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2018 ; 4
(3
): eaao5928
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Oxygen isotopic evidence for accretion of Earth s water before a high-energy
Moon-forming giant impact
#MMPMID29600271
Greenwood RC
; Barrat JA
; Miller MF
; Anand M
; Dauphas N
; Franchi IA
; Sillard P
; Starkey NA
Sci Adv
2018[Mar]; 4
(3
): eaao5928
PMID29600271
show ga
The Earth-Moon system likely formed as a result of a collision between two large
planetary objects. Debate about their relative masses, the impact energy
involved, and the extent of isotopic homogenization continues. We present the
results of a high-precision oxygen isotope study of an extensive suite of lunar
and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts
show a 3 to 4 ppm (parts per million), statistically resolvable, difference in
?(17)O. Taking aubrite meteorites as a candidate impactor material, we show that
the giant impact scenario involved nearly complete mixing between the target and
impactor. Alternatively, the degree of similarity between the ?(17)O values of
the impactor and the proto-Earth must have been significantly closer than that
between Earth and aubrites. If the Earth-Moon system evolved from an initially
highly vaporized and isotopically homogenized state, as indicated by recent
dynamical models, then the terrestrial basalt-lunar oxygen isotope difference
detected by our study may be a reflection of post-giant impact additions to
Earth. On the basis of this assumption, our data indicate that post-giant impact
additions to Earth could have contributed between 5 and 30% of Earth's water,
depending on global water estimates. Consequently, our data indicate that the
bulk of Earth's water was accreted before the giant impact and not later, as
often proposed.