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This close-up of canyons on Charon was taken by NASA’s New Horizons probe during its flyby last July. The color-coded picture shows elevation data. Serenity Chasma’s depth can exceed 4 miles in some places. In comparison, the Grand Canyon’s maximum depth is a mere mile. (Credit: NASA / JHUAPL / SwRI)

Scientists say the patterns of ice in canyons on Charon, Pluto’s largest moon, look as if some the frozen water was once liquid. And that suggests Charon had a subsurface ocean in ancient times.

The evidence comes from close analysis of images and elevation data collected last July when NASA’s New Horizons probe zoomed past Pluto and its moons. Even before the flyby, scientists speculated that Charon may have harbored liquid water, and that some water may still flow beneath its icy surface. The stereo measurements from New Horizons’ Long Range Reconnaissance Orbiter, or LORRI, are consistent with that hypothesis.

The clues come in the form of stretch marks in the ice around Serenity Chasma, a canyon that’s 4.5 miles deep in some places.

“Charon’s tectonic landscape shows that, somehow, the moon expanded in its past, and – like Bruce Banner tearing his shirt as he becomes the Incredible Hulk – Charon’s surface fractured as it stretched,” the science team said in an image advisory on Thursday.

Such an expansion would have occurred as liquid water turned to ice. As anyone who has left a can of soda in the freezer knows, water (and Coke) expands when it freezes.

Pluto and its moons aren’t the likeliest places to find liquid water: The surface temperature is thought to be around 380 degrees below zero Fahrenheit. But New Horizons’ data already has delivered some intriguing observations of water ice on Pluto and Charon.

Pluto’s 11,000-foot-high mountains made of frozen H2O and ice volcanoes suggest there’s an active subsurface geology, capable of pushing up ice from below. Pluto and Charon, like Earth, probably had (and may still have) radioactive elements in their cores that keep the interior warm enough for water to flow.

Could liquid water, and perhaps even life, still exist beneath the surface there? That may be going too far, but if future explorers need to fill up their canteen on the way to Alpha Centauri, they know where to go.

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