Geology Reference
In-Depth Information
Maps of the Moon (
Fig. 4.23
) show that basins are ran-
domly distributed over the entire surface and are not pref-
erentially located in any one hemisphere, although the north
polar region appears to have a paucity of impact features
> 300 km in diameter, as described by NASA scientist Herb
Frey (
2011
). Samples returned by the Apollo astronauts,
coupled with photogeologic mapping, show that most of
the basins formed prior to 3.8 Ga ago. Basin-related geol-
ogy dominates the lunar surface in several ways, including
(a) mountain ranges that are segments of basin rings, (b)
lunar crustal fractures that controlled the eruptions of some
lunar lava flows, and (c) ejecta of deposits that blanket
much of the older surface.
Given the critical role of basins in the evolution of the
lunar surface, many of the Apollo and Luna landing sites
were selected to provide insight into these structures,
including one of the most prominent features, the
Imbrium basin. The American geologist G. K. Gilbert
was keenly interested in the geology of the Moon and
studied its surface telescopically, especially the area
around Imbrium. As noted in
Section 2.2
, he also con-
ducted experiments to simulate impact processes. His
results led Gilbert to suggest that the Imbrium basin was
a huge impact scar, some 1,140 km across. This feature
dominates the near side of the Moon, and Gilbert recog-
nized the distinctive radial grooves and furrows, which
he termed the Imbrium sculpture (
Fig. 4.24
). Geologic
mapping shows that the Imbrium basin includes three
rings, with the main ring de
ned by the Apennine
Mountains. An intermediate ring 850 km across is
marked by the lunar Alps, part of the Sinus Iridium
rim, and smaller isolated mountains such as La Hire.
An inner ring about 570 km across is de
ned only by
arc-like ridges seen on the mare lavas, which have been
interpreted as representing
flooding by lava
ows over
now-buried basin structure.
Ejecta deposits from the Imbrium impact are spread
over much of the lunar near side. Its distinctive appear-
ance led to the led
nition by early lunar geologic mappers
of the Fra Mauro Formation (
Fig. 4.11
), which is as thick
as 1 km some 600 km from the basin. This unit serves as
one of the primary index markers, or datum planes, for
lunar stratigraphy, in which other units are dated relatively
by superposition and cross-cutting relations. Because of
its critical importance, the Fra Mauro Formation. was
targeted as an Apollo site early in the series of landings.
Samples returned from Apollo 14 showed that the forma-
tion consists of highly brecciated rocks and provided a
date for the Imbrium impact of 3.85 Ga.
(b)
Figure 4.22. (cont.)
4.5 Geomorphology
The original two-fold classi
cation of the lunar surface into
smooth, dark maria and rugged, light highlands or terrae is
still valid, but sub-units are now recognized, including
terrains associated with impact basins, highlands plains,
and various features on the maria. In addition, features
associated with tectonic and gradation processes are seen
throughout the lunar surface.
4.5.1 Impact craters and basins
Impact craters are the dominant landform on the Moon;
interest in them ultimately contributed to the foundation of
planetary geology as a discipline. Lunar craters range in
size from features only microns across, seen on lunar
samples, to the enormous
“
basins
”
that are hundreds of
kilometers across. Lunar craters serve as the basis for
describing the morphology of impact craters in general,
although the geometries (such as the depth-to-diameter
ratios,
Fig. 3.31
) vary with planetary environment as
described in
Section 3.4.3
.
Planetary scientists Bill Hartmann and Gerard Kuiper of
Arizona coined the term basins for large impact structures
on the Moon. In addition to the large size (> 220 km), lunar
basins are typi
ed by concentric rings of mountain ranges.
