Washington, Feb 20 (IANS): Pluto’s largest moon, Charon, once had a sub-surface ocean that has long since frozen and expanded - pushing outward and causing the moon’s surface to stretch and fracture on a massive scale, new images from NASA’s New Horizons mission have revealed.

Charon is characterised by a system of “pull apart” tectonic faults, which are expressed as ridges, scarps and valleys - the latter sometimes reaching more than 6 km deep.

Charon’s tectonic landscape shows that somehow, the moon expanded in its past and 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,” the US space agency said in a statement.

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 - lifting the outermost layers of the moon and producing the massive chasms we see today.

The top portion of this image shows part of the feature informally named Serenity Chasma, part of a vast equatorial belt of chasms on Charon.

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.

This image was obtained by the Long-Range Reconnaissance Imager (LORRI) on New Horizons.