Models of the Mars polar cap energy balance suggest that the south cap has two possible configurations at the current epoch. It can be covered all year by CO2 frost, as was observed by the Viking spacecraft, or the CO2 frost can disappear at midsummer to expose an underlying surface. The difference between the two states depends on the role of energy conducted into the subsurface: If the cap was exposed the previous year, conducted energy will cause less CO2 frost to condense in winter, and the frost will disappear completely in summer; if the cap was covered the previous year, there will be no conducted energy, and it will stay covered again. Subtle effects in the atmosphere can perturb the cap energy balance sufficiently to cause the cap to jump between states. This behavior might provide an explanation for the large southern summer water vapor abundances observed in 1969. It also explains the observed differences between the north and south residual caps as resulting in part from a difference in the role in the annual energy balance of energy conducted into the subsurface. ¿ American Geophysical Union 1990