Photos taken by a NASA spacecraft in July 2015 suggest that Pluto's biggest moon, Charon, once had a subsurface ocean.

Its outer layer could have been warm enough to cause the ice to melt deep down, creating a subsurface ocean.

But as the moon cooled over time, the ocean would have frozen and expanded, producing the chasms.

The side viewed by the probe has a system of tectonic faults, including ridges more than 6.5 km deep.

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