from pristine to slightly polluted (35). If this
is true, it means that the preindustrial globe
should be considered differently from today’s
globe. At least over the oceans, the coverage
of warm clouds should be regarded as having
been much smaller than it is today.
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The research leading to these results received funding from the
European Research Council (ERC) under the European Union’s
Seventh Framework Programme (FP7/2007-2013)/ERC Grant
agreement no. 306965 (CAPRI).
Materials and Methods
Figs. S1 to S7
24 February 2014; accepted 5 May 2014
Identification of the giant impactor
Theia in lunar rocks
Daniel Herwartz,1,2 Andreas Pack,1 Bjarne Friedrichs,1 Addi Bischoff 3
The Moon was probably formed by a catastrophic collision of the proto-Earth with a
planetesimal named Theia. Most numerical models of this collision imply a higher portion
of Theia in the Moon than in Earth. Because of the isotope heterogeneity among solar
system bodies, the isotopic composition of Earth and the Moon should thus be distinct.
So far, however, all attempts to identify the isotopic component of Theia in lunar rocks have
failed. Our triple oxygen isotope data reveal a 12 T 3 parts per million difference in D17O
between Earth and the Moon, which supports the giant impact hypothesis of Moon
formation. We also show that enstatite chondrites and Earth have different D17O values,
and we speculate on an enstatite chondrite–like composition of Theia. The observed
small compositional difference could alternatively be explained by a carbonaceous
chondrite–dominated late veneer.
Earth’s Moon is distinct among the >150 moons of our solar system (1). Most other moons are either captured planetesimals, or they formed along with the planet in a common accretion disc. In contrast, it is
hypothesized that our satellite formed ~4.5 billion
years ago from the debris of a giant collision
between the proto-Earth and another smaller
proto-planet [giant impact hypothesis (2, 3)].
Some of the distinct features of the Moon—such
as the depletion in moderately volatile elements
and water, the small lunar core, and the angular
momentum of the Earth-Moon system—are interpreted as products of the energetic collision
with Theia (1).
Most numerical models of the collision as-
sume that Theia was about the size of Mars
and collided with the proto-Earth at an oblique
angle. These classic collision models predicted
that the Moon is made of 70 to 90% Theia
[mass fraction of impactor (Mi)] and only 10
to 30% proto-Earth [mass fraction of proto-
Earth (M⊕)] material (2, 3). Such a large frac-
tion of Theia in the Moon, however, is difficult
to reconcile with the observed isotopic simi-
larity between the Moon and Earth (4). Recent
simulations take this into account and aim to
decrease the compositional difference between
Earth and the Moon (see below).
Measurements of isotope ratios of terrestrial,
martian, and asteroidal samples show that the
bodies in the early solar system were isotopically
heterogeneous (5). It is therefore expected that
Theia and proto-Earth were isotopically distinct.
If the Moon formed predominantly from fragments of Theia, as predicted by most numerical
models, the Moon and Earth should differ in
their isotopic composition. However, no isotopic
differences between Earth and the Moon have
yet been recognized; for instance, for O (6–10), Ti
(11), Ca (12), Si (13), or W (14). We argue herein
that careful reinvestigation of the published data
sets for O and Ti hint at small variations between
Earth and the Moon.
Three explanations exist for the paradox of
identical isotopic compositions of Earth and the
Moon: (i) formation of proto-Earth and Theia at
similar heliocentric distances from the same
isotopic reservoir, resulting in identical compositions of proto-Earth and Theia (8); (ii) isotopic
reequilibration in the aftermath of the giant
impact that has obliterated the initial heterogeneity (4); or (iii) less compositional difference
between Earth and the Moon than predicted by
classic numerical simulations (15–17).
Recent collision models that aim to be consistent with isotope measurements proposed a
larger (15), smaller (16), or faster impactor (16, 17).
In models with small impactors that assume a fast-spinning proto-Earth (16), the Theia component in
1Georg-August-Universität Göttingen, Geowissenschaftliches
Zentrum, Abteilung Isotopengeologie, Goldschmidtstraße 1,
37073 Göttingen, Germany. 2Universität zu Köln, Institut
für Geologie und Mineralogie, Zülpicher Straße 49a, 50674
Köln, Germany. 3Westfälische Wilhelms-Universität
Münster, Institut für Planetologie, Wilhelm-Klemm-Straße 10,
48149 Münster, Germany.
*Corresponding author. E-mail: firstname.lastname@example.org