What keeps Pluto’s ocean from freezing?

Explore humanity’s first moon landing through newly discovered and restored archival footage. CNN Films “Apollo 11” premieres Sunday, June 23 at 9 p.m. ET/PT.

When NASA’s New Horizons spacecraft flew past Pluto in 2015, researchers hoped that its data would help them unravel some of the dwarf planet’s mysteries. Instead, the discoveries made during the close-up look at Pluto and its moon Charon revealed more questions that needed answering.

One of the big revelations from the flyby was the discovery of an ocean beneath the icy shell encapsulating Pluto. The ice shell was thin in a spot near the equator that’s about the size of Texas, known as Sputnik Planitia, which helped researchers notice Pluto’s odd topography and suggest the ocean’s existence.

But this created a conundrum. Given the age of Pluto, estimated to be between 4.4 billion and 4.6 billion years, the ocean should have frozen hundreds of millions of years ago. And rather than forming a bubble, the ice shell should have flattened over the frozen ocean.

July 14: NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. See NASA release: https://www.nasa.gov/mission_pages/newhorizons/main/index.html

Related article Winter is coming to destroy Pluto's atmosphere by 2030, study says

But Pluto’s ocean isn’t frozen, so researchers used New Horizons data and computer simulations to model what might keep the ocean from freezing. Their study was published Monday in the journal Nature Geoscience.

The most likely scenario is gas between the ocean and the ice shell that acts like an insulating later. This layer of gas, probably methane that originated in Pluto’s rocky core when the dwarf planet was forming, would be thick and have low thermal conductivity.

Related article Ancient scars on Pluto and Charon reveal distant space objects

The researchers believe the layer exists in the form of gas hydrates, in which water molecules create lattice-like cages that trap gas molecules, according to the study.

And this insulating gas layer would have caused the ice shell to take more than 1 billion years to achieve uniform thickness.

The Mountainous Shoreline of Sputnik Planum: In this highest-resolution image from NASA's New Horizons spacecraft, great blocks of Pluto's water-ice crust appear jammed together in the informally named al-Idrisi mountains. "The mountains bordering Sputnik Planum are absolutely stunning at this resolution," said New Horizons science team member John Spencer of the Southwest Research Institute. "The new details revealed here, particularly the crumpled ridges in the rubbly material surrounding several of the mountains, reinforce our earlier impression that the mountains are huge ice blocks that have been jostled and tumbled and somehow transported to their present locations."

Related article Pluto has 'Earth-like characteristics,' study says

Pluto’s atmosphere is rich in nitrogen and low on methane. The gas layer between the ice shell and the ocean might explain where the methane is.

The implications extend to other icy ocean worlds in our solar system, like Jupiter’s moon Europa and Saturn’s moon Enceladus, as well as exoplanets. If insulating gas layers can keep subsurface oceans from freezing, there could be a wealth of ocean worlds out there, the researchers said.

“This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” said Shunichi Kamata, study author and associate professor in the Department of Earth and Planetary Sciences at Hokkaido University in Japan, in a statement.