Radar Measurements of Martian Topography

Abstract

During the 1967 opposition, the planet Mars was observed with the Haystack radar, operated at a fre- quency of 7840 MHz (3.8-cm wavelength). Sufficient sensitivity was available to measure for the first time, in terms of round-trip echo delay, the variation in elevation of the Martian surface as the planet rotated on its axis. The topography around the parallel of latitude was found to vary by about 12 km. Expectations of others that there might be a simple correspondence between the locations of dark visible surface markings and topographical elevations or depressions appear to be unfulfilled. The distribution of the echo power both as a function of frequency and of delay, showed the reflections within a few degrees of the center of the Martian disk to be principally quasispecular, corresponding typi- cally to mean surface slopes of about 3 , and to a probable lower bound on the dielectric constant of about 3.5. The contribution of this quasispecular component to the cross section of Mars varied with longitude from less than 0.006 to about 0.05 ira2, where a is the planetary radius. In addition, a more diffuse compo- nent of the echo was noted which contributes a "background" cross section of approximately 0.1 ira2 largely independent of longitude. This wideband component may have been underestimated in previous cross-sec- tion determinations. No significant correlation was obtained between radar reflectivity and elevation of the Martian surface; the significance of the observed correlation between high radar reflectivity and visually dark regions is questionable.