Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11e1624w&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11E-1624
Astronomy and Astrophysics
Astronomy
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites, [6949] Radio Science / Radar Astronomy, [6969] Radio Science / Remote Sensing
Scientific paper
The typical Saturn icy satellite RADAR observation occurs at large distances, usually about 100,000 km, but sometimes as far as 400,000 km. Ostro et al. (Icarus 2010) have reported 2 cm-wavelength radar albedo estimates and diffuse scattering parameters for 73 radar tracks acquired through April 2008 across eight of Saturn's moons. We reprocess this data, together with more recently acquired data (bringing the total to more than 91 tracks), using an alternative processing approach, where we project the weighted received power onto the surface rather than model the power spectral shape. This allows us to create an extensive set of consistently processed measurements that we can use to understand the distribution of 2 cm-wavelength scattering characteristics across each moon's surface, as well as between the moons. We focus on data acquired during several close targeted flybys occurring at distances smaller than 45,000 km. The targeted flyby data enable us to carefully measure the backscatter response for regions on Rhea, Iapetus, and Enceladus. We model the backscatter response with a diffuse cosine power law to estimate the backscatter parameters A and N, where A is a measure of the reflection strength and N is a measure of the directivity of the backscatter power pattern. Isotropic surfaces will have N=1, whereas Lambertian surfaces have N=2. Our close flyby measurements span the range from A=0.41 for Iapetus' dark leading hemisphere to A=3.51 for Enceladus' bright trailing hemisphere, representing some of the darkest and the brightest surfaces in the Saturnian system. The measured N values reveal that Iapetus appears more Lambertian (N=1.81) and Enceladus looks more isotropic (1.23). The Rhea measurements are in between the two extremes, but are closer to the bright end of the spectrum (A=2.14, N=1.35). The Iapetus observation exhibits a small quasispecular rise at angles less than 20 degrees, a backscatter response similar to those measured for features on Titan. We fit a Hagfors and an exponential quasispecular model to the low-angle backscatter, and the results imply a low effective dielectric constant, possibly indicative of porous solid hydrocarbons or porous carbon dioxide ice, and moderate large-scale (>>2.2 cm) facet slopes. This is the first detection of a quasispecular component on an icy moon other than Titan, indicating the presence (albeit small) of a surface scattering mechanism in addition to the more prominent diffuse volume scattering mechanism that tends to dominate the backscatter of icy surfaces. We also form low resolution real aperture backscatter images of Enceladus, Rhea, and Iapetus and compare these to co-located high resolution synthetic aperture backscatter images. We analyze the resolution effects on the appearance of the backscatter responses.
Cassini RADAR Team
Wye Lauren
Zebker Howard A.
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