Abstract
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
modelling, that the upper mesosphere of Venus is sufficiently cold that
both H2O and CO2 can condense to form particles. In fact, we show that
there is likely to be a competition between the direct nucleation of
particles from the gas phase (homogeneous nucleation) and the nucleation
on meteoric smoke particles (MSPs, heterogeneous nucleation). Amorphous
solid water particles (ASW) are likely to nucleate first, resulting in
clouds of nano-scaled particles at around 120 km globally. The
temperatures can then become sufficiently low that CO2 particles can
nucleate either on MSPs or on ASW particles (>30% of the
time poleward of 60°). Since the main component of the atmosphere is CO2
these particles will grow and sediment on a timescale of 10-20 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.