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Conel M O'D Alexander
Of the potential volatile sources for the terrestrial planets, the CI and CM carbonaceous chondrites are closest to the planets' bulk H and N isotopic compositions. For the Earth, the addition of approximately 2-4 wt% of CI/CM material to a volatile-depleted proto-Earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. Two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt ('classical' model) or in the outer Solar System (5-15 AU in the Grand Tack model)...
May 28, 2017: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences
Kaveh Pahlevan, Alessandro Morbidelli
The Moon is generally thought to have formed from the debris ejected by the impact of a planet-sized object with the proto-Earth towards the end of planetary accretion. Models of the impact process predict that the lunar material was disaggregated into a circumplanetary disk and that lunar accretion subsequently placed the Moon in a near-equatorial orbit. Forward integration of the lunar orbit from this initial state predicts a modern inclination at least an order of magnitude smaller than the lunar value--a long-standing discrepancy known as the lunar inclination problem...
November 26, 2015: Nature
Seth Root, Luke Shulenburger, Raymond W Lemke, Daniel H Dolan, Thomas R Mattsson, Michael P Desjarlais
The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in Earth's mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories' Z Machine and extensive quantum calculations using density functional theory (DFT) and quantum Monte Carlo (QMC) methods...
November 6, 2015: Physical Review Letters
W F Bottke, D Vokrouhlický, S Marchi, T Swindle, E R D Scott, J R Weirich, H Levison
The inner solar system's biggest and most recent known collision was the Moon-forming giant impact between a large protoplanet and proto-Earth. Not only did it create a disk near Earth that formed the Moon, it also ejected several percent of an Earth mass out of the Earth-Moon system. Here, we argue that numerous kilometer-sized ejecta fragments from that event struck main-belt asteroids at velocities exceeding 10 kilometers per second, enough to heat and degas target rock. Such impacts produce ~1000 times more highly heated material by volume than do typical main belt collisions at ~5 kilometers per second...
April 17, 2015: Science
Alessandra Mastrobuono-Battisti, Hagai B Perets, Sean N Raymond
Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo (giant impactor) and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic compositions show them to be highly similar. In contrast, the compositions of other Solar System bodies are significantly different from those of the Earth and Moon, suggesting that different Solar System bodies have distinct compositions...
April 9, 2015: Nature
Thomas S Kruijer, Thorsten Kleine, Mario Fischer-Gödde, Peter Sprung
According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth's core. Core formation should have removed the highly siderophile elements (HSE) from Earth's primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected. One explanation for this overabundance is that a 'late veneer' of primitive material was added to the bulk silicate Earth after the core formed...
April 23, 2015: Nature
Nicolas Dauphas, Christoph Burkhardt, Paul H Warren, Teng Fang-Zhen
Geochemical evidence suggests that the material accreted by the Earth did not change in nature during Earth's accretion, presumably because the inner protoplanetary disc had uniform isotopic composition similar to enstatite chondrites, aubrites and ungrouped achondrite NWA 5363/5400. Enstatite meteorites and the Earth were derived from the same nebular reservoir but diverged in their chemical evolutions, so no chondrite sample in meteorite collections is representative of the Earth's building blocks. The similarity in isotopic composition (Δ(17)O, ε(50)Ti and ε(54)Cr) between lunar and terrestrial rocks is explained by the fact that the Moon-forming impactor came from the same region of the disc as other Earth-forming embryos, and therefore was similar in isotopic composition to the Earth...
September 13, 2014: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences
Kaveh Pahlevan
Ever since the Apollo programme, isotopic abundances have been used as tracers to study lunar formation, in particular to study the sources of the lunar material. In the past decade, increasingly precise isotopic data have been reported that give strong indications that the Moon and the Earth's mantle have a common heritage. To reconcile these observations with the origin of the Moon via the collision of two distinct planetary bodies, it has been proposed (i) that the Earth-Moon system underwent convective mixing into a single isotopic reservoir during the approximately 10(3) year molten disc epoch after the giant impact but before lunar accretion, or (ii) that a high angular momentum impact injected a silicate disc into orbit sourced directly from the mantle of the proto-Earth and the impacting planet in the right proportions to match the isotopic observations...
