HR: 1330h
AN: P42A-0425    [PDF]
TI: Climate Transition on Mars: CO2 Exchange Process Between Atmosphere and Ice Caps Under Paleoenvironment
AU: Kuramoto, K
EM: keikei@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Kita-10 Nishi-8 Kita-ku, Sapporo, Hokkaido, 060-0810 Japan
AU: * Yokohata, T
EM: yokohata.tokuta@nies.go.jp
AF: National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba, Ibaraki, 305-8506 Japan
AU: Odaka, M
EM: odakker@gfd-dennou.org
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Kita-10 Nishi-8 Kita-ku, Sapporo, Hokkaido, 060-0810 Japan
AB: Several lines of evidence suggest that climate warming and cooling have repeated on Mars in its history. In order to study the stability and evolution of Martian climate, we construct a 2-dimensional (horizontal-vertical) energy balance climate model. The long-term CO2 mass exchange processes between the atmosphere and CO2 ice caps are investigated with particular attention to the albedo effects of planetary ices (H2O and CO2) on the climate stability. We make numerical simulations of the climate transition between warm and cold climate states taking into account the evolution of CO2 ice cap topography (areal extent and altitude). Our results are summarized as follows. A few bars of CO2 atmosphere presumed for early ($\sim$ 3.8 Ga) warm and wet Mars possibly begins to condense irreversibly onto polar caps when H2O ices would cover more than several tens % of the Martian surface. This is because the effect of ice albedo causes polar cooling and thus promotes the condensation of atmospheric CO2. Once such condensation is triggered, rapid transition into cold climate state (with lower atmospheric pressure like the present one) occurs within a time scale of about 1000 years. This is caused by the acceleration of CO2 condensation associated with further polar cooling due to the atmospheric pressure drop. We call this transition process "collapse condensation". If the collapse condensation of a few bars of CO2 atmosphere occurs, the resultant polar CO2 ice caps extend toward latitudes as low as 80-70 degrees with mean thickness of several 100 meters. This areal extent is consistent with the distribution of the present polar layer deposits (PLD), and thus PLD may have some information of such CO2 glaciation in the past. On the other hand, the low atmospheric pressure buffered by the CO2 ice caps is possibly unstabilized when the CO2 ice albedo decreases or the Martian obliquity increases. If the ice caps contain a large amount of CO2 (e.g. formed by the collapse condensation), such instability causes the climate jumps to a warm state with high atmospheric pressure owing to complete evaporation of the ice cap. We call this process "runaway evaporation". The time scale for the completion of runaway evaporation is similar to that of the collapse condensation.
DE: 5407 Atmospheres--evolution
DE: 5462 Polar regions
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting