever see. Nguyen and McGovern created mathematical models to explain the cracks and dents in the ice that covers Pluto’s Sputnik Platina basin, where a meteor struck billions of years ago. Their calculations suggest that the ocean in this area lies beneath a 40 to 80 km thick layer of water ice, a protective layer that likely prevents the inner ocean from completely freezing over.

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

As Nguyen explained, this density would explain the frequency of fractures on the surface. If the ocean were significantly less dense, the ice shell would collapse, causing many more fractures than are actually observed. If the ocean were much denser, there would be fewer breaks. “We estimated a kind of Goldilocks zone where density and shell thickness are just right,” he said.

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