One of the most striking features of Pluto is its heart, also known as the Tombaugh Regio. Its left lobe, the Sputnik Planitia, is a smooth, young region as wide as Texas and some researchers believe it might hide a liquid ocean underneath its icy surface.
Just how such an ocean could exist is the focus of a new paper published in Nature Geoscience. The origin and age of Sputnik Planitia is still a matter of debate. It might have formed due to a dramatic asteroid impact. Its mostly craterless surface indicates that it could be as young as just a few hundred thousand years old, but its eroded rims suggest an age of billions of years. The young surface is not the only piece of evidence suggesting an ocean. There's also a gravitational anomaly that could be explained with the subsurface ocean hypothesis.
If the basin actually is ancient and an ocean formed in the crater, then it should have frozen completely hundreds of millions of years ago. Researchers have now simulated how a liquid ocean might persist in such an environment, and they think the solution is in a gas layer.
In their model, a layer of clathrate hydrates might be key. These hydrates are gaseous molecules, such as methane, trapped in a molecular water cage. They are very viscous and have very low thermal conductivity, making them an ideal substance to separate the ocean from the icy surface.
Thanks to the hydrates, the liquid ocean would be insulated and can maintain the heat it had at its formation. The surface too could slowly change. Without the layer, a thick uniform crust would have formed in about 1 million years. With the insulating layer of clathrate hydrates instead, the evolution is much longer, expected to exceed 1 billion years.
The researchers suggest that the possible component of the gas layer is methane originating from the core of the dwarf planet. This is consistent with the general composition of the Plutonian atmosphere, which is nitrogen-rich and methane-poor.
While it doesn’t fully explain the observations seen on Sputnik Planitia, the simulation supports the plausibility of a sub-surface ocean. And, interestingly, not just on Pluto. The same mechanism could work on large enough but minimally heated objects such as icy moons and dwarf planets.
“This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible,” lead author Shunichi Kamata, from Hokkaido University said in a statement.
All the detailed knowledge we have of Pluto is thanks to NASA’s New Horizons mission, which flew past the dwarf planet in 2015. The spacecraft passed its second target, MU69, a few months ago and we're now receiving data from this second observation.