Atmospheric Nitrogen When Life Evolved on Earth

The amount of nitrogen (N₂) present in the atmosphere when life evolved on our planet is central for understanding the production of prebiotic molecules and, hence, is a fundamental quantity to constrain. Estimates of atmospheric molecular nitrogen partial surface pressures during the Archean, however, widely vary in the literature. In this study, we apply a model that combines newly gained insights into atmospheric escape, magma ocean duration, and outgassing evolution. Results suggest <420 mbar surface molecular nitrogen at the time when life originated, which is much lower compared with estimates in previous works and hence could impact our understanding of the production rate of prebiotic molecules such as hydrogen cyanide. Our revised values provide new input for atmospheric chamber experiments that simulate prebiotic chemistry on the early Earth. Our results that assume negligible nitrogen escape rates are in agreement with research based on solidified gas bubbles and the oxidation of iron in micrometeorites at 2.7 Gyr ago, which suggest that the atmospheric pressure was probably less than half the present-day value. Our results contradict previous studies that assume N₂ partial surface pressures during the Archean were higher than those observed today and suggest that, if the N₂ partial pressure were low in the Archean, it would likely be low in the Hadean as well. Furthermore, our results imply a biogenic nitrogen fixation rate from 9 to 14 Teragram N₂ per year (Tg N₂/year), which is consistent with modern marine biofixation rates and, hence, indicate an oceanic origin of this fixation process.