Introduction

Three decades after the Viking missions, which failed to detect any biorelics, not even a slight trace of organic activity, the question as to Mars having harboured habitable conditions, if not life, has been dramatically reopened. A key ingredient, liquid water, might have covered large fractions of early Mars over sustained periods, as indicated by the ongoing space missions. This chapter presents our understanding of the evolution over time of the Martian water reservoirs.

It took centuries for Mars to evolve (in human minds) from a ‘planet of death’ to a ‘world of life’: its colour no longer referred to blood (thus its being named after the God of war) but to rust; rust: thus water; water: thus life. These later syllogisms have persisted until very recently, translating the transcendental quest of life far beyond the scientific sphere. And yet: is Mars actually covered by ferric material? If so, is liquid water responsible for the oxidation? More importantly still, would that be sufficient for life to have emerged on Mars? Without direct means to address (and possibly answer) such questions, Mars has always been viewed as the closest and most favourable planet to have harboured extraterrestrial life. A variety of similarities between Mars and the Earth could support the ‘plurality of worlds’ that was conceived as the operational dogma.