FIGURE 8 The distribution of known terrestrial craters. [R. Grieve, Geological Survey of Canada. Adapted from their web page at http:// gdcinfo.agg.emr.ca/crater/world craters.html.]

geologic time. Comparison with cratering on other bodies in the solar system shows that craters less than about 20 km in diameter are largely missing on the earth, presumably having been removed by erosion. Thus the observed record is biased toward younger, larger craters. More craters will be discovered on the earth in the future, but the smaller the original crater, and the older it is, the greater the likelihood that erosion has removed it forever.

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