Study Notes on the Origin of the Moon

Origin of the Moon

Authorship and Publication Details

  • Contributors:

    • Robin M. Canup, Kevin Righter, Nicolas Dauphas, Kaveh Pahlevan, Matija Ćuk, Simon J. Lock, Sarah T. Stewart, Julien Salmon, Raluca Rufu, Miki Nakajima, Tomáš Magna

  • Institutions:

    • 1. Planetary Sciences Directorate, Southwest Research Institute, Boulder, CO, U.S.A.

    • 2. NASA Lyndon B. Johnson Space Center, Houston, TX, U.S.A.

    • 3. Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, IL, U.S.A.

    • 4. Carl Sagan Center for the Study of Life in the Universe, SETI Institute, Mountain View, CA, U.S.A.

    • 5. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, U.S.A.

    • 6. Earth and Planetary Sciences, University of California, Davis, CA, U.S.A.

    • 7. Space Studies Department, Southwest Research Institute, Boulder, CO, U.S.A.

    • 8. Department of Earth and Environmental Sciences, University of Rochester, NY, U.S.A.

    • 9. Center for Lithospheric Research, Czech Geological Survey, Prague, Czech Republic

  • Published: Accepted for publication fall 2020; to appear in “New Views of the Moon II”

1. INTRODUCTION

  • The Earth-Moon system is characterized by several unique features:

    • The Moon has a radius approximately ¼ that of Earth, which gives this system a larger satellite-to-planet size ratio than all known satellites (with the exception of Pluto's Charon).

    • The Moon has a small core, contributing to merely ~1% of its mass, in contrast to Earth's core, which constitutes nearly 30% of its total mass.

    • The Earth-Moon system possesses a high total angular momentum, indicating that Earth was spinning rapidly at the time the Moon formed.

    • The early Moon likely had a hot and at least partially molten state along with a substantial magma ocean.

  • The challenge of identifying a viable lunar origin model that satisfactorily explains these characteristics has driven decades of research.

2. LUNAR ORIGIN THEORIES

  • Historically, lunar origin theories (before the Apollo era) included:

    • Capture: Suggesting the Moon was captured by Earth's gravity.

    • Fission: Proposing the Moon fissioned off from the Earth due to rapid rotation.

    • Co-accretion: Where the Earth and Moon formed together as a joint system from the primordial disk.

    • Collisional ejection: Indicating the Moon formed from debris after an impact event.

  • Leading Hypothesis: The giant impact hypothesis gained prominence following discussions from the 1984 “Origin of the Moon” conference, as this theory best accounted for several existing empirical observations:

    • Earth's rapid early spin

    • The Moon's small core and similarities in isotopic compositions between Earth and Moon

    • The Moon's hot starting state

  • Early impact simulations were limited by computational capabilities, but eventually showed that a giant collision could potentially produce an iron-poor, Earth-orbiting disk. Later models sought to refine the impact scenarios to match physical properties of the Earth-Moon system, including considerations of angular momentum and mass constraints of the Moon.

3. NEW DATASETS IN LUNAR RESEARCH

3.1. Volatile Elements Assessment: H, C, S, N, Cl, F, Alkalis, and Volatile Metals

  • Initial discussions around lunar volatiles began in the 1970s demonstrating that:

    • The Moon has lower abundances of volatile elements compared to Earth's mantle, which was detailed in studies conducted by Ringwood and Kesson (1976) and Wolf et al. (1979).

    • The Moon was considered extremely dry by comparison, despite theoretical bulk mantle values suggesting up to 600 ppm of H2O might be possible (Abe et al. 2000; Righter and O’Brien 2011).

  • Recent measurements over the past decade, however, revealed that the Moon contains measurable levels of volatile elements in various forms, including:

    • Lunar glasses

    • Apatites

    • Nominally anhydrous minerals like plagioclase

  • Key findings reveal that the bulk composition of the silicate Moon suggests substantial evidence for water-bearing regions. Estimates of volatile content for bulk silicate Moon (BSM) indicate levels of H2O, C, S, Cl, and F are significantly less abundant compared to Earth's mantle (bulk silicate Earth):

    • Estimates for BSM:

    • H2O: ~3 to 292 ppm

    • C: 14 to 570 ppb

    • S: 75 to 80 ppm

    • Cl: 0.145 to 129 ppm

    • F: 4.5 to 60 ppm

    • For comparison, Earth's bulk silicate values estimate ~1200 ppm (H2O), ~100 ppm (C), 200 ppm (S), 30 ppm (Cl), and 25 ppm (F).

  • Emerging data imply some areas of the early Moon were likely water-bearing and provide critical constraints for models regarding lunar origin and accretion.

3.2. Importance of Isotopes

  • Geological processes such as melting and alteration show predictable mass-dependent isotopic variations, which provide insights into the established chemical composition variations within planetary bodies.

  • Anomalies in isotopic data signify significant differences inherited from the protoplanetary disk or other non-standard processes and are useful in determining the origins of planetary materials' compositions.

  • Isotopic ratios are typically calculated concerning a reference material and expressed in delta notation (𝛿), with:
    egin{equation} ext{𝛿}^{17,18} ext{O} ( ext{‰}) = rac{( ext{O}^{17,18} / ext{O}^{16}){ ext{Sample}} - ( ext{O}^{17,18}/ ext{O}^{16}){ ext{Standard}}}{( ext{O}^{17,18}/ ext{O}^{16})_{ ext{Standard}}} \times 1000

  • O isotopes are highlighted in studies comparing lunar samples with meteoritic materials derived from various celestial bodies. Disparities in isotopic signatures contribute to ongoing evaluations of the Moon's formation and its material origins.

4. DISTINGUISHING ISOTOPES IN MOON AND EARTH

4.1. O, H, Ti, Ca, and Cr Isotopic Systems

  • Examination of oxygen isotopes from lunar rock samples first demonstrated a significant alignment with the terrestrial mass fractionation line, showing minimal identified discrepancies.

  • Historical work revealed that lunar samples had similar delta-oxygen isotopic values when compared with terrestrial samples. Recent findings continue to support the hypothesis of isotopic similarity between Earth and Moon, albeit with intriguing proposals of deeper discrepancies warranting further exploration.

  • Isotopic systems demonstrate both small variations and larger inferred differences, culminating in discussions about planetary origins, accretion, and the shared history of celestial bodies in the solar system.

… [Truncated]