Evaluation of the bandwidths and spatial resolutions achievable with near-infrared observations of Venus below the cloud deck

Abstract

We identified potential atmospheric windows with which to study the Venusian surface on the nightside and calculated the approximate scattering footprint as would be seen by a prospective aerial mission. The emission from the surface, its scattering through the atmosphere, and the emission from the atmosphere were calculated from the surface to various heights beneath the cloud deck. We explored the effects of different sensor altitudes, varying surface emissivity, variation in elevation causing changes in surface temperature, and variations in the temperature profile, on the possible surface viewing atmospheric windows. The effects of surface emissivity and surface elevation dominated, and those of sensor altitude and variation in temperature profile are relatively small. The prospective windows are largely expanded versions of previously identified windows that have been exploited by satellites and ground-based observation with an additional window centered at 1.27 μm. Any mission that exploits the improved visibility beneath the cloud decks should make use of 0.7–1.0, 1.1, and 1.27 μm windows, as these provide a comprehensive ability to extract information about the surface. We explored the effect of sensor altitude and surface elevation on the scattering footprint. The scattering footprint beneath the cloud deck varies from ~15 to 5 km depending on wavelength at our nominal altitude of 40 km, a significant improvement over the 50–100 km footprint for orbiting satellites. Lowering the sensor altitude only mildly reduces the scattering footprint, as most scattering is caused by the dense lowermost atmosphere. Increased surface elevation reduces the scattering footprint. We concluded that regions elevated above the mean planetary radius represent the best targets as these have the greatest amount of the electromagnetic spectrum identified as surface viewing atmospheric windows and the smallest scattering footprint. These windows have the potential to elucidate questions about the composition and redox state of the surface of Venus, even for low emissivity materials (e.g., fine-grained hematite at ε~0.5), which have important implications for the evolution of the planet.