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Asteroid impact has produced a significant number of medium and large craters on the earth in comparatively recent geologic time, and the rate of impact can be interpreted to have remained fairly steady for at least the last half-billion years. By extrapolation of this rate, the age of major stratigraphic units on the moon may be estimated from the number and distribution of superimposed primary impact craters. With appropriate modification, the same principle should be applicable to Mars when detailed photographs become available for photogeologic mapping.
A second potential method of interplanetary correlation depends on the actual transport of impact debris from other planets to the earth, where the debris becomes incorporated in the terrestrial stratigraphic record. Some rock debris is ejected at escape velocity by asteroid impact on the Moon and probably also on Mars; part of the lunar ejecta must land on earth and a very small fraction of Martian ejecta is probably also swept up by Earth. Some tektites are probably formed by ablation of ejects thrown into orbit around the earth. It may be possible to identify the craters from which ejecta are derived at some advanced stage of lunar and planetary exploration and thus tie the age of these craters directly to the terrestrial time scale. A ray crater in the size range from Aristarchus o Tycho is the probable source of the ejecta from which the australites and associated Pleistocene tektites were formed.
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