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The moon-forming collision. A giant impact of large planetsimal with the primordial Earth. A small fraction of the ejected debris went into orbit around the Earth and formed the moon.Copyright William K Hartmann
Satellite meeting organised by Professor David Stevenson FRS and Professor Alex Halliday FRS
Our understanding of the origin of Earth’s moon is challenged by recent isotopic data, simulations of physical processes for giant impacts and evolution of the resulting disk, and new spacecraft studies. This meeting follows on from a Royal Society meeting in London on the same topic by focusing on the unsolved problems and assessing the prospects for future directions of research.
A poster session was held throughout this meeting alongside the schedule of presentations and discussion.
Professor Francis Albarede, Dr Mahesh Anand, Dr James Day, Professor Hidenori Genda, Dr William Hartmann, Dr Martin Jutzi, Professor Bernard Marty, Kaveh Pahlevan, Professor David Rubie, Dr Julien Salmon, Professor Richard J Walker, Dr Mark Wieczorek and Professor Jack Wisdom.
Programme available here
Professor David Stevenson FRS, Caltech, USAOrganiser and discussion leader
David Stevenson is the Marvin L. Goldberger Professor of Planetary Science at the California Institute of Technology. A native of New Zealand, his early work was in the condensed matter physics of planetary interiors, especially giant planets, but his wide ranging career has included contributions to the interpretation of planetary magnetic fields, the formation of planetary cores, melt migration, the origin of the Moon and numerous aspects of planetary and satellite formation, evolution and structure. He is involved in the Cassini mission and in the Juno mission, scheduled to arrive at Jupiter in 2016. Awards include Fellowship in the Royal Society, membership of the National Academy of Sciences, the Urey Prize (American Astronomical Society) and Hess Medal (American Geophysical Union).
Professor Alex Halliday FRS, University of Oxford, UKOrganiser and discussion leader
Professor Alex Halliday has been Head of the Mathematical, Physical and Life Sciences Division at Oxford University since October 2007.
Before coming to Oxford, he spent twelve years as a professor at the University of Michigan and then six years in Switzerland, where he was Head of the Department of Earth Sciences at the ETH in Zürich. In 2004 he took up the Chair of Geochemistry at Oxford, where his research involves the use of isotopic methods to study Earth and planetary processes.
Professor Halliday is a former President of the Geochemical Society and of the European Association for Geochemistry. He has experience with a range of top science boards and advisory panels including those of the Natural Environment Research Council, the Natural History Museum London, the Max Planck Society, the Royal Society and the American Geophysical Union.
An enthusiast for technological innovation, most of Professor Halliday's recent research is in developing and using new mass spectrometry techniques to shed light on the origin and early development of the solar system and recent earth processes, such as continental erosion and climate. However, he has also been engaged in other studies, such as the mechanisms of volcanic eruptions, and the formation of mineral and hydrocarbon deposits.
Professor Halliday's scientific accomplishments have been recognised with awards including the Murchison Medal of the Geological Society, the Bowen Award of the American Geophysical Union and the Urey Medal of the European Association of Geochemistry. He was elected a Fellow of the Royal Society in 2000.
Dr William Hartmann, Planetary Science Institute, USAGiant impact origin of the moon- forty years on
William K. Hartmann was first author of the 1975 Icarus paper that introduced the modern concept of lunar origin by giant impact. He is known for early work on lunar regolith, having coined the term "megaregolith" and development of a system of crater chronometry useful in dating features of the moon and Mars. He is currently involved in issues of cratering history during the first 600 Ma of solar system history. He is known also as a writer and painter.
Inspirations for the first presentations of the modern giant impact theory of the origin of the moon included discovery of the Orientale impact basin (Hartmann & Kuiper, 1962) and study of the work of Safronov in the later 1960s, as well as failure of the three primary lunar origin theories in vogue during the Apollo era. This raised questions about the largest impactors to hit Earth during its accretion. The work was presented by Hartmann and Davis in 1974 (Cornell satellite conference) and 1975 (Icarus paper). Discovery of the equality of lunar and terrestrial O isotope, a few years later, was originally seen as support for the impact hypothesis (although, ironically, it is now cited as a contradiction or “crisis” for the impact hypothesis). A 1984 conference on lunar origin, held in Hawaii, marked the first wide acceptance of the model, after which numerical models of giant impact processes evolved rapidly.
