Age dating crater counting
The oldest terrestrial rocks, found in the Precambrian shield of Greenland, are about 3.8 billion years old. The youngest extensive stratigraphic units dated by isotopic methods are the mare basalts, which range in age from about 3.3 to 3.8 billion years.
Most of Earth's surface (the ocean basins) was formed by seafloor spreading during the last 200 million years (about the last 5 percent of geologic history). Rocks recovered from the lunar highlands are even older, and ages in excess of 4.3 billion years have been measured.
Much work remains to be done, however, to refine the accuracy of these age estimates based on crater densities.
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).
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.
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.An assessment of mineral composition can be made from spectrophotometric observations, and plausible densities and masses can then be assigned to well-observed small bodies ().Cratering rates are estimated from the collision rates and from the masses and impact velocities of the colliding bodies by means of either empirical crater scaling laws or by more elaborate computer calculation of crater formation (Shoemaker, 1977).] Significant uncertainties are associated with each of these steps, particularly with the assignment of masses and with the calculation of crater sizes.In the case of asteroidal bodies that collide with Earth, it has been shown that these bodies are closely related to asteroidal objects that impact the other terrestrial planets.It has also been demonstrated that the flux of comets in the neighborhood of the terrestrial planets is closely linked to their flux in the neighborhood of Jupiter (Shoemaker and Helin, 1977).