FIGURE 4.1 Relative ages of major geological events in Mars’s history, derived from stratigraphic analyses of geological units and corresponding crater densities (right: expressed as number of craters greater than the indicated diameter per 10 of surface area), grouped here by geological process (SOURCE: J. The density of superposed craters provides a means of estimating absolute chronology, but this technique is dependent on imperfect models of the cratering rate at Mars through time.
The problem becomes intricate if more than one event that affected the radiogenic isotope systems has occurred during the evolution of the rock.
Two basic types of dating are possible: absolute and relative.
Absolute age dating determines the "calendar" time at which a rock, surface, or feature formed; relative age dating determines the order-but not the time-of formation. If the rocks have remained as closed isotopic systems, it is possible to calculate their age by measuring the amount of radiogenic isotopes relative to the amount of stable isotopes now present.
Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.
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.