Mars: The case against permanent CO_2 frost caps

Leighton and Murray have argued that there is a polar reservoir of solid CO_2 on Mars that lasts throughout the year and whose vapor pressure determines the mean partial pressure of CO_2 in the atmosphere. This model is discussed in the light of recent data, and several difficulties emerge. First, such a system might be unstable, owing to the tendency of poleward heat transport to increase with atmospheric pressure. Second, the annual retreat of the CO_2 frost cover would be slower according to the model than that observed. Moreover, the observations seem to indicate that the residual polar cap that lasts throughout the year is composed of water ice rather than CO_2. Finally, observations of water vapor in the atmosphere appear to be inconsistent with a permanent CO_2 cold trap in continuous existence for many years. These difficulties hold also for a CO_2 reservoir buried by water ice and for a hydrated CO_2 clathrate. If Leighton and Murray's model does not apply, several alternatives remain. First, the total accumulated CO_2 may simply be equal to that observed in the atmosphere. Second, there may be a buried reservoir of CO_2 that is not in vapor equilibrium with the atmosphere. Third, adsorption of CO_2 and water in the Mars regolith may control the amounts of these compounds observed in the atmosphere at present. Unfortunately, not one of these alternatives provides a satisfactory quantitative theory for the present CO_2 partial pressure in the atmosphere