The initial 1974/75 work focused on the second-largest body from the terrestrial feeding zone to strike Earth during Earth’s accretion. We thus suggest that the modern isotope data as constraining the nature of the putative Earth-zone impactor, rather than by assuming a priori that the impactor must have had different isotope ratios, and suggesting a conflict with the hypothesis. Kortenkamp and Hartmann are investigating the idea of Belbruno and Gott (2005) that such an object (with terrestrial isotope ratios) may have been temporarily trapped at terrestrial Lagrangian point. (Further work has proposed to NASA Planet. Geol. & Geophys. program in 2013).
As for recent objections to the giant impact model in terms of lunar water content, it appears that production of OH and H2O on silicate dust grains (during accretion of the moon) should be considered, not to mention likely early scattering of water-rich outer-solar-system material into the terrestrial zone, possibly during lunar accretion.
Belbruno, Edward and J. R. Gott III 2005. Where did the moon come from? Astron. J. 129: 1724-1745.
Hartmann, W. K. and G. P. Kuiper 1962. Concentric Structures Surrounding Lunar Basins. Comm. Lunar and Planetary Lab., 1, 51-66.
Hartmann, W. K. and D. R. Davis 1974. Satellite-sized planetesimals, I.A.U. Colloquium 28, “Planetary Satellites,” August 18-21, Cornell University, Abstract.
Hartmann, W. K. and Donald R. Davis 1975. Satellite-sized planetesimals and lunar origin. Icarus, 24, 504-515.
Dr Martin Jutzi, Physics Institute, Center for Space and Habitability, University of Bern, Switzerland How many impacts to form the Moon?
Martin Jutzi is a senior researcher at the University of Bern, Switzerland. In 2012, he was awarded the Ambizione Research Fellowship from the Swiss National Science foundation. Before that, he spent two years at the University of California in Santa Cruz as a postdoctoral researcher. He received his PhD degree in Physics in 2009 from the University of Bern and the Observatoire de la Côte d’Azur, France. He is using and developing numerical models to study impacts and (giant) collisions between small bodies, moons and planets.
The nearly identical isotopic composition of the Moon and the Earth’s mantle suggest a common origin. In recent numerical studies, the formation of a prelunar accretion disc of appropriate chemical composition was demonstrated in the case of a small impact in a fast rotating Earth (Cuk and Stewart, Science 338, 2012) or by involving very large impactors (Canup, Science 338, 2012). An alternative scenario was presented by Reufer et al. (Icarus 221, 2012), who considered hit-and-run collisions. We will present a follow-up study of this work, which includes also impactors with different initial compositions.
As suggested recently, a fraction of the material in the Moon forming disk could have accreted in one of the Trojan points, forming a smaller second moon (Jutzi and Asphaug, Nature 476, 2011). Such a two-Moon configuration could be stable for tens of millions of years after the giant impact (Cuk and Gladman, Icarus 199, 2009). The likely fate of the companion moon would be to collide with the Moon at low (subsonic) speed; a scenario which was suggested to have caused the lunar dichotomy (Jutzi and Asphaug, Nature 476, 2011). We will discuss various aspects and consequences of such Moon-companion moon collisions.
Professor Jack Wisdom, Department of Earth, Atmospheric, and Planetary Sciences, MIT Cambridge, USACoupled Thermal-Orbital Evolution of the Early Earth-Moon System
Jack Wisdom is Professor of Planetary Science in the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology. He graduated in Physics from Rice University in 1976, and earned a PhD in Physics from the California Institute of Technology in 1981. He is a MacArthur Fellow and a member of the National Academy of Sciences (USA). He has coauthored two books with Gerald Jay Sussman: "Structure and Interpretation of Classical Mechanics" and "Functional Differential Geometry."
His principal research interests are in the dynamics of the solar system. He pioneered the study of chaos in the solar system. He developed a family of integration algorithms based on nonlinear dynamics that are at the core of essentially all long-term studies of planetary motion. These include the Wisdom-Holman symplectic map for the n-planet problem. With Jihad Touma, he discovered that the obliquity of Mars evolves chaotically. With Gerald Jay Sussman, he confirmed by direct numerical integration that our solar system evolves chaotically. This work shattered the long-held view of the clockwork evolution of our solar system.
