From the calculated present cratering rates and the observed history of cratering in the Earth-Moon system, it can be shown that the period of early heavy bombardment probably ended 3.5 billion years ago on each of the terrestrial planets (Hartmann, 1972; Soderblom et al., 1974).On Mars, the analysis indicates that volcanism and plains formation extended through much of the post-heavy bombardment period.These objects include small bodies of asteroidal appearance and the nuclei of comets.
More extensive telescopic observations are needed to improve our knowledge of the physical properties and collision rates of the planet-crossing bodies, and computer models must be refined to estimate [Another method that has been used successfully on the Moon to estimate absolute ages involves the correlation of the morphology of small craters (1 km in diameter) with the absolute age of a surface determined from isotopic measurements (Shoemaker, 1966).
The technique depends on an erosion model that relates the shape of a crater to the integrated flux of meteoroids and secondary debris that have impacted the surface since the crater was fresh.
The method provides a means of estimating absolute surface ages in areas not sampled by the Apollo missions and suggests that some mare regions may be as young as about two billion years.
The Earth-Moon system also provides the essential record needed to determine the past variation of this cratering rate (Hartmann, 1972a).
If the cratering history is known for one planet or planet-satellite system, then, in principle, it can be derived for other planets and satellites, provided that the bodies impacting the various planets and satellites are dynamically related.