A Lower Limit of Atmospheric Pressure on Early Mars Inferred from Nitrogen and Argon Isotopic Compositions (1708.03956v1)

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.

Analysis of Early Mars Atmospheric Pressure via Nitrogen and Argon Isotopic Compositions

The paper provides a comprehensive assessment of the evolution of the Martian atmosphere, specifically tracing its pressure evolution using nitrogen (N) and argon (Ar) isotopic compositions. By deploying a one-box atmosphere-hydrosphere model, the authors integrate diverse atmospheric processes such as impact erosion, volcanic degassing, photochemical escape, and atmospheric replenishment by both asteroids/comets and interplanetary dust particles (IDPs).

Key Findings and Numerical Results

1. Atmospheric Pressure Estimation: The authors conclude that to maintain a less fractionated isotopic composition within the Martian atmosphere at around 4.0 billion years ago (Ga), the atmospheric pressure had to exceed approximately 0.5 bar. This estimation surpasses the pressure threshold required to avert an atmospheric collapse, substantiated by isotopic evidence from the Martian meteorite Allan Hills 84001.
2. Impact and Escape Dynamics: The paper models the stochastic nature of impacts during the Noachian epoch, indicating that intense bombardment could dynamically affect atmospheric pressure. This was crucial during the early stages where the pressure variance was instrumental in influencing isotopic fractionation timescales.
3. Model Calibration for Isotopic Preservation: Through a Monte Carlo approach, the team simulates numerous evolutionary tracks that align well with present-day atmospheric data from exploration missions and Martian meteorites. By adjusting parameters like the volatile content of impactors (X_gas), they identify conditions needed for isotopic preservation under varying atmospheric thickness.
4. Volcanic Degassing and Exogenous Contributions: A pivotal conclusion indicates that while volcanic degassing contributed significantly to Mars's atmospheric nitrogen, noble gases like Ne, Ar, and Kr primarily arose from external sources, namely comets and IDPs. The model necessitates volcanic degassing to uphold N levels, but exogenous input is essential for noble gases.
5. Isotopic Analysis: The paper successfully replicates the isotopic signatures of N, Ne, Ar, and Kr of the current Martian atmosphere, save for Xe, which hints at additional ionization-induced fractionation processes. Notably, the Xe isotopic fractionation resumes historical discussions about its primordial atmospheric remnants.

Implications and Future Directions

The implications reach beyond mere historical reconstruction; they offer insight into atmospheric retention factors, critical when modeling the climate conditions needed for liquid water stability on Mars. Understanding these dynamics aids in tailoring future Mars exploration missions to probe ancient climate conditions and test existing hypotheses on atmospheric dynamics.

Future work should capitalize on the MAVEN mission outputs, which could refine escape rate estimations and the interaction of Mars's atmosphere with solar activity. Efforts to bridge the temporal gaps in isotopic records from diverse Martian meteorites could further refine atmospheric modeling accuracy. Moreover, elucidating isotopic trends within Xe and tracing new findings from Martian regolith meteorites should remain a priority, enabling robust atmospheric evolution elucidation.

Ultimately, the paper provides a robust framework for modeling atmospheric dynamics on early Mars, informing both theoretical and methodological advancements in planetary science. This work enhances our understanding of planetary atmosphere evolution, offering a detailed insight into past Martian conditions that could have supported different climatic states.