Cometary Impacts on the Biosphere

  • D. Steel


There is now a wealth of evidence to link impacts by comets and asteroids with catastrophic disruptions of the biosphere and mass extinctions. Such evidence includes impact craters formed close to boundary events, iridium and other siderophile element/isotope anomalies, microtektites, and shocked quartz. Mass extinctions, crater excavations, and various geologic upheavals all seem to follow a common periodicity of ~30 Myr. Another cycle of ~250 Myr supports a link with the solar motion about the galaxy, this being its orbital period, whilst 26 Myr-32 Myr is its half-period for oscillations about the galactic disk. Oort cloud disturbances and therefore comet showers produced as the solar system passes through the disk seem the most likely explanation for the observed phenomena, although there is currently a problem with understanding the phase of the showers. This may be reconciled if the cyclicity is, in fact, ~15 Myr. An alternative (which does not explain the 250 Myr cycle) is the solar companion star or “Nemesis” theory, invoking Oort cloud disturbances when this hypothetical star passes perihelion every 30 Myr. The fact that the biosphere may be significantly affected by comets through means other than actual impacts (e.g., dust veiling of the atmosphere) is emphasized, such considerations leading to an understanding of the structure of boundary events through prolonged periods of influx to the Earth of meteoroids and dust. The possible contemporary modulation of the biosphere by cometary decay products is mentioned. Finally, the role of large impacts in panspermia—the spreading of life from one planet to another—is briefly discussed.


Solar System Mass Extinction Galactic Plane Impact Crater Oort Cloud 
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© Springer Science+Business Media New York 1997

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  • D. Steel

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