Mars radar scattering: Arecibo/Goldstone results at 12.6- and 3.5-cm wavelengths

Abstract

Dual-polarization CW radar observations of Mars from the 1988 and 1990 oppositions are used to study the spatial variation and wavelength dependence of the echo. The observations were made with the Arecibo S-band ( λ = 12.6 cm) and the Goldstone X-band ( λ = 3.5 cm) radars. This is the first multiwavelength radar study of Mars to include the entire echo in both circular polarizations. Although the percentage depolarization of the diffuse echo is approximately wavelength-independent, some wavelength dependence has been found in the amplitudes of diffuse/depolarized features. The greatest difference was found over Tharsis, where the diffuse scattering enhancement is found to be significantly weaker at the shorter wavelength. This offers support for the hypothesis that the rough scattering surfaces on Tharsis are buried under a dust mantle a few decimeters in depth. A similar but weaker effect was found for Elysium. Positional variations in quasispecular roughness (rms slope) measured between 3°S and 25°S latitude are reported and compared with earlier results over this region. Some wavelength dependence of the quasispecular parameters has been found, including a tendency for the scattering surface to be rougher and less reflective at the shorter wavelength. Comparison of the new data with earlier Arecibo data gives improved estimates of the source locations of enhanced diffuse/depolarized scatter. Preliminary maps of depolarized reflectivity have been constructed by applying a “Doppler imaging” algorithm to the Doppler spectra. At 12.6 cm the strongest center of depolarized scatter is located in southern Tharsis (Arsia Mons and the Daedalia Planum region southwest of Arsia Mons). The depolarized anomaly from the south pole, first discovered in Goldstone/VLA bistatic observations, has been identified in the Goldstone monostatic data. Doppler tracking places the center of this feature at 40.8°W, 87.20°S, near the center of the residual south polar icecap. The radar cross section of this feature is 3 × 10 4 km 2 at an incidence angle of 62°, which corresponds to a reflectivity of order unity.