Surface processes in the Venus highlands: Results from analysis of Magellan and Arecibo data

The highlands of Venus are characterized by an altitude-dependent change in radar backscatter and microwave emissivity, likely produced by surface-atm osphere weathering reactions. We analyzed Magellan and Arecibo data for the se regions to study the roughness of the surface, lower radar-backscatter a reas at the highest elevations, and possible causes for areas of anomalous behavior in Maxwell Montes. Arecibo data show that circular and linear rada r polarization ratios rise with decreasing emissivity and increasing Fresne l reflectivity, supporting the hypothesis that surface scattering dominates the return from the highlands. The maximum values of these polarization ra tios are consistent with a significant component of multiple-bounce scatter ing. We calibrated the Arecibo backscatter values using areas of overlap wi th Magellan coverage, and found that the echo at high incidence angles (up to 70 degrees) from the highlands is lower than expected for a predominantl y diffuse scattering regime. This behavior may be due to geometric effects in multiple scattering from surface rocks, but further modeling is required . Areas of lower radar backscatter above an upper critical elevation are fo und to be generally consistent across the equatorial highlands, with the sh ift in microwave properties occurring over as little as 500 m of elevation. These surfaces are not simply characterized, however, by the absence of a highly reflective component. Surface morphology and radar-scattering proper ties suggest that a mantling deposit forms at the highest elevations, most likely by in situ erosion of the original rock. In Ovda Regio, this process mantles or has removed surface festoon structure at the 1- to 10-m scale, implying a significant depth for the weathered layer. Similar radar-dark ar eas occur in Maxwell Montes but are apparently unrelated to the current top ography of the region. Possible reasons for these observations include mass wasting from areas of steep slopes, compositional or age differences withi n the montes, vertical tectonic shifts of relict contacts, local topographi c effects on surface temperature, or errors in the Magellan topography data in the rugged terrain. While there is evidence for some of these effects i n the existing data, no single model at present appears to satisfy all occu rrences of high-altitude, radar-dark terrain. New measurements of the surfa ce and lower atmosphere chemistry of Venus are needed to further refine the se conclusions.