Heavy heart may mask Pluto’s deep salty ocean

Reject planet Pluto continues to prove that it is one of the most interesting objects in the Solar System with news that it may have a salty ocean beneath its icy surface that is more than 100 km (60 miles) deep.

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with colour data from the Ralph instrument to create this enhanced color global view of Pluto. It clearly shows the heart-shaped feature, one lobe of which is thought to cover an underground salty ocean. Image Credit: NASA/JHUAPL/SwRI

That is the conclusion of planetary scientists who studied a striking heart-shaped feature on Pluto and deduced that it was partly caused by the impact of a giant asteroid 200 km (120 miles) or more wide.

It is now 14 months since NASA’s New Horizons probe flew by Pluto in July, 2015, giving us our first close-up views of this small world that was only discovered in 1930, and named by remarkable English girl Venetia Burney, and which we now know stands as gatekeeper to a zone of icy worlds called the Kuiper Belt.

After 76 years as the ninth planet, Pluto was unceremoniously downgraded to the new category of dwarf planet by the International Astronomical Union in 2006, just months after New Horizons had launched on its 5 billion km (3 billion mile), nine-year journey to pay a visit.

But however it is labelled, Pluto astonished scientists when the New Horizons flyby sent back a treasure trove of images and other data. They showed a showed a surprisingly varied and complex terrain. And as New Horizons approached and began to pick out detail, a bright plain came into view that was shaped like a heart. The left side, named Sputnik Planum, showed no sign of cratering across it, suggesting it formed relatively recently. It is filled with volatile nitrogen and carbon monoxide ices. (Other features have been named after favourite scifi series, including Star Trek and Doctor Who.)

A new study led by geologist Brandon Johnson, of Brown University, Rhode Island, has used computer modelling of impact dynamics to find out how Sputnik Planum was formed. It concludes that the western lobe of the heart shape, a 900 km (550 mile) wide basin called Sputnik Planum, was created by the impact of that huge asteroid, which is likely to have been another Kuiper Belt Object (KBO).

Johnson, an assistant professor in Brown’s Department of Earth, Environmental and Planetary Sciences, said in a statement about Pluto’s deep salty ocean: “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. We’ve been able to put some constraints on its thickness and get some clues about composition.”

Pluto and its largest moon, Charon, were found to be tidally locked – in other words they show each other the same face as they orbit around their common centre of gravity. Sputnik Planum sits exactly on the tidal axis that links Pluto and Charon, so that it always points towards that moon. This alignment suggested to the scientists that the impact basic has a positive mass anomaly, meaning that it has more mass than the rest of Pluto’s crust, on average.

Pluto and its largest moon Charon,left. They are tidally locked, with Sputnik Planum always facing Charon. Image credit: NASA-JHUAPL-SWRI

The puzzle for the researchers was that the impact of a giant cosmic missile might have been expected to be less massive than the rest of the crust. The nitrogen ice that has been found to fill the crater can only a little more of the mass that it has. The rest is thought to be generated by Pluto’s deep salty ocean.

Johnson commented: “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.”

The team’s computer modelling results, reported in the journal Geophysical Research Letters, suggests that water, which is denser than ice, may have welled up following the Sputnik Planum impact to form Pluto’s deep salty ocean.

Johnson said: “This scenario requires a liquid ocean. 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.”

The impact simulations were modelled to assume various depths of underground ocean. The one that best fitted the observed crater size and other conditions assumed an ocean that was more than 100 km (60 miles) deep and with a salinity of 30 per cent, or in other words as salty as the Dead Sea on Earth.

Johnson and his co-researchers – Timothy Bowling of the University of Chicago and Alexander Trowbridge and Andrew Freed from Purdue University – will continue to study data sent back by New Horizons to learn more about the likely ocean.

Johnson added: “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”

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