Geologic investigations of the Venusian surface using Magellan radar imagery, altimetry, and radiometry.

Abstract

A comprehensive study of Magellan multiple-Cycle synthetic aperture radar (SAR) data from Venus reveals morphological, surface roughness, and dielectric variations valuable in understanding emplacement mechanisms of fluidized ejecta blanket (FEB) craters, nearby plains and lava flows. FEB deposits develop variable channel morphologies related to parameters such as crater diameter and flow length. 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 geologic materials such as: (1) specific backscatter (σₒ) behavior; (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 either transitional between a'a and pahoehoe-like or more pahoehoe-like. Increasing FEB roughnesses 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 either gravity flow-like styles or 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. Computer modeling of FEB flows over topography was performed using modified programs previously applied to Mt. St. Helens' flows. These models demonstrate for the three FEB craters studied that the flows require relatively low initial velocities as well as low values of yield strength and viscosity. Geologic mapping of USGS quadrangle Barrymore (V59) shows evidence of an extensive plains formation event obscuring older local tectonic and volcanic structures, followed by regional and localized compression, forming wrinkle ridges and ridge belts. Application of terrestrial ERS-1 SAR data of the Channeled Scabland region shows backscatter values indicative of surfaces somewhat rougher than terrestrial a'a flows.