Global radar units on Venus derived from statistical analysis of Pioneer Venus Orbiter radar data

Abstract

Unsupervised, linear‐discriminant cluster analysis of Pioneer Venus reflectivity and rms‐slope data has produced a classification of 15 radar units. Examination of four hypsometric provinces previously established for Venus has been used to show that, although the highland (6053 to 6055.5 km) and mountainous (greater than 6055.5 km) provinces are distinctly different in their radar unit compositions from each other and from the lowlands (less than 6051 km) and rolling plains (6051 to 6053 km) provinces, the lowlands and rolling plains provinces are very nearly the same in their radar properties. This observation indicates that hypsometry alone is not a reliable criterion for quantitative examination of the surface geology on Venus. A classification unit encompassment scheme has been developed that shows the presence of three distinct trends: two toward elevations higher than the planetary mean elevation, and the other toward elevations lower than the mean. Venera lander data and earth‐based radar images have been used to infer that seven units within the two higher‐elevation unit encompassment trends represent materials of mostly basaltic composition that differ only in their surface morphologies. The physical characteristics of these materials indicate that they form terrains ranging from banded through rift valley to relatively flat and that the materials are composed of bedrock and “soil” in variable ratios. Three other radar classification units appear to be morphologically transitional forms between units of these two higher‐elevation unit encompassment trends. The five remaining units are associated with the lower‐elevation unit encompassment trend; their geologic significance cannot be determined with any certainty with the existing Pioneer Venus radar data. The radar units that correspond to those of an existing radar unit map, produced by supervised analysis, display similar areal distributions on Venus. However, our radar unit map shows more detail and is relatively unbiased in terms of unit definition. This global radar classification image of the Venusian surface provides a working data base from which more exacting geologic interpretations can be made when higherresolution imagery becomes available.