Sujoy Mukhopadhyay (Dept. of Earth & Planetary Sciences)
The composition of the Earth’s early atmosphere is of great importance in understanding the nature of chemical reactions occurring on our planet’s surface. For example, an early reducing atmosphere has been shown to be significantly more conducive to prebiotic chemistry than an oxidized H2O-CO2 atmosphere. The composition of the earliest atmosphere is, however, a strong function of whether atmospheric gases were liberated from the Earth’s mantle or delivered via late accreting chondritic planetesimals that also brought in the platinum group elements found in the Earth’s mantle. I will present new observations from mid-ocean ridge basalts and mantle plumes that show that the primordial Xe isotopes throughout the mantle are distinct from air and the Ar/Xe ratio in the Earth’s mantle is near-chondritic. As a result, I will show that the Earth’s atmosphere cannot be generated by hydrodynamic escape of an early atmosphere followed by outgassing of either the shallow or deep mantle. The atmospheric noble gases must have been delivered by late accreting chondritic planetesimals after the last major equilibration between the mantle and the surface reservoirs, the giant impact that formed the Moon. Impact outgassing of late accreting chondritic meteorites would lead to a more reduced atmosphere than obtained through outgassing of the mantle. Hence, the noble gases require that the composition of the early atmosphere following the Moon-forming impact must have been more reduced than indicated by the oxidation state of the Earth’s mantle.