Geology Reference
In-Depth Information
6
ANSMET Meteorites from the Moon
Randy L. Korotev
1
and Ryan A. Zeigler
2
“The occurrence of secondary craters in the rays extending as
much as 500 km from some large craters on the moon shows that
fragments of considerable size are ejected at speeds nearly half the
escape velocity from the moon (2.4 km/sec). At least a small
amount of material from the lunar surface and perhaps as much
or more than the impacting mass is probably ejected at speeds
exceeding the escape velocity by impacting objects moving in
asteroidal orbits. Some small part of this material may follow
direct trajectories to the earth, some will go into orbit around the
earth, and the rest will go into independent orbit around the sun.
Much of it is probably ultimately swept up by earth.”
Shoemaker et al
. [1963]
As a result, lunar meteorites are more diverse, particu-
larly in terms of chemical composition, than meteorites
from any other solar system body. The Moon is differen-
tiated both vertically and laterally. Vertically, it has a
small, most likely metallic core, a thick mantle (which
itself may be layered) consisting mainly of olivine and
pyroxene, and a crust that is rich in the anorthite extreme
of plagioclase feldspar [
Wieczorek et al
., 2006]. Laterally,
many of the huge impact basins that formed early in
lunar history are filled with basalt from volcanism:
melting of the mantle from the heat of radioactive decay
with the magmas rising to fill the low spots in the crust.
This lateral heterogeneity is very evident, even from
Earth, with the light-colored feldspathic “terra” or “high-
lands” pockmarked by dark circular “maria“ (seas;
singular: “mare;” Figure 6.1). Chemically, the maria are
rich in iron, magnesium, and in some places titanium
(olivine, pyroxene, ilmenite), whereas the highlands are
rich in aluminum and calcium (anorthite). Although the
mare-highlands dichotomy has been recognized for at
least hundreds of years, it was not until samples were
collected during the
Apollo
missions (1969-1972) and
the composition of the lunar surface was systematically
mapped by the Lunar Prospector mission (1998-1999)
that a second lateral heterogeneity was discovered. The
geochemically incompatible elements such as K, REE
(rare earth elements), P, Zr, Th, and U, which do not par-
tition into the major minerals of the lunar crust (plagio-
clase, pyroxene, olivine, and ilmenite), are concentrated
in the northwest quadrant of the nearside in a region
known as the Procellarum KREEP Terrane (Figure 6.2).
The last basin-forming impact to occur on the nearside
of the Moon, Imbrium, occurred in the Procellarum
KREEP Terrane (Figure 6.2), spreading KREEP-rich
rocks (i.e., rocks with high concentrations of Th and
other incompatible elements) over the surface of the
Moon [
Haskin
, 1998]. The lunar meteorites reflect the
6.1. INTRODUCTION
Despite the prediction of
Shoemaker et al
. [1963], it
took another 19 years before the first lunar meteorite was
recognized, Allan Hills 81005, a 31-g stone discovered
during an ANSMET search in January of 1982 (Table 6.1)
[
Marvin et al
., 1983;
Cassidy
, 2003]. As of 2013, ANSMET
has collected 24 lunar meteorite stones representing,
when pairings are taken into account, 14 different mete-
orites (Table 6.1). In total, 22% of the known lunar mete-
orites have been found in Antarctica,17% by ANSMET
and 7% by searches done by the NIPR (National Institute
of Polar Research) of Japan; the rest have been found in
hot deserts (Australia: 2%, Africa 46%, and Oman: 29%).
Unlike the asteroids, the Moon did not accrete directly
from dust and planetesimals. Instead, the Moon is the
likely product of a giant impact between two differenti-
ated bodies [
Hartmann and Davis
, 1975;
Cameron and
Ward
, 1976;
Canup and Asphaug
, 2001]. It subsequently
experienced further differentiation and a more prolonged
period of igneous geologic activity than any asteroid.
1
Department of Earth and Planetary Sciences and
McDonnell Center for the Space Sciences, Washington
University
2
Astromaterials Research and Exploration Science Directorate,
Acquisition and Curation, NASA Johnson Space Center
