Prebiotic nitrogen fixation by lightning in carbon dioxide-nitrogen-hydrogen mixtures relevant to the early Earth's atmosphere
Abstract
Nitrogen is the most abundant element in the present Earth's atmosphere and also appears to be present in significant amounts in the mantle. Recent advances in the cycling between these two reservoirs suggest that N$ _{2}$ in the atmosphere was readily reduced and dissolved into the molten surface forming NH$ _{4}$$ ^{+}$ minerals after the last global magma ocean. Consequently, the atmospheric levels of N$ _{2}$ and H$ _{2}$ were much lower and higher, respectively in the early Earth. Nitrogen was an essential element for the origins of life. However, N$ _{2}$ was in a kinetically stable form in the atmosphere. Its conversion into reactive forms, such as nitric oxide (NO) required high temperatures, such as those produced by lightning. Here we report laser-induced plasma experiments that examine the efficiency of nitrogen fixation by lightning in CO$ _{2}$ N$ _{2}$ atmospheres with or without H$ _{2}$. The production of NO by lightning in CO$ _{2}$-N$ _{2}$ mixtures exhibited an asymmetric peak shape with a maximum yield at 7.4x10$ ^{11}$ g NO per year at 0.77 CO$ _{2}$/(CO$ _{2}$+N$ _{2}$) mole ratio. In contrast, the productions of NO in CO$ _{2}$-N$ _{2}$ mixtures in the presence of H$ _{2}$ showed symmetric peak shapes with maxima at 0.5 CO$ _{2}$/(CO$ _{2}$+N$ _{2}$) mole ratio. Surprisingly, the maximum production of NO by lightning increased to 1.2x10$ ^{12}$ and 2.0x10$ ^{12}$ g NO per year in 10%\ and 20%\ H$ _{2}$, respectively. This enhancement is counter-intuitive because H$ _{2}$ is a reducing agent and not likely to increase the rate of nitrogen oxidation. Nevertheless, its presence led to a faster cooling of the shock wave raising the freeze out temperature of NO resulting in an enhanced yield. Therefore, the production of NO by lightning was more efficient in early Earth's atmospheres with lower N$ _{2}$ partial pressures and the presence of H$ _{2}$. NO played a key role as a source of fixed nitrogen for prebiotic reactions and the origins of life. We acknowledge support from the Universidad Nacional Autónoma de México (PAPIIT IN111619 and PAPIME PE102319),and the Centre National d'Études Spatiales.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1928N