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June 4th, 2013: Giant impacts, magma oceans and the origin of the early atmosphere

Posted on Jun 5, 2013 in Seminars

Sujoy Mukhopadhyay (Harvard)

Earth’s violent accretion likely generated multiple magma oceans. In particular, the Moon-forming giant impact is often thought to have produced a whole mantle magma ocean, which would have homogenized any pre-existing chemical heterogeneity within the mantle. The ratio of primordial 3He to primordial 22Ne in the mantle preserves a record of magma oceans on the early Earth. Importantly, the 3He/22Ne ratio of the Earth’s shallow depleted mantle is significantly higher than the deep mantle. To explain this observation, I propose that at least two giant impact-induced atmospheric blow-off and magma ocean degassing episodes are required and that the last giant impact did not generate a whole mantle magma ocean. New Xe isotopic data indicate that the catastrophic mantle outgassing and atmospheric blow-off events inferred from3He/22Ne ratios were accomplished between ~30 to 55 Myrs after the start of the Solar System. Therefore, outgassing associated with giant impacts, including the Moon-forming impact, must have occurred within this time window. Previous calculations of impact-induced atmospheric erosion have, however, found that it is difficult to completely remove the atmosphere from a body as large as Earth by a giant impact. The need for atmospheric loss inferred from the noble gas data could be reconciled with the dynamics of giant impacts by considering the new high-spin Moon formation hypothesis. I will further show that the current inventory of primordial noble gases in the atmosphere must largely be derived from late accreting planetesimals, a conclusion that has implications for the composition of the early atmosphere.