Scientists may have solved the mystery of how Pluto maintains a subsurface ocean, casting new light on why similar bodies of liquid may persist on other icy worlds throughout the solar system—and beyond.
In 2015, NASA’s New Horizons spacecraft made observations of Pluto, identifying a 1,000 kilometer (621 miles)-wide basin dubbed Sputnik Planitia, which indicated the likely presence of a subsurface ocean below an ice shell of varying thickness.
These observations have proved to be somewhat problematic for scientists. To maintain an ocean, Pluto would require some sort of mechanism to retain heat inside—such as a thick insulating layer of ice—to prevent the liquid from freezing.
However, to maintain the large variations in the thickness of its ice shell—as observed by New Horizons—the shell needs to be cold. If the ice shell is too warm, its viscosity would be low, eliminating any of the contrasts in thickness that are seen in different regions of the minor planet.
Now, though, a team of Japanese scientists say they have found a potential solution to this conundrum. In a study published by the journal Nature Geoscience, the researchers propose that a thin layer of ice containing trapped gas molecules at the bottom of the ice shell could insulate the ocean and shell from one another.
This layer is composed of what the authors describe as gas hydrates—molecules of gas in a lattice of water ice molecules. By calculating how Pluto’s temperature and ice shell thickness would have changed over time with such a layer, the team concluded that the gas hydrates would be sufficient to maintain both a subsurface ocean and thickness variation in the ice.
“We show that the presence of a thin layer of gas hydrates at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts,” the authors wrote in the study. “[Gas] hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile.”
The researchers suggest that the gas in the hydrate layer is most probably methane, which could have originated from the comet-like material that formed Pluto, chemical reactions in the minor planet’s rocky core, or a combination of both.
These results have important implications for our understanding of how other worlds maintain subsurface oceans, the authors say.
“The formation of a thin [gas] hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects,” the authors wrote.
“Liquid water oceans are thought to exist inside icy satellites of gas giants such as Europa and Enceladus and the icy dwarf planet Pluto,” they said. “Understanding the survival of subsurface oceans is of fundamental importance not only to planetary science but also to astrobiology.”