The Moon 237
FIGURE 10 Oblique view of crater Copernicus, 93 km in
diameter, showing a central-peak complex and well-developed
slump terraces on the inner walls. (Courtesy of NASA,
AS17-151-23260.)
mountain rings having a typical relief of about 3 km with
steep inward-facing scarps. These formed in a few minutes
after the impact of a body perhaps 50 km in diameter. The
central portion has been flooded with mare basalt. Thirty
FIGURE 11 The transition between central-peak craters and
peak-ring craters. The large central basin is Schr ̈odinger (320 km
in diameter), which has a well-developed peak ring. Antoniadi
(135 km in diameter), southeast from Schr ̈odinger, has both a
central peak and a peak ring. The small crater immediately
southwest of Antoniadi has a central peak only. (Courtesy of
NASA, Orbiter IV-8M.)
such basins have been recognized on the Moon (Fig. 14),
with another 14 probable. There is much controversy over
the origin of multiring basins. One possibility proposes that
the crust is fluidized by the impact and the rings form like
ripples on a pond into which a stone has been dropped.
The most likely explanation is that the mountain rings are
fault scarps, formed by collapse into a deep transient crater
formed by the initial impact.
The depth of excavation of the lunar basins decreases
with increasing basin diameter. A transient cavity forms dur-
ing the initial stage of the impact, but most excavated ma-
terial comes from shallower depths. Thus, no unequivocal
lunar mantle material has been recognized in the returned
samples from the lunar highland crust, and the transient
depth of excavation of the largest basins do not appear to
have exceeded 50 km. Ejecta blankets incorporate much
local material as they travel across the surface in a manner
analogous to a base surge. Apart from the ejecta blankets,
numerous blocks from large impacts travel with sufficient
velocity to produce secondary craters. These must be care-
fully distinguished from primary craters to avoid confusion
in the dating of lunar surfaces by crater counting.
Shock pressures up to 100 GPa (1 GPa=10 kbar) cause
a variety of effects from the development of planar fea-
tures in minerals (>10 GPa) to whole-rock melting (50–
100 GPa). Above about 150 GPa, the rocks are vaporized.
Vapor masses of a few times projectile mass and melt masses
about 100 times the projectile mass may be formed. Impact
melts compose 30–50% of all samples returned from the
lunar highlands.6.2 Lunar Cratering History and the Lunar Cataclysm
The intense cratering of the lunar highlands and the ab-
sence of a similar heavily cratered surface on the Earth were
long recognized as due to an early “pregeological” bombard-
ment. In contrast, the lightly cratered basaltic mare sur-
faces, on which the cratering rate is about 200 times less, had
escaped this catastrophe and were clearly much younger.
The ages of the mare surfaces, dated from the sample re-
turn to be between 3.3 and 3.8 billion years old, showed that
the cratering flux was similar, within a factor of 2, to that
observed terrestrially. It also established that the intense
cratering of the highlands occurred more than 3.8 billion
years ago. Most highland samples have ages in the range
3.8–4.3 billion years. The radiometric ages of the ejecta
blankets from the large collisions tend to cluster around 3.9
billion years, with the dates for the Imbrium collision being
3.85 billion years and that for Nectaris, 3.90 or 3.92 billion
years. This is a surprisingly narrow range and indicates a
rapid increase in the cratering flux just before 3.8 billion
years. This clustering has led to the concept of a “lunar
cataclysm” or a spike in the collisional history at that time.