Age dating crater counting Loli camchat
Many rocks have complex histories, and the challenge in isotopic age determination is to unravel and date not one, but each of the events that affected their evolution.] To date, only terrestrial, lunar, and meteoritic samples have been dated by isotopic methods.
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.
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.
One vexing problem is that, although comets have been and remain important impacting bodies, our knowledge of the sizes and densities of their nuclei remains especially poor (Wilkening, 1982).
Furthermore, some surfaces, such as those of the icy satellites of Jupiter and Saturn, may not yield rocks that are datable by current isotopic techniques.
On the other hand, most solid bodies in the solar system display a record of accumulated impact cratering on their surfaces.
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.
This method is not subject to the constraints of traditional crater counting methods using visible images.
Ten kilometer diameter craters are produced on Earth at the rate of ~2 X 10 (Shoemaker et al., 1979).
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).
Because the lunar impact cratering rate is directly related to interactions among near-Earth objects and main belt asteroids, our results will provide a new platform for testing various dynamical hypotheses about the evolution of the asteroid belt and interactions within it.
] One of the major goals of planetary exploration is to determine the surface histories of the solid planets and satellites.