Detecting surface change on Venus from Magellan and VERITAS radar images

Abstract

Detection of surface changes due to recent volcanism on Venus is a major focus of upcoming orbiting radar missions like VERITAS (Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy). The longest time baseline is to the Magellan mission, but these images will differ in wavelength (3.8 cm for VERITAS and 12.6 cm for Magellan), and often in incidence angle coverage. We explore the range of backscatter differences due to self-affine scaling properties of surface topography, using a model derived from terrestrial measurements. For moderately rough surfaces at least a 3-dB difference in backscatter coefficient above (rougher new flows) or below (smoother new flows) a model-scaled Magellan echo is required for confident change detection given uncertainty in the Hurst exponent of the topography. As roughness increases, echo saturation occurs due to diffuse scattering, and detectable change is due only to smoothing of the surface between the two missions. Quasi-specular echoes make change detection more challenging when the Magellan incidence angle is <30o (corresponding to latitudes below 35oS or above 55oN). At higher latitude, stronger correlative evidence from geologic context and more attention to local slope is required to infer surface change. There will be significant differences in the extent of mapped surficial mantling units between the two wavelengths. Further radar image data linked with terrestrial field measurements at the few-cm scale can validate the empirical model that relates surface roughness to wavelength dependence in radar scattering.