Organiser and discussion leader
Professor David Stevenson FRS, Caltech, USA
Giant impact origin of the moon- forty years on
Dr William Hartmann, Planetary Science Institute, USA
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.
How many impacts to form the Moon?
Dr Martin Jutzi, Physics Institute, Center for Space and Habitability, University of Bern, Switzerland
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.
Coupled Thermal-Orbital Evolution of the Early Earth-Moon System
Professor Jack Wisdom, Department of Earth, Atmospheric, and Planetary Sciences, MIT Cambridge, USA
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.
Water and volatile elements in the Moon
Professor Francis Albarede, Ecole Normale Supériere de Lyon, France
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.