Abiotic Input of Fixed Nitrogen by Bolide Impacts to Gale Crater During the Hesperian: Insights From the Mars Science Laboratory

Molecular hydrogen (H₂) from volcanic emissions is suggested to warm the Martian surface when carbon dioxide (CO₂) levels dropped from the Noachian (4100 to 3700 Myr) to the Hesperian (3700 to 3000 Myr). Its presence is expected to shift the conversion of molecular nitrogen (N₂) into different forms of fixed nitrogen (N). Here we present experimental data and theoretical calculations that investigate the efficiency of nitrogen fixation by bolide impacts in CO₂-N₂ atmospheres with or without H₂. Surprisingly, nitric oxide (NO) was produced more efficiently in 20% H₂ in spite of being 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. We estimate that the nitrogen fixation rate by bolide impacts varied from 7 × 10^-4 to 2 × 10^-3 g N·Myr^-1·cm^-2 and could imply fluvial concentration to explain the nitrogen (1.4 ± 0.7 g N·Myr^-1·cm^-2) detected as nitrite (NO₂^-) and nitrate (NO₃^-) by Curiosity at Yellowknife Bay. One possible explanation is that the nitrogen detected in the lacustrine sediments at Gale was deposited entirely on the crater's surface and was subsequently dissolved and transported by superficial and ground waters to the lake during favorable wet climatic conditions. The nitrogen content sharply decreases in younger sediments of the Murray formation suggesting a decline of H₂ in the atmosphere and the rise of oxidizing conditions causing a shortage in the supply to putative microbial life.