Isotopic Evidence that Ancient Mars’ Atmosphere was More Earth-Like

Credit: NASA.

A fundamental question in the solar system’s history is whether the early martian atmosphere could have been thick enough to support liquid water and potentially life. Key constraints on the evolution of a planet’s atmosphere come from the ratios of various isotopes of atmospheric gases, which record atmospheric origins from volcanic outgassing and atmospheric losses to space. Results from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, measuring atmospheric loss and accounting for fractionation histories of critical atmospheric gases such as N2, suggest that early Mars may have had a CO2-rich atmosphere with several bars of pressure (Earth’s atmospheric surface pressure is one bar), which was subsequently mostly lost to space. However, this prediction of a thick greenhouse-like atmosphere fails to match outgassing models, constrained by mapping Mars’ volcanic history, which suggest relatively low atmospheric pressures.

To resolve this discrepancy, Renyu Hu (NASA JPL/Caltech) and Trent Thomas (NASA JPL/University of Washington) modeled nitrogen isotopes (i.e., the ratio of abundance of 14N to 15N) in the martian atmosphere. Nitrogen isotopes are a key constraint on a planet’s atmospheric evolution over time. Considering time-dependent models, which do not assume steady-state conditions between atmospheric loss and outgassing rates for nitrogen, the atmosphere of ancient Mars was more like our own than previously thought. Multi-bar CO2-rich atmospheres, which previous models suggested, are not required to match escape and loss predictions from the MAVEN mission. These results are compatible with volcanic outgassing rates indicated by geologic observations and measurements of atmospheric 36Ar/38Ar ratios. These new results allow for several hundred millibars of N2, which may be sufficient to allow for a substantially warmed surface contemporaneous with the time of valley network formation. Over time, N2 may have been preferentially lost relative to CO2, possibly a common process on planets lacking magnetic fields. READ MORE