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

Present-day Mars is cold and dry, but mineralogical and morphological evidence shows that liquid water once existed on the surface of ancient Mars. This motivates the following open questions: What was the size and composition of the ancient Martian atmosphere that supported this water? And what happened to this atmosphere? We must answer these questions in order to understand Mars’s surface environments through time and assess their habitability. We address these questions by analyzing the information recorded in stable isotopes in Mars’s atmosphere. Processes that shape Mars’s atmosphere on long timescales (e.g., atmospheric escape, volcanic outgassing) have distinct impacts on the isotopic composition of the atmosphere. We model the evolution of the pressure and isotopic composition of Mars’s atmosphere according to our theoretical and empirical understanding of these planetary processes. We include the three most abundant atmospheric species on modern Mars — CO₂, N₂, and Ar — in our self-consistent, coupled model. We identify a range of potential atmospheric evolution scenarios that simultaneously reproduce the modern pressure and isotopic composition of all three atmospheric species. Thus, the models presented here are a comprehensive treatment of Mars’s atmospheric evolution. By statistically analyzing our model, we place quantitative constraints on the pressures of CO₂, N₂, and Ar at 3.8 Ga, when most of the valley networks formed, and their subsequent evolution to the present-day. We will discuss the following observations: (1) the amount of carbonate deposition at early times critically determines the ancient atmospheric pressure, (2) a large atmospheric nitrogen reservoir at 3.8 Ga is consistent with the fully coupled model, and (3) model solutions are consistent with a reduced Martian mantle, which may pave the way for warming mechanisms that involve volcanically sourced reducing gases (e.g., H₂). The results presented here help us move toward reconciling evidence for liquid water on ancient Mars with the ancient atmospheric size and composition.