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 four 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.

Because New Horizons took multiple views of Charon as it passed through the Pluto system, it is possible to measure these depths and heights via parallax. The largest of these gouges along the equatorial belt of Charon is now called Serenity Chasma. At a length of 1,800 kilometers, it runs four times longer and several times deeper than the Grand Canyon on Earth. The chasm's size is all the more remarkable because the moon is only about as big as Texas.