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The Provenances of Asteroids, and Their Contributions to the Volatile Inventories of the Terrestrial Planets

Science  10 Aug 2012:
Vol. 337, Issue 6095, pp. 721-723
DOI: 10.1126/science.1223474

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Constraining the Birthplace of Asteroids

Many primitive meteorites originating from the asteroid belt once contained abundant water that is now stored as OH in hydrated minerals. Alexander et al. (p. 721, published online 12 July) estimated the hydrogen isotopic compositions in 86 samples of primitive meteorites that fell in Antarctica and compared the results to those of comets and Saturn's moon, Enceladus. Water in primitive meteorites was less deuteriumrich than that in comets and Enceladus, implying that, in contradiction to recent models of the dynamical evolution of the solar system, the parent bodies of primitive meteorites cannot have formed in the same region as comets. The results also suggest that comets were not the principal source of Earth's water.

Abstract

Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn’s moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth’s volatiles.

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