Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p22a..01o&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P22A-01
Other
5422 Ices, 5464 Remote Sensing, 5470 Surface Materials And Properties, 6280 Saturnian Satellites
Scientific paper
Operating in its scatterometry mode, the Cassini radar has obtained 2.2-cm-wavelength echo power spectra from Phoebe on the inbound and outbound legs of its flyby (subradar points at W. Long, Lat. = 245,-22 deg and 328,+27 deg), from Iapetus' leading side (66,+39 deg) and trailing side (296,+44 deg) on the inbound and outbound legs of orbit BC, from Enceladus during orbits 3 (0,0 deg) and 4 (70,-13 deg), and from Rhea during orbit 11 (64,-77 deg). Our echo spectra, obtained in the same linear (SL) polarization as transmitted, are broad, nearly featureless, and much stronger than expected if the echoes were due just to single backreflections. Rather, volume scattering from the subsurface probably is primarily responsible for the echoes. This conclusion is supported by the strong anticorrelation between our targets' radar albedos (radar cross section divided by target projected area) and disc brightness temperatures estimated from passive radiometric measurements obtained during each radar flyby. Taking advantage of the available information about the radar properties of the icy satellites of Saturn and Jupiter, especially the linear- and circular-polarization characteristics of groundbased echoes from the icy Galilean satellites (Ostro et al. 1992, J. Geophys. Res. 97, 18227-18244), we estimate our targets' 2.2-cm total-power (TP) albedos and compare them to Arecibo and Goldstone values for icy satellites at 3.5, 13, and 70 cm. Our four targets' albedos span an order of magnitude and decrease in the same order as their optical albedos: Enceladus/Rhea/Iapetus/Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, whose extremely low electrical loss at radio wavelengths permits the multiple scattering responsible for high radar albedos. Plausible candidates for contaminants causing variations in radar albedo include ammonia, silicates, and polar organics. Modeling of icy Galilean satellite echoes indicates that penetration to a few meters is adequate to produce anomalously high TP albedos (Black et al. 2001, Icarus 151, 167-180; Peters 1992, Phys. Rev. B 46, 801-812.). In terms of average TP albedo, Enceladus at 2.2 cm resembles Europa at 3.5 and 13 cm, Rhea at 2.2 (and at 13 cm; Black and Campbell 2004, BAAS 36, 1123) resembles Ganymede at 3.5 and 13 cm, and Iapetus at 2.2 cm resembles Callisto at 3.5 and 13 cm. For Iapetus, the 2.2-cm albedo is dramatically higher on the trailing side than the leading side, requiring that to depths of at least several decimeters, the water ice is significantly "dirtier" on the leading side than the trailing side. By contrast, at 13 cm, little hemispheric asymmetry was seen by Black et al. (2004, Science 304, 553). Moreover, our average 2.2-cm TP albedo is several times larger than the average 13-cm value. This strong 2.2-to-13-cm albedo drop, which is reminiscent of Europa's 13-to-70-cm albedo drop (Black et al. 2001, Icarus 151, 160-166), suggests that efficiently scattering 13-cm-scale structure (e.g., particles or other heterogeneities) within the radar-penetrated surface is much less abundant than efficiently scattering 2.2-cm-scale structure. Perhaps contamination of water ice is limiting radar penetration to less than a meter on both the leading and trailing sides.
Anderson Yanhua Z.
Boehmer Rudy A.
Gim Yonggyu
Hamilton Gary A.
Janssen Michael A.
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