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Impact of Novel H₂O(v) Absorption Cross-Sections on the Present-Day Martian Atmosphere via Caltech-JPL’s 1D Photochemical Model

Presentation #213.03 in the session Martian Aurora, Atmosphere, Winds, and Dust (Poster + Lightning Talk)

Published onOct 23, 2023
Impact of Novel H₂O(v) Absorption Cross-Sections on the Present-Day Martian Atmosphere via Caltech-JPL’s 1D Photochemical Model

Mars’ early atmosphere was likely warm and wet, much different from the present cold and dry atmosphere. This is supported by the present-day large enrichment in atmospheric heavy isotopes (due to escape), as well as its surface geomorphology and geochemistry (seemingly modified by large volumes of surface water). The escape of water to space begins with photolysis near the surface, and subsequent HOx reactions produce H2. That hydrogen will then diffuse upwards and split into atomic H, via ion chemistry and finally escapes to space. Ranjan et al. (2020) found that water absorption cross sections at wavelengths ≥ 205 nm were underestimated in photochemical models, and they discussed the effect of the new (larger) absorption cross sections on the photochemistry at early-Earth-like atmospheres. In this work, we adapt KINETICS, the Caltech/JPL 1D Photochemical and Transport Model, to the current Martian atmosphere to learn if the water cross sections modify present-day Mars atmospheric photochemistry. We implement two “original” H2O(v) absorption cross section datasets, from Pinto et al. (2021) and Mills (1998), and in two additional cases we extrapolate the datasets to include the results of Ranjan et al. (2020) at wavelengths > 205 nm. KINETICS outputs the steady-state concentration profiles and solves thermal hydrogen and photochemical oxygen escape rates for these four cases. We find that the photolysis rates may not largely influence evolution, as the escape rates of species in the upper atmosphere (especially H) are not largely affected. Moreover, we investigate H escape during dust storms which is faster due to enhanced stratospheric water (Chaffin et al, 2017); we find H escape remains relatively unchanged by the larger water photolysis cross sections. In our steady state runs, we find that faster water photolysis influences the near-surface photochemistry due to a larger supply of HOx. Near-surface CO decreases by 25% and 12% from the starting datasets, respectively. This result is driven by increased OH production, causing CO to recycle to CO2 faster via: CO + OH -> CO2 + H. HNO3 increases by 50% and 18%, correspondingly. This result is, again, driven by increased OH production, as HNO3 production is dominated by the reaction NO2 + OH + M -> HNO3+ M. Changes in near-surface chemistry are important to understand the present-day Mars climate and also have implications for Mars’ habitability and early chemistry, particularly for the formation of nitrates in icy climates.

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