13:45 〜 14:00
[PPS06-11] Modeling the evolution of water on Mars over the past millions of years
キーワード：火星、水環境、地下氷、全球気候モデル、全球氷床モデル、ミランコビッチサイクル
Mars is an extremely cold planet with limited surface water reservoirs in the polar caps. However, a series of Mars exploration missions have revealed that a large amount of water ramains beneath the surface. Recently, HiRISE (High Resolution Imaging Science Experiment) onboard the MRO (Mars Reconnaissance Orbiter) captured high-resolution images of ejected ice and ice-exposing scarps, and SHARAD (SHAllow RADar) provided detailed views of the structure and extent of ice, especially in the polar caps and buried at mid-latitudes (e.g., ~1.43×105 km3 of water ice remains beneath Utopia Planitia at 40°~50°N). In addition, MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) onboard the Mars Express revealed liquid water beneath the south polar ice cap, and extensive subsurface ice deposits in equatorial regions such as the Medusae Fossae Formation (e.g., up to 3.96×105 km3). Several modeling studies assuming a subsurface ice table have suggested that water ice is unstable at the pressure-temperature conditions found at the surface or subsurface of low/mid-latitude Mars, and the subsurface ice in low to mid-latitudes is considered to be the remnants of past high obliquity periods. To quantify why water ice still exists in the low and mid-latitude subsurface of Mars, it is necessary to calculate the evolution of the subsurface environment over a long time series with varying planetary and orbital conditions.
Here, we have newly developed a fully coupled global water circulation model for the atmosphere, hydrosphere, and cryosphere down to a depth of 1 km in the subsurface. We performed millions of years of time series simulation using an iterative time integration scheme with changing Martian obliquity, eccentricity, and perihelion longitude over the last few million years using the Laskar et al. (2004) dataset. Our model implemented a water exchange scheme between the atmosphere and the regolith/crust for different porosity, grain size, and thermal property. As an initial water reservoir, we assumed the north polar layer deposit.
Our simulations show that during epochs with obliquity greater than 30°, substantial water transport from the polar caps to the subsurface occurred, leading to the formation of porous ice even in mid-latitudes above 30°. The water in the regolith showed a gradual diffusion from shallow to deeper layers, reaching a volume of about 105 km3 in the subsurface. This effect was particularly pronounced when the northern hemisphere summer solstice coincided with perihelion, resulting in an increased atmospheric water vapor humidity and a consequent increase in water transfer to the regolith. The near-surface layers were strongly affected by the atmospheric variation associated with the approximately 120,000-year cycle of obliquity variations. However, deeper underground, beyond the influence of these near-surface environmental changes, our results indicate that the ice deposited in the mid-latitudes remained relatively stable over long periods of time. While no significant vertical transport of water into the deep subsurface was observed in the high latitudes, active vertical diffusion of water to depths of 100 meters was found in regions of latitude below 30°, as observed by SHARAD and MARSIS. Although our modeling study covered a total simulation period of 1 million years, further extension of the time integration is expected to allow a quantitative calculation of the timing for the formation of substantial subsurface ice deposits in the low and mid-latitude regions.
Here, we have newly developed a fully coupled global water circulation model for the atmosphere, hydrosphere, and cryosphere down to a depth of 1 km in the subsurface. We performed millions of years of time series simulation using an iterative time integration scheme with changing Martian obliquity, eccentricity, and perihelion longitude over the last few million years using the Laskar et al. (2004) dataset. Our model implemented a water exchange scheme between the atmosphere and the regolith/crust for different porosity, grain size, and thermal property. As an initial water reservoir, we assumed the north polar layer deposit.
Our simulations show that during epochs with obliquity greater than 30°, substantial water transport from the polar caps to the subsurface occurred, leading to the formation of porous ice even in mid-latitudes above 30°. The water in the regolith showed a gradual diffusion from shallow to deeper layers, reaching a volume of about 105 km3 in the subsurface. This effect was particularly pronounced when the northern hemisphere summer solstice coincided with perihelion, resulting in an increased atmospheric water vapor humidity and a consequent increase in water transfer to the regolith. The near-surface layers were strongly affected by the atmospheric variation associated with the approximately 120,000-year cycle of obliquity variations. However, deeper underground, beyond the influence of these near-surface environmental changes, our results indicate that the ice deposited in the mid-latitudes remained relatively stable over long periods of time. While no significant vertical transport of water into the deep subsurface was observed in the high latitudes, active vertical diffusion of water to depths of 100 meters was found in regions of latitude below 30°, as observed by SHARAD and MARSIS. Although our modeling study covered a total simulation period of 1 million years, further extension of the time integration is expected to allow a quantitative calculation of the timing for the formation of substantial subsurface ice deposits in the low and mid-latitude regions.