Radar Observations of Mercury

Earth-based radar observations have provided information on the equatorial topography, sul3Pace scattering properties, and rotation of Mercury. The recent observational program at Arecibo Observatory has been devoted primarily to the measurement of altimetric profiles using the delay-Doppler technique. These profiles, which were derivedfrom observations made over a 6 yr period, provide fairly extensive coverage over a restricted equatorial band. The data have suficient resolution and accuracy to permit the identijication of radar signatures for features as small as 50-km diameter craters and km high arcuate scarps. More importantly, they have been used to identify large-scale topographic features such as smooth plains subsidence zones and major highlands regions. In this sense the radar data complement spacecraft images, as quantitative altimetry from the latter is limited to shadow measurements of small, high-relief features such as crater rims. Radar data such as that obtained at Arecibo also provide most of what little information is available on the unimaged hemisphere. Measurements of radar cross section, Doppler spectrum shape, and depolarization provide information on the dielectric properties and roughness of the sul3Pace. These results can be readily compared with similar measurements for other planets.