Prior to any serious examination of Martian Paleoclimatology one has to agree on terms, especially broad terms of planetary ages. There are two extant age systems for Mars. The first is based on crater density and has three ages, Noachian, Hesperian, and Amazonian. An alternate minerological timeline has been proposed, also with three ages, Phyllocian, Theikian, and Siderikian.
Recent observations and modeling is producing information not only about the present climate and atmospheric conditions on Mars but also about its past. The Noachian-era Martian atmosphere had long been theorized to be carbon dioxide rich. Recent spectral observations of deposits of clay minerals on Mars and modeling of clay mineral formation conditions [5] have found that there is little to no carbonate present in clay of that era. Clay formation in a carbon dioxide rich environment is always accompanied by carbonate formation, though once formed they are susceptible to destruction by volcanic acidity.
The discovery of water-formed minerals on Mars including Hematite and jarosite by the Opportunity rover, and goethite by the Spirit rover has led to the conclusion that climatic conditions in the distant past allowed for free flowing water on Mars. The morphology of some crater impacts on Mars indicate that the ground was wet at the time of impact.[citation needed] However, chemical analysis of martian meteorite samples suggests that the ambient near-surface temperature of Mars has most likely been below 0 C° for the last four billion years
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