Ephemeral Ice Clouds in the Upper Mesosphere of Venus

The conditions in Venus' upper mesosphere at around 120 km have some similarities to the upper mesosphere of Earth and Mars where ice clouds form. Here we show, using published satellite products and numerical modeling, that the upper mesosphere of Venus can be sufficiently cold that both H2O and CO2 may condense to form particles. We show that amorphous solid water particles (ASW) are likely to nucleate both heterogeneously on meteoric smoke and also homogeneously, resulting in clouds of nano-scaled particles at around 120 km that will occur globally. The temperatures may then become sufficiently low, below similar to 90 K, that CO2 particles can nucleate on ASW particles. Given the uncertainty associated with retrievals of temperature in the upper mesosphere, it is unclear how frequently this occurs, but it could be >30% of the time poleward of 60 degrees. Since the main component of Venus' tenuous atmosphere is CO2, any CO2 crystals that form will grow and sediment on a timescale of 10-20 min. 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 vapor from the very upper mesosphere of Venus through the growth and sedimentation of cloud particles, and possibly reducing the loss of water to space.