The isotopic similarity of the Earth and Moon has motivated a recent investigation of the formation of the Moon with a fast-spinning Earth.
Angular momentum was found to be drained from the system through the evection resonance, a resonance between the Moon and Sun. However, tidal heating within the Moon was neglected. Here we explore the coupled thermal-orbital evolution of the early Earth-Moon system, taking account of tidal heating within the Moon. We find that as the eccentricity rises once the evection resonance is reached, tidal heating within the Moon becomes especially strong. The large tidal heating in the Moon significantly lowers the tidal Q/k_2 in the Moon, with consequent early escape from the evection resonance and decay of the orbital eccentricity. Insufficient angular momentum is withdrawn from the system to be consistent with the current configuration of the Earth-Moon system.
Professor Francis Albarede, Ecole Normale Supériere de Lyon, FranceWater and volatile elements in the Moon
Francis Albarède is Professor of Geochemistry at the Ecole Normale Supérieure in Lyon. He is an isotope and trace element geochemist who dedicated his early career to igneous geochemistry, hydrothermal vents, palæoceanography, geochronology and Earth’s evolution. He proposed novel approaches to geochemical modeling and authored a book Introduction to Geochemistry, which was broadly used to teach geochemical modeling. He pioneered applications of MC-ICP-MS and metal isotopes to geochemistry, planetary sciences, history and now medicine. He authored more than 200 publications. Francis Albarede was Executive Editor of Earth and Planetary Science Letters from 1993 to 2000 and the Senior Editor of the Journal of Geophysical Research Solid Earth from 2000 to 2004. He is a Geochemistry and an AGU Fellow. He received the 2000 Bowen Award of the VGP Section of the American Geophysical Union and the 2008 Goldschmidt Award of the Geochemical Society.
For most of the post-Apollo decades, the lunar interior has been deemed extremely dry. This lack of water can be explained both by the low volatile of the impactor and by the gravitational escape from the lunar gravity field. However, lunar apatite contains enough to support the claim that the lunar mantle holds more water than initially expected. In situ analyses of pyroclastic glass beads also show that melt inclusions in olivine crystals contain hundreds of ppm of water. Evidence of fractionated Zn isotopes, another volatile element, adds to the conundrum. We examine K, Rb, Zn and Ge concentration data on lunar basalts and extrapolate the depletion trend of the to ~300 K, the freezing point of water and show that the water content of the lunar mantle must be in the sub-ppm range. The high water contents in lunar apatites and pyroclastic glasses can be reconciled by the concentrating effects of extreme magmatic differentiation and are also consistent with a dry lunar mantle.
Dr Mark Wieczorek, Institut de Physique du Globe de Paris, FranceGeophysical constraints on lunar composition and origin
Mark Wieczorek has been at the French Institut de Physique du Globe de Paris since 2002, where he is currently the director of the Planetary and Space Sciences group. Much of his scientific research has focused on using geophysical and remotely sensed geochemical data to decipher the interior structure and geologic evolution of the Moon, with an emphasis on planetary topography, gravity, and magnetic fields. He was a co-investigator of the orbiting SMART-1 and Chandrayaan-1 X-ray fluorescence spectrometers, and is currently a co-investigator of NASA's lunar gravity mapping mission GRAIL. In addition to his work with the Moon, he is working with NASA's geophysical mission to Mars, Insight, and the laser altimeters on ESA's missions to Mercury and Ganymede. He is currently the editor-in-chief of the Journal of Geophysical Research Planets.
