A nitrogen-rich atmosphere on ancient Mars consistent with isotopic evolution models

TL;DR

This study uses time-dependent models to show that ancient Mars could have had a high nitrogen atmosphere without requiring a multi-bar CO2 atmosphere, explaining isotopic data and possibly aiding early warming.

Contribution

It introduces a family of evolutionary scenarios with non-steady state nitrogen levels that align with isotopic measurements and volcanic evidence, challenging previous steady-state assumptions.

Findings

Estimated ancient N2 partial pressure was 60-740 mbar.

High nitrogen levels could have contributed to early Mars warming.

Models align with geological and isotopic constraints.

Abstract

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.