Daniel Herwartz^1,2, Andreas Pack¹, Bjarne Friedrichs¹ and Addi Bischoff³

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.

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 ± 3 parts per million difference in Δ¹⁷O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation. We also show that enstatite chondrites and Earth have different Δ¹⁷O 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.

Reference
Herwartz D, Pack A, Friedrichs B and Bischoff A (in press) Identification of the giant impactor Theia in lunar rocks. Science 344:1146.
[doi:10.1126/science.1251117]
Reprinted with permission from AAAS

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Tim Elliott

School of Earth Science, University of Bristol, Queen’s Road, Clifton BS8 1RJ, UK.

As in many building booms, planets were put together pretty rapidly. Transforming nebular dust to fully formed planets took less than ~100 million years of the ~4.5 billion years of solar system history. Accurate determination of the rates of planetary growth is key for understanding these tumultuous beginnings of the solar system, but obtaining high-precision ages on short-lived events that happened so long ago is a formidable challenge. On page 1150 of this issue, Kruijer et al. (1) determine with remarkable accuracy that planetary core formation began less than 1 million years after the first solids condensed—extraordinarily fast on geological time scales.

Reference
Elliot T (in press) Speed metal. Science 344:1086.
[doi:10.1126/science.1254943]
Reprinted with permission from AAAS

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T. S. Kruijer^1,2, M. Touboul³, M. Fischer-Gödde¹, K. R. Bermingham³, R. J. Walker³ and T. Kleine¹

¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, DE-48149 Münster, Germany.
²ETH Zürich, Inst. of Geochemistry and Petrology, Clausiusstrasse 25, CH-8092 Zürich, Switzerland.
³Department of Geology, University of Maryland, College Park, MD 20742, USA.

Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in ¹⁸²W, resulting from the decay of now-extinct ¹⁸²Hf, among five magmatic iron meteorite groups. These ¹⁸²W variations indicate that core formation occurred over an interval of ~1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently ~0.1 to 0.3 million years after the formation of Ca-Al–rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.

Reference
Kruijer TS, Touboul M, Fischer-Gödde M, Bermingham KR, Walker RJ and Kleine T (in press) Protracted core formation and rapid accretion of protoplanets. Science 344:1150.
[doi:10.1126/science.1251766]
Reprinted with permission from AAAS

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Seungwon Lee^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aJet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA

We present sub-millimeter observations of the ground-state rotational transition (1₁₀–1₀₁) of water vapour from comet C/2002 T7 (LINEAR) obtained with the MIRO Instrument on the ESA Rosetta Spacecraft (s/c) Orbiter on April 30, 2004. At the time of the observations, the comet was at a distance of 0.63 AU from the Sun, 0.68 AU from the MIRO telescope, and about 7.5 days after its perihelion. The ground state rotation transition of ortho-water at 556.936 GHz was observed and integrated for ∼ 8 hours using a frequency switched radiometer to provide short and long term stability. The MIRO beam size is 7.5 arcmin in terms of full width half maximum, corresponding to a radius of 1.1×10⁵ km at the comet location. The observed signal line area of the water line spectrum is 4.3±0.8 K km/s. Using a molecular excitation and radiation transfer model and assuming the spherically symmetric and constant radial expansion of gas in the coma, we estimate that the production rate of water is (1.0±0.2)x10³⁰ molecules/s and the expansion velocity is 1.1±0.2 km/s at the time of the MIRO observation. The present estimation of the water outgassing rate of the comet is in good agreement with other observation-based estimations when the outgassing rates with respect to the time after perihelion are compared. The Doppler-correctd center velocity of the observed line was red-shifted by 0.67±0.13 km/s, of which only 0.18 km/s shift is explained by the model and attributed to a self-absorption effect. The potential sources of the additional red shift are discussed.

Reference
Lee et al. (in press) Sub-millimeter Observation of Water Vapor at 557 GHz in Comet C/2002 T7 (LINEAR). Icarus
[doi:10.1016/j.icarus.2014.05.004]
Copyright Elsevier

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