Effect of Venus surface illumination on photographic image texture

Abstract

Radar images from Magellan have provided tremendous opportunities to study the geology and bulk dielectric properties of Venus, but many questions can only be addressed through direct photography or imaging spectroscopy of the surface. The thick atmosphere creates a situation of nearly isotropic illumination, so true shadows cannot exist for even very rough terrain. We carried out ray-tracing simulations for synthetic surfaces under both isotropic and directional illumination conditions. Relative to similar surfaces viewed under directional illumination, venusian terrain will exhibit an equal or greater range of mean brightness with changing roughness, but a concurrent reduction in the maximum possible standard deviation of brightness across any region of given roughness. The local variability of an image is often referred to as texture, and these results indicate that Venus geologic units will appear more homogeneous in brightness than the same surfaces viewed under favorable directional illumination. In addition, the lack of a preferred lighting direction makes identification of surface structural patterns difficult. A similar situation may occur on Titan.