Pluto’s Heart May Throb With A Salty Ocean

This view of Pluto features its famous "heart," the left or western half of which was created by an ancient impact, offers clues about a possible subsurface ocean. Credit: NASA/APL/SwRI
This view of Pluto features its famous “heart,” the left or western half of which, called Sputnik Planum, was created by an ancient impact. The nitrogen ice covered plain offers clues about a possible subsurface ocean.
Credit: NASA/APL/SwRI

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.

This photo was taken by New Horizon on July 11, 2015 on approach to Pluto. The hemisphere facing Charon (at left)
This photo was taken by New Horizons on July 11, 2015 on approach to Pluto. Sputnik Planum, which faces Charon, neatly lines up along  the tidal axis connecting the two objects. Tides are tugs induced on both Pluto and Charon by their mutual gravitational attraction. Credit: NASA/APL/SwRI

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.

The close up shows the nitrogen ice-covered Sputnik Planum, the likely site of a giant impact in the distant past. The surface temperature is  -380 F (-229 C). Credit:
The close up shows the nitrogen ice-covered Sputnik Planum, the likely site of a giant impact in the distant past. The surface temperature here is about 380 below zero Fahrenheit (-229 C). A 62-mile deep ocean may exist beneath this region which spans 650 miles by 500 miles. Credit: NASA/APL/SwRI

“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.

These are possible models of the interiors of Pluto (left) and Charon. Credit:
These are possible models of the interiors of Pluto (left) and Charon. Credit:

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.

2 Responses

  1. Marko Tomovski

    this is extremely interesting, i am curious though how could you prove that there is water beneath the surface if we never have gone or landed on Pluto? And the ice that filled the basin is not heavy enough to tip the planet how would that conclude to there being water under the surface?

    1. astrobob

      Marko,
      We can’t know for absolute certain that water is there, but it makes the most sense given the alignment with Charon, the extra density of the Sputnik Planum region and the likelihood that it was created by a large impact. Water under the surface by itself wouldn’t be enough to tilt Pluto Charon-ward, but when you add the layer of nitrogen ice we see in Sputnik Planum, it works. Still, we won’t know for sure unless and until there are future missions to Pluto. In the meantime, scientists are making a case using the data at hand to explain Pluto’s behavior.

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