In July 2015, NASA's New Horizons arrived at Pluto and spent three months investigating the dwarf planet, including its famous heart, a bright formation near the equator. Scientists have now determined there's likely a slushy, viscous ocean of water ice beneath this heart-shaped region, which is named Tombaugh Regio, according to a new paper published Wednesday in Nature.
A subsurface ocean could explain a mystery that's bothered scientists for some time: why Pluto's bright heart permanently points away from the planet's moon, Charon.
"Pluto and Charon are tidally locked to each other," MIT professor and New Horizons co-investigator Richard Binzel, who is an author of the paper, told me in an interview. The planet and its small moon are in a sort of eternal staring contest, with the same side of Charon always pointing at Pluto, and vice versa. But weirdly, the Tombaugh Regio region—and particularly a region called Sputnik Planitia, often called the heart's left ventricle—is "almost exactly opposite to Charon," Binzel said. "It's a perfection of placement."
That left ventricle, which seems to be a giant impact basin, is apparently filled with frozen nitrogen ice (which explains why it's extra-bright). The ice crust there seems to be thinner. "The viscous liquid from below [pushes] its way up to make a bulge," Binzel said, adding "a little extra mass there" that contributes to the planet's surprising alignment.
As for Pluto's ocean, scientists believe it's between 150 and 200 km beneath the surface, he said, with a water layer that "might be 100 km thick." It isn't known whether it's a global ocean, but it's possible, Binzel acknowledged.
Other bodies in our solar system are believed to harbour subsurface oceans. Geysers have been observed on Saturn's moon Enceladus, and water vapour plumes appear to spew from the surface of Jupiter's Europa. Why is there so much water everywhere? "The most common elements in the universe are hydrogen, helium, and oxygen," Binzel said. Helium is inert, leaving the other two—which are, of course, the compounds that make up water.
"H20 is a compound that the universe is very ready, willing, and able to form," he said.
That's good news for the search for life in other places, although of course we've no clue yet whether anything could live on Pluto: NASA considers water to be a key ingredient for life to form. Meanwhile, New Horizons is zooming off to the Kuiper Belt, a chaotic area beyond our solar system about which little is known.
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