Predicting Nitrogen Isotope Fractionation in Nitrate Deposition on Early Mars

Noachian and early Hesperian Mars were likely warm and wet, with an atmosphere abundant in molecular nitrogen. The recent discovery of nitrate deposits in the Yellowknife Bay mudstones at Gale Crater confirm the existence of nitrogen oxides (NO X ) on Noachian Mars. The processes responsible for the production of these nitrates would fractionate nitrogen isotopes: nitrogen oxides will have different isotopic signatures depending on how they were formed—lightning, solar energetic particles (SEPs) and galactic cosmic rays, or photolysis. We used the Caltech–JPL 1D photochemical and transport model KINETICS to simulate nitrogen isotope fractionation in the formation of nitrogen oxide species. At the surface, where deposition occurs, we predict a depletion of δ 15 N = −0.845‰ relative to the isotopic composition of atmospheric N 2 . Near 200 km altitude, photolysis contributes to positive fractionation. From 300 km to the top of the modeled atmosphere, mass fractionation in the negative direction becomes relevant and produces a depletion of 15 N above 450 km relative to source N 2 . Our study predicts a depletion of 15 N in atmospherically derived NO X relative to the assumed background ratio, which is critical knowledge for constraining the formation history of nitrate on Mars, and whether nitrogen isotopic fractionation could be used as a biosignature.