¹J. Shawcross,^2,3D. J. Adams,^3,4M. L. Wong,^1,5,6K. J. Smith,^1,7Y. L. Yung
Journal of Geopyhsical Research: Planets (in Press) Open Access Link to Article [https://doi.org/10.1029/2025JE009146]
¹Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Departmentof Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
³NHFP Sagan Fellow, NASA HubbleFellowship Program, Space Telescope Science Institute, Baltimore, MD, USA
⁴Earth and Planets Laboratory, CarnegieInstitution for Science, Washington, DC, USA
⁵Department of Water Resources Management, Environmental EngineeringProgram, Central State University, Wilberforce, OH, USA
⁶Department of Planetary Sciences, The University of Arizona,Tucson, AZ, USA, 7NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Published by arrangement with John Wiley & Sons

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 δ¹⁵N = −0.845‰ relative to the isotopic composition of atmospheric N₂. 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 ¹⁵N above 450 km relative to source N₂. Our study predicts a depletion of ¹⁵N 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.