[P-PS09] 火星と火星衛星

[PPS09-08] The effects of regolith-atmosphere water exchange on the Martian water cycle evaluated from anticorrelations between the pressure and normalized water vapor column

*古林 未来^1,2、黒田 剛史^1,3、Forget François²、鎌田 有紘¹、黒川 宏之^4,5、青木 翔平⁶、風間 暁^1,6、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.LMD/IPSL, Sorbonne Université、3.東北大学新領域創成部、4.東京大学大学院総合文化研究科、5.東京大学大学院理学系研究科地球惑星科学専攻、6.東京大学大学院新領域創成科学研究科)

キーワード：火星、水循環、レゴリス

Water plays a crucial role in understanding planetary environments. The Martian atmosphere contains a modest but significant amount of water, and its global water cycle is closely linked to the surface and subsurface water. The water vapor exchange between the regolith and atmosphere through adsorption, which is the physical accumulation of water molecules, is suggested to affect the water cycle (Fanale & Cannon, 1971; Zent et al., 1993, 1995). Considering the regolith-atmosphere interaction in 1-D model simulations results in consistent local-time variations in relative humidity near the surface, which align with observations from landers (Savijaervi et al., 2016, 2019, 2020, 2021, 2024). However, the regolith-atmosphere interaction is not generally considered in Mars Global Climate Models (MGCMs), and its effects have not been thoroughly evaluated by using a regolith-atmosphere fully coupled model. This study investigates its effects on the water cycle with global simulations and considers whether the observation of these effects with orbiters is possible. We use an MGCM coupled with a regolith model. Our MGCM traces the Martian diurnal/seasonal water cycle, including water ice caps and frost formation, turbulent fluxes in the atmospheric boundary layer (Kuroda et al., 2005, 2013), and simple cloud formation (Montmessin et al., 2014). The regolith model calculates water vapor diffusion, adsorption, and condensation in the regolith, using an adsorption coefficient as a free parameter (Kobayashi et al., JGR-Planets, in press). The regolith is initialized with the subsurface water amount obtained from a spin-up run without the regolith for about thousands of years. We examine several adsorption coefficients including zero (only considering pore ice) and the inhomogeneous adsorption coefficient (Kobayashi et al., JGR-Planets, in press). To examine the atmospheric water vapor column abundance (AWVC) without the effect of topography, we normalize the AWVC to a fixed pressure of 610 Pa (Smith, 2002; Fouchet et al., 2007). As a result, the normalized AWVC seems to be controlled by the global circulation with strong diurnal/seasonal variations rather than the local water exchange near the surface. Our results show that the surface pressure and normalized AWVC exhibit very weak to weak correlations from northern spring to summer but moderate anti-correlations from northern fall to winter, regardless of adsorption coefficient. This tendency is also suggested by observations from the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) (Smith, 2002). In northern fall to winter, the main water source for the atmosphere shifts south, and the near-surface wind blows from the northern lowlands, resulting in the anti-correlation. The normalized AWVC shows a weaker correlation. The normalized AWVC shows a weaker correlation (stronger anti-correlation) in northern summer (in northern winter) during the night and early morning. During the daytime, the water supply via desorption and sublimation moistens the atmosphere, which mitigates the effect of topography. However, it is difficult to discover the difference in the local-time variations of the normalized AWVC from its spatial distribution because wind lessens the effect of water supply so that the normalized AWVC varies only about 1 pr-um at maximum in a day. On the other hand, one can distinguish between the water flux from the regolith and frost sublimation. This is because the regolith stores water from 5 pm to 8 am and releases water from 9 am to 4 pm, while the surface frost sublimates from 8 am to 11 am in the low latitudes and forms around 6 pm. Therefore, the local increase in water vapor near the surface between noon and 4 pm should be caused by the regolith-atmosphere water exchange, and tracking local-time changes in the water vapor near the surface can provide important insights into the regolith-atmosphere water exchange.