Far distant Charon may have once had a large subsurface ocean

Charon's Serenity Chasma is highlighted, along with color-coded highlights to illustrate its extreme topography. (credit: NASA/JHUAPL/SwRI)

Pluto's moon Charon looks dark, foreboding, and dry in pictures taken by the New Horizons spacecraft last summer. A brownish feature near its pole appeared so ominous that scientists actually named it "Mordor." But after further analysis of Charon's fractured surface, scientists now think they've discovered evidence of an ancient, subsurface ocean.

Today, the gray outer layer of Charon is composed primarily of water ice. More than 4 billion years ago, however, even though Charon and Pluto were far from the Sun, decaying radioactive elements and other sources of internal heat would have kept them relatively warm. In fact, there probably would have been enough heat to melt water near Charon's core.

As Charon cooled over billions of years, this chthonic ocean would also have cooled, and then finally frozen, causing its volume to expand. Scientists looking at data collected by New Horizons believe this may explain a set of features they see on the surface of Charon today: a series of ridges, scarps, and valleys that sometimes plunge more than six kilometers deep.

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