MERGING MAGELLAN EMISSIVITY AND SAR DATA FOR ANALYSIS OF VENUS SURFACE DIELECTRIC-PROPERTIES

Integration of Magellan datasets for Venus is an important part of characterizing the geology of the surface. We analyze the global average behavior of radar backscatter and emissivity and find that for incidence angles greater than 30 degrees the two parameters can be modeled by the behavior of a single-dielectric surface (epsilon similar to 4.15) with variable roughness. Based on this result, we propose a model which relates the HH backscatter and H-polarized emissivity to values for a relative roughness fraction and dielectric constant. Maps of the model solutions are presented for the region 34 degrees S-54 degrees N. Removal of the contribution of roughness to the emissivity permits rapid identification of regions with anomalous composition or density. In smooth areas, the model typically yields dielectric constant values in good agreement with those calculated from the altimeter-derived reflectivities, but it appears that the model is more reliable than the Hagfors results for rough surfaces. The permittivity in highland regions rises with altitude, and the data for these areas are more consistent with a single high-dielectric interface (single and multiple scattering) than with internal volume scattering. Higher dielectric constants are associated with both smooth and rough terrains, but the general tendency is toward greater roughness with higher permittivity. This may suggest that a mechanical weathering process occurs in the formation of high-dielectric mineral phases. There are variations in dielectric constant within lava flow complexes (up to values of 9), which may reflect differences in the chemical content of the rock. Three major types of mantling deposits are identified: (1) high-permittivity (epsilon = 7-8) units associated with parabolic crater features and interpreted to be fine-grained material with a high proportion of metallic phases; (2) low-permittivity (epsilon = 2-3) crater-related splotches or haloes, interpreted to be fine-grained material with minimal or weathered metallic minerals; (3) low-permittivity deposits with no associated crater, tentatively attributed to soil or pyroclastic/ignimbrite eruptions. Within each type of deposit there is a range of surface roughness and radar backscatter strength, which are related to the original surface roughness and the depth of the mantling material. (C) 1994 Academic Press, Inc.