Constraints on the Size and Composition of the Ancient Martian Atmosphere from Coupled CO2-N2-Ar Isotopic Evolution Models

TL;DR

This study models the coupled isotopic evolution of CO2, N2, and Ar in ancient Mars's atmosphere to constrain its size and composition, revealing a likely thicker, warmer atmosphere that supported liquid water.

Contribution

It introduces a self-consistent coupled isotopic evolution model for Mars's atmosphere, integrating multiple gases and isotopes to refine ancient atmospheric conditions.

Findings

Ancient Mars had 0.3-1.5 bar CO2 and 0.1-0.5 bar N2 at 3.8 Ga.

Carbonate deposits critically influenced atmospheric composition.

Results support a reduced early mantle and a potentially H2-rich atmosphere.

Abstract

Present-day Mars is cold and dry, but mineralogical and morphological evidence shows that liquid-water existed on the surface of ancient Mars. In order to explain this evidence and assess ancient Mars's habitability, one must understand the size and composition of the ancient atmosphere. Here we place constraints on the ancient Martian atmosphere by modeling the coupled, self-consistent evolution of atmospheric CO2, N2, and Ar on Mars from 3.8 billion years ago (Ga) to the present. Our model traces the evolution of these species' abundances and isotopic composition caused by atmospheric escape, volcanic outgassing, and crustal interaction. Using a Markov-Chain Monte Carlo method to explore a plausible range of parameters, we find hundreds of thousands of model solutions that recreate the modern Martian atmosphere. These solutions indicate that Mars's atmosphere contained 0.3-1.5 bar CO2 and 0.1-0.5 bar N2 at 3.8 Ga. The global volume of deposited carbonates critically determines the ancient atmospheric composition. For example, a ~1 bar CO2 ancient atmosphere with 0.2-0.4 bar N2 requires ~0.9 bar CO2 deposited in carbonates primarily in open-water systems. With the joint analysis of C, N, and Ar isotopes, we refine the constraints on the relative strengths of outgassing and sputtering, leading to an indication of a reduced early mantle from which the outgassing is sourced. Our results indicate that a CO2-N2 atmosphere with a potential H2 component on ancient Mars is consistent with Mars's geochemical evolution and may explain the evidence for its past warm and wet climate.