The Effect of Underlying Hazes on Sub-Cloud Nighttime Near-IR Imaging of Venus’s Surface Features

Nighttime near-infrared surface imagery provides unique constraints on surface composition which, due to chemical weathering processes in the high-temperature (~740K), high atmospheric pressure (~92 bar) environment, provides constraints on the age of surface materials, as employed by Smrekar et al (2010, Science 328, 605-608, 2010) via Venus Express orbiter VIRTIS near-infrared imagery to reveal that Venus has been volcanically active in geologically recent times. However, due to the diffusive scattering effects of the intervening thick (τ > 20) near conservative-scattering clouds, the spatial resolution achievable from orbit is restricted to >50 km, severely limiting views of surface processes and geologic history. Our work on the clarity of surface views that can be achieved from just below the clouds at an altitude of ~47 km (~100 K) shows that for such a thick Rayleigh scattering atmosphere, the potential spatial resolution of surface images is limited only by (1) atmospheric seeing, and (2) instrumental characteristics (e.g., aperture size, jitter, exposure time for S/N, etc). Seeing equivalent to the diameter of Jupiter as observed from Earth (~.75 arc-minute) translates to 10-meter surface resolution, likely achievable viewing through the relatively stable environment of Venus’s lower atmosphere. Assessments of a 3-color (1.01, 1.09, and 1.18 μm) imager supported by an IMU moving over the surface with the prevailing zonal winds at altitude indicate that images incorporating 10-meter spatial sampling (20 meter resolution) are achievable from an altitude of 47 km. We have recently included the effects of the sub-cloud haze layer profiled by Knollenberg and Hunten (1980, JGR 85, 8039-8058; hereafter KH) in our analysis. This optically-thin aerosol layer (τ ~ 0.1), extending from 31 to the 47-km viewing altitude, can potentially blur surface images and limit surface contrast. Our preliminary analysis shows that the effect of the KH sub-cloud aerosol profile is minimal, reducing the Rayleigh-only contrast and surface resolution by a few percent. Taking variability due to “weather” into account, our initial results for aerosol profile abundances up to at least 5 times the KH profile indicate that 20-meter surface resolution is still achievable with our present camera design, with sufficient contrast to map 10% surface emissivity variations from point-to-point and from wavelength-to-wavelength. Co-adding pixels to enhance S/N can be used to provide more sensitive spectral sensitivity and/or achieve satisfactory views if heavier “weather” is encountered.