Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars
Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars
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DOI：
10.1038/ngeo831
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发表时间：
2010-05-01
影响因子：
18.3
通讯作者：
Milliken, Ralph E.
中科院分区：
文献类型：
--
作者：
Hurowitz, Joel A.;Fischer, WoodwardW.;Milliken, Ralph E.
来自现场实验和行星轨道器的观察表明，在子午线平原发现的沉积岩，火星是在酸性表面沃茨的存在下形成的（1-4）。这些水被认为是地下水上升流带到地表的（5，6），可能是火星上广泛存在的液态水的最后遗迹。然而，目前还不清楚为什么表面沃茨是酸性的。在这里，我们使用地球化学计算，从火星探测漫游者机遇号（7-10）的化学和矿物学数据的约束，以表明铁氧化和氧化的铁（Fe（3+））矿物的沉淀产生过量的酸相对于岩石中的碱阴离子的量存在于露头。我们认为，近中性pH值和富含Fe（2+）的地下沃茨在暴露于O（2）时被氧化，或者在火星表面被紫外线辐射光氧化，从而迅速酸化。火星表面酸度的时间变化可能受到液态水的控制，因此，低pH值的液体可能是火星表面干旱化的自然结果。最后，由于子午线的铁氧化会产生大量的气态H（2），最终来自H（2）O的还原，我们得出结论，表面地球化学过程会影响早期火星大气的氧化还原状态。
Observations from in situ experiments and planetary orbiters have shown that the sedimentary rocks found at Meridiani Planum, Mars were formed in the presence of acidic surface waters(1-4). The water was thought to be brought to the surface by groundwater upwelling(5,6), and may represent the last vestiges of the widespread occurrence of liquid water on Mars. However, it is unclear why the surface waters were acidic. Here we use geochemical calculations, constrained by chemical and mineralogical data from the Mars Exploration Rover Opportunity(7-10), to show that Fe oxidation and the precipitation of oxidized iron (Fe(3+)) minerals generate excess acid with respect to the amount of base anions available in the rocks present in outcrop. We suggest that subsurface waters of near-neutral pH and rich in Fe(2+) were rapidly acidified as iron was oxidized on exposure to O(2) or photo-oxidized by ultraviolet radiation at the martian surface. Temporal variation in surface acidity would have been controlled by the availability of liquid water, and as such, low-pH fluids could be a natural consequence of the aridification of the martian surface. Finally, because iron oxidation at Meridiani would have generated large amounts of gaseous H(2), ultimately derived from the reduction of H(2)O, we conclude that surface geochemical processes would have affected the redox state of the early martian atmosphere.