Oxygen Isotopes and the Moon-Forming Giant Impact

Wiechert, Uwe; Halliday, Alex N.; Lee, D.-C.; Snyder, G. A.; Taylor, L. A.; Rumble, Douglas

doi:10.1126/science.1063037

Cited by 386 publications

(236 citation statements)

References 16 publications

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“…Yet, in the giant impact model, the Moon forms from a mixture of material derived from two separately formed objects: the impactor and the target protoearth. It had been suggested that an impactor formed in an orbit close to that of the Earth could have had the same oxygen composition as the Earth, because observed oxygen compositions vary with heliocentric position [12,13]. This seemed to agree well with the canonical impact (see below) that requires a low impact velocity and therefore an impactor orbit similar to that of the Earth.…”

Section: (A) Constraintssupporting

confidence: 74%

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Lunar-forming impacts: processes and alternatives

Canup

2014

Phil. Trans. R. Soc. A.

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The formation of a protolunar disc by a giant impact with the early Earth is discussed, focusing on two classes of impacts: (i) canonical impacts, in which a Mars-sized impactor produces a planet-disc system whose angular momentum is comparable to that in the current Earth and Moon, and (ii) high-angularmomentum impacts, which produce a system whose angular momentum is approximately a factor of 2 larger than that in the current Earth and Moon. In (i), the disc originates primarily from impactorderived material and thus is expected to have an initial composition distinct from that of the Earth's mantle. In (ii), a hotter, more compact initial disc is produced with a silicate composition that can be nearly identical to that of the silicate Earth. Both scenarios require subsequent processes for consistency with the current Earth and Moon: disc-planet compositional equilibration in the case of (i), or large-scale angular momentum loss during capture of the newly formed Moon into the evection resonance with the Sun in the case of (ii).

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Section: (A) Constraintssupporting

confidence: 74%

“…The Earth and Moon have identical oxygen isotope compositions to within measurement precision, which are quite distinct from those of most meteorites and Mars [12]. Yet, in the giant impact model, the Moon forms from a mixture of material derived from two separately formed objects: the impactor and the target protoearth.…”

Section: (A) Constraintsmentioning

confidence: 99%

Lunar-forming impacts: processes and alternatives

Canup

2014

Phil. Trans. R. Soc. A.

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“…Because the formation of the Moon requires a relatively low-velocity impact between Theia and the proto-Earth, it had previously been argued that Theia likely resided in an orbit very similar to the proto-Earth and may have formed from a similar region of the planetesimal disk (Wiechert et al 2001). If Theia and Earth formed from the same pool of planetesimals, they may have a similar composition, while Mars and Vesta have distinctly different oxygen signatures, owing to their different feeding zones in the planetesimal disk.…”

Section: Theia and The Moon-forming Impactmentioning

confidence: 99%

The feeding zones of terrestrial planets and insights into Moon formation

Kaib

Cowan

2015

Icarus

105

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The final stage of terrestrial planet formation consists of several hundred approximately lunar mass bodies accreting into a few terrestrial planets. This final stage is stochastic, making it hard to predict which parts of the original planetesimal disk contributed to each of our terrestrial planets. Here we present an extensive suite of terrestrial planet formation simulations that allows quantitative analysis of this process. Although there is a general correlation between a planet's location and the initial semi-major axes of its constituent planetesimals, we concur with previous studies that Venus, Earth, and Mars analogs have overlapping, stochastic feeding zones. We quantify the feeding zone width, ∆a, as the mass-weighted standard deviation of the initial semi-major axes of the planetary embryos and planetesimals that make up the final planet. The size of a planet's feeding zone in our simulations does not correlate with its final mass or semi-major axis, suggesting there is no systematic trend between a planet's mass and its volatile inventory. Instead, we find that the feeding zone of any planet more massive than 0.1M ⊕ is roughly proportional to the radial extent of the initial disk from which it formed: ∆a ≈ 0.25(a max − a min ), where a min and a max are the inner and outer edge of the initial planetesimal disk. These wide stochastic feeding zones have significant consequences for the origin of the Moon, since the canonical scenario predicts the Moon should be primarily composed of material from Earth's last major impactor (Theia), yet its isotopic composition is indistinguishable from Earth. In particular, we find that the feeding zones of Theia analogs are significantly more stochastic than the planetary analogs. Depending on our assumed initial distribution of oxygen isotopes within the planetesimal disk, we find a ∼5% or less probability that the Earth and Theia will form with an isotopic difference equal to or smaller than the Earth and Moon's. In fact we predict that every planetary mass body should be expected to have a unique isotopic signature. In addition, we find paucities of massive Theia analogs and high velocity moon-forming collisions, two recently proposed explanations for the Moon's isotopic composition. Our work suggests that there is still no scenario for the Moon's origin that explains its isotopic composition with a high probability event.

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“…The lunar-forming impact was probably also the last major event in the Earth's accretion: substantial addition of material after the Moon-forming impact would tend to make the Moon too iron rich (Canup 2004a), and even if such later accretion was limited to the Earth, it would probably cause the Earth's O-isotope composition to diverge from that of the Moon (e.g. Wiechert et al 2001).…”

Section: The Moon-forming Impactmentioning

confidence: 99%

“…Wiechert et al 2001). This appears to require either that the impactor and proto-Earth had identical compositions (unlikely, given the compositional variation predicted by accretion models) or that extensive mixing between proto-lunar and proto-Earth material after the impact but prior to the Moon's formation allowed compositions to equilibrate (Pahlevan & Stevenson 2007).…”

Section: (C ) Implications and Open Issuesmentioning

confidence: 99%

Accretion of the Earth

Canup

2008

Phil. Trans. R. Soc. A.

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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. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 458 and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

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Oxygen Isotopes and the Moon-Forming Giant Impact

Cited by 386 publications

References 16 publications

Lunar-forming impacts: processes and alternatives

Lunar-forming impacts: processes and alternatives

The feeding zones of terrestrial planets and insights into Moon formation

Accretion of the Earth

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