Surface‐Height Variations on Venus and Mercury

Abstract

Radar time‐delay and Doppler observations have been used to infer surface‐height variations near the equator of Venus. One method, applicable along the apparent equator within a few degrees from the subradar point, has a resolution of about 25 km in longitude and 200 km in latitude and has disclosed an elevated region with a height of about 2 km. Within the latitudinal‐resolution cell, the region extends 150 km in longitude and has a radar cross section enhanced by about 4 db above average. A second method employs measurements of the round‐trip delay to the subradar point on the target planet. These data, accumulated over a number of years, cover the entire equatorial region on Venus, although nonuniformly. Each observation has a resolution along the surface of about 1000 km. Comparison of the measured values with predictions that assume Venus to be spherically symmetric, but that take into account all other significant effects, shows systematic trends in the residuals as a function of the longitude on Venus of the subradar point. A model in which the equator is assumed to be elliptical (semi‐major axis, a; semi‐minor axis, b) with center offset by Δrhov from the center of mass allows the systematic trends to be removed. With all relevant parameters estimated simultaneously, we find Δp = 1.5±0.3 km and a ‐ b = 1.1±0.4 km (formal standard errors). Typically, the actual uncertainty is several times the formal error. The direction of Δp, determined with a formal standard error of 10°, points approximately away from the earth at inferior conjunction. The first method has not been applied to Mercury because of its weaker echo signal. The second method, using data that cover the entire equatorial region, yields time‐delay residuals which show no systematic trend when displayed as a function of the longitude on Mercury of the subradar point; the estimates for Δp and a ‐ b are both less than their respective formal standard errors of 0.4 and 0.2 km. Thus, both Venus and Mercury possess surface height variations far smaller in magnitude than those of either the earth or Mars.