Four graphs from the paper.
Top panel (a) shows color-coded latitudinal and temporal (Ls) coverage of ACS solar occultations from the start of TGO mission in Martian Year 34 through the beginning of Martian Year 36. All ACS solar occultations are shown in gray. The lower panel shows mean vertical profiles of ozone (b) and water vapor (c) retrieved from ACS/MIR, and of temperature (d) retrieved from concurrent ACS NIR observations, with the same color code as in (a). The shaded regions represent one standard deviation of the averaged profiles. Overall, the lower panel reveals that water vapor abundance is controlled by atmospheric temperature, and that water vapor and ozone are anti-correlated. Credit: Olsen et al. [2022], Figure 8
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Planets

Ozone (O3) and water vapor (H2O) play a key role in the atmospheric chemistry of Mars. Photolysis of H2O generates ozone-destroying radicals (the HOx family), which in turn regenerate photo-dissociated CO2 in the upper atmosphere through the reaction CO + OH à CO2 + H. Therefore, the anticorrelated coupling between O3 and H2O is critical to stabilize the composition of the Mars CO2 atmosphere. While this anticorrelation has been long predicted from models, coincident orbital retrievals of both species have been lacking until recently.

Olsen et al. [2022] use measurements in solar occultation mode from the mid-infrared channel of the Atmospheric Chemistry Suite (ACS/MIR) on board the ExoMars Trace Gas Orbiter (TGO) to measure coincident profiles of O3, H2O and temperature. Then, the authors analyze the relations between these three profiles as function of elevation and time. They find that water vapor abundance is controlled by atmospheric temperature, and that, as expected, O3 and H2O are anti-correlated. Water vapor condenses when the atmosphere cools, allowing O3 to build up as the production of ozone-destroying radicals is reduced. Conversely, warmer temperatures lead to H2O enhancements and ozone loss.

Interestingly, comparisons with the LMD Mars Global Climate Model reveal that the observed O3 abundance is larger by factors between 2 and 6, indicating important differences in the rate of odd-hydrogen photochemistry on Mars as compared to that used in models. This discrepancy has important implications for the photochemical cycles of H2O and CO, as well as trace gases such as HCl and CH4, and it may arise from the use of the model without heterogeneous chemistry (i.e., without the inclusion of heterogeneous uptake of OH, HO2 and H2O2 on water ice aerosols).

In combination with previous coincident observations of O3 and water vapor H2O from SPICAM on board Mars Express (Lefèvre et al. [2021] and references therein), and from the NOMAD-UVIS instrument on board TGO (Patel et al. [2021]), the results in Olsen et al. [2022] represent a significant step in defining current model-data disagreements in our understanding of Mars’s photochemistry.

Citation: Olsen, K. S., Fedorova, A. A., Trokhimovskiy, A., Montmessin, F., Lefèvre, F., Korablev, O., et al. (2022). Seasonal changes in the vertical structure of ozone in the Martian lower atmosphere and its relationship to water vapour. Journal of Geophysical Research: Planets, 127, e2022JE007213.

—Germán Martínez, Associate Editor, Journal of Geophysical Research: Planets

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