Saturn’s moon Hyperion has an oblong shape and a very unusual surface that makes it look like a sea sponge in images sent back from the spacecraft Cassini. The ice-rich moon has a very low density, which means it’s light in weight. It must be half-empty, riddled with hollow pockets.

There are many pits on the surface with dark deposits in their centers. On Earth, similar features appear in high-altitude glaciers. Dark material collects in a small cavity. The dark material absorbs sunlight and heats up faster than its light-colored surroundings. The dark material melts the ice around it and forms a feature called a “sun cup”. The sun cups can get rather deep with high-edged sides. Rims of sun cups can intersect and form honeycomb-like patterns. “Ultimately, a bizarre surface relief can develop where deep pits are surrounded by jagged icy walls and pinnacles” says Cassini team member Paul Helfenstein of Cornell University. This would explain the unusual sponge-like appearance of Hyperion’s surface. The honeycomb pattern could account for the empty spaces and Hyperion’s low density.