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RE: Theia
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Are we getting closer to understanding where the moon actually came from?

This week researchers at UCLA provided new evidence in the journal Science that the collision was head-on, and so powerful that materials from both bodies mixed completely before settling into the Earth-moon system we know today.
To come to that conclusion, the researchers analyzed seven lunar rocks collected by the Apollo 12, 15 and 17 missions, as well as six volcanic rocks that include material from Earth's mantle. Specifically, they wanted to see if the ratio of oxygen isotopes in lunar rocks was the same as that in the terrestrial rocks.

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Title: The Tethered Moon
Author: Kevin J. Zahnle, Roxana Lupu, Anthony Dobrovolskis, Norman H. Sleep

We address the thermal history of the Earth after the Moon-forming impact, taking tidal heating and thermal blanketing by the atmosphere into account. The atmosphere sets an upper bound of ~100 W/m˛ on how quickly the Earth can cool. The liquid magma ocean cools over 2-10 Myrs, with longer times corresponding to high angular-momentum events. Tidal heating is focused mostly in mantle materials that are just beginning to freeze. The atmosphere's control over cooling sets up a negative feedback between viscosity-dependent tidal heating and temperature-dependent viscosity of the magma ocean. While the feedback holds, evolution of the Moon's orbit is limited by the modest radiative cooling rate of Earth's atmosphere. Orbital evolution is orders of magnitude slower than in conventional constant Q models, which promotes capture by resonances. The evection resonance is encountered early, when the Earth is molten. Capture by the evection resonance appears certain but unlikely to generate much eccentricity because it is encountered early when the Earth is molten and Q_Earth >> Q_Moon. Tidal dissipation in the Earth becomes more efficient (Q_Earth << Q_Moon) later when the Moon is between ~20 R_Earth and ~40 R_Earth. If lunar eccentricity grew great, this was when it did so, perhaps setting the table for some other process to leave its mark on the inclination of the Moon.

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Moon-Forming Impact
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Title: Melting and Mixing States of the Earth's Mantle after the Moon-Forming Impact
Author: Miki Nakajima, David J. Stevenson

The Earth's Moon is thought to have formed by an impact between the Earth and an impactor around 4.5 billion years ago. This impact could have been so energetic that it could have mixed and homogenized the Earth's mantle. However, this view appears to be inconsistent with geochemical studies that suggest that the Earth's mantle was not mixed by the impact. Another plausible outcome is that this energetic impact melted the whole mantle, but the extent of mantle melting is not well understood even though it must have had a significant effect on the subsequent evolution of the Earth's interior and atmosphere. To understand the initial state of the Earth's mantle, we perform giant impact simulations using smoothed particle hydrodynamics (SPH) for three different models: (a) standard: a Mars-sized impactor hits the proto-Earth, (b) fast-spinning Earth: a small impactor hits a rapidly rotating proto-Earth, and (c) sub-Earths: two half Earth-sized planets collide. We use two types of equations of state (MgSiO3 liquid and forsterite) to describe the Earth's mantle. We find that the mantle remains unmixed in (a), but it may be mixed in (b) and (c). The extent of mixing is most extensive in (c). Therefore, (a) is most consistent and (c) may be least consistent with the preservation of the mantle heterogeneity, while (b) may fall between. We determine that the Earth's mantle becomes mostly molten by the impact in all of the models. The choice of the equations of state does not affect these outcomes. Additionally, our results indicate that entropy gains of the mantle materials by a giant impact cannot be predicted well by the Rankine-Hugoniot equations. Moreover, we show that the mantle can remain unmixed on a Moon-forming timescale if it does not become mixed by the impact.

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Moon's origin
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Multiple studies address riddles of the Moon's origin

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Moon-forming impactor
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Title: A primordial origin for the composition similarity between the Earth and the Moon
Author: 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 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 composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to also have differed in composition from the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion, in order to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are systematically more similar to the planets they impact. A significant fraction of planet-impactor pairs have virtually identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.

