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.

The first global atmospheric model of Ni (WACCM-Ni) has been developed to understand recent observations of the mesospheric Ni layer by ground-based resonance lidars. The three components of the model comprise: the Whole Atmospheric Community Climate Model (WACCM6); a meteoric input function derived by coupling an astronomical model of dust sources in the solar system with a chemical meteoric ablation model; and a comprehensive set of neutral, ion-molecule and photochemical reactions pertinent to the chemistry of Ni in the upper atmosphere. In order to achieve closure on the chemistry, the reaction kinetics of three important reactions were first studied using a fast flow tube with pulsed laser ablation of a Ni target, yielding k(NiO + O) = (4.6 +/- 1.4)e-11; k(NiO + CO) = (3.0 +/- 0.5)e-11; and k(NiO2 + O) = (2.5 +/- 1.2)e-11 cm3 molecule-1 s-1 at 294 K. The photodissociation rate of NiOH was computed to be J(NiOH) = 0.02 s-1. WACCM-Ni simulates satisfactorily the observed neutral Ni layer peak height and width, and Ni measurements from rocket-borne mass spectrometry. The Ni layer is predicted to have a similar seasonal and latitudinal variation as the Fe layer, and its usually broad bottom-side compared with Fe is caused by the relatively fast NiO + CO reaction. The quantum yield for photon emission from the Ni + O, observed in the nightglow, is estimated to be between 6 and 40%.