ASTEROID AND COMET BOMBARDMENT OF THE EARTH

Two classes of solid bodies large enough to be detected by telescopes occur in orbits that overlap that of the Earth. These bodies are the Earth-crossing asteroids and comet nuclei. Although their orbits only rarely intersect the Earth's, the probabilities of their collision with the Earth are nevertheless finite and calculable. Systematic telescopic surveys carried out over the past two decades show that the flux of asteroids and comet nuclei in the Earth's neighborhood is sufficiently high that the effects of occasional collisions should be recognizable in the geological record. During these same two decades, an intensive international search for ancient impact structures has gone forward. The actual rate of bombardment of the Earth during the last half-billion years has been found to be roughly consistent with the present rate predicted from astronomical observations. Within a factor of about two, the average rate of bombardment of the Earth during the last half­ billion years also appears to be consistent with the average rate of bombardment of the Moon over the last 3.3 billion years. Spectacular new lines of study have developed in recent years leading to the recognition of rare large impact events that produce geochemical anomalies on a global scale. The possible effects of these large impacts on the Earth's biota have become the subject of vigorous debate. In this paper, I first review the astronomical and geologic evidence concerning the history of bombardment and then discuss the physical effects of large impacts, as they may apply to both the inorganic and organic worlds.