Climate Change of Mars: Effects of Obliquity

Introduction: Behaviors of the Martian climate system has been investigated in several studies [e.g., 1-4]. If the Martian climate was warm owing to the greenhouse effect of CO 2 in the past, a drastic climate change (climate jump or climate collapse) due to decrease in the amount of CO 2 in the system must have occurred during the evolution from a warm condition to the present cold condition [1, 3]. On the other hand, it is suggested that pulses of CO 2 injected into the atmosphere could place the atmosphere back into a warm state with higher atmospheric pressure [2]. However, these previous studies considered the climate system under the condition of the present obliquity. It is known that the obliquity of Mars could have changed greatly (from 0° to 60°) on short timescales [e.g., 5]. The obliquity change would have profound effects on the climate system of Mars because it alters the latitudinal distribution of the solar radiation. François et al. [6] argued that permanent CO 2 ice caps are stable only under the low obliquity. They proposed that, if the obliquity increases, the atmospheric pressure increases and, therefore, the permanent ice caps become unstable due to enhanced heat transport from the low latitude. Although it is one of the controlling factors for the stability of the permanent CO 2 ice, other factors may play important roles. For example, when the obliquity becomes large, the solar radiation income onto the pole increases. Also, if the atmospheric pressure increases, the planetary albedo, the greenhouse effect, and the freezing point of CO 2 should change. The energy balance at the pole will depend on all these factors. In this study, we evaluate these effects and discuss the stability of the permanent CO 2 ice more quantitatively from the viewpoint of the energy balance. We also discuss effects of low solar luminosity on behaviors of the Martian climate. Model: We adopt a time-dependent latitudinally-one-dimensional energy balance climate model for Mars based on the model developed by [4]. We improved the previous model to treat the energy balance of the ground and the atmosphere separately. The model considers the latitudinal temperature distribution , the areal extent of CO 2 ice, and the meridional heat transport. We also consider seasonal changes of the amount of CO 2 ice and the latent heat owing to sublimation and condensation of CO 2. The atmospheric pressure should …