NASA’s New Horizons spacecraft has baffled scientists with new mysteries ever since it flew past the icy planet Pluto in July 2015. Now, far in the western hemisphere, scientists have discovered what looks like a giant “bite mark” on Pluto’s surface. Scientists suspect it may be caused by a process known as sublimation – the transition of a substance from a solid to a gas.

The methane ice-rich surface on the dwarf planet may be sublimating away into the atmosphere, exposing a layer of water-ice underneath, Nasa said. The image captured by Nasa indicate that the cratered plateau uplands south of Piri Rupes are rich in methane ice.

Scientists speculate that sublimation of methane may be causing the plateau material to erode along the face of the cliffs, causing them to retreat south and leave the plains of Piri Planitia in their wake.

The data also show that the surface of Piri Planitia is more enriched in water ice than the higher plateaus, which may indicate that Piri Planitia’s surface is made of water-ice bedrock, just beneath a layer of retreating methane ice. Because the surface of Pluto is so cold, the water ice is rock-like and immobile.

The light/dark mottled pattern of Piri Planitia in the left inset is reflected in the composition map, with the lighter areas corresponding to areas richer in methane—these may be remnants of methane that have not yet sublimated away entirely.

According to NASA, the image measures approximately 450km long by 410km wide and was obtained by New Horizons at a range of approximately 33,900 miles from Pluto, about 45 minutes before the spacecraft’s closest approach to Pluto on July 14, 2015. The LEISA data was gathered when the spacecraft was about 47,000km from Pluto.