Pluto’s icy surface could be hiding a 100km-deep ocean of liquid water, according to analysis aided by NASA’s New Horizons space craft.
Geologists were able to predict the ocean’s depth after investigating the impact dynamics of a huge crater photographed by the probe.
The crater originally courted scientists’ attention because it sits on the axis linking Pluto with its largest moon Charon.
The pair are tidally locked together, meaning the 900km-wide Sputnik Planum constantly faces the same point on Charon as they rotate through space.
That the crater directly faces the moon suggested it had been pulled into its axis because it had an unusually high mass.
But why a hollowed out crater was heavier than the rest of the dwarf planet’s surface perplexed scientists.
Brown University geologist Brandon Johnson said: “An impact crater is basically a hole in the ground.
“You’re taking a bunch of material and blasting it out, so you expect it to have negative mass anomaly, but that’s not what we see with Sputnik Planum. That got people thinking about how you could get this positive mass anomaly.”
When an asteroid smashes into a planet, it causes a dent before the planet rebounds. As it bounces back, material from deep inside the planet is pulled towards the surface. But only when the deeper material is denser than the impacted material, does the crater end up with the same mass as before.
In Pluto’s case, scientists presumed this would only have happened if there was water beneath the icy crust as water is denser than ice.
After the impact, a layer of nitrogen ice settled on the surface of the crater. This contributed to the crater’s greater mass.
“This scenario requires a liquid ocean,” Johnson said. “We wanted to run computer models of the impact to see if this is something that would actually happen. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It’s also sensitive to how salty the ocean is, because the salt content affects the density of the water.”
Scientists simulated a number of models to fit the crater’s observed size, before concluding that it was most likely located above an ocean layer 100km thick, with a salinity of around 30% - equivalent to the Dead Sea.
“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 kilometers has to be there,” Johnson said. “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”
The study was published in the journal Geophysical Research Letters.