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During past ages, there was rain and/or snow on Mars; especially in the Noachian and early Hesperian epochs. 〔Baker, V. R. (1982), The Channels of Mars, 198 pp., Univ. of Tex. Press, Austin.〕 Some moisture entered the ground and formed aquifers. That is, the water went into the ground, seeped down until it reached a layer that would not allow it to penetrate (such a layer is called impermeable), and then water piled up forming a layer that was saturated with water. In an aquifer water occupies open space (pore space) that lies between rock particles. This layer would spread out, eventually coming to be under most of the Martian surface. The top of this layer is called the water table. Calculations show that the water table on Mars was for a time 600 meters below the surface. Several prominent features on the planet have been produced by the action of groundwater. When water rose to the surface or near surface, various minerals were deposited and sediments became cemented together. Some of the minerals were sulfates that were probably produced when water dissolved sulfur from underground rocks, and then became oxidized when it came into contact with the air. While traveling through the aquifer, the water passed through the igneous rock basalt that contains sulfur. ==Layered terrain== Some locations on the Red Planet show groups of layered rocks. Rock layers are present under the resistant caps of pedestal craters, on the floors of many large impact craters, and in the area called Arabia. In some places the layers are arranged into regular patterns. 〔Lewis, K. W., O. Aharonson, J. P. Grotzinger, A. S. McEwen, and R. L. Kirk (2010), Global significance of cyclic sedimentary deposits on Mars, Lunar Planet. Sci., XLI, Abstract 2648.〕 It has been suggested that the layers were put into place by volcanoes, the wind, or by being at the bottom of a lake or sea. Calculations and simulations show that groundwater carrying dissolved minerals would surface in the same locations that have abundant rock layers. According to these ideas, deep canyons and large craters would receive water coming from the ground. Many craters in the Arabia area of Mars contain groups of layers. Some of these layers may have resulted from climate changes. The tilt of the rotational axis of Mars has repeatedly changed in the past. Some changes are large. Because of these variations of climate, at times the atmosphere of Mars will be much thicker and contain more moisture. The amount of atmospheric dust also has increased and decreased. It is believed that these frequent changes helped to deposit material in craters and other low places. The rising of mineral-rich ground water cemented these materials. The model also predicts that after a crater is full of layered rocks; additional layers will be laid down in the area around the crater. So, the model predicts that layers may also have formed in intercrater regions; layers in these regions have been observed. Layers can be hardened by the action of groundwater. Martian ground water probably moved hundreds of kilometers, and in the process it dissolved many minerals from the rock it passed through. When ground water surfaces in low areas containing sediments, water evaporates in the thin atmosphere and leaves behind minerals as deposits and/or cementing agents. Consequently, layers of dust could not later easily erode away since they were cemented together. On Earth, mineral-rich waters often evaporate forming large deposits of various types of salts and other minerals. Sometimes water flows through Earth's aquifers, and then evaporates at the surface just as is hypothesed for Mars. One location this occurs on Earth is the Great Artesian Basin of Australia. On Earth the hardness of many sedimentary rocks, like sandstone, is largely due to the cement that was put in place as water passed through. Wikiesp 035896 1845crommelinbutte.jpg|Butte in Crommelin Crater, as seen by HiRISE under HiWish program. Location is Oxia Palus quadrangle. Wikiesp 035896 1845crommelinhollows.jpg|Layers in Crommelin Crater, as seen by HiRISE under HiWish program. Location is Oxia Palus quadrangle. Wikiesp 035896 1845crommelinfaults.jpg|Layers in Crommelin Crater, as seen by HiRISE under HiWish program. Arrow indicates fault. Location is Oxia Palus quadrangle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Groundwater on Mars」の詳細全文を読む スポンサード リンク
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