[P-PS06] 惑星科学

[PPS06-14] Water-ice content estimation on the Martian subsurface based on the permittivity

*小林 真輝人¹、宮本 英昭² (1.東京大学理学系研究科地球惑星科学専攻、2.東京大学工学系研究科システム創成学専攻)

キーワード：比誘電率、火星、水氷、レーダーサウンダー

The ancient Martian surface has retained a large amount of water, which formed large-scale aqueous-related structures, interacted with the rocks and atmosphere, and attributed to the evolution of surface conditions (e.g., Baker, 1992; Carr, 2006; Head and Carr, 2010; Ehlmann and Edwards, 2014). During late-Hesperian to Amazonian, the surface water has dramatically vanished associated with capture into polar region and subsurface (e.g., Holt et al., 2010; Dundas et al., 2018), as well as thermal and nonthermal atmospheric loss (e.g., Jakosky, 2021), and chemical weathering of the regolith (e.g., Ehlmann et al., 2009). However, the amount of vanished water is ambiguous, which is clues to understand the evolution of the Martian environment. Some water could exist on the subsurface at even present, which has been detected by the radar sounders such as MARSIS and SHARAD (e.g., Plaut et al., 2007; Usui et al., 2015; Orosei et al., 2018, 2020). Radar sounders utilize the electrical properties, especially permittivity, to visualize the subsurface structures and make it possible to estimate the amount of water on the subsurface. As the permittivity depends on various parameters such as temperature, density, chemical composition, and so on, it is difficult to estimate the accurate value of the subsurface materials. Indeed, the detection of water-ice is based on the consistency of the topography, that is, the basal topography can be duplicated assuming the permittivity value of water-ice when utilizing the radargram. However, the value also can be explained by the combination of air, water-ice, and dust materials. Then, we calculate the possible composition of air, water-ice, and rocks using the laboratory experimental data under the Martian environment. For the calculation, we use the value of the Martian simulant measured at the HF band, which is utilized for the radar sounders, and the mixing equation (Looyenga, 1965), which is appliable to the low-temperature materials (Hickson et al., 2020), and the possible subsurface vertical structure (e.g., Clifford et al., 2010). As a result, some model that has different the ration of water-ice/dust can explain the permittivity value estimated by radar data. For instance, the ratio of dust and water-ice on the subsurface of the Medusae Fossae Formation may have no change vertically, which indicates that the estimated amount of water-ice might be about 50 % or less. With consideration of subsurface structures in detail, our method is the potential to estimate the amount of water-ice on the Martian subsurface to apply to the radar data globally.