Geology of the Venus equatorial region from Pioneer Venus radar imaging

Abstract

Pioneer Venus radar data has provided images (resolution 20- to 40-km) of approximately 50% of the total surface of Venus in a band between 45 ° N to 15 ° S. These data are used to map the broad radar characteristics of the equatorial region on the basis of radar brightness and texture. Seven radar units are defined and are used to assess the geologic character of the equatorial region. These units fall into two distinct classes, those that are radar-bright (35% of the equatorial region) which correspond to highlands and zones of intense tectonic deformation, and radar-dark units, corresponding primarily to plains (65% of the equatorial region). The correspondence between features in the 15 ° region of overlap between the Pioneer Venus and Venera 15/16 images is examined and used to extend units mapped in the northern high latitudes into the equatorial region. On the basis of the distribution of the radar units, properties of RMS slope, reflectivity, the scattering behavior of the surface, and topographic signature, seven physiographic units are mapped in the equatorial region and are identified by increasing complexity as plains (undivided), dark halo plains, upland rises, upland plateaus, interhighland tectonic zones, tectonically segmented linear highlands, and tectonic junctions. The physiographic units are distributed in a nearly continuous interconnecting zone of volcanic rises and tectonic features that extends for nearly 360 ° around the equator of the planet. The distribution of large circular structures interpreted as coronae is also examined and it is concluded that the abundances of the largest structures, diameters greater than 500 km, is less than in the northern high latitudes with a notable absence of smaller coronae. The absence of small coronae may be due to the resolution limit of the Pioneer Venus data since analyses of higher resolution Arecibo and Goldstone imagery suggests that a number of corona-like features not identified in the PV data are present.