The Moon 247
contains about 25% metallic Fe; the Moon, less than about
2–3%. However, the bulk Moon contains 12–13% FeO,
50% more than in current estimates of 8% FeO in the ter-
restrial mantle. Along with its depletion in iron, the Moon
also has a low abundance of siderophile elements that are
depleted in order of their metal-silicate distribution coeffi-
cients. This observation indicates that these elements have
been segregated into a metallic core. However, this pattern
may have been established in precursor planetesimals or
in the impactor from which most of the Moon appears to
have been derived, rather than, or as well as, into a lunar
core.
The other major element abundances are mostly model-
dependent. Si/Mg ratios are commonly assumed to be chon-
dritic (CI), although the Earth and many meteorite classes
differ from this value. The lunar Mg value is generally es-
timated to be about 0.80, lower than that of the terrestrial
mantle value of 0.89.
The Moon is probably enriched in refractory elements
such as Ti, U, Al, and Ca, a conclusion consistent with geo-
physical studies of the lunar interior. This conclusion is rein-
forced by the data from theGalileo,Clementine, andLunar
Prospectormissions, which indicate that the highland crust
is dominated by anorthositic rocks. This requires that the
bulk lunar composition contains about 5–6% Al 2 O 3 , com-
pared with a value of about 3.6% for the terrestrial mantle
and so is probably enriched in refractory elements (e.g.,
Ca, Al, Ti, U) by a factor of about 1.5 compared to the
Earth.
In the light of the caveats already given, the bulk com-
position of the Moon is only known to a first approximation.
Data for the bulk composition of the Moon are given in
Table 2 compared to CI, the terrestrial mantle abundances
and to the bulk Earth.
Both the Cr and O isotopic compositions are identical in
the Earth and Moon, probably indicating an origin in the
same part of the nebula, consistent with the single impact
hypothesis that derives most of the Moon from the silicate
mantle of the impactor, Theia.
Clearly the Moon has a composition that cannot be made
by any single-stage process from the material of the primor-
dial solar nebula. The compositional differences from that
of the primitive solar nebula, from the Earth, from Phobos
and Deimos (almost certainly of carbonaceous chondritic
composition), and from the satellites of the outer planets
(rock-ice mixtures with the exception of lo) thus call for a
distinctive mode of origin.
9.1 Lunar Minerals
Only about 100 minerals have been identified in lunar
samples, in contrast to the several thousand species that
have been identified on Earth. This lunar paucity is due to
the dry nature of the Moon and the depletion in volatile
and siderophile elements. Extensive summaries of lunar
mineralogy can be found in Frondel (1975), Heiken et al.
(1991), and Papike et al. (1999).9.2 Lunar Meteorites
Our understanding of the lunar crust has been aided by
the discovery of lunar meteorites of which about 20 are
known. From their feldspar-rich and KREEP-poor com-
position, many appear to be from the lunar farside; they
are distinct from the nearside highland samples returned
byApollo 14, 15 , 16 , and 17 andLuna 20. However, their
major element composition is close to that of estimates of
the average highland crust. They confirm, as do theGalileo,
Clementine, andLunar Prospectormissions, the essentially
anorthositic nature of the lunar highland crust.9.3 Tektites
The notion that tektites were derived from the Moon en-
joyed considerable support before the Apollomissions.
However, the controversy that had raged, particularly in the
1960s, over a lunar versus a terrestrial origin was settled in
favor of the latter source by the first sample return from the
Moon in 1969. It has been decisively established from iso-
topic and chemical evidence that tektites are derived from
the surface of the Earth by meteoritic or asteroidal impact.
Because the debate still surfaces occasionally, readers in-
terested in these glassy objects will find a useful review of
the evidence for a terrestrial origin in Koeberl (1994).10. The Origin of the Moon
10.1 The Nature of the Problem
Hypotheses for the origin of the Moon must explain the
high value for the angular momentum of the Earth–Moon
system, the strange lunar orbit inclined at 5.09◦to the plane
of the ecliptic, the high mass relative to that of its primary
planet and the low bulk density of the Moon, much less than
that of the Earth or the other inner planets. The chemical
age and isotopic data revealed by the returned lunar sam-
ples added additional complexities to these classic problems
because the lunar composition is unusual by either cosmic
or terrestrial standards. It is perhaps not surprising that pre-
vious theories for the origin of the Moon failed to account
for this diverse set of properties and that only recently has
something approaching a consensus been reached.
Hypotheses for lunar origin can be separated into five
categories:
1.Capture from an independent orbit
2.Formation as a double planet
3.Fission from a rapidly rotating Earth
4.Disintegration of incoming planetesimals
5.Earth impact by a Mars-sized planetesimal and cap-
ture of the resulting debris into Earth orbit