Seasonal and spatial variability of carbon monoxide in the Mars atmosphere as observed by NOMAD LNO

Carbon monoxide (CO) is a minor trace gas of the Martian atmosphere that plays a key role in the photochemical cycle of carbon. CO is produced in the upper atmosphere by the carbon dioxide (CO₂) photolysis, while its recycling through CO₂ reformation occurs via catalytic reaction with the odd hydrogen radicals (HO_x) produced by the water vapor (H₂O) photolysis (see [1, 2] and references therein). As non-condensable species, CO column-averaged volume mixing ratio (VMR) is expected to show seasonal and spatial gradients that closely follow the seasonal CO₂ condensation/sublimation processes at the poles and related surface pressure variation. Given its relatively long lifetime (~6 years), CO is often used as a tracer of the atmospheric dynamical patterns.

Mars CO column abundance observations from space are relatively sparse, with Mars Express OMEGA [3] and PFS instruments [4, 5], and CRISM aboard the Mars Reconnaissance Orbiter [6, 7]. More recently, the ExoMars Trace Gas Orbiter NOMAD-LNO spectrometer has provided a climatology of CO VMR at a global scale covering slightly more than a full Martian Year (MY 34, L_s=150° to MY 35, L_s=241°; March 2018-July 2020) of observations [8]. The globally averaged CO VMR derived from these data sets is found to range from ~800 to 1000 ppm with corresponding uncertainties of 20-45%.

Here we present the seasonal and spatial CO column abundance distributions derived from NOMAD-LNO dayside nadir observations over the full mission period (MY 34-38). LNO (Limb Nadir and solar Occultation) is one of the two echelle grating infrared spectrometers installed on the NOMAD (Nadir and Occultation for MArs Discovery) instrument aboard the ESA ExoMars Trace Gas Orbiter (TGO) [9]. In these spectrometers, the echelle grating is combined with an Acousto-Optic Tunable Filter (AOTF) for the spectral window selection [10]. As for our LNO H₂O column retrieval [11], the LNO nadir CO column-averaged VMRs are retrieved using the Optimal Estimation scheme [12] implemented in the ASIMUT-ALVL radiative transfer tool [13]. ASIMUT-ALVL is applied separately to LNO reflectance factor spectra from grating orders 186-191 covering the ~4180-4330 cm^-1 wavenumber range. Altitude, pressure, temperature, CO, dust, and water ice a priori vertical profiles are extracted at the spectra locations from the GEM-Mars General Circulation Model [14]. Scattering by dust and water ice particles is taken into account in the forward simulations by using the LIDORT radiative transfer model [15]. Our retrieval also includes the new evaluations of the LNO AOTF function and its central position temperature dependence [11].

In this presentation, we will first discuss the consistency of the retrieval results between the selected diffraction orders and the impact of the CO absorption lines saturation on the retrieval results. The retrieved CO VMR seasonal, latitudinal, and inter-annual patterns will be then assessed over the full mission period and our data set will be compared to other available observational data sets (CRISM, PFS, and LNO from [7], [5], and [8], respectively), and to output from the GEM-Mars General Circulation Model. The correlation between LNO CO and H₂O column abundances will be also investigated.

Acknowledgements

The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA) with co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). This project acknowledges funding by: the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000140753, 4000140863); by the Spanish Ministry of Science and Innovation (MCIU) and European funds (grants PGC2018-101836-B-I00 and ESP2017-87143-R; MINECO/FEDER), from the Severo Ochoa (CEX2021-001131-S) and from MCIN/AEI/10.13039/501100011033 (grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00); by the UK Space Agency (grants ST/V002295/1, ST/V005332/1, ST/X006549/1, ST/Y000234/1 and ST/R003025/1); and by the Italian Space Agency (grant 2018-2-HH.0).

References

[1]McElroy M. B., and T. M. Donahue (1972), Stability of the Martian atmosphere, Science 177, 986–988, DOI: 10.1126/science.177.4053.986

[2]Vandaele, A. C., et al. (2024), Composition and Chemistry of the Martian Atmosphere as Observed by Mars Express and ExoMars Trace Gas Orbiter, Space Science Reviews, 220:75, https://doi.org/10.1007/s11214-024-01109-7

[3]Encrenaz, T., et al. (2006), Seasonal variations of the martian CO over Hellas as observed by OMEGA/Mars Express, Astronomy and Astrophysics, 459, 265–270, DOI: 10.1051/0004-6361:20065586

[4]Sindoni, G., et al. (2011), Observations of water vapor and carbon monoxide in the Martian atmosphere with the SWC of PFS/MEX. Planetary and Space Science, 59, 149-162, DOI: 10.1016/j.pss.2010.12.006

[5]Bouche, J., et al. (2021), Seasonal and spatial variability of carbon monoxide (CO) in the Martian atmosphere from PFS/MEX observations, Journal of Geophysical Research: Planets, 126, e2020JE006480, https://doi.org/10.1029/2020JE006480

[6]Smith, M. D., et al. (2009), Compact Reconnaissance Imaging Spectrometer observations of water vapor and carbon monoxide, J. Geophys. Res., 114, E00D03, https://doi:10.1029/2008JE003288

[7]Smith, M. D., et al. (2018), The climatology of carbon monoxide and water vapor on Mars as observed by CRISM and modeled by the GEM-Mars general circulation model, Icarus, 301, 117-131, https://doi.org/10.1016/j.icarus.2017.09.027

[8]Smith, M. D., et al. (2021), The climatology of carbon monoxide on Mars as observed by NOMAD nadir-geometry observations, Icarus, 362, 114404, https://doi.org/10.1016/j.icarus.2021.114404

[9]Vandaele, A. C., et al. (2015), Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission, Planetary and Space Science, 119, 233-249, https://doi.org/10.1016/j.pss.2015.10.003

[10]Thomas, I. R., et al. (2022), Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 2 – The Limb, Nadir and Occultation (LNO) channel, Planetary and Space Science, 218, 105411. https://doi.org/10.1016/j.pss.2021.105410

[11]Hendrick, F., et al. (2026), New climatology of Martian water vapor column abundance derived from NOMAD LNO nadir observations over Martian Years 34–38, Journal of Geophysical Research: Planets, 131, e2025JE009592, https://doi.org/10.1029/2025JE009592

[12]Rodgers, C. D. (2000), Inverse methods for atmospheric soundings: Theory and practice, World Scientific

[13]Vandaele, A. C., et al. (2006), Modeling and retrieval of atmospheric spectra using ASIMUT, European Space Agency, (Special Publication) ESA SP

[14]Neary, L., and F. Daerden (2018), The GEM-Mars general circulation model for Mars: Description and Evaluation, Icarus, 300, 458–476, https://doi.org/10.1016/j.icarus.2017.09.028

[15]Spurr, R. J. D. (2006), VLIDORT, A linearized pseudo-spherical vector discrete ordinate radiative transfer code for forward model and retrieval studies in multilayer multiple scattering media, J. Quant. Spectrosc. Radiat. Transfer, 102(2), 316-342, doi:10.1016/j/jqsrt.2006.05.005