The present day interior structure of the Moon has been elucidated by a number of geophysical measurement techniques. These include data collected from the surface (such as seismic, heat flow, and magnetic field measurements), data collected from orbit (such as topographic, gravimetric and magnetic field measurements), and data collected from Earth-based observatories (such as lunar laser ranging data). Though our current knowledge of the Moon's interior is imperfect, these measurements provide critical constraints on the Moon's bulk composition, initial differentiation, and subsequent thermal evolution. In this presentation, the geophysical approaches that have been used to decipher the Moon's interior structure will be reviewed, the shortcomings of these techniques discussed, and the consequences that these measurements have for lunar origin highlighted. It will be shown that despite problems in interpreting these data, tight constraints exist on the size, composition, and thermal state of the lunar core, the thickness of the Moon's crust, and the bulk silicate abundance of refractory elements. Investigations of the Moon's thermal evolution and magnetic field generation will be shown to offer the possibility of constraining the initial thermal state of the Moon. A future geophysical mission to the Moon's surface would resolve many resolving problems, and a potential mission scenario that is in the realm of a NASA discovery-class or ESA M-class mission will be outlined
Dr Mahesh Anand, The Open University, UK Understanding the origin and evolution of water in the Moon through lunar sample studies
Mahesh Anand is a Lecturer in Planetary and Space Sciences at the Open University in Milton Keynes, UK. Mahesh obtained a PhD in Earth Sciences from the University of Cambridge, UK in 2001 followed by two postdoctoral appointments at the University of Tennessee, USA and the Natural History Museum, UK, respectively, before joining the Open University in 2005. His research interests include understanding fundamental processes involved in the differentiation and evolution of terrestrial bodies such as the Earth, Moon and Mars through laboratory-based investigations of planetary samples. Mahesh’s current research is focussed on investigating the abundance, distribution, and isotopic composition of water in the Moon by analysing lunar samples using modern analytical instrumentation such as a NanoSIMS.
Several recent lunar sample studies have demonstrated that the lunar interior contains heterogeneously distributed but appreciable quantities of water although estimates for the bulk-water content of the Moon vary over an order of magnitude. In addition, the origin and evolution of this lunar water remain unresolved with a range of possibilities including retention of primordial water during lunar accretion phase to later addition of water after the crystallisation of the putative Lunar Magma Ocean (LMO). Thus far, water (measured as OH) contents (and in some cases hydrogen isotopic composition) of volcanic glasses, apatite, agglutinates, and nominally anhydrous minerals (e.g., plagioclase) have been measured in lunar samples utilising the latest advancements in analytical instrumentation and techniques. In this contribution we review the existing database on water contents and its hydrogen isotopic composition in a variety of lunar samples and evaluate different hypotheses for understanding the history of water in the Moon.
Mahesh Anand1,2*, Romain Tartèse1, Jessica J Barnes1,2
1Planetary and Space Sciences, The Open University, Milton Keynes, MK7 6AA, UK
2Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
Dr James Day, Scripps Institution of Oceanography, USAZinc isotope constraints on lunar formation
James Day is an Assistant Professor at Scripps Institution of Oceanography in San Diego, California. James obtained a PhD in Earth Sciences from Durham University in 2004 followed by two postdoctoral appointments at the University of Tennessee and the University of Maryland, before joining Scripps in 2010. His research interests broadly revolve around understanding planet formation processes through petrology and isotope geochemistry. James holds an US Antarctic Medal for his service to meteorite collection and was awarded the Houtermans Award from the European Association of Geochemistry in 2013.
Zinc isotopes are a powerful tracer of planetary volatiles because fractionation can occur during volatilisation. Recent work has shown that mare basalts have mean d66Zn values of ~1.3‰, and are distinct from carbonaceous chondrites (0-0.5‰), as well as terrestrial (0.28±0.05‰) and martian basalts (0.25±0.03‰) [1,2]. These results suggest volatile depletion of the Moon through evaporation, and are consistent with an impact origin for the Earth-Moon system.
Here we examine why these differences have become manifested using correlations with volatile element abundances and enrichments of the highly siderophile elements (HSE). One explanation requires an episode of melting and evaporation that was subsequently overprinted on Earth by more extensive late accretion than for the Moon. Mass-balance requires ~12% late accretion to account for terrestrial Zn, which is much greater than ≥0.5% estimated from the HSE. This discrepancy can potentially be explained through stochastic accretion of massive impactors combined with partial vaporization and metal-silicate segregation leading to apparent late accretion of moderately volatiles to the HSE of >10:1. Alternatively, oxidation conditions on planetary bodies determine Zn volatility. The mantles of Earth and Mars are more oxidised than the Moon. Combining this evidence with recent work suggesting delivery of chondritic hydrogen into the lunar mantle from late accretion, as well as inheritance from the proto-Earth , it is possible that Earth’s mantle was already oxidized prior to the Moon-forming event.
