The seasonal behavior of water on Mars.

Seasonal and spatial variations in the column abundance of water vapor in the Martian atmosphere are due to the combined effects of exchange of water with non-atmospheric reservoirs and transport of water within the atmosphere. The non-atmospheric reservoirs include the seasonal polar caps, the residual polar caps, adsorbed water within the regolith, and possible surface or near-surface ground ice. Atmospheric transport occurs via the global circulation. Transport is primarily as vapor, although some transport as condensate occurs. The mechanism which drives most of these processes is the solar heating; latitudinal variations in insolation drive the atmospheric circulation, while seasonal changes drive the exchange with the non-atmospheric reservoirs. Observations of the seasonal changes in atmospheric water content, combined with quantitative numerical models of the exchange and transport processes, have led to our present understanding of the importance of each process. These observations and models are summarized, along with the inferences concerning the behavior of the seasonal water cycle.