Early Nitrogen Budget of the Martian Atmosphere and Its Evolution

In 1976, Viking measured a N-14/N-15 ratio of 170 +/- 15 in the Martian atmosphere, and in 2012, MSL measured a ratio of 173 +/- 11. This type of enrichment of the heavier isotope can only be explained by a significant loss of N to space, with the lighter isotope escaping more efficiently. A quantitative understanding of this nitrogen evolution of the Martian atmosphere requires analyzing all of the sources, sinks and exchanges of N-2. We employ the processes of sputtering, photochemical escape by dissociative recombination and photodissociation, volcanic outgassing and impact processes to understand the evolution of N-2 through time. We use a forward time-marching model starting at initial abundance at 4 Gya to calculate the N-2 abundance and N-15/N-14 enrichment over the history of Mars. We also incorporate a simultaneous CO2 evolution, which decays in either a power-law or linear function to the present-day value of 6 mbar. We constrain the simulated evolutionary tracks to the present day N-2 abundance and N-15/N-14 ratio of the Martian atmosphere to explore the initial conditions of the early atmosphere. Our model indicates a modal value of 120 mbar of initial N-2 pressure, with an upper limit of 3.6 bars, for an initial CO2 pressure of 1 bar. This modal value is in good agreement with the N-2 budget of Earth's atmosphere when scaled to the mass of Mars, which amounts to similar to 110 mbar. Plain Language Summary There is evidence that Mars has had flowing surface water early in its history; until similar to 2 billion years after its formation. This would have required a thick atmosphere similar to that of Earth during that time. However, as the atmosphere of Mars today is thin and cold, it is important to understand how it evolved to this state through time. One piece of this puzzle is nitrogen, which is the second most abundant gas in the atmosphere today. We looked at different input and output processes that would have affected the amount of nitrogen in the atmosphere with time. By constraining the evolutionary tracks to reach the present day-measured abundance and nitrogen isotopic ratio, we figure out a range of possible nitrogen budgets 4 billion years ago.