Shock Modification and Chemistry and Planetary Geologic Processes

The second half of the twentieth century has seen a revolution in the Earth and planetary sciences. One radically new concept is that hypervelocity impact is a major process at work in the solar system. Only in the last few decades has it become fully accepted that the Earth, the other terrestrial planets, the Moon, the icy satellites, and the asteroids all have surfaces that are heavily modified by a history of impact. Such modification is evident from the cratered surfaces of these solar system objects, the micro­ structures of many meteorites, and the properties of the lunar regolith, all of which show evidence of repeated exposure to impacts and resulting shock waves. Along with the appreciation of the importance of impact as a surface process, accretionary models for planetary formation have been advanced and accepted. For example, it is now clear that shock heating was a primary contributor to thc thermal state of planets as they grew. In the last decade, hypotheses have been embraced that require impacts of various sizes to explain events ranging from the origin of the Moon to the Cre­ taceous-Tertiary mass extinction to the ejection of rocks (thc SNC meteor­ ites) from the surface of Mars. Serious hypotheses have included impact as the cause of phenomena as varied as periodic biologic extinctions and magnetic pole reversals.