Thermal structure of the Venusian atmosphere from the sub-cloud region to the mesosphere as observed by radio occultation

Zonally and temporally averaged temperature and static stability distributions in the Venusian atmosphere from 40 km to 85 km altitudes are obtained by Venus Express and Akatsuki radio occultation measurements. At latitudes equatorward of 75$^\circ$, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements; there is a low-stability layer at altitudes of 50-58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. At latitudes poleward of 75$^\circ$, on the other hand, a low-stability layer extends down to 42 km, which has not been unreported by any previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the Earth's troposphere, which generally has a deeper low-stability layer at low-latitudes than at mid- and high-latitudes.