Our understanding of the role that water has played in Mars' history has evolved dramatically over the past 40 years (Carr, 1996). Early spacecraft investigations revealed a cold, dry world with little evidence of liquid water ever having played a dominant role. Mariner 9 and Viking observations of the surface geology led to the paradigm of an early wet and warm Mars which transitioned into its present cold, dry state by the end of the Noachian. Data from MGS, Odyssey, and MEx reveal that water has played an important role up to recent times (Figure 7.1), although whether this water has been primarily in the liquid or ice form is still debated. While the atmosphere and polar caps are the most obvious locations of H2O today, the majority of Mars' water resides in the subsurface, primarily in the form of ice. The distribution of these subsurface H2O reservoirs is only now being revealed through instruments such as Odyssey's GRS and the ground-penetrating radars on MEx and MRO.
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