Letter

Nature 448, 50-56 (5 July 2007) | doi:10.1038/nature05779; Received 15 January 2007; Accepted 23 March 2007

Hyperion's sponge-like appearance

P. C. Thomas1, J. W. Armstrong2, S. W. Asmar2, J. A. Burns1, T. Denk3, B. Giese4, P. Helfenstein1, L. Iess5, T. V. Johnson2, A. McEwen6, L. Nicolaisen7, C. Porco8, N. Rappaport2, J. Richardson1, L. Somenzi5, P. Tortora9, E. P. Turtle10 & J. Veverka1

  1. Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853, USA
  2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
  3. Institut für Geologische Wissenschaften, Freie Universität, 12249 Berlin, Germany
  4. Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
  5. Dipartimento di Ingegneria Aerospaziale ed Astronautica, Università La Sapienza, via Eudossiana 18, 00184 Roma, Italy
  6. Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721, USA
  7. Physics and Astronomy Department, University of California at Los Angeles, Box 951547 Los Angeles, California 90095, USA
  8. Space Science Institute, 4750 Walnut Street, Boulder, Colorado 80301, USA
  9. DIEM-II Facoltà di Ingegneria, Università di Bologna, Via Fontanelle, 4047100 Forlì, Italy
  10. Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA

Correspondence to: P. C. Thomas1 Correspondence and requests for materials should be addressed to P.C.T. (Email: pct2@cornell.edu).

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 plusminus 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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