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Science 25 February 2005:
Vol. 307. no. 5713, pp. 1237 - 1242
DOI: 10.1126/science.1107981
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Research Articles

Cassini Imaging Science: Initial Results on Phoebe and Iapetus

C. C. Porco,1* E. Baker,1 J. Barbara,2 K. Beurle,3 A. Brahic,4 J. A. Burns,5 S. Charnoz,4 N. Cooper,3 D. D. Dawson,6 A. D. Del Genio,2 T. Denk,7 L. Dones,8 U. Dyudina,9 M. W. Evans,3 B. Giese,10 K. Grazier,11 P. Helfenstein,5 A. P. Ingersoll,9 R. A. Jacobson,11 T. V. Johnson,11 A. McEwen,6 C. D. Murray,3 G. Neukum,7 W. M. Owen,11 J. Perry,6 T. Roatsch,10 J. Spitale,1 S. Squyres,5 P. C. Thomas,5 M. Tiscareno,5 E. Turtle,6 A. R. Vasavada,11 J. Veverka,5 R. Wagner,10 R. West11

The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on Iapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture. Iapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown.

1 Cassini Imaging Central Laboratory for Operations, Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA.
2 Goddard Institute for Space Studies, NASA, 2880 Broadway, New York, NY 10025, USA.
3 Astronomy Unit, Queen Mary, University of London, London, E1 4NS, UK.
4 Centre d'Etudes de Saclay, Université of Paris 7, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France.
5 Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA.
6 Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721, USA.
7 Institut für Geologische Wissenschaften, Freie Universität, 12249 Berlin, Germany.
8 Department of Space Sciences, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA.
9 Division of Geological and Planetary Sciences, California Institute of Technology, 150-21, Pasadena, CA 91125, USA.
10 Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany.
11 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.

* To whom correspondence should be addressed. E-mail: carolyn{at}ciclops.org

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