  The ratio of nitrogen isotopes in the Martian atmosphere is a key constraint
on the planet's atmospheric evolution. However, enrichment of the heavy isotope
expected due to atmospheric loss from sputtering and photochemical processes is
greater than measurements. A massive, multi-bar early CO2-dominated atmosphere
and recent volcanic outgassing have been proposed to explain this discrepancy,
and many previous models have assumed atmospheric nitrogen rapidly reached a
steady state where loss to space balanced volcanic outgassing. Here we show
using time-dependent models that the abundance and isotopic composition of
nitrogen in the Martian atmosphere can be explained by a family of evolutionary
scenarios in which the initial partial pressure of nitrogen is sufficiently
high that a steady state is not reached and nitrogen levels gradually decline
to present-day values over 4 billion years. Our solutions do not require a
multi-bar early CO2 atmosphere and are consistent with volcanic outgassing
indicated by both geologic mapping and the atmospheric 36Ar/38Ar ratio. Monte
Carlo simulations that include these scenarios estimate that the partial
pressure of N2 was 60 - 740 mbar (90% confidence, with a median value of 310
mbar) at 3.8 billion years ago when the valley networks formed. We suggest that
such a high nitrogen partial pressure could have contributed substantially to
warming on early Mars.
