Long-term variations in the impact cratering rate on Earth
Authors: Shoemaker Eugene M. 
Published: September 2007

Extended abstract

Lunar and terrestrial impact craters, the terrestrial stratigraphic record of impact events, and astronomical observations of Earth-crossing asteroids and comets provide the primary evidence for evaluating the past and present flux of bodies impacting Earth. Variations in this flux on ∼10
5
, ∼10
6
, ∼10
7
and ∼10
8
year time scales are expected on theoretical grounds; ∼10
6
, ∼10
7
, and ∼10
8
year variations appear to be reflected in the crater and stratigraphic records. The available evidence suggests that, late in geological time, comets have produced about half the impact craters on Earth >20 km in diameter and nearly all craters >100 km in diameter.

Systematic imaging of the Moon by the Clementine mission has enabled a thorough reexamination of the stratigraphic classification of Eratosthenian (3.2 to 0.8 Ga) and Copernican (0.8 Ga to the present) craters (McEwan
et al.
1998). Further, it is now clear that the age of Copernicus, ∼0.8 Ga, probably is close to the Eratosthenian-Copernican boundary. The mean rate of production of craters >20 km diameter in the past 0.8 Ga may have increased by ∼40% over the mean rate in the previous 2.4 Ga, although the difference is within the statistical uncertainties (Table 1). A larger increase in the cratering rate is suggested by the spatial density of small craters on the ejecta blanket of Copernicus.

The record of Proterozoic craters in Australia, the most complete on Earth, yields an average Proterozoic cratering rate very close to that estimated from Eratosthenian craters on the Moon, when appropriate scaling and other calculations are used to find the cratering rate on Earth corresponding to that on the Moon (Table 1). In contrast, the Phanerozoic record of craters >10 km diameter on the Mississippi lowland of the United States and the record of craters ≥20 km diameter and younger than 120 Ma on the North American and European cratons (Grieve & Shoemaker 1994) suggest that the cratering rate may have increased late in geological time, perhaps as much as ∼50% or more (Table 1). Again, the differences, individually, are well within the statistical uncertainties. However, the consistency between the lunar and terrestrial data sets strengthens the suggestion of a late increase in the long-term cratering rates. Moreover, the cratering rates derived from the late crater records are consistent with the present crater production rate estimated from astronomical observations of Earth-crossing asteroids and comets (Table 1).
The frequency of large craters formed in the last 100 Ma appears particularly anomalous when compared with the 3.2 Ga crater record of the Moon. The young age of the 170 km diameter, 65 Ma Chixulub crater is especially noteworthy. Only one crater equal to or larger than Chicxulub has been formed in the past 3.2 Ga on the Moon. On the basis of the average 3.2 Ga lunar cratering rate, there would be about a 35% chance that a crater the size of Chicxulub was formed on Earth in the last 65 Ma. The odds that such a crater was formed on the continents is ∼11%. Similarly, the chance that the 100 km diameter Popigai crater was formed on the continents in the last 36 Ma is only about 20%; the chance that the 85-km diameter crater Tycho was formed on the Moon in the last 100 Ma is about 25%. While the statistics are small, the ages of these large craters collectively indicate a substantial late increase in the rate of production of large craters.
Short-term variations in the cratering rate are suggested by the distribution of terrestrial crater ages and the stratigraphic record of impact events. The distribution of ages of craters >5 km diameter over the last 220 Ma appears to contain a periodic component with a mean period near 31 or 32 Ma. About half of the ages may be distributed periodically and half randomly. The significance of the apparent periodicity has been challenged, but significance tests for the periodic component are model dependent. The last three peaks in the cratering rate, which occur at ∼1, 35, and 65 Ma are strongly reflected in independent data on ages and stratigraphic distribution of impact glass, noble metal anomalies, and shocked quartz grains.
A major problem in assessing the present flux of impacting bodies from astronomical observations lies in the difficulty of determining the sizes of comet nuclei. In addition, there are severe selection effects in the discovery of comets, particularly periodic comets, the vast majority of which exhibit no activity detectable under the usual circumstances of comet search. Within present observational uncertainties, it is entirely possible that comet impact has contributed to the production of about half or more of the craters larger than 20 km on Earth.

Order of magnitude and larger variations in the past flux of comets probably have occurred as a result of comet showers and changes in the motion of the sun through the galaxy. Comet showers due to tidal disruption of large, short-period comets have expected half-lives of about 0.5 × 10
5
years; showers caused by encounter of the sun with stars and other massive objects have half-lives of order 10
6
years. Varying galactic tidal perturbation of the Oort comet cloud modulates the delivery of long-period comets to Earth-crossing orbits, which leads to peaks in the comet flux about every 30 to 35 × 10
6
years (Matese
et al.
1995).

The apparent periodic component in the distribution of ages of terrestrial impact structures and impact glass now appears understandable in terms of the variation in the tidal perturbation of the orbits of Oort Cloud comets. Both the mean period and timing of peaks are roughly consistent with plausible times of crossing of the sun through the galactic plane. The non periodic component present in the distribution of crater ages is consistent with the estimated production of craters by impact of Earth-crossing asteroids.

The most likely explanation for a long-term increase in the cratering rate is a decrease in the amplitude of the motion of the sun normal to the galactic plane and a consequent increase in the average flux of comets derived from the Oort Cloud. Combined evidence from lunar and terrestrial craters suggests that a major increase in the long-term comet flux occurred at about 200 to 300 Ma. It is unlikely that this surge is due to a change in the flux of asteroids, as a pulse in the flux of Earth-crossing asteroids decays with a half-life of 2 to 3 × 10
7
years. A previous long-term surge in the cratering rate may have occurred in the Neoproterozoic.