Surface Property Variations in Venusian Fluidized Ejecta Blanket Craters

Abstract

A comprehensive study of Magellan Cycles 1 and 2 radar data from Venus reveals surface roughness and dielectric variations associated with fluidized ejecta blanket (FEB) craters that help illuminate styles of flow ejecta emplacement. This study develops new procedures of digital unit mapping and polygon-filling algorithms using Magellan SAR, altimetry, and radiometry data. These techniques allow the extraction of radiophysical information for FEB crater materials, nearby plains, and lava flows such as (1) their specific backscatter (σ o) behavior (relative to each other and to terrestrial lava surfaces); (2) average calculated values of emissivity, rms slopes, corrected reflectivity, and the diffuse component of reflectivity; and (3) variations in radar properties along longitudinal traverses that are best explained by surface roughness trends at several spatial scales and/or dielectric variations. Backscatter curve slopes of the FEBs studied here are consistent with surface textures that are transitional between a'a and pahoehoelike. Average surface property values of ejecta units are relatively similar for a given crater, but are discernibly different from other craters. Individual crater ejecta reflectivity and emissivity values are relatively similar to those for the surrounding plains, which may suggest a link between plains material and ejecta dielectric properties. Increasing FEB roughness downflow are interpreted to be associated with more lava-like flows, while decreasing roughnesses are more similar to trends typical of gravity (pyroclastic-like or debris-like) flows. Most commonly, FEB crater flow materials exhibit transitions from proximal, lava/melt-like flow styles to distal, gravity flow-like styles. Some FEBs show more complicated behavior, however, or appear to be more dominated by dielectric differences downflow, as inferred from correlations between the data sets. Such transitions may result from changes in local topography or from overlapping of flow lobes during FEB emplacement.