Kaveh Pahlevan
Planetary Scientist

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In the widely accepted giant impact hypothesis, the Earth-Moon system originated from a collision between two planetary-sized bodies toward the end of Earth accretion. Following such a collision, a circumplanetary disk of molten and vaporized material surrounded the Earth from which the Moon rapidly formed. Our understanding of what happened during this fluid stage of the evolution is poor. The goal of my work here is to forge a connection between the formation process and the lunar composition as observed in the isotopes and chemistry of lunar samples.

Earth-Moon equilibration
Over the course of the past two decades, isotope geochemists have observed an increasingly precise match in isotopic abundances of oxygen [1], titanium [2], silicon [3], and tungsten [4] between rocks derived from Earth's mantle and Moon against a background isotopic heterogeneity among sampled Solar System bodies. The similarity is such as to leave little doubt that these two bodies are derived from the same reservoir [5]. But if the Moon is the result of a collision between two distinct planetary bodies, where is the isotopic evidence for the impacting planet? The scenario that I co-developed is that Earth's magma ocean and the proto-lunar magma disk underwent an episode of isotopic equilibration through exchange with a common vapor atmosphere in the energetic aftermath of the giant impact while the system existed in a fluid state [6].

Figure 1 Cross-sectional view of the post-impact Earth and melt-vapor proto-lunar disk generated by the giant impact. Turbulent convection and exchange with a common atmosphere may homogenize the silicate Earth-Moon system. This scenario requires that lunar accretion is delayed by ~100 years. From [6].

Silicate Earth-Moon differences
If turbulent mixing - or any other process - was responsible for deriving the lunar material from Earth's mantle, what then is the origin of the chemical differences between these two reservoirs? More recent work has focused on understanding the FeO enrichment of the lunar mantle [7], the presence of water in the lunar interior [8, 9], and the depletion of the proto-lunar disk in volatile elements due to hydrodynamic outflows [10]. A major new development is the measurement of stable isotopic differences between the silicate Earth and Moon [e.g., 11] and this is a topic of current study.
  1. Wiechert, U. et al. (2001) Oxygen isotopes and the Moon-forming giant impact, Science 294, 345-348.
  2. Zhang, J. et al. (2012) The proto-Earth as a significant source of lunar material, Nature Geosciences 5, 251-255.
  3. Armytage, R. et al. (2012) Silicon isotopes in lunar rocks: Implications for the Moon's formation and the early history of the Earth, Geochem. Cosmochem. Acta 77, 504-514.
  4. Touboul, M. et al. (2007) Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals, Nature 450, 1206.
  5. Pahlevan, K. (2018) Telltale tungsten and the Moon. Nature Geoscience, 11, 16-18. link
  6. Pahlevan, K., Stevenson, D.J. (2007) Equilibration in the aftermath of the lunar-forming giant impact, Earth and Planetary Science Letters, 262 438-449. link News and Views: Isotopic Lunacy link
  7. Pahlevan, K. et al. (2011) Chemical fractionation in the silicate vapor atmosphere of the Earth, Earth and Planetary Science Letters 301, 433-443. link
  8. Saal, A. et al. (2008) Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior, Nature 454, 192-195.
  9. Pahlevan, K., Karato, S., Fegley, B. (2016) Speciation and dissolution of hydrogen in the proto-lunar disk, Earth and Planetary Science Letters 445, 104-113. link
  10. Pahlevan, K. (2023) Thermal Dissociation and Hydrodynamic Outflows from Post-Giant Impact Atmospheres, American Geophysical Union. link
  11. Wang, K., Jacobsen, S. (2016) Potassium isotopic evidence for a high-energy giant impact origin of the Moon, Nature 538, 487-490.