Studies of the chronology of martian meteorites can distinguish various events in the history of the meteorites and give ground truth to martian geology. Different isotopes date distinct events. Long-lived radiogenic isotopes date planetary differentiation and igneous formation ages; ⁴⁰Ar/³⁹Ar can sometimes distinguish a separate ancient impact age. Cosmogenic isotopes date both the exposure age of the rocks in space as small (meter-sized) objects, which may also define the ejection time from Mars, as well as the terrestrial ages, which begin with the arrival of the rocks on Earth.

Radiogenic isotope studies of the shergottites yield a whole-rock model age of about 4.5 Ga. This suggests that planetary differentiation on Mars took place immediately after accretion and that the core, mantle and crust were isolated thereafter. The single ancient martian meteorite, ALH84001, has a formation age of about 4.5 Ga, an apparent shock age of 4.0 Ga, and an exposure age of 16 Ma. It represents the product of igneous melting and crystallization in the earliest martian crust. It is surprising that chance should cause us to find as meteorites only a piece of the martian crust that is as old as the oldest Apollo lunar samples, and pieces as young as the youngest martian surfaces, but no samples of intermediate age.

The nakhlites and Chassigny have formation ages of 1.3 Ga, much younger than ALH84001, and exposure ages of about 12 Ma (Figure 2). These two distinct rock types appear to be tied to the same geologic province. Similarly most of the basaltic and lherzolitic shergottites share a formation age of about 170 Ma and an exposure age of about 3 Ma. Basaltic shergottite EETA79001 has the same formation age, but a lower exposure age (0.7 Ma). Attempts to model a relationship between the shergottites and nakhlites-Chassigny based on mineralogy, and major and trace element compositions have shown that they are not simply related to the same magma or mantle source. Thus we conclude that partial melting in the mantle produced mafic magmas like those parental to the martian meteorites at intervals throughout Mars' history.

Clustering of martian meteorites into three groups based only on rock type, formation age and exposure age (Figure 2) suggests that these three groups represent sampling of three distinct martian sites by at least three distinct impacts in the last 15 Ma. The site for ALH84001 is presumably somewhere in the ancient cratered highlands of Mars' southern hemisphere and the two sites for samples having young formation ages (<1.3 Ga) probably are in the northern volcanic plains of Mars. However, integrating interpretations based on sample chemistry with those based on impact crater modeling and orbital mechanics is not straightforward. Two leading mechanisms for ejecting material off Mars, without melting or severely shocking the meteoroids, involve spallation from a free surface and gas drag resulting from vapor clouds generated by the impact. These mechanisms allow ejection of small, meteorite-sized fragments off Mars from relatively small craters. Recent modeling suggests that small rocky fragments ejected from Mars can be delivered to Earth in time periods encompassed by the meteorite exposure ages. If these models are accurate, then the numerous small craters on martian surface suggest that many fragments have been placed in solar orbit, and the question becomes "why is the range of crystallization ages and cosmic ray exposure ages so restricted?"