Surface characteristics of Venus derived from Pioneer Venus altimetry, roughness, and reflectivity measurements

The three primary data sets for the Pioneer Venus orbiter radar experiment (topography, roughness, and reflectivity) contain important information about the geological and textural characteristics of the surface of Venus. We have subdivided the range of roughness and reflectivity values into three categories as follows: roughness, in degrees rms slope: relatively smooth (1°–2.5°), transitional from smooth to rough (2.5°–5°), and relatively rough (>5°); and Fresnel reflectivity: surfaces dominated by soil or porous material (<0.1), surfaces dominated by rock material (0.1–0.2), and surfaces with a significant percentage of anomalously high dielectric material (>0.2). We have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties (e.g., rough rocky surfaces, smooth soil surfaces). We then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high‐resolution images from spacecraft and earth‐based observatories. We find that the majority of Venus is covered by regionally contiguous rock and bedrock surfaces. Many of the smooth surfaces we interpret to be of volcanic origin, most likely lava flows, while rougher surfaces are locally characterized by tectonic deformation of several types. Soil surfaces cover less than about 27% of the planet and are generally patchy in their distribution. On the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread, topographically controlled erosion, lateral transport, and sedimentation. The small percentage of the surface of Venus characterized by high‐dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high‐dielectric materials, such as metal or metal oxides (e.g., Theia Mons in Beta Regio), and exposure of high‐dielectric materials by tectonic deformation (e.g., Maxwell Montes in Ishtar Terra). These global data set correlations provide a fundamental frame‐work for understanding the nature of the surface of Venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context.