 Paniello et al. 2012. Nature, 490, 376-379.  Chen et al. 2012. EPSL, 369-370, 34-42  Saal et al. 2013. Science, 340, 1317-1320.
James M.D. Day1 and Frederic Moynier2
1Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA. email@example.com
2Department of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, MO 63130, USA. firstname.lastname@example.org
Dr Hidenori Genda, Tokyo Institute of Technology, JapanN-body and SPH Simulations of the Formation of the Terrestrial Planets and Moon
Hidenori Genda is a Scientific Researcher in Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology. He received his undergraduate training in Physics at Keio University and his PhD degree in Planetary Science at the University of Tokyo in 2004. After serving for 3 years as JSPS Research Fellow for Young Scientists and one and half years as an Assistant Professor at Tokyo Institute of Technology, in 2009 he moved to the University of Tokyo. He worked there for 3.5 years, and moved to Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology in 2013. His research focuses on the comparative planetary science, especially origin and early evolution of planets, atmospheres and ocean.
We have developed the hybrid code that can handle both the long-term orbital evolutions of objects (N-body code) and the short-term collision processes of objects (SPH code). Here, we apply this hybrid code to the last stage of terrestrial planet formation. During this stage, several tens of Mars-sized protoplanets collide each other, that is, giant impacts. Giant impacts are the energetic events, and related to the origin of the large satellite like the Moon. N-body simulation of giant impact stage and SPH simulation of moon-forming impacts have been well investigated independently. Our hybrid code can handle histories of giant impacts such as change in core-mantle ratio, spin axis, spin velocity of protoplanets and so on. From the results of several tens of numerical simulations for giant impact stage, we will statistically discuss formation probability of Moon-like satellite, typical size and composition (also Oxygen isotopic composition) of satellite, and so on.
Dr Julien Salmon, Southwest Research Institute, USAModeling the accretion of the Moon from the protolunar disk
Julien Salmon was born in Avignon, France. He graduated in 2007 from the Ecole Nationale Supérieure des Mines of Saint-Etienne, France, with majors in Energetics and Image Processing. He then joined the Institut de Recherche sur les lois Fondamentales de l’Univers (IRFU) in Saclay, France, to help with the development of scientific software in connection with the study of the images form the ISS instrument onboard the Cassini spacecraft. In 2010, he got his PhD in Astronomy and Astrophysics from the University Paris 7 Denis Diderot, on the topic of the dynamics of dense planetary rings, and particularly Saturn’s rings and satellites. He joined the Southwest Research Institute in 2011, where he is developing numerical models to study the evolution of the protolunar disk and the accretion of the Moon
We have developed a new numerical model to investigate the accretion of the Moon from the protolunar disk. Material inside the Roche limit is represented by a uniform slab of material that spreads viscously, based on the Thompson & Stevenson (1988) disk model. Beyond the Roche limit, the disk is modeled by a collection of individual particles tracked with the N-body code Symba. The latter interact with the Roche-interior disk at the strongest Lindblad resonances. We find that the Moon accretes in 3 consecutive steps: 1) material in the outer disk rapidly accretes and forms a lunar “core” on a timescale of ~1 yr, similar to what found by pure N-body simulations (e.g. Ida et al. 1997, Kokubo et al. 2000); 2) the inner disk, which has been confined by outer objects in phase 1), slowly spreads outward; 3) after ~20 years, material from the inner disk is brought through the Roche limit, forms new moonlets that recoil from the disk and collide with the Moon, prolonging its accretion over ~200 years. The forming Moon scatters and traps into mean motion resonance some of the objects formed at the Roche limit, resulting in an expansion its semi-major axis. For canonical disks, the Moon has a semi-major axis of ~6Re by the end of its formation, compared to an average of 3.8Re found in pure N-body simulations (e.g. Ida et al. 1997, Kokubo et al. 2000). We will present the range of outcomes produced by initial disk profiles inspired from both canonical (e.g. Canup 2008) and non-canonical impacts (Canup 2012, Cuk & Stewart 2012), and discuss what the dynamics of accretion imply regarding the Earth and Moon’s compositional similarities. For non-canonical disks, we will discuss how the Moon’s initial orbital parameters may affect the likelihood of its capture into the evection resonance with the Sun.
