Thermal anomaly on Venus’s mesosphere

The Venus mesosphere (~60 - 100 km) is bounded below by the massive, cloudy, super-rotating troposphere and above by the thermosphere/cryosphere. Despite the fact that Venus has negligible obliquity (< 4K) and that the mesosphere is subjected to intense solar radiative forcing, most of this atmospheric layer has an anomalous, reversed pole-to-equator thermal structure with polar temperatures up to 10 K warmer than those over the equator [1]. This thermal structure implies net (solar – thermal) radiative heating in the Venus mesosphere. Here, we employ the Spectral Mapping Radiative Transfer (SMART) model to estimate the net heating rates. We used updated constraints on the atmospheric thermal structure and composition from ground-based observations and results from the Venus Express mission. The net heating is thought to be maintained by a thermally-indirect meridional circulation driven by interactions between the zonal super-rotation and atmospheric thermal tides [2]. Also, we updated Sulfur dioxide concentration to reach global heat balanced. The meridional circulation could play a critical role in the transport of trace gases and the production of sulfuric acid aerosols at levels throughout the Venus mesosphere. This circulation may imply the lofting of sulfuric acid aerosols containing enriched HDO, thereby explaining the dramatic enhancement of the D/H ratio[3] in the upper atmosphere of Venus > 100 km. [1] Limaye, S.S., Grassi, D., Mahieux, A. et al.Venus Atmospheric Thermal Structure and Radiative Balance. Space Sci Rev 214, 102 (2018).[2] Crisp, D. (1986). Radiative forcing of the Venus mesosphere: I. solar fluxes and heating rates. Icarus, 67(3), 484-514.[3] Mahieux, Arnaud, Sébastien Viscardy, Roger Vincent Yelle, Hiroki Karyu, Sarah Chamberlain, Séverine Robert, Arianna Piccialli et al. "Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere." Proceedings of the National Academy of Sciences 121, no. 34 (2024): e2401638121.