Outgassing and Escape of the Martian Atmosphere: Implications for Mars' Atmospheric and Water History
The study "Outgassing History and Escape of the Martian Atmosphere and Water Inventory" by Lammer et al. delves into the complex interactions and evolutionary processes impacting the Martian atmospheric composition and water inventory over geological timescales. The investigation targets the dichotomy of Mars' atmospheric history into early and late evolutionary epochs, highlighting significant influences from solar extreme ultraviolet (EUV) radiation, impact processes, and surface interactions like carbonates and serpentinization.
Early Atmospheric Dynamics and Protoatmosphere Loss
Mars' atmospheric evolution is initially governed by its protoatmosphere, characterized by a hydrogen-rich composition, a legacy of the solar nebula. The young Sun's copious EUV output contributed significantly to hydrodynamic escape, leading to rapid hydrogen loss and accompanying heavier species such as oxygen and carbon. This phase lasted approximately 500 million years (Myr), dramatically diminishing Mars' primary atmosphere.
Simultaneous to atmospheric thinning, outgassing from volcanic activities and impacts likely recharged the Martian atmosphere with CO₂, forming a secondary atmosphere purportedly ranging from tens to hundreds of millibars around 4 to 4.3 billion years ago (Gyr). This period marks the cusp of the Noachian epoch, setting the stage for later atmospheric dynamics.
Secondary Atmospheric Evolution and Escape Mechanisms
The evolution from the Noachian to the present is defined by a complex interplay of various non-thermal atmospheric escape processes, ongoing volcanic outgassing, and impacts, which interact with surface and subsurface geological processes such as carbonate formation and hydration reactions (serpentinization).
Numerical studies suggest that volcanic outgassing could have produced substantial volumes of CO₂ and H₂O, but much of these volatile substances were likely lost either to space or geochemically sequestered. The influences of solar-induced high energy processes, primarily driven by EUV radiation, led to significant thermal and non-thermal escape losses, shaping the Mars' atmosphere to the present state with a notably reduced CO₂ atmosphere.
Impacts and Implications
The paper critically examines the role of impact-related erosion and accretion of volatiles. The Late Heavy Bombardment (LHB) period, characterized by an elevated flux of cosmic impacts, had a significant impact on Mars’ potential atmospheric density, though the efficiency of this process remains debated, critically dependent on Mars’ ability to absorb, rather than lose atmospheric constituents via impacts.
Simulations and theoretical models infer impacts and other loss mechanisms have exacted a toll on the initial CO₂ reservoir. The study presents evidence suggesting interactions between volcanism, carbonate formation, and atmospheric erosion may have led to sequestration of CO₂ in mineral formats, thus offsetting escapes to space.
Implications for Water Inventory
Hydrological features and isotopic measurements suggest an ancient Martian surface with substantial water presence, but today’s water on Mars is significantly relegated to polar caps and subterranean reservoirs. Outgassing, aqueous alteration, and potential storage within the crust as clathrates or hydrates might suggest that significant Mars' water has been trapped in geological formations, possibly placing constraints on the paleoclimatic conditions and liquid water stability at the planet's surface over eons.
Conclusion and Prospects
The investigation suggests while the initial conditions favored a water-rich environment with substantial atmosphere conditions, loss mechanisms, solar influences, and geological sequestration have dramatically reshaped these conditions. Remaining questions, including the abundance and role of subsurface carbonates and remnants of a potentially dense early atmosphere, signal areas for ongoing exploration and research. Mars presents a compelling case study in planetary evolution, offering critical insights into planetary processes impacting habitability and the dynamic interactions among planetary atmospheres, volatiles, and the solar environment.