The Hazard of Near-Earth Asteroid Impacts on Earth

Frontiers ^ | 4 March 2004 | Clark R. Chapman

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The actual damage that a NEA impact might cause on Earth was concretely described by Baldwin, a leading advocate for the impact origin of lunar craters. Later, Opik... proposed that NEA impacts might account for mass extinctions in the Earth's paleontological record. Around the same time, Shoemaker firmly established the impact origin of Meteor Crater in Arizona... [I]t was not only a cultural but a scientific shock when Mariner 4's first photographs of the Martian surface revealed it to be covered by craters; a decade later, Mariner 10 found the same on Mercury... In 1979 and 1980, the Voyagers first encountered Jupiter and Saturn, demonstrating that cratered surfaces extended from Mercury at least out through the giant planets' satellite systems... Even after discovery of the Chicxulub impact structure in Mexico and its temporal simultaneity with the Cretaceous –Tertiary (K– T) boundary and mass extinctions, it has taken some earth scientists a while to recognize and accept the statistical inevitability that Earth is struck by asteroids and comets. Each impact, occurring on timescales of tens to hundreds of Myr, liberates tens of millions to billions of megatons (Mt, TNT-equivalent) of energy into the fragile ecosphere, which must have had dramatic consequences every time. A few researchers still consider the Chicxulub impact to be only one of several contributing factors to the K– T extinctions... Some geoscientists still think of asteroid impacts as ad hoc explanations for paleontological changes and they resist the logic that earlier, even greater impact catastrophes surely occurred. If the great mass extinctions are not attributed to impacts (e. g., explained instead by episodes of volcanism or sea regressions), one must ask how the huge impacts that must have occurred failed to leave dramatic evidence in the fossil record.

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• NASA Mariner 4 page

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