Venus global radar reflectivity and correlations with elevation

The global distribution of the reflectivity of the surface of Venus as determined by the Pioneer Venus orbital radar instrument has been analyzed in a geological context and statistically correlated with elevation. In addition, a comparison between the reflectivity and rms slope (roughness) correlations with elevation permits radar‐geologic topographic zones to be identified. The radar reflectivity ρ at normal incidence and at a wavelength of 17 cm (1.76 GHz) is a model‐dependent measure of the bulk dielectric constant κ of surfaces dominated by dry rocks and soils and depends on surface material properties such as porosity and conductivity. Only the quasi‐specular component of the radar echo was used in determining the reflectivity values analyzed in this study, and for very rough surfaces the absolute magnitude of ρ may be underestimated by 10–15%. Empirically derived relationships between ρ, κ, and bulk density γ are used to interpret geologically the ρ distribution. The global mean ρ of 0.13 is significantly greater than the average lunar and typical martian values of ∼0.07, suggesting the absence of a continuous soil mantle on Venus. The ρ distribution is well described by a two‐stage Gaussian distribution with modes at 0.11 and 0.14. The close proximity of these modes suggests that there is no fundamental dichotomy of surfaces on Venus insofar as their ρ properties, in contrast with roughness. Less than 15% of Venus has ρ values low enough to indicate a major soil component on the surface. Approximately 27% of the surface is dominated by low‐porosity materials such as bedrock, and less than 15% is enriched in high dielectrics. The rest of the surface (43%) is most simply envisioned as partially mantled bedrock, perhaps an extension of the types of terrain viewed by the Venera 10 and 13 landers. The most plausible model for the highest ρ (and thus highest κ) materials requires enrichment in Ti and Fe (e.g., minerals such as rutile, ilmenite, and magnetite). High‐titanium basalts such as those found on the moon would produce the required enrichment, as would pyrites. Since surface geochemical measurements demonstrate that there are basalts in the Venusian plains, a model in which high‐titanium basalts are exposed at the highest elevations (Maxwell, Theia, Atla, Ovda) is favored. Possible weathering of ilmenite in such basalts to produce rutile could explain the high‐κ materials in less elevated regions. When correlated with elevation, κ exhibits a complex nonmonotonic trend in which both decreases and increases are observed. A major decrease in ρ of ∼0.02 km −1 in the upper plains contrasts with the almost 1° rms km −1 increase in surface roughness over the same interval and may be an expression of increased soil production, perhaps due to enhanced breakdown of silicates into carbonates in the lower highlands. Alternately, the decrease could be due to increased centimeter scale roughening, perhaps caused by the increase in regional slope in the highlands. A major increase in ρ (0.05 km −1 ) in the middle highlands correlates with a rapid rise in rms slope. Unlike rms slope, however, there is no overall correlation of ρ with either elevation or regional slope, suggesting that it may be a more locally controlled parameter. Hierarchical clustering analysis of the ρ, roughness, and regional slope properties of Venus demonstrates that distinct subregions are best defined on the basis of topographic zones in which the radar parameters follow well‐defined trends. The lowland plains are extremely smooth at scales from centimeters to 100 km. In general, no single radar parameter (e.g., ρ) serves to subdivide the surface into distinct geologic regions, but taken together, the radar parameters can be statistically correlated to define meaningful radar geologic units. Such units appear to correlate with the kinds of surfaces that can be seen on Venus at kilometer resolution.