Hyperion's sponge-like appearance

Saturn's Hyperion, the largest known irregularly-shaped satellite and the only moon observed to undergo chaotic rotation1-3, shows a sponge-like appearance at scales of a few km. This unique appearance, distinct from surfaces of other small, airless objects subject to shaping by impact cratering, suggested the possibility of unexpected modification of surface features (D. P. Cruikshank et al. in preparation). Here we analyze images of Hyperion to show that the appearance is the result of a high surface density of relatively well-preserved 2-10 km impact craters, we determine the size and mass of Hyperion, and obtain a mean density indicative of porosity >40%. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained4,5 and thus be the crucial factor in this unusual landscape