Root-Mean-Square Surface Slopes of Phoenix Landing Sites with 75-Cm Bistatic Radar Received By Mars Odyssey

Abstract

Between August and December 2005, we conducted 76 oblique-incidence scattering experiments using the SRI International 46-m antenna in the Stanford foothills to illuminate Mars for 20-min periods with an unmodulated, 75-cm λ, circularly polarized wave. The direct signal and a Martian surface echo, separated by differential Doppler frequency shifts, were received simultaneously by the one-bit receiver on board the Mars Odyssey spacecraft. Four of the proposed Phoenix landing regions, denoted A–D, were probed by at least one experiment. The surface echoes are characterized by both fluctuating amplitude and varying spectral width, which are responses to surface reflectivity and roughness variations. Our analysis of the echo data is based on quasi-specular scattering theory and makes use of high-resolution Mars Orbiter Laser Altimeter (MOLA) topographic maps to model the scattering surface in three dimensions along the specular track of the echo. We find MOLA topography sufficient to model scattering at 75-cm λ, indicating that the effective horizontal scales being sensed by the radar are at or larger than the MOLA grid spacing of 150–500 m. We find that the RMS surface slopes for the proposed Phoenix landing regions, as determined using the Hagfors scattering law and applicable to those scales, rarely exceed 1°, except in the presence of large craters and other surface irregularities.