Kaveh Pahlevan
Planetary Scientist

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Models of the Moon-forming giant impact extensively melt and partially vaporize the silicate Earth and deliver metal to the core [e.g. 1,2]. Subsequent evolution of the terrestrial magma ocean and primordial atmosphere over the ensuing ~105-6 years has been constrained by theoretical models [e.g. 3,4] with remnant signatures from this epoch remaining elusive.

Primordial outgassing
We have calculated the behavior of hydrogen and deuterium during the magma ocean and subsequent water ocean epochs to determine the extent to which hydrogen isotopes reflect the evolution of this early epoch. In analogy with modern silicate Earth, the magma ocean-primordial atmosphere system is often assumed to be chemically oxidized (logfO2~QFM) with H2O and CO2 the dominant atmospheric species. However, the terrestrial magma ocean - having held metallic droplets in suspension - may also be much more reducing (logfO2~IW-2) such that equilibrium with the overlying atmosphere renders H2 and CO the dominant H- and C-bearing species [5]. This variable - the redox state of the magma ocean - is critical to the evolution of the early (Hadean) Earth but has not been previously constrained.

Figure 2 Schematic evolution of the behavior of water and hydrogen during and immediately after the magma ocean. Solidification leads to outgassing of terrestrial volatiles, whose evolution generates the earliest oceans and atmosphere. The deuterium content of the oceans reflects the chemical composition (e.g. H2/H2O) of the primordial atmosphere. From Pahlevan et al. (submitted to EPSL).

We have identified the hydrogen isotopic composition of the terrestrial oceans as a tracer of magma ocean outgassing processes [6]. We have found that the redox state of the magma ocean controls not only the chemical composition of the primordial atmosphere but also the hydrogen isotopic composition of the oceans following primordial hydrogen escape. Water concentrates deuterium and, to the extent that H2 was a significant species in the primordial atmosphere, significant deuterium-enrichment in the Hadean oceans would have been generated. The Earth-chondrite "match" in D/H (to within ~10-20%) constrains the amount of H2 in the primordial atmosphere. We have developed climate models to determine the temperature of ocean-atmosphere equilibration. This has allowed us to constrain the H2/H2O of the primordial atmosphere to < 0.3. The D/H of the Hadean oceans is an oxybarometer for magma ocean outgassing and constrains the oxygen fugacity of last equilibration to >IW+1. This constitutes the first observational constraint on the composition of Earth's outgassed atmosphere.

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.M. (2012) Magma ocean influence on early atmosphere mass and composition, Earth and Planetary Science Letters 341-344, 48-57.
  6. Pahlevan, K., Schaefer, L., Hirschmann, M.M. (2018) Magma ocean outgassing and hydrogen isotopic constraints on the Hadean Earth (submitted to EPSL)