Pluto's moon Charon may have expanded when a sub-surface ocean froze, according to NASA. Images recorded by the New Horizons probe's flyby show "pull apart" faults on Charon's surface, suggesting the moon expanded in its past, fracturing the surface.

Charon's outer layer is mostly water ice. When the planet was younger, the water was probably kept liquid by heat from radioactive elements and Charon itself, as it formed. If Charon was warm enough to cause water to melt below the surface, it may have created an ocean. But as the space rock cooled, the ocean would freeze — and expand. That would account for the chasms observed by the flyby, some of which are more than 4 miles (6.5 kilometers) deep. That's four times the depth of the Grand Canyon.

NASA/JHUAPL/SwRI

The photo above shows the Serenity Chasma, part of a belt that encircles Charon's equator. The system it belongs to runs for at least 1,100 miles (about 1,800 kilometers) — one of the longest such chasm belts in the solar system, NASA says. Measurements of the Serenity Chasma hinted to scientists that Charon once had at least partially liquid water, which then refroze.

Correction: A previous version referred to New Horizons as an orbiter in the second paragraph; it was a flyby.