Encyclopedia of the Solar System 2nd ed

246 Encyclopedia of the Solar System

FIGURE 20 Major lunar crustal

terranes. The Procellarum KREEP

Terrane (PKT) is on the nearside,

with Th greater than 3.5 ppm. The

South Pole–Aitken Terrane (SPAT)

has an outer region corresponding

to basin ejecta. The Feldspathic

Highland Terrane (FHT)

corresponds to the thickest part of

the crust, concentrated on the

lunar farside. FHT,O consists of

those regions where basin ejecta or

cryptomare obscure the feldspathic

surface. (Courtesy of Brad Joliff,

Washington University.)

lack of compensation indicates that the crust and interior
have been strong enough to support this structure for most
of lunar history. There is little sign of the residual KREEP
component in this location, despite the depth of excavation.
This reinforces the notion that the residual KREEP melt
was not uniformly distributed. Figure 21 shows the man-
tle uplift beneath the South Pole–Aitken Basin as well as
that partially superimposed later uplift resulting from the
excavation of theApollobasin.

9. Lunar Composition

The Moon is bone-dry and highly reduced, no indigenous

H 2 O having been detected at ppb levels, and lacks ferric

iron. It is strongly depleted to volatile elements (e.g., K, Pb,

Bi) by a factor of about 50 compared to the Earth, or 200 rel-

ative to primordial solar nebula abundances. Compared to

the Earth, the most striking difference is in the abundance

of iron that is reflected in the low lunar density. The Earth

NE

Megaregolith

SE

Upper Crust

Lower Crust

South Pole-Aitken

Basin Rim

Basin Ejecta

Fall-back Breccia

Cryptomare

Maria Crater Ejecta

Apollo Basin

Mantle Uplifted

by Later Apollo Event

Mantle Uplift

CENTER

FIGURE 21 The South Pole–Aitken Basin (2500 km diameter and 12 km deep) on which are
superimposed two later impact basins.