Remember Pluto’s big, white, icy heart? A new study, led by Brown University geologist Brandon Johnson, says there’s a good chance that beneath that ice, there’s 62 miles (100 km) of liquid water. And it’s super salty, similar to the Dead Sea.
The research focused on Sputnik Planum, a basin 560 miles (900 km) across that makes up the western half of the famous heart-shaped feature that appears in so many photos taken during the New Horizons flyby in July 2015. The distinctive, curved basin appears to have been created by the impact of an object at least 125 miles across.
The story of how the basin relates to a potential Plutonian ocean starts with the basin’s position on the planet relative to Pluto’s largest moon, Charon. Pluto and Charon are tidally locked with each other, so that they always show each other the same face. One hemisphere of the Pluto faces Charon 24/7 while the opposite hemisphere doesn’t see the moon at all!
Sputnik Planum sits directly on the tidal axis, a line connecting each object’s mutual tug on the other, linking the two worlds. That position suggests that the basin has what’s called a positive mass anomaly — it has more mass than average for Pluto’s icy crust. As Charon’s gravity pulls on Pluto, it would pull more on areas of higher mass, which would tilt the planet until Sputnik Planum became aligned with the tidal axis.
But there’s something not quite right here, since the huge impact that dug out the basin excavated what is now Sputnik Planum would remove lots of material from the region, decreasing the amount of matter there and leading to just the opposite — a negative anomaly and no tilt in that direction.
“An impact crater is basically a hole in the ground,” said Brandon Johnson, Brown University geologist and lead author of a new paper on the topic. “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.”
New Horizons discovered that the bright, white surface is primarily nitrogen ice. At the chill temperatures found on Pluto’s surface, nitrogen ice is mobile. It moves like glaciers move. After the impact, some of that ice partially filled the basin, but it’s not thick enough to tip the planet in the direction of Charon. That’s why Johnson and his team think that liquid water must lurk beneath the surface.
A large impact from a comet or asteroid blasts away crust on a planet to carve a crater but also compresses rock and other materials, followed by a rebound. With Pluto, that rebound could have pulled material upward from deep in the planet’s interior. If denser than what was blasted away by the impact, the crater would end up with about as much mass after as before the cataclysm.
Now if there were a layer of liquid water deep under Pluto’s skin, it could have welled up following the Sputnik Planum smackdown, evening out the crater’s mass. Recall that water is denser than ice. It plus the nitrogen ice, deposited later, would be enough to seal the deal and make Sputnik Planum a positive anomaly. So long as you don’t forget to pass the salt.
Salt content affects the density of ocean water. The computer models simulated the impact of an object large enough to create a basin of Sputnik Planum’s size hitting Pluto at a speed expected for that part in the solar system. The simulation assumed various thicknesses of the water layer beneath the crust, from no water at all to a layer 200 kilometers thick. The one that fit best had an ocean layer more than 62 miles (100 km) thick with a 30% salt content.
Amazing, isn’t it, that 3.7 billion miles from the sun, there could be a salty, dark ocean rumbling around in Pluto’s belly.