Images from NASA’s New Horizons mission suggest that Pluto’s moon Charon once had a subsurface ocean that has long since frozen and expanded, pushing outward and causing the moon’s surface to stretch and fracture on a massive scale, according to NASA.

The side of Pluto’s largest moon viewed by NASA’s passing New Horizons spacecraft in July 2015 is characterized by a system of “pull apart” tectonic faults, which are expressed as ridges, scarps and valleys—the latter sometimes reaching more than 4 miles (6.5 kilometers) deep. 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 outer layer of Charon is primarily water ice. This layer was kept warm when Charon was young, by heat provided by the decay of radioactive elements, as well as Charon’s own internal heat of formation. Scientists say Charon could have been warm enough to cause the water ice to melt deep down, creating a subsurface ocean. But as Charon cooled over time, this ocean would have frozen and expanded (as happens when water freezes), lifting the outermost layers of the moon and producing the massive chasms we see today, according to NASA. 

The top portion of this image shows part of the feature informally named Serenity Chasm, part of a vast equatorial belt of chasms on Charon. This system of faults and fractures runs at least 1,100 miles (about 1,800 kilometers) long and in places there are chasms 4.5 miles (7.5 kilometers) deep. By comparison, the Grand Canyon is 277 miles (446 kilometers) long and just over a mile (1.6 kilometers) deep.

The lower portion of the image shows color-coded topography of the same scene. Measurements of the shape of this feature tell scientists that Charon’s water ice layer may have been at least partially liquid in its early history, and has since refrozen.