Nitrogen fixation by lightning and its role for early life on Earth and exoplanets

Nitrogen is an essential building block of DNA, RNA, and proteins and, subsequently, it must have been bioavailable since the origin of life. On modern Earth, biological sources are mostly responsible for making nitrogen bioavailable via N₂ fixation with only a few percent coming from abiotic sources. On early Earth, before the origin of life and the onset of biological nitrogen fixation, these abiotic sources such as lightning must have been the dominant producer of bioavailable nitrogen. Previous experiments have shown that in N₂-dominated atmospheres lightning leads to the formation of nitrate (NO₃^-) and nitrite (NO₂^-), which could not only have facilitated the origin of life but also sustained the earliest ecosystems. This hypothesis has been difficult to test with the available rock record because geochemical fingerprints of this fixed nitrogen source have not been developed. We present new results from spark discharge experiments in varying atmospheric compositions corresponding to different points of time in Earth’s evolution. We find substantial amounts of nitrate are produced in an N₂/CO₂ atmosphere. Furthermore, we investigate the effect of lightning on the isotopic composition of the resulting nitrogen oxides in solution. Our fixed nitrogen is depleted in heavy ¹⁵N in comparison to atmospheric N₂, in line with rock samples older than 3.2 billion years. For the first time we can assess to what degree lightning chemistry may have influenced the origin and early evolution of life. However, the spark in our experiment is much smaller and cooler than lightning channels in Earth’s atmosphere. To extrapolate our experimental results to full-scale planetary atmospheres we plan to complement them with simulations of the atmospheric chemistry of exoplanets and Earth. This will allow us to extend our experiments to real lightning conditions and develop observable tracers for lightning chemistry in exoplanetary atmospheres. Being able to predict the bioavailability of nitrogen on other worlds will be another factor determining the potential habitability of these worlds.