Ephemeral Ice Clouds in the Upper Atmosphere of Venus

Venus is well known for extreme heat at its surface and being shrouded in clouds composed of sulphuric acid. However, there are regions of Venus’ atmosphere around 120 km that are cold enough to harbour ice clouds, under conditions similar to the upper mesospheres of Earth and Mars where ice clouds form. In this presentation we will show, using published satellite products and numerical modelling, that the upper mesosphere of Venus can be cold enough for both H₂O and CO₂ to condense and form particles. Amorphous solid water particles (ASW) are likely to nucleate both heterogeneously on meteoric smoke (formed from the condensation of the metallic vapours which ablate from cosmic dust particles entering  Venus’ atmosphere) and also homogeneously, resulting in clouds of nano-scaled particles at around 120 km that will occur globally. The temperatures may become cold enough (below ~90 K) that CO₂ particles nucleate on ASW particles. Taking account of the uncertainty associated with retrievals of temperature in the upper mesosphere (using the SOIR instrument on Venus Express), CO₂ ice cloud formation could occur more than 30% of the time poleward of 60^o. Since the main component of Venus’ tenuous atmosphere is CO₂, any CO₂ crystals that form will grow and sediment on a timescale of a few minutes. Mie calculations show that these Venusian mesospheric clouds (VMCs) should be observable by contemporary satellite instruments, although their short lifetime means that the probability of detection is small. We suggest that VMCs are important for the redistribution of meteoric smoke and may serve as a cold-trap, removing some water vapour from the very upper mesosphere of Venus through the growth and sedimentation of cloud particles, and possibly reducing the loss of water to space.