Water and climates on Mars

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 the planet Mars. More and more data are available, but many of these observations appear to be contradictory. In such condition, there is no consensus on what really happened on Mars, its climate evolution, the presence of liquid water, and the possibility of life. We know that there have been multiple kinds of climates on Mars, because the planet environment strongly varied with the oscillations of the planet's orbital and rotational parameters as well as with the evolution of the content of its atmosphere. I personally would conclude that, although Mars appears to have enjoyed conditions suitable for sustained liquid water on its surface 3.7-4.2 billion years ago, it seems that afterward the Martian climates did not allow surface liquid water except during episodical events. For several billions years, Mars has probably not been very suitable for life as we know it, except maybe in the deep subsurface. However, one can expect that new discoveries will soon, prove or disprove this opinion, bring new insight to the problems, and probably question some of the "certain facts" presented in this chapter. 2007 Springer-Verlag Berlin Heidelberg.