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Title: Dynamical Evolution of the Earth-Moon Progenitors - Whence Theia?
Author: Billy Quarles, Jack J. Lissauer

We present integrations of a model Solar System with five terrestrial planets (beginning ~30-50 Myr after the formation of primitive Solar System bodies) in order to determine the preferred regions of parameter space leading to a giant impact that resulted in the formation of the Moon. Our results indicate which choices of semimajor axes and eccentricities for Theia (the proto-Moon) at this epoch can produce a late Giant Impact, assuming that Mercury, Venus, and Mars are near the current orbits. We find that the likely semimajor axis of Theia, at the epoch when our simulations begin, depends on the assumed mass ratio of Earth-Moon progenitors (8/1, 4/1, or 1/1). The low eccentricities of the terrestrial planets are most commonly produced when the progenitors have similar semimajor axes at the epoch when our integrations commence. Additionally, we show that mean motion resonances among the terrestrial planets and perturbations from the giant planets can affect the dynamical evolution of the system leading to a late Giant Impact.

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Title: On the origin and composition of Theia: Constraints from new models of the Giant Impact
Author: Matthias M.M. Meier (1 and 2), Andreas Reufer (3), Rainer Wieler (2) ((1) CRPG CNRS Nancy, France (2) ETH Zurich, Department of Earth Sciences, Zurich, Switzerland (3) School of Earth & Space Exploration, Arizona State University, USA)

Knowing the isotopic composition of Theia, the proto-planet which collided with the Earth in the Giant Impact that formed the Moon, could provide interesting insights on the state of homogenization of the inner solar system at the late stages of terrestrial planet formation. We use the known isotopic and modeled chemical compositions of the bulk silicate mantles of Earth and Moon and combine them with different Giant Impact models, to calculate the possible ranges of isotopic composition of Theia in O, Si, Ti, Cr, Zr and W in each model. We compare these ranges to the isotopic composition of carbonaceous chondrites, Mars, and other solar system materials. In the absence of post-impact isotopic re-equilibration, the recently proposed high angular momentum models of the Giant Impact ("impact-fission", Cuk & Stewart, 2012; and "merger", Canup, 2012) allow - by a narrow margin - for a Theia similar to CI-chondrites, and Mars. The "hit-and-run" model (Reufer et al., 2012) allows for a Theia similar to enstatite-chondrites and other Earth-like materials. If the Earth and Moon inherited their different mantle FeO contents from the bulk mantles of the proto-Earth and Theia, the high angular momentum models cannot explain the observed difference. However, both the hit-and-run as well as the classical or "canonical" Giant Impact model naturally explain this difference as the consequence of a simple mixture of two mantles with different FeO. Therefore, the simplest way to reconcile the isotopic similarity, and FeO dissimilarity, of Earth and Moon is a Theia with an Earth-like isotopic composition and a higher (~20%) mantle FeO content.

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Traces of another world found on the Moon

Researchers have found evidence of the world that crashed into the Earth billions of years ago to form the Moon.
Analysis of lunar rock brought back by Apollo astronauts shows traces of the "planet" called Theia.
The researchers claim that their discovery confirms the theory that the Moon was created by just such a cataclysmic collision.

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Earth and Moon Formation
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NASA Lunar Scientists Develop New Theory on Earth and Moon Formation

New research, funded by the NASA Lunar Science Institute (NLSI), theorises that our early Earth and moon were both created together in a giant collision of two planetary bodies that were each five times the size of Mars.
This new theory about how Earth's moon formed is challenging the commonly believed "giant impact hypothesis," which suggests that Earth's moon formed from a colossal impact of a hypothetical planetary embryo, named Theia, with Earth, early in our Solar System's history.

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Massive Planetary Collision May Have Zapped Key Elements from Moon

Fresh examinations of lunar rocks gathered by Apollo mission astronauts have yielded new insights about the moon's chemical makeup as well as clues about the giant impacts that may have shaped the early beginnings of Earth and the moon.
Geochemist James Day of Scripps Institution of Oceanography at UC San Diego and colleagues Randal Paniello and Frédéric Moynier at Washington University in St. Louis used advanced technological instrumentation to probe the chemical signatures of moon rocks obtained during four lunar missions and meteorites collected from the Antarctic. The data revealed new findings about elements known as volatiles, which offer key information about how planets may have formed and evolved.

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