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some portions of the lunar crust by a large number of collisions in a short
time, less than 200 million years.
To better characterize the cause, mechanism, and extent of LHB, we
can resort to the recent progress in extensive asteroid surveys that have
revealed fine structures of the size-frequency distribution (SFD) of main
belt asteroids (MBAs) and near-Earth asteroids (NEAs). These data can be
compared with the SFD of the crater projectiles on the Moon and on other
planets. In this paper, we provide compelling new evidence that the source
of the LHB impactors was the main asteroid belt, and that the dynamical
mechanism that caused the LHB was unique in the history of the solar
system and distinct from the processes producing the flux of objects that
currently hit planetary surfaces.
5
2. Crater SFDs
Throughout this manuscript, SFDs of craters and asteroids are expressed
by the so-called
R
-plot, which expresses differential SFD of crater/asteroid
populations relative to
D
−
3
,where
D
denotes diameter.
6
Since many popu-
lations of inner solar system small bodies and craters have differential SFDs
(
dN/dD
) more or less proportional to
D
−
3
, it is reasonable to normalize
them by
D
−
3
so that we can see their differences in detail. Also,
R
values
of craters are generally divided by the surface area
A
, where we count the
number of craters in order to estimate the number density of craters;
R
is
defined as
R
(
D
3
/A
)
dN/dD
.
Expressing the SFD of lunar and planetary craters by
R
-plot, clearly we
see two distinctive SFD populations (Fig. 1). The first crater population is
what is typically observed on the oldest lunar highlands; LHB craters as old
as
≡
×
4 Ga. This crater population is characterized by a wavy
R
curve as in
Fig. 1(a), which is also seen on the oldest highlands on Mercury (Fig. 1(b))
as well as on Mars (Fig. 1(c)).
There is another crater population characterized by rather flat
R
pat-
terns. These craters are younger than the LHB craters, and their number
density is lower. In general, these craters have a wide variety of ages, indi-
cating they have been formed over a timespan as long as Gyr. A typical
example of this population is seen on young and smooth northern martian
hemisphere where there are a lot of relatively new craters. The
R
curve
for these craters is almost flat, showing
R
∼
D
−
3
,asinthelowerpart
of Fig. 1(c). Another example of this crater population is observed on the
Moon as Class-1 craters with quite fresh morphology. These craters also
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