Gas layer may be keeping Pluto’s subsurface ocean from freezing

Laurel Kornfeld

A layer of gas located beneath Pluto’s icy shell could be keeping the dwarf planet’s subsurface ocean from freezing, according to a new study published in the journal Nature Geoscience.

Researchers led by Shunichi Kamata of Japan’s Hokkaido University used a combination of data collected by NASA’s New Horizons spacecraft during its July 2015 Pluto flyby and computer simulations to determine how a small world so far from the Sun maintains a liquid underground ocean.

New Horizons found the ice on Sputnik Planitia—the left side of Pluto’s “heart” feature located near its equator—to be particularly thin. This led scientists to conclude the thin ice layer sits on top of a liquid ocean beneath Pluto’s surface.

However, unlike Jupiter’s moon Europa, Saturn’s moon Enceladus, and other solar system worlds known to harbor subsurface oceans, Pluto does not experience tidal forces from a large parent planet to keep its underground water warm. According to scientists, heating produced by the radioactive decay of elements is insufficient to have prevented a total freezing of the ocean in the approximately 4.6 billion years since Pluto’s formation.

Pluto’s ocean should have frozen several hundred million years ago beneath a flattened surface ice shell, yet neither of those things happened. Instead of flattening, the ice shell formed a bubble beneath Sputnik Planitia.

Kamata and his research team hypothesize a layer of gas beneath Pluto’s surface ice shell is insulating the ocean beneath it, preventing it from freezing. This gas takes the form of gas hydrates, crystalline solids composed of water and gas that look and act like ice but contain high levels of methane.

New Horizons found only low levels of methane in Pluto’s extremely thin atmosphere, which is composed largely of nitrogen. One explanation for this is that the methane is trapped in the subsurface gas layer.

Water molecules within gas hydrates trap gas molecules and could be responsible for Pluto’s ice shell having attained uniform thickness over more than a billion years. The process keeps heat within Pluto’s interior from escaping by slowing the ice shell’s ability to remove that heat from the water.

The same mechanism could preserve subsurface oceans on other solar system worlds and on exoplanets that do not experience tidal heating from a parent planet.

“This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” Kamata said.

Further exploration of this possibility could be done by studying meteorites and comets, Kamata said. While no return mission to Pluto is currently underway, New Horizons principal investigator Alan Stern has proposed a followup orbiter mission, which could potentially confirm the gas hydrates’ presence.