<P> Recent observations and modeling are 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, although the carbonate may later be dissolved by volcanic acidity . </P> <P> 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 (earlier than about 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 . </P> <P> Some scientists maintain that the great mass of the Tharsis volcanoes has had a major influence on Mars's climate . Erupting volcanoes give off great amounts of gas, mainly water vapor and CO . Enough gas may have been released by volcanoes to have made the earlier Martian atmosphere thicker than Earth's . The volcanoes could also have emitted enough H O to cover the whole Martian surface to a depth of 120 m (390 ft). CO is a greenhouse gas that raises the temperature of a planet: it traps heat by absorbing infrared radiation . So Tharsis volcanoes, by giving off CO, 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 temperatures above 0 ° C at any point in its history, although this may simply reflect problems in accurately calibrating such models . </P> <P> Mars' temperature and circulation vary from year to year (as expected for any planet with an atmosphere). Mars lacks oceans, a source of much inter-annual variation on Earth . Mars Orbiter Camera data beginning in March 1999 and covering 2.5 Martian years show that Martian weather tends to be more repeatable and hence more predictable than that of Earth . If an event occurs at a particular time of year in one year, the available data (sparse as it is) indicate that it is fairly likely to repeat the next year at nearly the same location, give or take a week . </P>

How mars' atmosphere relates to its temperature
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