Evidence is mounting for an ocean at Pluto, buried beneath its frozen heart.
Scientists on Wednesday said that Pluto may have rolled over on its axis eons ago, the result of tidal forces with jumbo moon Charon.
The extra weight of an underground sea is the most likely explanation, they said.
These latest findings are based on observations by NASA's New Horizons, which made an unprecedented flyby of Pluto last year.
The spacecraft is now 365 million miles from Pluto and en route to a 2019 close approach of another faraway orb.
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Published in this week's journal Nature, the studies focus on a 600-mile basin in the left lobe of Pluto's heart-shaped region.
This basin is known as Sputnik Planitia, named after the Russian satellite that launched the Space Age in 1957.
Sputnik Planitia is aligned with Pluto's tidal axis, so much so that it's unlikely to be coincidence, according to the researchers. More likely, the nitrogen ice-coated basin has extra mass - below the surface - to cause Pluto to reorient itself and have Sputnik Planitia on the opposite side of the dwarf planet as Charon.
"It's a big elliptical hole in the ground, so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that," lead author Francis Nimmo of the University of California, Santa Cruz, said in a statement.
Nimmo suspects the ocean is primarily water with some ammonia or other "antifreeze" thrown in. Slow refreezing of this ocean would conceivably crack the planet's shell - a scenario consistent with photos taken by New Horizons.
Subsurface oceans may also be on other similarly sized worlds orbiting in the Kuiper Belt, a so-called twilight zone on the fringes of our solar system, according to Nimmo.
"They may be equally interesting, not just frozen snowballs," he noted.
