A Nitrogen-Rich Atmosphere on Ancient Mars Indicated by Isotopic Evolution

Nitrogen's isotopic ratio was the first indicator of substantial atmospheric evolution on Mars and its value has been recently measured with high precision. Loss to space in the sputtering and photochemical processes would have enriched the heavy isotope more than observed, and previous models proposed a massive, multi-bar early \ce{CO2} atmosphere or large recent volcanic outgassing as mitigations. A common feature of these models is that the atmospheric nitrogen has been in a steady state -- loss to space balances outgassing -- from $\sim3$ billion years ago. Here we show a previously unknown family of evolutionary solutions that explain the size and isotopic composition of Mars nitrogen. In this family of ``dynamical'' solutions, the nitrogen is almost never in a steady state, and its partial pressure gradually descends to the present-day value over the past 4 billion years. The dynamical solutions do not require a multi-bar early \ce{CO2} atmosphere and are fully consistent with the amount of volcanic outgassing indicated by both geologic mapping and the atmospheric $^{36}$Ar/$^{38}$Ar ratio. Monte Carlo simulations that include the dynamical solutions indicate that the pressure of \ce{N2} at 3.8 billion years ago was 60 -- 740 mbar (90\% confidence), with 310 mbar being the median value and the spread mostly due to the uncertainty in the sputtering loss rate. Nitrogen thus likely contributed substantially to the warming and the formation of sustained surface melting on early Mars.