Interactive Evolution of Multiple Water-Ice Reservoirs on Mars: Insights from Hydrogen Isotope Compositions
Interactive Evolution of Multiple Water-Ice Reservoirs on Mars: Insights from Hydrogen Isotope Compositions
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DOI：
10.2343/geochemj.2.0407
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发表时间：
2015-11
期刊：
影响因子：
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通讯作者：
H. Kurokawa;T. Usui;Masahiko Sato
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文献类型：
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作者：
H. Kurokawa;T. Usui;Masahiko Sato
来自轨道飞行器任务的遥感数据表明，火星上目前可能存在地面冰，尽管数量仍然不确定。最近对火星陨石的分析表明，水的水库至少有三种不同的氢同位素组成（D/H比）：原始和高D/H比，分别与地球上的海水大致相同和六倍，以及新发现的中间D/H比，大约比地球上的海水高出两到三倍。通过模拟火星上多个水库之间的氢同位素交换和大气逃逸，计算了D/H比和水库体积的演变。最上面的薄表面冰层的D/H比略高于大气中的D/H比，这是因为升华导致的同位素分馏，而在可交换的最上面的表面层下面的水冰水库保留了在火星陨石中发现的中间D/H特征。我们提出了两种可能的模型，考虑到所观察到的D/H比和古海洋的地貌估计约束的地面冰的体积。第一种假设大气损失主要是由金斯逃逸引起的。在这种情况下，地面冰的体积应该大于可观察到的表面冰的总体积，表面冰主要以极地分层冰沉积的形式出现。另一个模型假设扩散有限的大气损失，其中多个水库的相互作用的演变自然占所观察到的D/H比。在这种情况下，大量的地面冰不一定存在于火星上，而不是根据最近的轨道飞行器任务提出的透视图。
Remote sensing data from orbiter missions have proposed that ground ice may currently exist on Mars, although the volume is still uncertain. Recent analyses of Martian meteorites have suggested that the water reservoirs have at least three distinct hydrogen isotope compositions (D/H ratios): primordial and high D/H ratios, which are approximately the same and six times that of ocean water on Earth, respectively, and a newly identified intermediate D/H ratio, which is approximately two to three times higher than that in ocean water on Earth. We calculate the evolution of the D/H ratios and the volumes of the water reservoirs on Mars by modeling the exchange of hydrogen isotopes between multiple water reservoirs and the atmospheric escape. The D/H ratio is slightly higher in the topmost thin surface-ice layer than that in the atmosphere because of isotopic fractionation by sublimation, whereas the water-ice reservoir just below the exchangeable topmost surface layer retains the intermediate D/H signature found in Martian meteorites. We propose two possible models for constraining the volume of the ground ice considering the observed D/H ratios and geomorphological estimates of Paleo-oceans. The first assumes that the atmospheric loss is dominated by the Jeans escape. In this case, the volume of ground ice should be larger than the total volume of the observable surface ice that mainly occurs as polar layered ice deposits. The other model assumes diffusion-limited atmospheric loss in which the interactive evolution of the multiple water reservoirs naturally accounts for the observed D/H ratios. In this scenario, a large volume of ground ice does not necessarily exist currently on Mars as opposed to the perspective view proposed on the basis of recent orbiter missions.