Pluto may have a vast ocean beneath its frozen crust, according to researchers studying a heart-shaped feature on the dwarf planet’s surface.
Captured in high resolution by the New Horizon’s spacecraft in 2015, the bright, heart-shaped expanse is known as Tombaugh Regio, the left lobe of which is formed by the 1,000-kilometre-wide Sputnik Planitia. This plain is thought to have arisen after a meteorite slammed into Pluto’s surface billions of years ago, producing a crater that later filled with nitrogen ice.
But scientists have been puzzled as to why Sputnik Planitia lies near Pluto’s equator and faces almost directly away from Pluto’s largest moon, Charon, pointing out there is only a 5% likelihood that it could have ended up in such a position by chance.
Now, in a pair of papers published in the journal Nature, two teams of researchers say they have the answer: Pluto tipped over , potentially aided by a vast ocean beneath its icy surface.
“Roughly speaking it has got the same volume as the Earth’s oceans, because it is a lot deeper – it might be 100 km deep – but Pluto is smaller,” said Francis Nimmo from the University of California, Santa Cruz, and a co-author of one of the studies, adding that the ocean likely stretches all the way around Pluto.
According to Nimmo, the meteorite impact on Pluto ejected a huge quantity of water ice, leaving a crater with only a thin layer of ice at its bottom. If an ocean existed beneath this layer, possibly kept liquid – or at least slushy- by the presence of ammonia, it would push upwards against the remaining thin crust, creating a bulge. And as water is denser than ice, this bulge would compensate for the mass of the ejected material.
As frozen nitrogen built up in the basin over time, adds Nimmo, the mass of the region would become greater than it was before the impact, causing Pluto to tip over.
“There are two things that are happening,” said Nimmo. “Pluto’s rotation is pulling that extra mass towards the equator and the gravitational effect of Charon is to push it towards either the point directly underneath Charon or the point directly opposite [it], whichever is closer.” The result is that Sputnik Planitia ended up in its current location, near Pluto’s equator and facing away from Charon.
Without the influence of an ocean the research suggests the nitrogen ice would need to be more than 40km thick before the change in mass would be enough to trigger Pluto’s reorientation – a scenario the authors describe as implausible.
The second study, from researchers in the US and Japan, also suggests that Pluto tipped over, revealing that the pattern of cracks in Pluto’s ice crust fits with its reorientation, together with a slow freezing of an ocean beneath its surface.
But, they add, there could be several explanations as to exactly why the planet tipped, besides the idea of a bulge of water. Among the possibilities is that material ejected by the meteorite impact might have simply fallen around the edge of the crater, in effect cancelling out the presence of the hole, with later build-up of nitrogen ice adding enough mass for Pluto to tip.
And the dwarf planet may yet be on the move. According to James Keane of the University of Arizona, a co-author of the second study, Sputnik Planitia might still be accumulating nitrogen ice, with seasonal variations potentially causing Pluto to wobble. “I think that Pluto has astonished almost everyone, even the New Horizons team and even those who have been studying Pluto for the last several decades, with how geologically active [it] is,” he said.
Wesley Fraser, a planetary astronomer from Queen’s University Belfast who was not involved in the research, pointed out that it is not the first time a subsurface ocean on Pluto has been mooted, but said that the new studies were convincing. “All in all actually I think the idea is perfectly sound, it kinda solves a weird thing,” he said.