Evolution of the Martian Climate System: Effects of Rotation-Axis Inclination

Greenhouse effect of CO₂ may have been the most important factor for the Martian surface environment throughout the history of Mars. Polar ice caps and surface regolith are considered to be large CO₂ reservoirs to exchange CO₂ with the atmosphere on the present Mars (atmosphere-ice caps-regolith (AIR) system). We construct a one-dimensional energy balance climate model (EBM) for the AIR system of Mars under the condition of seasonal changes of solar radiation income. In this study, we investigate behaviors of the EBM under the condition of seasonal change of the solar incident flux and variations in the obliquity. Steady state solutions derived from the EBM can be classified into four cases: (i) a solution which has residual ice caps in summer (residual-cap solution), (ii) a solution which does not have residual ice caps, but has seasonal ice caps during the winter (seasonal-cap solution), (iii) a solution which has no ice caps throughout the year (no-ice-cap solution), and (iv) a solution which has a residual cap in one pole and a seasonal ice cap in another pole. The solutions (i) and (iv) are combined into (A) ``residual-cap regime" in which the annual mean atmospheric pressure is constant irrespective of the total amount of CO<sub>2</sub> in the AIR system. The solutions (ii) and (iii) are combined into (B) ``no-residual-cap regime" in which the atmospheric pressure depends on the amount of CO₂ in the AIR system. We found that the Martian climate could have been changed abruptly and drastically during its history due to decrease in the amount of CO₂ in the AIR system. This is a ``climate jump" during the long-term evolution (10⁸ ~ 10⁹ years) of the Martian climate. On the other hand, it is known that obliquity of Mars could have changed continually between 0^o and 60^o on short time-scale (10⁵ ~ 10⁶ years). Our numerical results show that obliquity change has profound effects on the climate system of Mars. The obliquity change could cause a climate jump in the Martian climate system on short timescale. We found that the climate jump could have occurred repeatedly in short-term cycles during the Martian history due to changes in the obliquity of Mars.