Dr Kaveh Pahlevan, Observatoire de la Côte d'Azur, FranceIsotopic constraints on physical models
Kaveh Pahlevan was born in Tehran in 1982. He got his Bachelor of Science in Astronomy at the University of Maryland working on the orbital evolution of the Galilean satellites. He continued his studies of satellites at the California Institute of Technology, where he obtained his Master of Science and Doctorate in Planetary Science working on the chemical and isotopic consequences of lunar origin. More recently, he has been a Bateman Postdoctoral Fellow at Yale University and a visiting scientist at the Southwest Research Institute continuing research into the chemical and isotopic signatures of lunar origin. He has recently started a new position at the Observatoire de la Côte d’Azur as a Poincaré Fellow in Nice, France.
Ever since Apollo, isotopic abundances have been used as tracers to study lunar formation, in particular, to study the sources of the lunar material. In the last decade, a number of increasingly precise observations have been made that give strong indications that the Moon and the Earth's mantle have a common heritage. To reconcile this observation with the origin of the Moon via the collision of two isotopically distinct bodies, two ideas have been put forward: (1) it has been proposed that the Earth-Moon system underwent isotopic homogenization into a single reservoir via turbulent mixing during the ~10^2 year molten disk epoch after the giant impact but before lunar accretion and (2) non-canonical impacts have been proposed in which the silicate disk injected into orbit is sourced directly from the mantle of the proto-Earth and the impacting planet in the right proportions to match the isotopic observations. Simultaneously, it has been recognized that liquid-vapor separation in the aftermath of the giant impact is capable of evolving small but measurable (mass-dependent) isotopic differences in the Earth-Moon system, making isotopic measurements sensitive not only to the sources but to the processes involved. I will discuss how ongoing interaction of theoretical models with isotopic observations can continue to inform our ideas about lunar origin.
Professor Bernard Marty, CRPG-CNRS, Université de Lorraine, FranceVolatile elements in Earth and Moon
Bernard Marty is professor of geochemistry at the Ecole Nationale Supérieure de Géologie (Nancy, France) and staff scientist at the Centre de Recherches Pétrographiques et Géochimiques (CRPG-CNRS; director from 2002 to 2008). His research interests include the isotope geochemistry of volatile elements, with application to the origin of isotopic anomalies in the Solar system, Early Earth geodynamics and environments, mantle geodynamics, and the geological carbon cycle. Marty holds a Ph. D. in Physics (Toulouse), and a Doctorat d'Etat (Université Pierre et Marie Curie, Paris; 1987). After a post-doctorate research at the University of Tokyo (1981-1984), he became research assistant at the CNRS (1986-1992) and then moved to Nancy. He chaired the Goldschmidt conference in Prague in 2011 (3,300 attendants), he is editor of Earth and Planetary Science Letters, and he co-chairs of the Reservoirs & Fluxes Directorate of the Deep Carbon Observatory. His record includes 148 publications (20 in Nature and Science) which have been cited >5000 times (h = 39). He was elected senior member of the Institut Universitaire de France (2008-), Fellow of the American Geophysical Union (2007) and of the Meteoritical Society (2012). He is Knight of the Order of Merit.
The discovery of indigenous lunar water renewed tremendous interest on the origin of atmophile/hydrophile elements, namely water, carbon, nitrogen, noble gases, in the terrestrial planets. The Moon is more depleted in moderately volatile elements such as K, Zn... than the Earth, so that retention of water in the lunar interior required specific processes and conditions. The terrestrial H, C, N, noble gas inventory is consistent with contribution of about 2(±1)% carbonaceous chondrite (CC) -type material. Halogens of the bulk Earth requires a comparable contribution of a few percents CC, despite their lower volatility. This implies that the Earth was dried up for atmophile/hydrophile elements at the end of the giant impact phase , as a result of impact erosion, and was contributed later on by wet material. The timing of this transition is constrained by the 129I-244Pu-129,131-136Xe closure age of the terrestrial atmosphere of 40-50 Ma (after correction for geological loss of atmospheric xenon). A similar closure age is obtained for the terrestrial mantle, also from I-Pu-Xe systematics. This was probably the time of the Moon-forming event.
