ANALYSIS OF VOLCANIC SURFACE-MORPHOLOGY ON VENUS FROM COMPARISON OF ARECIBO, MAGELLAN, AND TERRESTRIAL AIRBORNE RADAR DATA

Arecibo Observatory and Magellan radar data for Venus are compared to airborne radar images for potential terrestrial analog surfaces. Volcanic deposits in Western Eistla Regio and northern Sedna Planitia am characterized. It is shown that the expected-sense circularly polarized echoes in the "dark plains" and broad flow aprons of Eistla Regio decrease rapidly with incidence angle. This angular scattering behavior implies surfaces no rougher than terrestrial pahoehoe flows. Polarization ratio (based on the ratio of the "unexpected"- to "expected"-sense circular echoes) comparisons show that the extensive lava flows in Western Eistla Regio and Sedna Planitia are generally consistent with the properties of terrestrial pahoehoe flows, with only limited occurrences of a'a morphology. Blocky textures on scales of several meters are not observed in these regions. This occurs despite the fact that Arecibo images are collected at 12.6-cm wavelength, while the airborne data are collected at 24-cm wavelength, shorter illuminating wavelengths tend to increase the perceived roughness of a given surface. Comparison of calibrated backscatter cross section values for Hawaiian lava flows and Magellan synthetic aperture radar data of the study areas supports these interpretations: only three major units of a'a morphology are identified (other radar-bright surfaces are likely rough pahoehoe flows). These results suggest three scenarios: (1) many of the large flow units in the two study regions were emplaced as complexes of low-effusion rate (presumably tube-fed) pahoehoe flows, rather than as higher eruption rate events which might be expected to produce a'a surface textures, (2) the long lava flows were originally emplaced as a'a but have since weathered to a smoother texture, or (3) a combination of atmospheric and magma compositional effects combine to inhibit a'a formation even at high volume eruption rates.