Radar properties as clues to relative ages of ridge belts and plains on Venus

[i] An ongoing issue regarding the crustal evolution of Venus is whether or not certain features and processes occurred in the same sequence and at the same time globally. This paper seeks to test one aspect of "directional" models for the geologic history; the inference that all ridge belts are older than regional plains. This test involves traditional geologic analysis, but also involves the use of radar properties in a unique way. We use a simplified model for the ridge shape, and an estimate of the diffuse scattering behavior of the Venus surface, to determine the "true" backscatter coefficient of the slope of the ridge tilted away from the radar, and ratio this value to the echoes from nearby plains units. In general, ridge-forming terrain in these belts appears to have greater inferred backscatter coefficients, and thus decimeter-scale roughness, than the regional plains. Plains units closely adjacent to many ridge belts have higher backscatter than plains at some distance, which we infer is generally due to aeolian deposition of debris characterized by centimeter-scale microdunes or ripples where the belt topography interferes with wind patterns. The rougher apparent surface texture within the belt-forming terrain is not consistent with the gentle folding that forms the ridges, and so is likely a result of weathering or tectonic processes operating prior to belt deformation. We thus conclude that the material incorporated in the ridge belts we have studied predates the surrounding regional plains.