Professor David Rubie, University of Bayreuth, GermanyOxygen isotope evolution during accretion of the terrestrial planets
Professor David Rubie works in the fields of mineral physics, geochemistry, and planetology. His research is based on experimental studies performed at the extreme pressures and temperatures of the deep interiors of planets and he applies the results to understanding the structure, composition and evolution of the Earth and the other terrestrial planets of the solar system. He has published 160 scientific research papers in leading international journals and has edited 4 books. He is a research professor at the Bayerisches Geoinstitut (University of Bayreuth, Germany) which is one of the leading institutions worldwide for high pressure research in the Earth sciences. David Rubie has been a full professor there since 1993 and was Institute Director for a total of 8 years.
David Rubie was the recipient of the 2008 Abraham-Gottlob-Werner-Medal of the German Mineralogical Society and the 2008 Schlumberger Medal of the Mineralogical Society of Great Britain.
The terrestrial planets and meteorite parent bodies formed with intrinsic oxygen fugacities approximately 5 orders of magnitude higher than that of a solar gas. A major cause of this was the oxidation of Fe to form a FeO component in silicates. The most likely oxidant was water. There is abundant evidence that the oxygen isotopic composition of water was high in 18O/16O and 17O/16O compared with other oxygen reservoirs in the early solar system, implying a link between the oxygen isotope ratios of planetary bodies and their oxidation state. We have investigated this link in the context of N-body accretion models for planet formation. We find that the composition of the cores and mantles of the Earth and other terrestrial planets are best reproduced with models in which Fe is significantly oxidized at heliocentric distances greater than 1 to 1.5 AU prior to accretion. The oxygen isotopic corollary is that the high 18O/16O and 17O/16O ratios of the planets relative to solar is also determined by the FeO component rather than by accretion of H2O ices. Exchange between Fe-bearing dust and water vapor enriched in the heavy oxygen isotopes prior to planet formation explains both the chemical and oxygen isotopic compositions of the planets. This exchange can be understood as mixing between water produced by photochemical dissociation of CO and oxygen of solar composition. In addition, in the Grand Tack accretion models, D17O values of all bodies above a certain mass tend to rise together. Consequently the oxygen isotopic composition of the Moon-forming impactor (Theia) could have been similar to that of the proto-Earth.
E.D. Young (UCLA, Los Angeles, USA)
S.A. Jacobson (OCA, Nice, France)
A. Morbidelli (OCA, Nice, France)
D.P. O’Brien (PSI, Tucson, USA)
Professor Richard J Walker, University of Maryland, USASiderophile Element Constraints on the Origin of the Moon
Richard J. Walker is a professor in the Department of Geology at the University of Maryland. He utilizes radiogenic isotopes and trace elements to conduct research in several areas of geo-and cosmo-chemistry including the chemical evolution of Earth’s mantle, the formation and crystallization histories of early solar system planetesimals, and the accretional and differentiation histories of Earth, Moon and Mars. His research group specializes in the application of siderophile elemental and isotopic data. He received a PhD in geology from the State University of New York at Stony Brook in 1984. He did postdoctoral work at the U.S. National Bureau of Standards, the Carnegie Institution of Washington’s Department of Terrestrial Magnetism, and the U.S. Geological Survey. He became a member of the faculty of the University of Maryland in 1990. By 2013, he has published more than 150 peer reviewed papers. He was the 1990 recipient of the Clarke Medal of the Geochemical Society. He is a Fellow of the American Geophysical Union, the European Association for Geochemistry and also the Geochemical Society.
The lunar mantle sources of volcanic glasses and basalts were depleted in highly siderophile elements (HSE) relative to the terrestrial mantle by at least a factor of 20. Limited Os isotopic data, however, suggest the HSE are not fractionated from chondritic relative abundances in the lunar mantle. Although there are other possibilities, the most likely explanations for the disparity between the Earth and Moon are either: 1) the Moon received a disproportionately lower share of late accreted materials than Earth, such as may have resulted from stochastic late accretion, or 2) the major phase of late accretion occurred prior to the Moon-forming event, and the putative giant impact led to little drawdown of HSE to the Earth’s core. High precision determination of the 182W isotopic composition of the Moon can help to resolve this issue.
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