Geoscience Reference
In-Depth Information
The lunar crust
The thickness and composition of the lunar high-
land crust indicate that the Moon is both a
refractory-rich body and an extremely well dif-
ferentiated body. The amount of aluminum in
the highland crust may represent about 40%
of the total lunar budget. This is in marked con-
trast to the Earth, where the amount of such
major elements as aluminum and calcium in
the crust is a trivial fraction of the total in the
planet. On the other hand the amount of the
very incompatible elements such as rubidium,
uranium and thorium in the Earth's crust is a
large fraction of the terrestrial inventory. This
dichotomy between the behavior of major ele-
ments and incompatible trace elements can be
understood by considering the effect of pressure
on the crystallization behavior of calcium- and
aluminum-rich phases. At low pressures these ele-
ments enter low-density phases such as plagio-
clase, which are then concentrated toward the
surface. At higher pressures these elements enter
denser phases such as clinopyroxene and garnet.
At still higher pressures, equivalent to depths
greater than about 300 km in the Earth, these
phases react to form a dense garnet-like solid
solution that is denser than such upper-mantle
phases as olivine and pyroxene. Therefore, in the
case of the Earth, much of the calcium and alu-
minum is buried at depth. The very incompatible
elements, however, do not readily enter any of
these phases, and they are concentrated in light
melts. The higher pressures in the Earth's magma
ocean and the slower cooling rates of the larger
body account for the differences in the early his-
tories of the Earth and Moon.
In the case of the Moon, the anorthositic
component is due to the flotation of plagio-
clase aggregates during crystallization of the
ocean. Later basalts are derived from cumulates
or cumulus liquids trapped at depth, and the
KREEP (K, REE, P rich) component represents the
final residual melt. The isotopic data (Pb, Nd,
Sr) require large-scale early differentiation and
uniformity of the KREEP component. About 50%
of the europium and potassium contents of the
Moon now reside in the highland crust, which
is less than 9% of the mass of the Moon. Esti-
mates of the thickness of the magma ocean are
be enriched in refractories. In these theories
the volatile-rich materials must be concentrated
toward the interior. In a cooling-gas model of
planetary formation, the refractories condense
before the volatiles, and it was therefore implied
that the Moon was made inside out! The stan-
dard geochemical model of terrestrial evolution
also invokes a volatile-rich interior, one that is
rich in
3
He, but there is no evidence for this. The
strange chemistry of the Moon is consistent with
condensation from a gas--dust cloud caused by a
giant impact on proto-Earth.
Large-ion refractory elements are concen-
trated in the lunar-mare basalts by several orders
of magnitude over the highland plagioclase-rich
material, with the notable exception of euro-
pium, which is retained by plagioclase. Compared
to the other rare-earth elements, europium is
depleted in basalts and enriched in anorthosites.
The ''europium anomaly” was one of the early
mysteries of the lunar sample-return program
and implied that plagioclase was abundant some-
where on the Moon. The predicted material
was later found in the highlands. Similarly, in
terrestrial samples, there are missing elements
that imply eclogitic and kimberlitic material at
depth.
The maria on the Moon are remarkably
smooth and level; slopes of less than one-tenth of
a degree persist for hundreds of kilometers, and
topographic excursions from the mean are gen-
erally less than 150 m. By contrast, elevation dif-
ferences in the highlands are commonly greater
than 3 km. The mean altitude of the terrace, or
highlands, above maria is also about 3 km. The
center of mass is displaced toward the Earth and
slightly toward the east by about 2 km.
Seismic activity of the Moon is much lower
than on Earth, both in numbers of quakes and
their size, or magnitude. Their times of occur-
rence appear to correlate with tidal stresses
caused by the varying distance between the Moon
and the Earth. Compared with the Earth, they
occur at great depth, about half the lunar radius.
The Moon today is a relatively inactive body. This
conclusion is consistent with the absence of obvi-
ous tectonic activity and with the low level of
stresses in the lunar interior implied by gravity
and moment-of-inertia data.
