The present climate of Mars is extremely cold and dry and is characterized by an extremely thin CO
2 atmosphere. However, geological evidence suggests that the Martian climate in the past might have been warm and wet. If this were the case, the atmospheric CO
2 pressure would have been several bars higher. Our numerical study suggests that permanent CO
2 ice caps (CO
2 ice sheets) would have played an important role in the evolution of Mars from having a warm climate with a dense CO
2 atmosphere to the present cold climate. Owing to the instability of the CO
2 system, which was triggered by impact erosion and/or sputtering of the atmosphere, the dense CO
2 atmosphere should have collapsed to form large polar caps. The collapse corresponds to a runaway condensation of the atmospheric CO
2 into large CO
2 ice caps (climate jump). Our investigation of the CO
2 ice caps by using an ice sheet model suggests that the temperature at the bottom of the CO
2 ice cap would exceed the melting point of CO
2 due to the geothermal heat flux prevalent in Mars. In that case, gaseous or liquid CO
2 produced by basal melting might have permeated and diffused into the subsurface through the pores of the Martian crust. It is possible that the CO
2 removed from the surface presently exists in the Martian subsurface as clathrate, carbonate, and/or liquid and gaseous phases.
View full abstract