North‐south roughness anisotropy on Venus from the Magellan Radar Altimeter: Correlation with geology

Abstract

Analysis of the Doppler spectrum of Magellan Radar Altimeter echoes has revealed the spatially coherent deflection of the echo center from the nadir point (Tyler et al., 1992). We present and analyze a global map of the Doppler centroid of the radar altimeter echo, which, for the plains of Venus, characterizes the degree of anisotropy of the surface roughness in the north–south direction at scales from decimeters to kilometers. We found a global hemispherical trend of roughness anisotropy: on average, equator‐facing slopes are steeper than pole‐facing slopes. We searched globally for correlations between contrasts in roughness anisotropy and surface geology. We found that sharp contrasts in anisotropy often coincide with boundaries of geological units, although not consistently. We found a number of examples where radar‐bright (rough) lava flows have higher asymmetry than their surroundings. Diffuse radar‐dark crater‐related parabolic features and halos often have a symmetric roughness signature, while typical regional plains away from these diffuse features often have an anisotropic roughness. There is not, however, any direct and uniform correlation between the roughness anisotropy signature and geomorphologic terrain type or appearance in the radar images. We believe that asymmetric topography of deposits of windblown material, e.g., microdunes or small‐scale wind streaks in the wind shadow of obstacles, is the main contributor to the observed roughness anisotropy.