Meteorite impacts on ancient oceans opened up multiple NH3 production pathways

Abstract

A recent series of shock experiments by Nakazawa et al. starting in 2005 (e.g. [Nakazawa et al., Earth Planet. Sci. Lett., 2005, 235, 356]) suggested that meteorite impacts on ancient oceans would have yielded a considerable amount of NH₃ to the early Earth from atmospheric N₂ and oceanic H₂O through reduction by meteoritic iron. To clarify the mechanisms, we imitated the impact events by performing multi-scale shock technique-based ab initio molecular dynamics in the framework of density functional theory in combination with multi-scale shock technique (MSST) simulations. Our previous simulations with impact energies close to that of the experiments revealed picosecond-order rapid NH₃ production during shock compression [Shimamura et al., Sci. Rep., 2016, 6, 38952]. It was also shown that the reduction of N₂ took place with an associative mechanism as seen in the catalysis of nitrogenase enzymes. In this study, we performed an MSST-AIMD simulation to investigate the production by meteorite impacts with higher energies, which are closer to the expected values on the early Earth. It was found that the amount of NH₃ produced further increased. We also found that the increased NH₃ production is due to the emergence of multiple reaction mechanisms at increased impact energies. We elucidated that the reduction of N₂ was not only attributed to the associative mechanism but also to a dissociative mechanism as seen in the Haber–Bosch process and to a mechanism through a hydrazinium ion. The emergence of these multiple production mechanisms capable of providing a large amount of NH₃ would support the suggestions from recent experiments much more strongly than was previously believed, i.e., shock-induced NH₃ production played a key role in the origin of life on Earth.