The key to any discussion of the past and future state of the climate on Venus is an understanding of the production and loss processes for atmospheric gases, from the interior, at the surface, and at the top of the atmosphere where it merges into space. The balance between all of these determines the total mass of the atmosphere, and hence the surface pressure. This, in turn, is the principal factor controlling the temperature and hence the habitability and other characteristics of the surface environment.

At the heart of the problem is an almost complete lack of understanding of the various factors, summarised in Figure 11.1, related to volcanic activity on Venus. Volcanic emissions not only contribute to the mass and composition of the atmosphere, they also fuel cloud formation as part of a complicated cycle of atmospheric and surface chemistry involving various sulphur compounds. The surface itself is mainly of volcanic origin, although this leaves plenty of scope for puzzling about its composition and its capacity for absorbing, as well as emitting, atmospheric gases. The crust is dry and thin but evidently supports some huge volcanic mountains despite apparently being too weak to do so without convective upwelling which, if present, should also drive plate tectonics, although the observational evidence for the latter is elusive.