Exchange of water vapor between the atmosphere and surface of Mars

A model for exchange of water from the atmosphere to condensing CO₂ caps is developed. The rate of water condensation in the caps is assumed to be proportional to the meridional heat flux. It follows that the amount of water condensed in the caps varies inversely with the amount of CO₂ condensed. The seasonal phase of the release of water from the caps is not consistent with observed variations in the abundance of atmospheric water. Seasonal variations of atmospheric water abundance are most consistent with vapor exchange between the atmosphere and permafrost in the subtropics. Although water condensation in semipermanent caps is normally very slow, it may take place at a much faster rate at unusually high atmospheric temperatures, such as those produced by absorption of solar radiation by airborne dust.