Reviews in Mineralogy & Geochemistry Vol. 60, pp. 365-518, 2006 4 Copyright © Mineralogical Society of America Thermal and Magmatic Evolution of the Moon Charles K. Shearer1, Paul C. Hess2, Mark A. Wieczorek3, Matt E. Pritchard4, E. Mark Parmentier2, Lars E. Borg1, John Longhi5, Linda T. Elkins-Tanton2, Clive R. Neal6, Irene Antonenko7, Robin M. Canup8, Alex N. Halliday9, Tim L. Grove10, Bradford H. Hager10, D-C. Lee11, Uwe Wiechert12 1Inst. of Meteorites, University of New Mexico, Albuquerque, New Mexico, U.S.A. 2Dept. of Geol. Sci., Brown University, Providence, Rhode Island, U.S.A. 3Institut de Physique du Globe de Paris, Paris, France 4Dept. of Earth & Atmospheric Sci., Cornell University, Ithaca, New York, U.S.A. 5Lamont-Doherty Earth Observatory, Palisades, New York, U.S.A. 6Dept. of Civil Eng. & Geol. Sci., Univ. of Notre Dame, Notre Dame, Indiana, U.S.A. 7University of Toronto, Toronto, ON, Canada 8Dept. of Space Studies, Southwest Research Institute, Boulder, Colorado, U.S.A. 9Dept. of Earth Sciences, University of Oxford, Oxford, United Kingdom 10Dept. of Earth, Atmospheric, & Planetary Sci., MIT, Cambridge, Massachusetts, U.S.A. 11Academica Sinica, Institute of Earth Sciences, Taipei, Taiwan 12Eidgenossische Technische Hochschule, Zurich, Switzerland Corresponding author e-mail: Charles K. Shearer <[email protected]> 1. INTRODUCTION As with all science, our continually developing concepts of lunar evolution are firmly tied to both new types of observations and the integration of these observations to the known pool of data. This process invigorates the intellectual foundation on which old models are tested and new concepts are built. Just as the application of new observational tools to lunar science in 1610 (Galileo’s telescope) and 1840 (photography) yielded breakthroughs concerning the true nature of the lunar surface, the computational and technological advances highlighted by the Apollo and post-Apollo missions and associated scientific investigations provided a new view of the thermal and magmatic evolution of the Moon. 1.1. Pre-Apollo view of the thermal and magmatic evolution of the Moon Many of the early views of the Moon manifested in mythology and art throughout the world were primarily tied to lunar and terrestrial cycles and the relationships between the Sun and the Moon. Prophetically, myths involving the lunar deities Mwuetsi from Zimbabwe and Coyolxauhqui from Mexico told of rather violent or catastrophic events in which the Moon was expunged from the Earth. Numerous ancient scientific observations were made about the nature of the Moon ranging from those uncovered in early Neolithic sites that correctly identified mare Crisium and mare Humorum to the insights made by Greek philosophers such as Anaxagoras (ca. 500-428 B.C.) and Democritus (ca. 460-370 B.C.), who attached terrestrial analogues to its character (stone, mountains). With the advent of the telescope (1610) and photography (1840) as scientific tools for lunar exploration, semiquantitative data could be collected that would provide an intellectual foundation for scientific interpretation. Initially, modern terrestrial geological analogs were extended to the Moon (lunar highlands, volcanic craters, seas). Combined with the rigors of 1529-6466/06/0060-0004$15.00 DOI: 10.2138/rmg.2006.60.4 366 Shearer et al. Thermal & Magmatic Evolution of the Moon 367 computational modeling, these observational data were extended to predict the original thermal state of the Moon and its thermal and magmatic history. Its proximity to the Earth made the Moon a prime candidate for the source of a wide range of meteorites (chondrites, eucrites) (Urey 1962, 1965; Duke and Silver 1967) and potential extraterrestrial materials (tektites) (Verbeek 1897). Numerous models were made for the ejection of material from the lunar surface (Arnold 1965). The possibility that the Moon was the source for these materials erroneously added “lunar sample” mineralogical and geochemical observations to the pre-Apollo computational models for its thermal and magmatic history. 1.1.1. Pre-Apollo view of the initial thermal state of the Moon. The conditions and processes under which the Moon formed had profound implications for its initial thermal state and its subsequent thermal and magmatic evolution. Pre-Apollo models for its origin fall within three groups (1) accretion or condensation along with the Earth as a double planetary system, (2) fission from a rapidly rotating Earth, and (3) capture by the earth of a fully formed body that was assembled elsewhere in the solar system. The co-accretion of the Earth and Moon had been a fairly popular model for the origin of this double planet system. In its simplest form it suffered from two major problems. It failed to explain the orbital relationship between the two bodies and their contrasting densities. More elaborate models appealed to fractionation during accretion or slightly different accretional environments to account for the differences in density. In the 19th century it was anticipated