The purpose of this chapter is to summarize the geologic history of Mars and the role water has played in the evolution of the surface so that subsequent chapters on more specific topics can be viewed in a broader context. It focuses mainly on surficial processes such as erosion, sedimentation, and weathering, rather than on primary terrain-building processes such as impact, tectonism, and volcanism, since surficial processes provide more information on surface conditions under which lakes could have formed. With a mean annual temperature of 215 K and a mean surface pressure of 6.1 mbar, liquid water can exist at the surface only locally and temporarily under anomalous conditions. Yet, geologic evidence for the widespread presence of liquid water is compelling, particularly for early Mars, and claims have also been made of present-day water activity. Martian surface features have been divided into three age groups—Noachian, Hesperian, and Amazonian—on the basis of intersection relations and the numbers of superimposed impact craters. A major change occurred at the end of the Noachian. The rates of impact, valley formation, weathering, and erosion dropped precipitously. On the other hand, volcanism continued at a relatively high rate throughout the Hesperian, resulting in the resurfacing of at least 30% of the planet. Large floods formed episodically, possibly leaving behind large bodies of water. The rate of formation of the ice-related features and possibly the gullies probably varied as changes in obliquity affected the ice-stability relations.
