A Lower Limit of Atmospheric Pressure on Early Mars Inferred from Nitrogen and Argon Isotopic Compositions

TL;DR

This study models the evolution of early Martian atmosphere using nitrogen and noble gas isotopic compositions, concluding that atmospheric pressure was above 0.5 bar at 4 billion years ago, influenced by impact events and escape processes.

Contribution

It introduces a comprehensive model accounting for impact erosion, volcanic degassing, and atmospheric escape to infer early Mars atmospheric pressure from isotopic data.

Findings

Atmospheric pressure on early Mars was likely above 0.5 bar at 4 Ga.

Impact events and escape processes significantly influenced atmospheric evolution.

Isotopic compositions suggest a less fractionated primitive atmosphere preserved in meteorites.

Abstract

We examine the history of the loss and replenishment of the Martian atmosphere using elemental and isotopic compositions of nitrogen and noble gases. The evolution of the atmosphere is calculated by taking into consideration various processes: impact erosion and replenishment by asteroids and comets, atmospheric escape induced by solar radiation and wind, volcanic degassing, and gas deposition by interplanetary dust particles. Our model reproduces the elemental and isotopic compositions of N and noble gases (except for Xe) in the Martian atmosphere, as inferred from exploration missions and analyses of Martian meteorites. Other processes such as ionization-induced fractionation, which are not included in our model, are likely to make a large contribution in producing the current Xe isotope composition. Since intense impacts during the heavy bombardment period greatly affect the atmospheric mass, the atmospheric pressure evolves stochastically. Whereas a dense atmosphere preserves primitive isotopic compositions, a thin atmosphere on early Mars is severely influenced by stochastic impact events and following escape-induced fractionation. The onset of fractionation following the decrease in atmospheric pressure is explained by shorter timescales of isotopic fractionation under a lower atmospheric pressure. The comparison of our numerical results with the less fractionated N ($^15$N/$^14$N) and Ar ($^38$Ar/$^36$Ar) isotope compositions of the ancient atmosphere recorded in the Martian meteorite Allan Hills 84001 provides a lower limit of the atmospheric pressure in 4 Ga to preserve the primitive isotopic compositions. We conclude that the atmospheric pressure was higher than approximately 0.5 bar at 4 Ga.