Kaveh Pahlevan
Planetary Scientist

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Evolution of the Magma Ocean
Models of the Moon-forming giant impact extensively melt and partially vaporize the silicate Earth and deliver metal to the Earth's core [e.g. 1,2]. The subsequent evolution of the terrestrial magma ocean and overlying vapor atmosphere over the subsequent ~105-6 years has been largely constrained via theoretical models [e.g. 3,4] with remnant signatures from this epoch proving somewhat elusive.

We have calculated equilibrium hydrogen isotopic fractionation between the magma ocean and overlying steam atmosphere to determine the extent to which H isotopes trace the evolution during this epoch. By analogy with the modern silicate Earth, the magma ocean-steam atmosphere system is often assumed to be chemically oxidized (log fO2 ~ QFM) with the dominant atmospheric vapor species taken to be water vapor. However, the terrestrial magma ocean - having held metallic droplets in suspension - may also exhibit a much more reducing character (log fO2 ~ IW) such that equilibrium with the overlying atmosphere renders molecular hydrogen the dominant H-bearing vapor species [5]. This variable - the redox state of the magma ocean - has not been explicitly included in prior models of the coupled evolution of the magma ocean-steam atmosphere system.

We have found that the redox state of the magma ocean influences not only the vapor speciation and liquid-vapor partitioning of hydrogen but also the equilibrium isotopic fractionation during the crystallization epoch. The liquid-vapor isotopic fractionation of H is substantial under reducing conditions and can generate measurable D/H signatures in the crystallization products but is largely muted in an oxidizing magma ocean and steam atmosphere. We have coupled equilibrium isotopic fractionation with magma ocean crystallization calculations to forward model the behavior of hydrogen isotopes during this epoch and have found that the distribution of H isotopes in the silicate Earth immediately following crystallization represents an oxybarometer for the terrestrial magma ocean [6]. Whether such endogenous isotopic heterogeneity would survive as an observable signature in the modern silicate Earth is an open question.

References
  1. Canup, R., Asphaug, E. (2001) Origin of the Moon in a giant impact near the end of the Earth's formation, Nature 412, 708-712.
  2. Canup, R. (2004) Simulations of a late lunar-forming impact, Icarus 168, 433-456.
  3. Hamano, K. et al. (2013) Emergence of two types of terrestrial planet on solidification of magma ocean, Nature 497, 607-610.
  4. Zahnle, K. et al. (2015) The tethered Moon, Earth and Planetary Science Letters 427, 74-82.
  5. Hirschmann, M. et al. (2012) Solubility of molecular hydrogen in silicate melts and consequences for volatile evolution of terrestrial planets, Earth and Planetary Science Letters 345-348, 38-48.
  6. Pahlevan, K., Schaefer, L., Elkins-Tanton, L., Desch, S., Karato, S. (2016) Hydrogen isotopic fractionation and the redox state of the terrestrial magma ocean (in prep.)