that planetary bodies were formed by the accretion of incandescent matter. Therefore, the Moon and the Earth were once molten and differentiated during the crystallization of these molten oceans (Thomson 1864). In the co-accretion model, hot or cold accretion of the Moon dictated its initial thermal state. In the fission model initially proposed by Darwin (1879), the Moon separated from the Earth by solar tidal forces and the Pacific Ocean basin was the resulting scar. The rationale for this model was that it accounted for the density differences between the Earth and the Moon because it implied that this event occurred following core formation and involved material derived from the upper mantle and crust of the Earth. The similarity of the composition of tektites to the terrestrial crust and the erroneous conclusion that were derived from the Moon (Verbeek 1897; Nininger 1943, 1947; O’Keefe 1963) added inaccurate geochemical evidence for this model. The fission model was initially criticized because it did not account for the orbital dynamics of the Earth-Moon system and because tides of the magnitude required to extract the Moon were thought to be mechanically impossible. The thermal consequences of the fission model for the Moon were not widely explored. The incorporation of a terrestrial crustal component enriched in U, Th, and K into the Moon would have resulted in melting of the lunar interior over a substantial period of time due to the release of radioactive heat. Although abandoned by many prior to Apollo missions, the primordial extraction of the Moon from the Earth by other mechanisms proved to be attractive following the Apollo missions. The perceived similarities of moons in the solar system, the uniqueness of the Earth- Moon system, and the dynamics of the Moon’s orbit around the Earth were the philosophical basis for the capture model. Capture of the Moon by the Earth was long advocated by Urey (1952, 1957, 1959) and the theoretical basis was provided by Gerstenkorn (1955), MacDonald (1964), and Goldreich (1966). The capture model in its pre-Apollo incarnations implies that a relatively undifferentiated and cool Moon was produced elsewhere in the solar system with a primitive composition similar to chondritic meteorites. The Moon as a source for chondritic meteorites was advocated by many (Urey 1959, 1962, 1965; Arnold 1965; Öpik 1966) until the return of the first Apollo mission to the Moon. Most of the capture models suggested that the Moon was captured during the very early history of the Earth. However, in a pamphlet that was privately printed in 1908, F.B. Taylor suggested that the Moon was captured as late as the Cretaceous. Fitting within this model, Urey (1952) calculated that the initial interior of the 366 Shearer et al. Thermal & Magmatic Evolution of the Moon 367 Moon was less than 600 °C and more likely was 300 °C. Consequently, magmatic evolution of the Moon primarily involved surface melting caused by impacts and basin formation. 1.1.2. Pre-Apollo view of the thermal and magmatic evolution of the Moon. The first observations related to the thermal and magmatic evolution of the Moon were made by Renaissance scientists in the 17th century. Drawing analogies to crater-producing volcanic processes on Earth, these early observers understandably attached a volcanic origin to the lunar craters. Dana (1846) bolstered these earlier views of a volcanic origin for lunar craters. Gilbert (1893) and Baldwin (1949) challenged this view by concluding that the large lunar craters were of impact origin. The debate continued until the Apollo missions. Besides the debate on the origin of lunar craters, the interpretation of the early magmatic history of the Moon hinged upon the nature of the lunar maria and highlands. Originally considered seas by the first lunar explorers during the Renaissance, 20th century observers prior to Apollo speculated that they were asphalt lakes (Wilson 1962), dust (Gold 1955), sedimentary rocks (Gilvarry 1968), impact derived melts (Urey 1952), and flood basalts (Baldwin 1949; Kuiper 1954; Fielder 1963). Each of these origins implies a distinctive thermal and magmatic history for the Moon. Proposed ages for the mare ranged from 4 billion years to tens of millions of years (Baldwin 1949; Hartmann 1965; Gault 1970). Drawing on the initial terrestrial analogy made by Galileo in 1610 for the lunar terra, the brighter reflectivity and the presumed lunar origin for tektites, most observers prior to Apollo equated the lunar highlands to terrestrial continental masses. Although their composition and origin were far less debated than the maria, the highlands were suspected to be largely volcanic or to represent more sialic rocks such as granites and rhyolites. Gilbert (1893) predicted that the Moon formed cold and remained cold throughout its history. The mathematical problem of the cooling of a sphere radioactively heated was solved by Lowan (1933) and first applied to the thermal history of the Moon in the 1950s (Urey 1952, 1955) and MacDonald (1959).