Abstract
Mars is a small planet with a thin atmosphere of almost pure carbon dioxide. To first order, the Martian meteorology can be compared with what one would expect on a cold, dry desert-like Earth. However, several phenomena make the Martian climate system more complex than it appears. First, as much as 30% of the carbon dioxide atmosphere condenses every winter at high latitude to form CO2 ice polar caps, inducing large surface pressure variations all over the planet and an atmospheric circulation without equivalent on Earth. Second, a highly variable amount of suspended dust lifted by the winds modifies the radiative properties of the atmosphere, with sometime global dust storms able to totally shroud the planet. Last, a peculiar water cycle occurs on Mars, with water vapor transported by the atmosphere between the polar caps and possibly subsurface reservoirs, allowing the formation of clouds, hazes and frost. Telescopic and spacecraft observations have shown us that this complex climate system is highly variable, seasonally and from year to year, but these variations remain poorly understood. In fact, the Martian climate system has probably experienced large variations related to the oscillations in the parameters of the orbit and rotation of Mars (obliquity) a few millions or even thousand of years ago. These oscillations affected surface temperatures and the water cycle, inducing the mobilization and accumulation of large ice deposits in various locations on the planets. In a much distant past, it is also likely that Mars may have been a completely different planet. The observations of the geology (dry riverbeds and deltas, lacustrine sediments) and mineralogy (clay, sulfate) of the oldest surface on Mars dating back to more than 3 billions years ago provide evidence that liquid water was then abundant on the surface, at least episodically. Mars may have been warmed by a thicker atmosphere containing greenhouse gas and clouds, high geothermal fluxes, or episodically by large asteroid impacts.
Highlights
The particular importance of Mars among the other planets results primarily from the existence of its atmosphere
A peculiar water cycle occurs on Mars, with water vapor transported by the atmosphere between the polar caps and possibly subsurface reservoirs, allowing the formation of clouds, hazes and frost
The new era of exploration of Mars that was initiated by NASA in 1997 and followed by ESA in 2003, is revolutionizing our understanding of the present and past of planet Mars
Summary
The particular importance of Mars among the other planets results primarily from the existence of its atmosphere. The atmosphere may have enabled the climatic conditions on Mars to have sometime been suitable for liquid water on its surface, and life. The surface of Mars is characterized by multiple geological evidences that suggest that liquid water existed at and near the Martian surface at various time in its history. The observation of the geology and mineralogy of the oldest surface on Mars (dating back to more than 3 billion years ago) provide evidence that the Martian climate was completely different with abundant liquid water on the surface, probably because of a thickest atmosphere or a higher geothermal flux. The purpose of this chapter is to briefly review what we know about the Mars climate and water on the red planet. I will present why it is believed that Mars early climate was so much different from what is its today
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