Pluto may contain an ocean spanning over 100 kilometres beneath its icy surface, with a salt content similar to that of the Dead Sea on Earth, a new study suggests. Ever since NASA’s New Horizons spacecraft flew by Pluto last year, evidence has been mounting that the dwarf planet may have a liquid ocean beneath its icy shell.
Now, by modelling the impact dynamics that created a massive crater on Pluto’s surface, researchers led by geologist Brandon Johnson from Brown University in the US have made a new estimate of how thick that liquid layer might be.
The study finds a high likelihood that there is more than 100 kilometres of liquid water beneath Pluto’s surface. The research also offers a clue about the composition of that ocean, suggesting that it likely has a salt content similar to that of the Dead Sea.
“Thermal models of Pluto’s interior and tectonic evidence found on the surface suggest that an ocean may exist, but it’s not easy to infer its size or anything else about it,” said Johnson.
The research focused on Sputnik Planum, a basin 900 kilometres across that makes up the western lobe the famous heart-shaped feature unveiled during the New Horizons flyby.
The basin appears to have been created by an impact, likely by an object 200 kilometres across or larger.
The story of how the basin relates to Pluto’s putative ocean starts with its position on the planet relative to Pluto’s largest moon, Charon.
As Charon’s gravity pulls on Pluto, it would pull proportionally more on areas of higher mass, which would tilt the planet until Sputnik Planum became aligned with the tidal axis.
“An impact crater is basically a hole in the ground,” Johnson said.
Part of the answer is that, after it formed, the basin has been partially filled in by nitrogen ice. That ice layer adds some mass to the basin, but it is not thick enough on its own to make Sputnik Planum have positive mass, Johnson said.
The rest of that mass may be generated by a liquid lurking beneath the surface.
Water is denser than ice. If there were a layer of liquid water beneath Pluto’s ice shell, it may have welled up following the Sputnik Planum impact, evening out the crater’s mass.
If the basin started out with neutral mass, then the nitrogen layer deposited later would be enough to create a positive mass anomaly.
“This scenario requires a liquid ocean,” Johnson said.
The scenario that best reconstructed Sputnik Planum’s observed size depth, while also producing a crater with compensated mass, was one in which Pluto has an ocean layer more than 100 kilometres thick, with a salinity of around 30 per cent.
“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly – and it appears as though it is – this ocean layer of at least 100 kilometres has to be there,” Johnson said.