The long and complex geological history of Mars, notably the history of its water, can be unraveled by understanding the relative and absolute ages of the planet’s geological units, which have been produced or deposited by the various geological processes that have operated throughout the planet’s history. Rocks of the Hesperian System overlie Noachian units and are characterized by the ridged plains materials of the northern lowlands.
Moreover, understanding the geology of a landing site, and therefore, that of samples examined in situ or returned to Earth, requires placing such samples in their appropriate time-stratigraphic geological contexts. Much of the northern lowlands consists of Hesperian age units.
Though little is currently known of the absolute chronology of Mars, fruitful future opportunities exist in this area. Notably, lava flows and eolian activity can partially fill or completely obliterate craters. Eolian activity can exhume a cratered surface that has been protected from cratering for an uncertain amount of time, implying that its crater density would indicate a somewhat younger age than its actual age. The factor-of-two uncertainty in age has relatively little effect on interpretation of the absolute age of Noachian terrains, expected to have been originally nearly saturated with craters. Similarly, the factor of two has relatively little effect on interpretation of the age of very young terrains, where a surface with a nominal age of ~10 million years (Myr) is young in any case.