New Cassini RADAR Results for Iapetus and Saturn's Other Icy Satellites
Résumé
During the past two years, radar tracks on the icy satellites have more than doubled the number of observations reported by Ostro et al. (2006, Icarus 183, 479-490). Our scatterometry has now yielded estimates of the 2.2-cm- wavelength radar albedo in the same linear (SL) polarization as transmitted for Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe. Our tracks have sampled widely separated subradar locations on most of the satellites, in some cases using beams comparable to or larger than the target's disc and in others using beams significantly smaller than the disc. Hence we can begin to assess the dispersion in these objects' distributions of radar reflectivity. The 2.2-cm SL radar albedo varies by at least several tens of percent on at least Dione, Rhea, Enceladus, and Iapetus. For Enceladus, a scatterometric observation centered on (29 S, 243 W) with a beamwidth of 1.2 Enceladus diameters gives the largest disc-integrated SL radar albedo obtained at any wavelength for any solar system object (including Europa), probably because of the extreme purity of at least the uppermost few decimeters of the water-ice regolith on much of the hemisphere facing the radar. For Iapetus, multi-beam, disc-resolved measurements disclose that the disparity in the 2.2-cm SL radar albedos between the optically bright and dark terrains is greater than that inferred by Ostro et al. (2006). During the Iapetus 49 flyby shortly after this meeting's abstract deadline, radar observations will include SAR imaging with 2-to-12-km surface resolution covering much of the visible disc, using beam sizes of about 120 km (less than one tenth of an Iapetus diameter), plus a short altimetric measurement with 35-m range resolution. The radar imaging will be mostly of the optically dark terrain and will include tracks across the equatorial ridge and the largest impact structures in Cassini Regio. The Iapetus SAR images will be the first of an icy satellite for which high-resolution optical images are available, and therefore will provide useful lessons for interpreting Titan SAR images.