Kaveh Pahlevan
Planetary Scientist

Home | Lunar Origin | Magma Oceans | Tidal History | Publications | Contact
 
Lunar Origin
In the widely accepted giant impact hypothesis, the Earth-Moon system originates 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 surrounds the Earth from which the Moon rapidly forms. Our understanding of what happens 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 observed lunar composition (in terms of isotopes and chemistry).

I. Earth-Moon isotopic homogeneity
Over the course of the past decade, 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 the Earth's mantle and Moon against a background isotopic heterogeneity among Solar System bodies. The similarity is such as to leave little doubt that these two bodies are derived from the same reservoir. But if the Moon is the result of a collision between two distinct planetary bodies, where is the isotopic evidence for the impacting planet? What happened to the isotopically exotic, non-terrestrial material? One possibility 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 [5].

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 Pahlevan and Stevenson (2007)


II. Earth-Moon chemical differences
If turbulent mixing is responsible for the isotopic similarities between Earth mantle and Moon, what is the origin of the chemical differences between these two silicate reservoirs? The clearest and most unambiguous of these differences is the relative dearth of volatile trace elements observed in the Apollo samples. Such elemental abundances were almost certainly established by liquid-vapor partitioning [6]. I am currently developing coupled physical-chemical models of the evolution of the proto-lunar disk with the goal of forging a connection between the processes accompanying lunar origin and the remnant observables.
 
III. Origin of Lunar Hydrogen
After decades of scientific thought maintaining that the Moon was essentially devoid of water, recent work has revealed the presence of indigenous hydrogen in the lunar material [7]. Such a discovery of indigenous lunar hydrogen raises questions about its origin: are we measuring a remnant of primordial lunar hydrogen inherited at birth or implantation of hydrogen via impacts [8] by water-rich impactors? Collaborators and I have investigated the behavior of hydrogen in the proto-lunar disk and have developed a new mechanism for the origin of the recently observed lunar water [9].
 
References
  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., Stevenson, D.J. (2007) Equilibration in the aftermath of the lunar-forming giant impact, Earth and Planetary Science Letters, 262 438-449. pdf News and Views: Isotopic Lunacy pdf
  6. Pahlevan, K. et al. (2011) Chemical fractionation in the silicate vapor atmosphere of the Earth, Earth and Planetary Science Letters 301, 433-443. pdf
  7. 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.
  8. Bottke, W. F. et al. (2010) Stochastic Late Accretion to Earth, the Moon and Mars, Science 330, 1527-1530.
  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. pdf