Modelling by astrobiologists Mark Bullock and David Grinspoon of Venus' atmospheric evolution suggests that the between the current 92-bar atmosphere and existing surface minerals, particularly calcium and magnesium oxides, is quite unstable, and that the latter could serve as a sink of carbon dioxide and sulfur dioxide through conversion to carbonates. If these surface minerals were fully converted and saturated, then the atmospheric pressure would decline and the planet would cool somewhat. One of the possible end states modelled by Bullock and Grinspoon was a atmosphere and surface temperature. To convert the rest of the carbon dioxide in the atmosphere, a larger portion of the crust would have to be artificially exposed to the atmosphere to allow more extensive carbonate conversion. In 1989, Alexander G. Smith proposed that Venus could be terraformed by lithosphere overturn, allowing crust to be converted into carbonates. Landis 2011 calculated that it would require the involvement of the entire surface crust down to a depth of over 1 km to produce enough rock surface area to convert enough of the atmosphere.