The Dust Cycle on

Ralph Lorenz, Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, USA.

Before the greenhouse effect due to carbon dioxide on Venus was fully recognized, the idea that wind friction and dust in the lower atmosphere (the 'aeolosphere') might warm the surface was advance by Öpik (1861), and a dust insulation model was explored by Hansen and Matsushima (1967). Like many "wrong" ideas in science, these hypotheses may yet contain an element of truth.

Measurements by the Venera and Pioneer Venus descent probes show that the Venus atmosphere below the main cloud deck is 'clear'. However, it should be recognized that this may be a somewhat relative term – the dynamic range of the instrumentation used is modest, and the fact that there is already so much opacity above 40km makes radiative-convective models somewhat insensitive to the introduction of dust in the lower atmosphere. Both Pioneer Venus and Venera measurements show variations in opacity in the lowest few kilometers – implying, perhaps, sources and sinks. Recently, Venera -13 and -14 electrical data have been suggested to be consistent with the presence of charged aerosols in the lowest 40km of the atmosphere (Lorenz, 2018).

It is striking that out of only two occasions (Venera 13 and 14) when multiple panoramas were taken at a landing site in the space of about an hour, sand and dust were observed to move. Such an event might typically take months or years to observe on , or ! It can therefore be argued (Lorenz, 2015) that surface windspeeds on Venus not infrequently exceed the transport (saltation) threshold and the possibility exists that surface dust-lifting may be common. Additionally, there is the prospect of injection of volcanic ash into the atmosphere. Some of these processes may also be associated with electrical activity. Compared to Earth and Mars, the effectiveness of snow-out or rain-out processes on Venus will be poor.

The possible dust generation, injection and removal processes on Venus will be reviewed and the extent and character of lower atmospheric dust considered. In-situ measurements (optical and/or electrical) on future missions will be needed to resolve the question.