The timing of the lunar impacts is poorly constrained as the ages of many of the lunar basins are unknown. In particular, the impact flux may not have declined monotonically (e.g., Ryder, 2002, 2003). Rather, the impact flux between 3.8 and 4.0 Ga may have been significantly higher than the impact flux before then. This terminal cataclysm is consistent with what is known about lunar basin ages, but the age of the largest basin, South Pole-Aiken, is unknown and the age of Nectaris has been subject to controversy.
The strongest evidence for a cataclysm involves impact melts whose ages give times that the Moon was hit. As expected from the cataclysm hypothesis, most of these ages fall within the interval of the postulated cataclysm. There are few older ages, which Ryder (2002, 2003) attributes to a peaceful period of low impact flux between the formation of the Moon and the cataclysm. He contends that this is not an artifact of poor preservation of impact melts older than the cataclysm because grossly similar igneous melts that are older than the putative cataclysm are well preserved and well sampled. Ryder's perspective is not universally accepted. Lunar argon-argon ages are spread between 3.7 and 4.1 Ga and older dates are present; they do not generally show a tight clustering in time (Bogard, 1995). Lunar Sr-Rb ages show a similar age distribution. Moreover, radiometric ages of the eucrite parent body show no sign of the cataclysm (Bogard, 1995) that should have affected Vesta at the same time that it affected the Moon.
Figure 7.1 compares various concepts of the lunar bombardment. The curve denoted "Wilhelms" is adapted from the recommended chronology based on Apollo dated surfaces as given by Wilhelms (1987, p. 160). It is illustrative of the aggressively extrapolated lunar cratering history favored by Shoemaker and others. For this curve the e-folding decay time of the late bombardment is 100 Ma. Shown for comparison is Neukum's standard lunar
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