Improving the Retrieval of Vertical Profiles of CO in the Martian Atmosphere from NOMAD Solar Occultation Observations

The ExoMars Trace Gas Orbiter (TGO) mission is a joint venture of the space agencies ESA and ROSCOSMOS which was launched in 2016 and carries onboard instruments dedicated to studying the trace gas compositions of the Martian atmosphere. NOMAD (Nadir and Occultation for MArs Discovery) is one such instrument that housed three observing channels named UVIS (the Ultra Violet and Visible Spectrometer), LNO (Limb Nadir Occultation) and SO (Solar Occultation) to scan the Martian atmosphere in nadir and limb geometries [1]. The SO channel of NOMAD operates in the IR (Infra-Red) region of the solar spectrum in the wavelength range 2.3 – 4.3µm. The SO spectrometer contains an echelle grating which can produce diffraction patterns of multiple orders but only one order is allowed to fall onto the detector selected by an AOTF (Acousto Optical Tunable Filter) filter. Spectral region of diffraction orders from 186 – 191 contains well-separated and strong absorption lines of CO. The NOMAD-SO channel is using diverse diffraction orders to monitor the CO due to its importance in understanding the dynamics and chemistry of the Martian atmosphere. CO is produced in the upper Martian atmosphere by the photolysis of CO₂ and destroyed by the hydroxyl (OH) radicals in the lower atmosphere. Hydroxyl radicals thus recycle CO into CO₂. The study of the CO vertical distribution is important to understand the photo-chemical stability of the atmosphere. CO not only links the chemistry of the carbon and odd hydrogen families but is a long-lived species which also serves as a dynamical tracer.

At IAA-CSIC we have developed a preprocessing scheme to clean the NOMAD calibrated data from a number of systematics and prepare them for inversion of different atmospheric species [2,3,4,5]. Those systematics are spectral shift of the absorption lines and spectral bending which occurs due to thermally induced mechanical stress on the detector [6]. The work presented here is in continuation with our previous work on the retrievals of CO [3] wherein the retrieval scheme has been described in detail. Our previous study reveals two crucial factors that need to be considered for a correct CO retrieval, one is the saturation of spectral lines in diffraction orders 186 and 190, those used in our work to derive CO. The second one is the use of observed temperature and pressure in the retrieval rather than the climatological T/P from GCMs (general circulation model). For order 190, the absorption lines become saturated below 70 km while for orders 186, the lines remain unsaturated for most of the atmospheric region below this altitude. In the altitudes above 70 km, the absorptions in 186 are dominated by random noise but the lines in 190, due to their strength remain clear. Due to this fact, an adequate combination of these two diffraction orders is recommended for performing CO inversions from TGO solar occultation data.

In this work, we will present the improved CO vertical densities using this strategy and the impact on the CO distribution.

References

[1] Vandaele, A. C., Lopez-Moreno, J. J., Patel, M. R., Bellucci, G., Daerden, F., Ristic, B., ... & NOMAD Team. (2018). NOMAD, an integrated suite of three spectrometers for the ExoMars trace gas mission: Technical description, science objectives and expected performance. Space Science Reviews, 214, 1-47.

[2] López‐Valverde, M. A., Funke, B., Brines, A., Stolzenbach, A., Modak, A., Hill, B., ... & NOMAD team. (2023). Martian atmospheric temperature and density profiles during the first year of NOMAD/TGO solar occultation measurements. Journal of Geophysical Research: Planets, 128(2), e2022JE007278.

[3] Modak, A., López‐Valverde, M. A., Brines, A., Stolzenbach, A., Funke, B., González‐Galindo, F., ... & Vandaele, A. C. (2023). Retrieval of Martian atmospheric CO vertical profiles from NOMAD observations during the first year of TGO operations. Journal of Geophysical Research: Planets, 128(3), e2022JE007282.

[4] Stolzenbach, A., López Valverde, M. A., Brines, A., Modak, A., Funke, B., González‐Galindo, F., ... & Vandaele, A. C. (2023). Martian atmospheric aerosols composition and distribution retrievals during the first Martian year of NOMAD/TGO solar occultation measurements: 1. Methodology and application to the MY 34 global dust storm. Journal of Geophysical Research: Planets, 128(11), e2022JE007276.

[5] Brines, A., López‐Valverde, M. A., Stolzenbach, A., Modak, A., Funke, B., Galindo, F. G., ... & Vandaele, A. C. (2023). Water vapor vertical distribution on Mars during perihelion season of MY 34 and MY 35 with ExoMars‐TGO/NOMAD observations. Journal of Geophysical Research: Planets, 128(11), e2022JE007273.

[6] Liuzzi, G., Villanueva, G. L., Mumma, M. J., Smith, M. D., Daerden, F., Ristic, B., ... & Bellucci, G. (2019). Methane on Mars: New insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration. Icarus, 321, 671-690.

Acknowledgements:

The IAA/CSIC team acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S and by grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00 all funded by MCIN/AEI/ 10.13039/501100011033. A. Brines acknowledges financial support from the grant PRE2019-088355 funded by MCIN/AEI/10.13039/501100011033 and by ’ESF Investing in your future’. ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University).