36th DPS Meeting, 8-12 November 2004
Session 4 Phoebe and Iapetus
Oral, Monday, November 8, 2004, 1:30-3:00pm, Lewis

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[4.04] Cassini RADAR Observations of Phoebe

S. J. Ostro, C. Elachi, Y. Anderson, R. Boehmer, P. Callahan, G. Hamilton, M. Janssen, W. Johnson, K. Kelleher, R. Lopes, L. Roth, S. Wall, R. West (JPL/Caltech), M. Allison (NASA/Goddard), R. Kirk (USGS), C. Wood (PSI), F. Posa (Politechio di Bari), E. Stofan (Proxemy Rsch.), H. Zebker (Stanford U.), R. Lorenz, J. Lunine (U. Arizona), G. Francescetti (U. Naples), G. Picardi, R. Seu (U. Rome La Sapienza), D. Muhleman (Caltech), P. Encrenaz (DEMIRM/Obs. de Paris), Cassini RADAR Science and Instrument Operations Teams

The Cassini RADAR instrument, operating in its scatterometry mode, obtained continuous-wave (cw) echo power spectra from Phoebe during the inbound and outbound legs of the flyby, 4 h before and 2.5 h after closest approach. Phoebe's distance and subradar coordinates were approximately (93,000 km, 247 deg W, 26 deg S) inbound and (56,000 km, 323 deg W, 26 deg N) outbound. The durations of the cw sequences were 6 and 5 minutes. Larger intervals in the RADAR windows were devoted to observations with a chirp waveform able to provide range as well as Doppler resolution, and to passive radiometry; those data are not yet reduced.

For Phoebe (and Dione, Mimas, Iapetus, Enceladus, Rhea, Hyperion, and Tethys), scatterometry aims to use estimates of radar albedo and angular scattering law to constrain the near-surface bulk density and/or the relative cleanliness of the icy regolith. The RADAR instrument's wavelength is 2.2 cm, vs. 3.5 cm or 13 cm for most groundbased radar astronomy, but Arecibo and Goldstone observations of the icy Galilean satellites and of asteroids give us no reason to expect significant wavelength dependence in this regime. Comparison of RADAR measurements of Titan and Iapetus with groundbased results will let us evaluate this expectation and will be key to calibrating both our measurements and their interpretation.

Our inbound and outbound Phoebe echoes indicate Lambertian scattering, which requires structural complexity at scales no smaller than a centimeter. However, despite Phoebe's prominent large-scale topography, our spectra are nearly featureless, suggesting that the radar roughness is sub-topographic. We probably are seeing a combination of single scattering and multiple scattering from surface and subsurface structure. At this writing, our calibration indicates that Phoebe's average radar albedo is much closer to that of Iapetus than to those of the icy Galilean satellites.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.