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Evidence for carbonates on Mars has remained elusive until recently. For example, most remote sensing instruments such as OMEGA and THEMIS that are sensitive to infrared emissivity spectral features of carbonates, had not suggested the presence of carbonate outcrops〔 〕 at 100 m or coarser spatial scales.〔 〕 Though ubiquitous, carbonates dominated by Magnesite (MgCO3) in Martian dust had mass fractions less than 5% and could have formed under current atmospheric conditions.〔 〕 Furthermore, with the exception of the surface dust component, carbonates had not been detected by any in situ mission, even though mineralogic modeling does not preclude small amounts of calcium carbonate in Independence class rocks of Husband Hill in Gusev crater〔 〕 (note: An IAU naming convention within Gusev is not yet established). The first successful identification of a strong infrared spectral signature from surficial carbonate minerals of local scale (< 10 km²) was made by the MRO-CRISM team.〔 〕 Their spectral modeling identified a key deposit in Nili Fossae dominated by a single mineral phase that was spatially associated with olivine outcrops. The dominant mineral appeared to be magnesite, while morphology inferred with HiRISE and thermal properties suggested that the deposit was lithic. Stratigraphically, this layer appeared between phyllosilicates below and mafic cap rocks above, temporally between the Noachian and Hesperian eras. Even though infrared spectra are representative of minerals to less than ≈0.1 mm depths〔 〕 (in contrast to gamma spectra which are sensitive to tens of cm depths),〔 〕 stratigraphic, morphologic, and thermal properties are consistent with the existence of the carbonate as outcrop rather than alteration rinds. Nevertheless, the morphology was distinct from typical terrestrial sedimentary carbonate layers suggesting formation from local aqueous alteration of olivine and other igneous minerals. However, key implications were that the alteration would have occurred under moderate pH and that the resulting carbonates were not exposed to sustained low pH aqueous conditions even as recently as the Hesperian. This increased the likelihood of local and regional scale geologic conditions on Mars that were favorable to analogs of terrestrial biological activity over geologically significant intervals. The absence of more extensive carbonate deposits on Mars was thought to be due to global dominance of low pH aqueous environments.〔Grotzinger, J. and R. Milliken (eds.) 2012. Sedimentary Geology of Mars. SEPM〕 Even the least soluble carbonate, siderite (FeCO3), precipitates only at a pH greater than 5.〔 〕〔 〕 Evidence for significant quantities of carbonate deposits on the surface began to increase in 2008 when the TEGA and WCL experiments on the 2007 Phoenix Mars lander found between 3–5wt% calcite (CaCO3) and an alkaline soil. In 2010 analyses by the Mars Exploration Rover Spirit, identified outcrops rich in magnesium-iron carbonate (16–34 wt%) in the Columbia Hills of Gusev crater, most likely precipitated from carbonate-bearing solutions under hydrothermal conditions at near-neutral pH in association with volcanic activity during the Noachian era. After Spirit Rover stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."〔http://www.sciencedaily.com/releases/2010/06/100603140959.htm〕〔 Richard V. Morris, Steven W. Ruff, Ralf Gellert, Douglas W. Ming, Raymond E. Arvidson, Benton C. Clark, D. C. Golden, Kirsten Siebach, Göstar Klingelhöfer, Christian Schröder, Iris Fleischer, Albert S. Yen, Steven W. Squyres. Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover. Science, June 3, 2010 〕 Carbonates (calcium or iron carbonates) were discovered in a crater on the rim of Huygens Crater, located in the Iapygia quadrangle. The impact on the rim exposed material that had been dug up from the impact that created Huygens. These minerals represent evidence that Mars once had a thicker carbon dioxide atmosphere with abundant moisture. These kind of carbonates only form when there is a lot of water. They were found with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter. Earlier, the instrument had detected clay minerals. The carbonates were found near the clay minerals. Both of these minerals form in wet environments. It is supposed that billions of years age Mars was much warmer and wetter. At that time, carbonates would have formed from water and the carbon dioxide-rich atmosphere. Later the deposits of carbonate would have been buried. The double impact has now exposed the minerals. Earth has vast carbonate deposits in the form of limestone.〔(Some of Mars' Missing Carbon Dioxide May be Buried )〕 ==Gallery== Image:Huygens Crater.jpg|Huygens Crater - circle shows location of carbonate deposit - representing a time when Mars had abundant liquid water on its surface (Scale bar = 259 km). Image:PIA19817-Mars-NiliFossae-CarbonateRichDeposit-20150902.jpg|Nili Fossae on Mars - largest known carbonate deposit. 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「Carbonates on Mars」の詳細全文を読む スポンサード リンク
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