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L.J. Harcke, H.A. Zebker, G.L. Tyler, R.A. Simpson (Stanford University), S.J. Ostro (Jet Propulsion Laboratory), J.K. Harmon (NAIC/Arecibo Observatory)
Speculation that Europa and Callisto possess liquid water oceans under an icy lithosphere has greatly increased interest in exploring the Galilean satellites of Jupiter. Earth-based radar observations using either Arecibo (13 cm) or the Goldstone/VLA bistatic system (3.5 cm) can constrain the physical properties of these satellites. We present radar images of Europa, Ganymede, and Callisto acquired using Arecibo during the 1999 and 2000 oppositions. Dual-polarization albedo maps with approximately 0.05" (75 km) resolution were obtained using the long-code delay-Doppler technique. Though a north-south hemisphere ambiguity is present in these images, the resolution is much finer than the 0.25" (360 km) resolution achievable with aperture synthesis imaging with the A-configuration of the Goldstone/VLA radar. Overlaying the radar images on the recent Galileo orbiter optical images of these satellites identifies particular surface features visible in the radar data. The spatially resolved data permit tentative identification of the terrains which produce enhanced backscatter from the surfaces of these icy moons, and might suggest candidate resurfacing processes. Unlike passive optical spectrometer data, which penetrates only fractions of millimeters into the ice, the decimeter-wavelength radar provides data on volume scattering from deeper within the regolith. Comparison of the spectrometer and radar observations might suggest whether the chemical properties of the outermost surface are maintained with depth, with possible implications for how the surface evolves with time.
This research is supported by the Planetary Astronomy Program of the National Science Foundation and by NASA.