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B. J. Buratti (Jet Propulsion Laboratory), D. P. Cruikshank (NASA/Ames), R. Clark (USGS), R. H. Brown (University of Arizona), J. M. Bauer (Jet Propulsion Labortory), D. P. Simonelli (National Research Council), R. Jaumann (DLR), K. Hibbitts, T. B. McCord (PSI), K Soderlund (Florida Institute of Technology), K. H. Baines (JPL), G. Bellucci (IAS, Italy), J.P. Bibring (Universite de Paris, Sud-Orsay), F. Capaccioni, P. Cerroni, A. Coradini (IAS, Italy), P. Drossart (Observatoire de Paris-Meudon), V. Formisano (National Research Council, Italy), Y. Langevin (Universite de Paris, Sud-Orsay), D. L. Matson (Jet Propulsion Laboratory), V. Mennella (Osservatorio Astronomico di Capodimonte, Italy), R. Nelson (Jet Propulsion Laboratory), P. D. Nicholson (Cornell University), B. Sicardy (Observatoire de Paris-Meudon), C. Sotin (Universite de Nantes)
Iapetus is perhaps the most enigmatic body in the solar system: One hemisphere is as dark as lampblack, and the other is almost as bright as snow. The models that have been offered to explain this dichotomy range from endogenously placed material (Smith et al., 1982, Science 215, 504), to material exogenously placed from Phoebe (Soter, 1974, IAU Colloq. 28), or other bodies (Owen et al., 2001, Icarus 149, 160; Buratti et al., 2002, Icarus 155, 375; Buratti et al., 2003, B.A.A.S, 915). No mechanism for the darkening process or purported source for the exogenic particles is entirely satisfactory. One key question is whether the process that led to the formation of the low-albedo hemisphere of Iapetus is unique, or whether the satellite has been subjected to a satellite alteration process in a more extreme form. Both Callisto and the outer satellites of Uranus show evidence for exogenic accretion of particles onto their leading sides.
A targeted flyby of Iapetus by Cassini, during which the spacecraft will approach the satellite to within 1000 km, is scheduled to occur in September 2007. An untargeted approach of 65,000 km to the satellite will occur on New Year's day 2005, and observations are planned for the period around closest approach. However, a "sneak peak" of the satellite was afforded by Cassini on July 19, 2004, during which the spacecraft approached to less than three million miles (the Voyager closest approach was 909,070 km). The first disk resolved spectra of Iapetus in the 0.4 to 5 micron region were obtained by the Cassini Visual Infrared Mapping Spectrometer (VIMS). We report the tentative identification of carbon dioxide on the low-albedo portion of the surface. A comparison of the spectrum of Iapetus to that obtained by VIMS during its flyby of Phoebe on June 11, 2004 will be made. Mixing models incorporating water ice, minerals, and organics can replicate the spectrum of the dark hemisphere.
Work performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.