Scientists have their first evidence that Pluto, located some 4 billion miles from the sun, has an underground ocean made of water and slushy ice.
The sea is believed to be buried 93 to 124 miles beneath Pluto's giant impact basin, known as Sputnik Planitia, which lies within the heart-shaped Tombaugh Regio.
The ocean is estimated to be 62 miles deep, said planetary scientist Francis Nimmo, with the University of California, Santa Cruz.
Evidence for the ocean, reported in a pair of papers in this week's Nature, comes from images and analysis of data collected by NASA's New Horizons spacecraft, which flew past Pluto, its primary moon Charon and four smaller moons in July 2015.
Scientists were trying to figure out how Sputnik Planitia formed and why it is located in its present position, roughly 18 degrees north of Pluto's equator.
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The analysis suggests that billions of years ago Pluto was hit by a comet or Kuiper Belt object, forming a basin on the surface. The depression then filled with nitrogen ice, which caused the dwarf planet to roll over, stressing and cracking the crust in the process. Eventually the faults caused Pluto's mountains and canyons to form.
Computer models show the best explanation for how this occurred is if Pluto has an underground layer of slushy water pushing up against the thinned crust.
"If you put an extra weight on the surface of Pluto, Pluto will roll over to place the weight on the equator at a point directly opposite Charon. That point is very close to the location of Sputnik Planitia. So Sputnik Planitia must represent an extra weight - even though it's a hole in the ground," Nimmo wrote in an email to Seeker.
"This type of gravity anomaly with a thin crust and this extra mass of dense slushy ocean pushing up seems to be the most efficient explanation," added Massachusetts Institute of Technology planetary scientist Richard Binzel.
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"We know there is water ice present at Pluto. We see it frozen on the surface. We see giant icebergs the size of mountains that have to be lifted up somehow, and know that water and water ice have to be active on Pluto. This is what gives us the idea of a slushy layer," Binzel told Seeker.
Pluto, which formed along with the rest of the solar system about 4.6 billion years ago, has enough internal heat left from the decay of naturally occurring radioactive materials to liquefy frozen water, the studies show.
The existence of a watery ocean raises the prospect of life, but Binzel said it is unlikely to be found on Pluto. The ocean is too deeply buried and not nearly as active as, for example, the global ocean that exists on Jupiter's moon Europa or Saturn's Enceladus.
Rather, the significance of an ocean on Pluto is really "water, water everywhere," Binzel said. "Every time we learn something new about a process in nature it helps us revisit other worlds and even how we understand how the Earth works.
Pluto's slow re-orientation may still be underway. The studies show that Sputnik Planitia eventually may be located directly on Pluto's equator.