Imaging Venus' surface at night in the near-IR from above its clouds: New analytical models for the effective spatial resolution, illustrated with new Parker Solar Probe data

Abstract

There are a handful of spectral windows in the near-IR through which we can see down to Venus' surface on the night side of the planet. The surface of our sister planet has thus been imaged by sensors on Venus-orbiting platforms (Venus Express, Akatsuki) and during fly-by with missions to other planets (Galileo, Cassini). The most tantalizing finding, so far, is the hint of possible active volcanism. However, the thermal radiation emitted by Venus' searing surface (c. 475 degrees C) has to get through the opaque clouds between 50 and 70 km altitude, as well as the sub-cloud atmosphere. In the clouds, the light is not absorbed but scattered, indeed, many times. This results in blurring the surface imagery to the point where the smallest discernible feature is roughly 100 km in size, full-width half-max (FWHM), and this has been reproduced using numerical models. We propose a new analytical modeling framework for predicting the width of the atmospheric point-spread function (APSF) that determines the effective resolution of surface imaging from space. Our best estimates of the APSF width for the 1-to-1.2 micron spectral range are clustered around 130 km FWHM. Interestingly, this is somewhat larger than the accepted value of 100 km based on both visual image inspection and detailed numerical simulations. Lastly, we apply the new modeling framework to the fly-by imaging by the Parker Solar Probe in a somewhat shorter wavelength band.