The Moon 239
FIGURE 14 The distribution of major impact basins on (a) the nearside and (b) the farside of
the Moon. (Courtesy of D. E. Williams.)
The noncataclysmic explanation is that the Imbrium and
Orientale Basins formed during the tail end of the accre-
tion of the planets and so represent the final sweep-up of
large objects. The problem with this scenario is that extrap-
olation from the rate at 3.8 billion years back to 4.5 billion
years results in the accretion of a Moon several orders of
magnitude larger than observed. It seems probable that ac-
cretion of the Moon was essentially complete and that the
Moon was at its present size by about 4450 million years ago,
at the time of the crystallization of the feldspathic highland
crust. Other arguments in favor of the cataclysm include
the scarcity of impact melts older than 4 billion years and
the lead isotope data, which indicate a major resetting of the
lead ages at 3.86 billion years. Although it is often argued
that the sampling from theApollomissions is dominated
by Imbrium ejecta, lunar meteorites have provided fresh
insights. These provide a random sampling of the surface
but display no impact melts older than 3.92 billion years,
supporting the notion of a “cataclysm” although the storage
for several hundred million years and supply of the mas-
sive impactors poses some interesting problems. Figure 15
shows a reconstruction of the lunar crater production rate
with time.
7. The Maria
The maria make up the prominent dark areas that form
the features of “the man in the Moon” (Figs. 1 and 16).
FIGURE 15 The production rate over geological time for lunar
craters greater than 4 km in diameter. This illustrates the very
high cratering prior to 3.8 billion years ago, but whether this
represents the tail end of accretion or spike (cataclysm), as
preferred here, in the bombardment history is unclear. The
terrestrial rate is for the past 200 million years. (Updated from
the Basaltic Volcanism Study Project, 1981.)