11:00 〜 11:15
[PPS05-07] Global simulations of water vapor exchange between the regolith and the atmosphere and its effects on the water cycle on Mars
キーワード：火星、レゴリス、水
The surface of Mars is covered by highly porous, hygroscopic regolith with a porosity of 20-50% (Squyres et al., 1992). It has long been considered that the exchange of water vapor between the regolith and the atmosphere plays a crucial role in the Martian water cycle (Bottger et al., 2005; Savijarvi et al., 2016; 2019; Steele et al. 2017). Recently, a previous study suggests that relative humidity observed by Rover Environmental Monitoring Station (REMS) on the Curiosity rover is explained by the water vapor exchange between the regolith and the atmosphere. (Steele et al., 2017). However, the global effects of water vapor adsorption by the regolith on Mars are highly uncertain.
Here, we implemented a regolith scheme into our Mars General Circulation Model (MGCM) to investigate diurnal and seasonal variations of the amount of water vapor in the atmosphere. Our MGCM, named DRAMATIC (Dynamics, RAdiation, MAterial Transport, and their mutual InteraCtions), solves a self-consistent global water cycle (Kuroda, 2017). Our newly developed regolith scheme is based on Zent et al. (1993), Bottger et al. (2005), and Steele et al. (2017). We ran DRAMATIC with two cases: an inert regolith-atmosphere interaction case and an active regolith-atmosphere interaction case. In the former, there were no regolith effects and in the latter, the model solved the exchange of water vapor between the regolith and the atmosphere, diffusion, adsorption, and condensation in the subsurface with globally-distributed 2 kg m-3 of subsurface ice at each grid point. Water vapor diffusion in the subsurface was calculated with a combination of Fickian and Knudsen diffusions. Regarding Knudsen diffusion, the grain size distribution was obtained from the thermal kinetics of the gas using the observed thermal inertia by the Thermal Emission Spectrometer (TES). The amount of adsorbed water was defined by an adsorption isotherm of Jakosky et al. (1997).
First, we show the diurnal variation in relative humidity and water vapor fluxes through the surface. In the active regolith-atmosphere interaction case, our model showed a consistent diurnal variation in relative humidity with REMS observations at Gale crater after 20 years of calculation. Regolith reduced sublimation fluxes due to the downward fluxes from the atmosphere to the regolith, which is consistent with previous studies (Zent et al., 1993; Bottger et al., 2005; Savijarvi et al. 2016; 2019).
Second, we discuss a seasonal variation in the water vapor column in the atmosphere. The global water vapor abundances of the atmosphere became larger in the northern summer (Ls~90 deg) and smaller in the southern summer (Ls~270 deg) compared with the inert regolith-atmosphere interaction case. This is because water vapor was transferred from the mid-latitude to the north pole and water ice accumulated in the subsurface of the northern hemisphere. In the case of uniform grain size, water accumulated in areas of low thermal inertia and the water vapor column didn't agree with observations. It is suggested that the grain size of the regolith is important for the active regolith-atmosphere interaction because of the Knudsen diffusion coefficient on surface water vapor fluxes. In addition, since these were strongly influenced by ground temperature, realistic consideration of surface and subsurface thermal properties was necessary.
These results suggest that water vapor exchange between the regolith and the atmosphere has considerable effects on the water cycle on Mars and it is affected by soil properties and thermal properties of the regolith and has more localized effects than circulation.
Here, we implemented a regolith scheme into our Mars General Circulation Model (MGCM) to investigate diurnal and seasonal variations of the amount of water vapor in the atmosphere. Our MGCM, named DRAMATIC (Dynamics, RAdiation, MAterial Transport, and their mutual InteraCtions), solves a self-consistent global water cycle (Kuroda, 2017). Our newly developed regolith scheme is based on Zent et al. (1993), Bottger et al. (2005), and Steele et al. (2017). We ran DRAMATIC with two cases: an inert regolith-atmosphere interaction case and an active regolith-atmosphere interaction case. In the former, there were no regolith effects and in the latter, the model solved the exchange of water vapor between the regolith and the atmosphere, diffusion, adsorption, and condensation in the subsurface with globally-distributed 2 kg m-3 of subsurface ice at each grid point. Water vapor diffusion in the subsurface was calculated with a combination of Fickian and Knudsen diffusions. Regarding Knudsen diffusion, the grain size distribution was obtained from the thermal kinetics of the gas using the observed thermal inertia by the Thermal Emission Spectrometer (TES). The amount of adsorbed water was defined by an adsorption isotherm of Jakosky et al. (1997).
First, we show the diurnal variation in relative humidity and water vapor fluxes through the surface. In the active regolith-atmosphere interaction case, our model showed a consistent diurnal variation in relative humidity with REMS observations at Gale crater after 20 years of calculation. Regolith reduced sublimation fluxes due to the downward fluxes from the atmosphere to the regolith, which is consistent with previous studies (Zent et al., 1993; Bottger et al., 2005; Savijarvi et al. 2016; 2019).
Second, we discuss a seasonal variation in the water vapor column in the atmosphere. The global water vapor abundances of the atmosphere became larger in the northern summer (Ls~90 deg) and smaller in the southern summer (Ls~270 deg) compared with the inert regolith-atmosphere interaction case. This is because water vapor was transferred from the mid-latitude to the north pole and water ice accumulated in the subsurface of the northern hemisphere. In the case of uniform grain size, water accumulated in areas of low thermal inertia and the water vapor column didn't agree with observations. It is suggested that the grain size of the regolith is important for the active regolith-atmosphere interaction because of the Knudsen diffusion coefficient on surface water vapor fluxes. In addition, since these were strongly influenced by ground temperature, realistic consideration of surface and subsurface thermal properties was necessary.
These results suggest that water vapor exchange between the regolith and the atmosphere has considerable effects on the water cycle on Mars and it is affected by soil properties and thermal properties of the regolith and has more localized effects than circulation.