IMPACT - A NATURAL HAZARD IN PLANETARY EVOLUTION

Bodies in the solar system have been bombarded by interplanetary debri s throughout time. In early solar-system history, collisions between p roto-planets occurred and one such impact of a Mars-sized object with the proto-Earth may have resulted in the formation of the Moon. For th e first 600 million years of geological time, impact was a dominant ge ological process and shaped the surface and crustal evolution of the t errestrial planets. On Earth, it may also have resulted in the erosion of the atmosphere and massive excursions in temperature that affected the viability of the biosphere. As this early intense flux died off w ith time, impact played less of a role in planetary evolution. Occasio nal large-impact events by bodies in the ten kilometre size range, how ever, continued through the Phanerozoic and at least one resulted in a mass extinction event in the biostratigraphic record. Smaller-impact events by bodies in the one kilometre size range, which occur less tha n every million years, are of little danger to the global biosphere. H owever these impacts could severely affect or even destroy the complex infrastructure of modern human civilizations through the equivalent o f a 'nuclear winter'.