Martian CO vertical distribution combining 4 Martian Years of TGO/ACS MIR solar occultation data

Carbon monoxide (CO) is one of the most important tracers of atmospheric circulation and photochemical activity in the Martian atmosphere. Because of its relatively long chemical lifetime, its spatial and vertical distribution is strongly linked to the large-scale transport processes that shape the Martian climate. At the same time, CO is directly connected to the CO₂ photochemical cycle, making it a valuable probe of the coupling between atmospheric dynamics and chemistry. Since its first detections through ground-based observations several decades ago [1,2], CO has been extensively investigated by orbiters and space missions dedicated to the exploration of Mars. These observations have revealed strong seasonal, latitudinal, and vertical variations, highlighting the complexity of the Martian atmosphere and the need for long-term, high-resolution measurements to better constrain atmospheric circulation and photochemical models.

The ExoMars Trace Gas Orbiter (TGO), launched in 2016, has significantly improved our capability to study trace species in the Martian atmosphere thanks to its high spectral resolution and global coverage. Among its scientific payload, two instruments are capable of observing CO: NOMAD and the Atmospheric Chemistry Suite (ACS) [3,4]. In particular, the ACS-MIR channel provides high-quality solar occultation measurements with exceptional sensitivity to the vertical structure of the atmosphere, allowing the retrieval of atmospheric profiles over a broad altitude range.

In this work, we analyze solar occultation spectra acquired by the ACS-MIR channel, focusing on the CO (2–0) overtone absorption band between 4150 and 4350 cm⁻¹. We describe in detail the preprocessing steps applied prior to the atmospheric retrievals, including corrections for spectral bending and wavelength shifts, as well as improvements in the characterization of the instrumental line shape [5]. Particular attention has been devoted to the refinement of the error budget in order to properly account for instrumental and spectroscopic uncertainties affecting the measurements. These improvements are essential to maximize the accuracy and stability of the retrievals, especially when combining information from lines with very different sensitivities throughout the atmosphere.

The spectra are analyzed using the KOPRA radiative transfer model coupled with the RCP inversion code [6] to retrieve vertical profiles of CO volume mixing ratio (VMR) and temperature. A detailed sensitivity analysis of the CO absorption lines at different atmospheric altitudes is also presented. Because the strongest CO lines become saturated below ~50 km, retrieving reliable information across the full atmospheric column requires a careful selection and combination of spectral lines probing different altitude regions. While saturated lines reduce sensitivity in the lower atmosphere, they remain crucial for constraining CO abundances in the upper atmosphere, particularly between ~80 and 120 km. To address this challenge, we developed a dedicated retrieval strategy that allows us to derive robust and continuous CO vertical profiles from the lower atmosphere up to the thermosphere.

In addition, we demonstrate for the first time the simultaneous retrieval of temperature and CO VMR from a dataset containing only CO absorption lines. This is possible because the full CO absorption band is observed: the overall shape of the band contains information about the atmospheric thermal structure, while the depth and relative intensity of the individual absorption lines constrain the CO abundance. The methodology developed for this simultaneous retrieval represents a significant step forward for the analysis of ACS-MIR observations, and a more detailed explanation of this approach will be presented in this work.

Using this method, we retrieve CO vertical profiles between ~7 and 120 km altitude and temperature profiles between ~7 and 100 km. We present temperature profiles for the first half of Mars Year (MY) 34 and compare them with other available datasets [8]. In addition, we show CO abundance maps covering MY 34 to MY 37, providing a multi-year overview of the seasonal and latitudinal variability of CO in the Martian atmosphere. The retrieved distributions are broadly consistent with previous studies [7,8], showing an approximately constant CO abundance below ~50 km and a marked increase at higher altitudes driven by photochemical production. Clear seasonal and latitudinal variations are also observed throughout MY 34–37, reflecting the influence of atmospheric transport and circulation processes. The implications of these results for our understanding of Martian atmospheric dynamics and photochemistry are discussed.

 

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