IMPACT RATE VARIATIONS OVER GEOLOGIC TIME: SPIKY OR SMOOTH?

Models of crater production functions used to date terrains assume that the impact rate has been smooth (constant or slowly changing) over the past ~3 Ga; the sparse Apollo dates for lunar craters do not contradict that assumption [1]. However, impact rates must be variable (spiky) due to major collisions in the asteroid belt and from perturbations of Oort comets. How much uncertainty do these variations introduce to crater-count age estimates? The observational evidence for spikes ~480 [2] and ~800 [3] Ma ago have not been reproduced by independent studies, or may be explained by single impact events. However, evidence for an increased cratering rate in the inner solar system over the past ~300 Ma has now been reported from studies of lunar [4], terrestrial [5, but see 6], and martian [7] craters. This idea is supported by comparing dated lunar craters to a model of the incoming asteroid flux [8]. Evidence for a recent increase based on astronomical surveys [9, 10] and lunar impact glass spherules [11] remain debated. There is also both observational and theoretical evidence for a long-term decline in the cratering rate from 3 to 0.5 Ga [10, 12]. A new study indicates a remarkably constant rate of small meteorite delivery to Earth over past 500 Ma, except at 466 Ma [13]. But these particles would make very small (<1 m) lunar craters, whereas the evidence for variability comes from >10 km craters; the variability may be greater for larger impactors [14]. A new surge in lunar exploration including sample return and in-situ age dating promises to resolve these debates [15]. Given an assumption of constant cratering rate over the past 3 Ga, errors in the crater retention ages of planetary landscapes may be as large as a factor of two, which should be acknowledged in geologic interpretations.

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