Extended Data Fig. 1: A model for the long-term evolution of the free nitrogen reservoir on Mars.

The free nitrogen reservoir is comprised of N₂ adsorbed in the regolith and ₂ in the atmosphere, and it changes over time due to sputtering loss, photochemical loss, ion loss, volcanic outgassing, and nitrate deposition. The regolith and the atmosphere are assumed to exchange isotopes over geologic timescales driven by the temperature variations due to orbital obliquity changes⁶³. We do not include impact additions or removal because the major impacts should have occurred before the modeled period^1,2 (from 3.8 Ga to present). Nor do we include the impact decomposition of near-surface nitrates⁶⁴ explicitly, as its rate is less than the present-day outgassing and photochemical escape rates by several orders of magnitude (Supplementary Information H).