The impact cratering rate in recent time


  • Richard A. F. Grieve


Data on the recent cratering rate in the earth-moon system is somewhat ambiguous with respect to whether the rate has been constant over the last ∼3 b.y. Rate estimates based on the present population of earth-crossing bodies are higher than estimates based on crater counts at the Apollo 12 and 15 sites, while some estimates based on terrestrial crater counts are intermediate in value. The terrestrial data base for structures with diameter (D)>20 km on the North American and European cratons has been investigated for variations in the rate with time. There appear to be two populations, with a 0–120 m.y. age group suggesting a higher cratering rate than an older 200–360 m.y. age group. Reexamination of the preservation levels of these structures indicates that the topography of a 20 km-sized structure may be removed by erosion in 34 to 273 m.y. and the geologic anomaly resulting from structural uplift in 79 to 630 m.y. This suggests that the relatively lower rate for the 200–360 m.y. group may reflect problems in crater recognition associated with crater retention. If rate estimates are restricted to the possibly more complete sample of 0–120 m.y. craters on the cratons, the recalculated rate is 5.4±2.7× 0−15 km−2 y−1 for D<20 km. This is comparable to that based on earth-crossers and reinforces previous suggestions that the recent cratering rate may be a factor of 2–3 higher than the average rate over the last ∼3.0 b.y. This rate estimate, when extrapolated back in time, is a better fit to lunar data from Tycho and Copernicus than a constant rate based on Apollo 12 and 15 data and may be interpreted to suggest that the relatively higher recent rate has been in effect for ∼1 b.y. Given the uncertainties, however, this must be regarded as speculative.