High altitude clouds on Venus and Mars

Curiosity Rover's Three-Frame Mosaic of Clouds

Recent missions reveal conditions where CO₂ and H₂O ice clouds can exist in the mesospheres of both Mars and Venus. Comparable ice clouds in Earth’s atmosphere are important for the redistribution of condensable materials, atmospheric chemistry and, in the case of denser ice clouds, radiative transfer. These cloud-atmosphere interactions depend critically on ice particle size and number, which is determined by the nucleation mechanism. However, our understanding of how ice particles in these clouds form in the mesospheres of Mars and Venus is in its infancy. In this project we will elucidate the nucleation pathways for these mesospheric clouds by: i) experimentally determining key parameters to quantify the

nucleation of solid CO₂ on H₂O ice, which we argue is of relevance for the highest clouds in both the atmospheres of Mars and Venus; ii) experimentally test the hypothesis that hygroscopic growth of very viscous sulphuric acid solution droplets in the upper haze layer of Venus leads to their freezing when they become sufficiently dilute and cold; iii) use 1-D models to understand the interplay between cooling rate and nucleation in order to predict hydrometeor size and number concentration and use the new contact angle parameterisations to define CO₂ nucleation in the Laboratoire de Météorologie Dynamique (LMD) Mars General Circulation Model (LMD-GCM) in order to better reproduce CO₂ ice cloud occurrence.

This project is funded by the UK Science and Technology Facilities Council. The investigators are John Plane, Ben Murray (U. Leeds, School of Earth & Environment), and Tom Mangan, together with Anni Määttänen (LATMOS, Sorbonne University, Paris)