Meteorites 271
FIGURE 14 Lithophile element concentrations (K vs. La) in
ordinary and Cl chondrites and samples of evolved bodies: lunar
samples from variousApollomissions and lunar meteorites;
terrestrial rocks; and martian meteorites. Data for HED
achondrites parallel and lie between the Earth and Moon lines.
but, in principle, could be much less; they probably orig-
inated from∼20 impacts forming lunar explosion craters
that are a few kilometers in diameter, and possibly as small
as 0.5 km.
Lunar meteorites total∼11.2 kg, much less than the
382 kg of Apollo and Luna material, but they provide very
important lunar information. Because Apollo and Luna
landing sites (all Nearside) were chosen for safety rea-
sons or as geologically interesting but unrepresentative,
their regional sampling of the Moon is biased. Lunar me-
teorites represent random (but unknown) impact sites. In-
deed, when compared with lunar spectral reflectance data
from theClementinespacecraft, the distribution of FeO
contents, KREEP-associated U and Th contents, and, in-
deed, the highlands nature of lunar meteorites themselves
parallel the overall lunar character. One meteorite, NWA
773, samples a Mare basalt region unlike any provided by
the Apollo or Luna missions. Much will doubtless be learned
about the Moon and its history from these lunar meteorites
and others, yet discovered.
4.2 Martian Meteorites
The 32 martian meteorites are unusual igneous meteorites
(of five different types), and all but ALH 84001 crystal-
lized from parent melts≤1.3 Ga ago (the youngest, 170 Ma
ago). This alone suggests a large parent because only a plan-
etary body could retain interior temperatures sufficient to
maintain igneous melts that recently. Asteroid-sized objects
could have been differentiated early but would have cooled
rapidly, crystallizing igneous rocks 4.5 Ga ago. That is the
age of ALH 84001, which must be a rare survivor of early
martian differentiation. It is, of course, linked by oxygen iso-
topic composition to the other 31 rocks in the SNC portion
of Fig. 11. They are linked to Mars specifically by gases
(e.g.,^20 Ne,^36 Ar,^40 Ar,^84 Kr,^131 Xe, N 2 ,CO 2 ) in shock-
formed glass in EET A79001, the only meteorite show-
ing a contact between two igneous regions. Contents of
these gases in EET A79001 match those in the martian
atmosphere measured in 1976 by theVikinglanders. The
Martian atmosphere apparently lost more light gases than
did the Earth.
Because martian escape velocity is higher than that of
the Moon, impacts intense enough to propel Martian me-
teorites Earthward must be greater, requiring larger ex-
plosion craters, 10–100 km in diameter. From cosmic ray
histories, the 32 martian meteorites apparently derive from
6–8 events. All solidified near, but below, the martian sur-
face; none were surface samples irradiated by cosmic rays
or heavily weathered so that Fe^2 +is present in them rather
than the red Fe^3 +of martian surface samples. Martian sedi-
mentary rocks and soil may be too friable to survive impact-
ejection.
Considering evidence for water flow on Mars’ surface,
Martian meteorites are surprisingly dry, and evidence for
desiccated salts is slight. Curiously, low initial (radiogenic)
Sr and Nd isotopic data indicate that parent magmas in
the martian mantle were depleted in heat-producing ra-
dionuclides (Fig. 14) relative to chondrites (Section 6.4).
Shergottites are also depleted in light REE. Not surpris-
ingly, we cannot specify sites from which martian meteorites
derive. From crystallization ages, ALH 84001 apparently
originated in the old (heavily cratered) southern highlands,
while the other 31 come from the young (less-cratered) vol-
canic northern plains areas.5. Chemical and Isotopic Constituents
of Meteorites
Earlier, we summarized meteorite compositions and ge-
netic processes as necessary to understand general me-
teoritic properties. Here, we focus upon these topics in
greater detail.5.1 Noble Gases
The chemical inertness of noble gases allows their ready
separation from all other chemical elements. Thus, gas mass