Heterogeneous Processes in the Atmosphere of Mars and Impact on H2O2 and O3 Abundances

• Published in: Journal of geophysical research. Planets 2023-12, Vol.128 (12), p.n/a
• Main Authors: Daerden, Frank, Crowley, John N., Neary, Lori, Smith, Michael D., Loeffler, Mark J., Clancy, R. Todd, Wolff, Michael J., Aoki, Shohei, Sagawa, Hideo
• Format: Article
• Language: English
• Subjects: Abundance; atmosphere; Atmospheric chemistry; Atmospheric circulation; Atmospheric models; chemistry; Clouds; Dust; GCM; General circulation models; H2O2; Hydrogen peroxide; Ice; Ice clouds; Imaging spectrometers; Laboratory experiments; Low temperature; Mars; Mars atmosphere; Mars dust; Mars surface; Modelling; Numerical models; Ozone; Photolysis; Sunlight; Three dimensional models; Water ice
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2023JE008014

Current models underestimate the highest observed ozone (O3) column densities on Mars. These estimates could be improved by including the uptake of odd hydrogen species (HOx) on water ice clouds, but the reported uptake coefficient of HO2 is likely overestimated for atmospheric conditions. This leaves a fundamental problem in Mars' atmospheric chemistry unsolved. Here, using the GEM‐Mars general circulation model, we explore a range of processes involving multiple phases (gas, adsorbed and solid) that may contribute to an alternative solution. First, we focus on hydrogen peroxide (H2O2) and discuss its physical states on Mars and its chemical impact. We also conjecture its photolytic destruction in ices with model simulations and Compact Reconnaissance Imaging Spectrometer for Mars observations. Then, we include in the model all relevant (for Mars) heterogeneous reactions, both on dust and water ice, recommended by the International Union of Pure and Applied Chemistry for terrestrial atmospheric studies. We find that only the uptake of HO2 and H2O2 on dust are efficient on Mars. Finally, we find that attenuation of sunlight by water ice clouds in the calculation of photolysis rates leads to increased O3 and H2O2 abundances below the ice clouds. The combination of the proposed processes leads to O3 increases without the need for strong uptake of HO2 on ice, but it remains difficult to find a good agreement with O3 and H2O2 observations on the global scale. We provide specific recommendations for future work in observations, laboratory experiments and modeling to advance our understanding of fundamental chemistry on Mars. Plain Language Summary After decades of observations and numerical modeling, there remain persistent problems in our understanding of atmospheric chemistry on Mars. One of these is the underestimation of the highest ozone abundances by models, on which we focus here. It was demonstrated that uptake of HO2 on water ice could improve this, but we argue that the experimentally obtained reaction rate is too large. We present a range of other processes that can contribute to solving the Mars ozone problem, and implement them in a 3D atmospheric model. We investigate the low temperature behavior of H2O2 on Mars and its impact on ozone, and demonstrate that H2O2 is destroyed in surface ices. This is supported by a first search for H2O2 in surface ices on Mars. We also test all heterogeneous reactions on dust and water ice that are recommended for