Current models for the formation of the abundant large closed depressions of Valles Marineris have serious shortcomings. Purely tectonic mechanisms are inconsistent with the morphology of many depressions, and removal of large quantities of ground ice from the canyon walls is inconsistent with the observed strength of the walls. Accordingly, we offer some alternatives. One possibility involves decay of ice-rich bodies occupying partially sediment-choked ancient graben that predated the overlying cratered and ridge plains. Other possibilities involve the removal of massive equatorial carbonate deposits storing much of the planet's CO2 inventory, generated during greenhouse conditions on early Mars. Solution by carbonic acid derived from the atmosphere (analogous to terrestrial karst) requires extensive recycling of the available water supply. Solution by various ground water acids, possibly derived from the Tharsis magmas, requires less water, especially if only the smaller closed depressions are due to carbonate decay. Alternatively, volume loss due to decarbonation of carbonate during early high heat flow or a later Tharsis-related heat pulse can produce extensive collapse, especially if the carbonates have high silica content. ¿ American Geophysical Union 1990