Hyperion’s hydrocarbons

Astronomers have confirmed the existence of frozen water on the surface of Saturn’s moon Hyperion and have also discovered solid carbon dioxide there. The evidence comes from spectra taken by the Cassini spacecraft during the first flyby of the moon, in September 2005. The craft’s observations of Hyperion’s craggy surface also reveal dark material that appears to be other hydrocarbons.

Hydrocarbons have been found on Titan and a few other Saturnian moons as well as on several of Jupiter’s icy moons. When ultraviolet light strikes these carbon-based, ice-embedded molecules, they produce more-complex compounds that might provide the ingredients of life, notes Dale Cruikshank of NASA’s Ames Research Center in Mountain View, Calif. He and his colleagues describe the findings in the July 5 Nature.

The frozen carbon dioxide isn’t found in a pure form but appears to be chemically bound to other material. That arrangement may allow the carbon dioxide to stay on the surface rather than slowly evaporate, Cruikshank notes.

In the same issue of Nature, another team using Cassini data attributes Hyperion’s distinctive surface, riddled with sharp-edged craters, to the moon’s low density. Peter Thomas of Cornell University and his colleagues suggest that material smacking into the moon’s porous outer layers forms craters by compressing material, rather than by blasting it off the surface. Any debris ejected by impacts probably escapes the moon instead of falling back into the craters. Together, these processes give the moon its unusual, spongelike appearance.