September 13, 2014: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences
Daniel Herwartz, Andreas Pack, Bjarne Friedrichs, Addi Bischoff
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 Δ(17)O between Earth and the Moon, which supports the giant impact hypothesis of Moon formation...
June 6, 2014: Science
Shun-ichiro Karato
One of the difficulties of the current giant impact model for the origin of the Moon is to explain the marked similarity in the isotopic compositions and the substantial differences in the major element chemistry. Physics of shock heating is analyzed to show that the degree of heating is asymmetric between the impactor and the target, if the target (the proto-Earth) had a magma-ocean but the impactor did not. The magma ocean is heated much more than the solid impactor and the vapor-rich jets come mainly from the magma-ocean from which the Moon might have been formed...
2014: Proceedings of the Japan Academy. Series B, Physical and Biological Sciences
Matija Ćuk, Sarah T Stewart
A common origin for the Moon and Earth is required by their identical isotopic composition. However, simulations of the current giant impact hypothesis for Moon formation find that most lunar material originated from the impactor, which should have had a different isotopic signature. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to that of the present day; however, Earth-mass planets are expected to have higher spin rates at the end of accretion. Here, we show that typical last giant impacts onto a fast-spinning proto-Earth can produce a Moon-forming disk derived primarily from Earth's mantle...
November 23, 2012: Science
Ralf I Kaiser, Alexander M Mebel
This tutorial review compiles recent experimental and theoretical studies on the formation of polyacetylenes (H(C≡C)(n)H) and cyanopolyacetylenes (H(C≡C)(n)CN) together with their methyl-substituted counterparts (CH(3)(C≡C)(n)H, CH(3)(C≡C)(n)CN) as probed under single collision conditions in crossed beam studies via the elementary reactions of ethynyl (CCH) and cyano radicals (CN) with unsaturated hydrocarbons. The role of these key reaction classes in the chemical evolution of Titan's orange-brownish haze layers is also discussed...
August 21, 2012: Chemical Society Reviews
Paul Hartogh, Dariusz C Lis, Dominique Bockelée-Morvan, Miguel de Val-Borro, Nicolas Biver, Michael Küppers, Martin Emprechtinger, Edwin A Bergin, Jacques Crovisier, Miriam Rengel, Raphael Moreno, Slawomira Szutowicz, Geoffrey A Blake
For decades, the source of Earth's volatiles, especially water with a deuterium-to-hydrogen ratio (D/H) of (1.558 ± 0.001) × 10(-4), has been a subject of debate. The similarity of Earth's bulk composition to that of meteorites known as enstatite chondrites suggests a dry proto-Earth with subsequent delivery of volatiles by local accretion or impacts of asteroids or comets. Previous measurements in six comets from the Oort cloud yielded a mean D/H ratio of (2.96 ± 0.25) × 10(-4). The D/H value in carbonaceous chondrites, (1...
October 13, 2011: Nature
Francis Albarède
Accretion left the terrestrial planets depleted in volatile components. Here I examine evidence for the hypothesis that the Moon and the Earth were essentially dry immediately after the formation of the Moon-by a giant impact on the proto-Earth-and only much later gained volatiles through accretion of wet material delivered from beyond the asteroid belt. This view is supported by U-Pb and I-Xe chronologies, which show that water delivery peaked approximately 100 million years after the isolation of the Solar System...
October 29, 2009: Nature
Robin M Canup
The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them...
November 28, 2008: Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences
M Touboul, T Kleine, B Bourdon, H Palme, R Wieler
The Moon is thought to have formed from debris ejected by a giant impact with the early 'proto'-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf-182W and 146Sm-142Nd chronometry, but these methods have yielded contradicting results. In earlier studies, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first approximately 60 million years of the Solar System...
December 20, 2007: Nature
U Wiechert, A N Halliday, D C Lee, G A Snyder, L A Taylor, D Rumble
We have determined the abundances of 16O, 17O, and 18O in 31 lunar samples from Apollo missions 11, 12, 15, 16, and 17 using a high-precision laser fluorination technique. All oxygen isotope compositions plot within +/-0.016 per mil (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation line within uncertainties. This observation is consistent with the Giant Impact model, provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components...
October 12, 2001: Science
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