Context Images for Venus Express Radio Occultations: a Search for a Dynamical-Convective Origin of Cloud-top UV Contrasts

Abstract:

It has been known for many decades that Venus shows strong contrasts when observed at UV wavelengths, as opposed to longer wavelengths in the visible and NIR, where Venus looks very homogeneous. This has been explained by the presence of a so-called UV absorber, which chemical identity is still unclear. Two hypothesis concerning the source and distribution of the UV-absorber have been put forward. For one of them the argument is that the absorbing substance is being transported from below the clouds up to the cloud top level by means of convection. This implies that in regions with more convection the absorber would be more abundant at the cloud-tops, thus resulting in lower brightness when observed in the UV. In the other scenario, it is haze-forming material which is brought to the cloud-tops by convection; in this case regions with stronger convection produce higher cloud tops would therefore show brighter in the UV.
The premise behind the current analysis is to combine data from the Venus Express Radio Science experiment (VeRa) and from the Venus Monitoring Camera (VMC), to search for any correlation between the temperature structure (Tz and static stability, Sz) as sounded on one specific location and the UV brightness of that same location.
Between 25 November and 31 December 2013 a special observing “South Polar Dynamics” campaign was performed with Venus Express. On each orbit one VeRa atmosphere sounding was acquired shortly after the pericentre passage, as well as a series of VMC-UV images capturing the very VeRa sounding location on that orbit before and after the sounding as it moves across the planet pushed by the zonal and meridional winds. Additional data was found from the another 42 orbits from entire mission Venus Express data set. The sounded latitudes varied between 0˚and -83º.
The question is whether or not we can identify any statistically significant correlations between the temperature structure at a given location and the relative UV-brightness at that same location. We compare Tz and Sz for levels between 50 and 80km altitude to the relative UV brightness for 30 orbits. No significant correlations have been found.