Assessing the Deuterium Kinetic Isotope Effect of Hydrogen Radical Reactions on Photochemistry in the Mars Atmosphere

Abstract:

Abundant geological evidence, including fluvial features and hydrated minerals, suggests the presence of surface liquid water at early Mars which would have required a thicker, warmer atmosphere than there is today. The present-day D/H enrichment of ~5-10xSMOW (standard mean ocean water) in the Martian atmosphere provides understanding of the history of Martian water and its escape to space. In general, the mass difference facilitates H to escape faster than D, leading to an increase in the atmospheric D/H ratio over time, but this ratio is also affected by the deuterium kinetic isotope effect (^DKIE) which causes deuterated reactions to proceed at different rates than their H-bearing counterparts. The influence of the ^DKIE on the Mars D/H ratio is poorly constrained, primarily because many deuterated reaction rates are not well known. This study investigates the influence of the ^DKIE on the Martian D/H ratio by (1) identifying which HO_x reactions the Martian atmosphere is most sensitive to in order to (2) improve constraints on their rates and ^DKIEs. We use KINETICS, the Caltech/JPL coupled photochemistry and transport code, to model the chemistry of 35 species (including 6 deuterated species) linked by 193 reactions. As a sensitivity test, we vary the rates of HO_x reactions, and we determine that a few reactions lead to substantial changes in the hydrogen concentration. We use semi-classical transition state theory to better estimate rates and ^DKIEs. Future work will continue similar sensitivity tests in order to compute and update key reaction rates for improved understanding of the role of kinetics on the Martian D/H ratio.