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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.Crater densities indicate that the present rates of formation of large craters on each of the terrestrial planets and on the Moon are approximately within a factor of 2 of the present cratering rate on Earth.Ten kilometer diameter craters are produced on Earth at the rate of ~2 X 10 (Shoemaker et al., 1979).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.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.
Surface histories tell us how these bodies evolved through time and provide information on the probable causes for observed differences.
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.