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.