[P-PS06] 火星と火星衛星

[PPS06-12] Global modeling of atmospheric water vapor exchange with inhomogeneous regolith on Mars

*古林 未来¹、鎌田 有紘¹、黒田 剛史¹、黒川 宏之²、青木 翔平³、中川 広務¹、寺田 直樹¹ (1.東北大学大学院理学研究科地球物理学専攻、2.東京大学大学院総合文化研究科広域科学専攻、3.東京大学大学院新領域創成科学研究科複雑理工学専攻)

キーワード：火星、水循環、地表面

Mars has a water cycle in which water vapor sublimates from polar caps of water ice in both hemispheres and is carried toward the other hemisphere along with the atmospheric circulation. Porous materials of “regolith,” which constitutes the shallow part of Mars, store water and supply water vapor to the atmosphere in response to the change in the ground temperature. Regolith is thought to influence the diurnal cycle on Mars from the observations at a few landing sites and modeling (Zent et al., 1993, 1997; Jakosky et al., 1997; Steele et al., 2017; Savijärvi et al., 2016, 2020). However, the diurnal and seasonal variations of water vapor exchange between the regolith and the atmosphere, as well as effects on the entire Martian water cycle, are still poorly understood. Previous models that explicitly calculated water vapor diffusion in the subsurface have assumed globally homogeneous regolith properties for the simplicity of the models (Böttger et al., 2005; Schorghofer & Aharonson, 2005; Steele et al., 2017). However, the Martian regolith properties have wide ranges globally, and Pommerol et al. (2009) have examined the adsorption efficiency of the Martian regolith analogs, finding that small grains have higher adsorption efficiency due to their larger specific surface areas than large grains. In this context, we have newly implemented a regolith scheme into a Mars Global Climate Model (MGCM), considering the inhomogeneity of regolith properties on the Martian water cycle to investigate the meso- and global-scale effects of water vapor exchange between the regolith and the atmosphere, especially the effects of inhomogeneous regolith properties on this surface interaction. The regolith scheme has been developed based on Zent et al. (1993), Böttger et al. (2005), and Steele et al. (2017). The inhomogeneous regolith properties of grain size, porosity, and the adsorption coefficients (as a function of the specific surface area) have been considered based on empirical equations (Presley & Christensen, 1997; Sizemore & Mellon, 2008; Bryson et al., 2008) with observational data from the Thermal Emission Spectrometer (TES). Our results of the diurnal variations of water vapor fluxes show that the upward fluxes, which release water vapor into the atmosphere, are small around Arabia Terra and Olympus Mons with large adsorption efficiencies. This is because the large adsorption efficiency results in less water vapor in the subsurface and less water vapor is supplied to the atmosphere. The southern hemisphere has lower adsorption efficiency and higher thermal inertia than the northern hemisphere, where the regolith with small grains and high adsorption efficiencies is concentrated. The water vapor exchange is more active in the south than in the north because relatively small diurnal variations of surface temperatures reduce the diurnal variations of atmospheric water vapor contents and make the large differences in water vapor content between the regolith and the atmosphere. In addition, our results show that the seasonal variations of the water vapor fluxes are 10 times larger in high latitudes than in low latitudes. The water vapor fluxes are maximized in summer at all latitudes. The water vapor exchange occurs all year round at the equator, though it is limited in summer in high latitudes. This study suggests that the adsorption coefficients influence the Martian water cycle, emphasizing the importance of considering inhomogeneous regolith properties in modeling the water vapor exchange between the regolith and the atmosphere.