Martian Paleoclimatology
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 mineralogical 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 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. Geomorphic observations of both landscape erosion rates and martian valley networks also strongly imply warmer, wetter conditions on Noachian-era Mars (approximately greater than 4 billion years ago). 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.
Some scientists maintain that the great mass of the Tharsis volcanoes has had a major influence on the climate of Mars. Erupting volcanoes give off great amounts of gas. The gases are usually water vapor and carbon dioxide. Estimates put the amount of gas released to the atmosphere from Martian volcanoes as enough to make the atmosphere thicker than the Earth's. In addition, the water vapor from the volcanoes could have made enough water to place all of Mars under 120 m (390 ft) of water. Carbon dioxide is a greenhouse gas that raises the temperature of a planet by trapping heat in the form of infrared radiation. So Tharsis volcanoes, by giving off carbon dioxide, could have made Mars more Earth-like in the past. Mars may have once had a much thicker and warmer atmosphere, and oceans and/or lakes may have been present. It has, however, proven extremely difficult to construct convincing global climate models for Mars which produce conditions above freezing at any point in its history, though this may simply reflect problems in accurately calibrating such models.
Read more about this topic: Climate Of Mars