Venus Express/VIRTIS observations of middle and lower cloud variability and implications for dynamics

Abstract

We present an analysis of Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data, carried out to characterize the morphological, geographical, and evolutionary trends of the middle and lower cloud features that are observed in the atmosphere of Venus as variations in brightness temperatures in specific near‐infrared wavelengths. In this preliminary study, we analyze only data collected over the span of 11 orbits. The mean radiance as a function of latitude is consistent with previous ground‐based observations, indicating that the overall global distribution of mean cloud cover is stable, at least on a 10‐ to 20‐year time scale. In contrast with the consistent level of radiance at high latitudes, a significant amount of variability to the radiance exists at lower latitudes, consistent with significant convective activity in the lower and middle cloud decks. The morphology of the holes tends from highly variable orientations of features with aspect ratios of nearly one at low latitudes, to very large aspect ratios and zonally oriented features at higher latitudes. The peak radiance of the holes appears not to demonstrate a latitudinal tendency. There is evidence of more variability to the morphology and radiance of features at lower latitudes. To investigate the evolution of the holes, we examine a sequence of images taken over a 5 h span of a single orbit. If this limited amount of data is representative, then the typical e‐folding time scale for the evolution of a hole is about 1 day.