Hyperion is Saturn’s largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation1,2,3. Previous work has identified Hyperion’s surface as distinct from other small icy objects4,5 but left the causes unsettled. Here we report high-resolution images that reveal a unique sponge-like appearance at scales of a few kilometres. Mapping shows a high surface density of relatively well-preserved craters two to ten kilometres across. We have also determined Hyperion’s size and mass, and calculated the mean density as 544 ± 50 kg m-3, which indicates a porosity of >40 per cent. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained6,7, and accordingly may be the crucial factor in crafting this unusual surface.
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Technical assistance was provided by B. Carcich, K. Consroe, P. Smith and P. F. Helfenstein. G. Patton checked the crater tabulation on Phoebe and Hyperion. This work was funded in part by the Cassini project. A portion of this work was done at the Jet Propulsion Laboratory, California Institute of Technology, under a contract from NASA. The work of L.I., P.T. and L.S. was funded in part by the Italian Space Agency. The manuscript benefited from reviews by K. Housen and E. Asphaug.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Thomas, P., Armstrong, J., Asmar, S. et al. Hyperion's sponge-like appearance. Nature 448, 50–53 (2007). https://doi.org/10.1038/nature05779
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