09:00 〜 09:25
★ [PPS02-01] 火星地下氷の水素同位体比と厚さ：マルチリザーバーモデルからの制約
キーワード：地下氷, 水素同位体比, 大気散逸
Martian surface ice is currently observed only as polar layered deposits (PLDs), whereas Mars Odyssey Gamma Ray Spectrometer (Boynton et al., 2002; Boynton et al., 2007) and Mars Express radar sounder observations (Mouginot et al., 2012) propose the presence of much larger amount of ground ice in the mid- to high-latitudes. The total volume of PLDs is 20-30 m in Global Equivalent Depth (Zuber et al., 1998; Plaut et al., 2007). Ground-ice region is expected to spread over a few tenths of percent of the total Martian surface, yet the thickness (i.e. volume) is poorly constrained (Mouginot et al., 2012).The thickness of the ground ice is related to the evolution history of the Martian water reservoirs. After ancient oceans became extinct (~ 4Ga), the oceanic water would become "surface ice", which currently occur as PLDs, and "ground ice" which would extend from high latitude to mid- or low-latitude. Atmospheric escape of hydrogen and oxygen through the Martian history causes decrease of the amount of the ice. The signature of the evolution history is recorded by hydrogen isotope ratio (D/H). Martin atmosphere and soil have D/H ratio of ~6 (relative to SMOW) (Owen et al., 1988; Webster et al., 2013), which is higher than the Martian primitive D/H ratio of ~1.3 (Usui et al., 2012).We constrain the hydrogen isotope ratio of surface ice and ground ice, and estimate the thickness of ground ice, using a multi-water-reservoir box model (see figure shown below). The model solves the evolution of water inventories and D/H ratio of atmosphere, surface ice, and ground ice during the ice age. Atmospheric escape and sublimation are considered as D/H fractionation processes. We adapt our model to the Matian ice age (4Ga to present). The initial D/H ratio is that of ancient ocean, which is informed by D/H data of the Martian meteorite ALH84001 formed at ~4.1Ga (Lapen et al., 2010): D/H = 2.2-4.0 (relative to SMOW) (Boctor et al., 2003,Greenwood et al., 2008).First, we show the results from two water-reservoir box model (ice and atmosphere). The ratio of atmospheric D/H and ice D/H is in a quasi-equilibrium state of the fractionation caused by atmospheric escape and sublimation. The ratio of the present Mars is mainly determined by the fractionation caused by sublimation. Second, we show the results from four water-reservoir box model (surface ice, ground ice, high-latitude atmosphere, and mid- to high-latitude atmosphere). Assuming the atmospheric condition of the present Mars, the mixing of two atmospheric reservoir is inefficient in D/H exchange between surface ice and ground ice, which results in the independent growth of D/H ratio of the surface ice and the ground ice. To fractionate the D/H ratio of the surface ice and the ground ice into ~6, the thickness of active ground ice which can exchange water with atmosphere is constrained. Thin active ice causes high deuterium concentration. The required thickness is a few hundred meters, which is distinctly large value compared to the thickness that HDO diffusion works (~10 m in 1 Gyrs). Nature of this active ground ice might be partially melted ice suggested by recent observations of recurring slope lineae (McEwen et al., 2014), hydrated clathrates in underground cryosphere, or breathing porous permafrosts.