Early Nitrogen Budget of the Martian Atmosphere and Its Evolution

Abstract

AbstractIn 1976, Viking measured a 14N/15N 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 N2. We employ the processes of sputtering, photochemical escape by dissociative recombination and photodissociation, volcanic outgassing and impact processes to understand the evolution of N2 through time. We use a forward time‐marching model starting at initial abundance at 4 Gya to calculate the N2 abundance and 15N/14N 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 N2 abundance and 15N/14N 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 N2 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 N2 budget of Earth's atmosphere when scaled to the mass of Mars, which amounts to ∼110 mbar.