Synthesis of nitrous oxide by lightning in the early anoxic Earth's atmosphere

Carbon dioxide (CO₂) was the main atmospheric component of the early Earth's atmosphere and exerted a key role in climate by maintaining a hydrosphere during a primitive faint Sun [1]; however, CO₂ was eventually removed from the atmosphere by rock weathering and sequestered in the Earth's crust and mantle [1]. Nitric oxide (NO) was fixed by lightning discharges at a rate of 1×10¹⁶ molecules J^-1 in CO₂ (50-80%) rich atmospheres [2]. As the levels of atmospheric CO₂ dropped to 20%, the production rate of NO by lightning rapidly decreased to 2×10¹⁴ molecules J^-1 and then slowly diminished to 1×10¹⁴ molecules J^-1 at CO₂ levels of about 2.5% [2]. In order to maintain the existence of liquid water in the early Earth, it is required to warm up the planet with other greenhouse gases such as methane (CH₄) [3]. Here we report an experimental study of the effects of lightning discharges on the nitrogen fixation rate during the evolution of the Earth's early atmosphere from 10 to 0.8 percent of carbon dioxide with methane concentrations from 0 to 1,000 ppm in molecular nitrogen. Lightning was simulated in the laboratory by a plasma generated with a pulsed Nd-YAG laser [2]. Our results show that the production of NO by lightning is independent of the presence of methane but drops from 3×10¹⁴ molecules J^-1 in 10% CO₂ to 5×10¹³ molecules J^-1 in 1% CO₂. Surprisingly, nitrous oxide (N₂O) is also produced at a rate of 4×10¹³ molecules J^-1 independent of the levels of CH₄ and CO₂. N₂O is produced by lightning in the contemporaneous oxygenated Earth's atmosphere at a comparable rate of (0.4-1.5)×10¹³ molecules J^-1 [4, 5], but was not detected in nitrogen-carbon dioxide mixtures in the absence of oxygen [6]. The only previously reported abiotic synthesis of N₂O was by corona discharges in rich CO₂ atmospheres (20-80%) with a production rate of 8×10¹² molecules J^-1 [6]; however at lower CO₂ (<20%) levels, N₂O is no longer produced. Therefore, lightning in the early Earth's atmosphere was the main source of N₂O in nitrogen dominated atmospheres. N₂O is not known to have played a role in abiotic synthesis. It is not incorporated by microorganisms, and hence may not have had a role in the supply of reactive nitrogen to the biosphere. However, it is a powerful greenhouse gas and may have had a role in warming up the early Earth's atmosphere [7]. Lightning activity is enhanced in a warmer climate [8] and so the production of N₂O by lightning may have had a positive feedback in increasing lightning activity resulting in more N₂O production. N₂O is also produced by microbial activity and has been suggested as a potential biosignature in the atmospheres of extrasolar planets [9]. Here we show that lightning can interfere with the remote detection of life using N₂O as a biosignature. [1] Kasting, J.F.: 1993, Science 259, 920; [2] Navarro-González, R., et al.: 2001, Nature 412, 61; [3] Tian, F., et al.: 2011, Earth Planet. Sci. Lett. 308, 417; [4] Levine, J.S. et al.: 1979, Geophys. Res. Lett. 6, 557; [5] [5] Hill, R.D. et al.: 1984, J. Geophys. Res. 89, 1411; [6] Nna Mvondo, D. et al.,: 2005, Origins Life Evol. Biosph. 35, 401; [7] Roberson, A.L. et al., Geobiology 9, 313; [8] Williams, E.R.: 2004, Atmos. Res. 76, 272; and [9] Rauer, H.S. et al., Astron. Astrophys. 529, A8.