Decimeter-Wavelength Polarimetric Radar Imaging of the Icy Moons of Jupiter

Introduction: Imaging radars with wavelengths in the range of 10 cm to 1 m can deeply penetrate the surface of an icy body, revealing details of the geo-morphology, local structure, and electrical properties of the upper layers. Radar studies of icy surfaces on Earth have used the polarization state of backscatter echoes at multiple frequencies to characterize the surface and subsurface properties of glaciers, showing relatively smooth surfaces on the scale of radar wavelengths , and subsurface scattering from volume scat-terers consistent with ice pipes and lenses [1, 2]. These volume scattering effects are evident in enhanced polarization ratios over a limited range of backscatter incidence angles. The Galilean satellites exhibit similarly enhanced polarization ratios and volumetric scattering effects [3], but the observations are limited in angular resolution, leading to ambiguity in interpreting the scattering mechanisms and their structural implications. Subsurface Imaging: Polarimetric radar observations of the Galilean satellites at decimeter-scale wavelengths are well-suited to addressing some of the poorly understood characteristics of the icy surfaces. An imaging radar has vastly improved resolution, allowing detailed association of scattering mechanisms with geomorphological structures. Where the surfaces are smooth and volumetric effects are small, a long wavelength imaging radar probes deep into the ice, allowing characterization of the spatial variability of ice layers. Where the volumetric effects dominate, presumably due to impact events, the radar can measure the depth and extent of the gardened layer. Fully polarimetric observations enable polarization synthesis needed to explore the unique scattering mechanism of these bodies. Bistatic Imaging: With powerful transmitters at the DSN capable of illuminating the Galilean satellites, or using the high power transmit capabilities of an orbiter in conjunction with a DSN receiver, probing the coherence properties of the surfaces can be enhanced. All observations to date, as well as conventional imaging radars, are monostatic, giving an incomplete picture of the volume scattering mechanisms. Bistatic observations of the complete scattering function would give valuable new insight into these mechanisms. Repeat-pass interferometry and volumetric response of the upper layers: When a radar satellite is maneuvered to follow exact repeat orbits, it is possible to observe a surface feature from exactly the same van