Mapping of Martian CO from NOMAD solar occultation measurements for MY35 and 36

Introduction: CO is one of the important trace gas species in the Martian atmosphere which is linked to the Martian photochemistry through its production in the upper atmosphere in the photolysis of CO₂ and its destruction in the lower altitudes in a chemical reaction with OH radicals. Thus it can be used as a proxy to study the HO_x chemistry. CO is also one of the long-lived species in the atmosphere and its global distribution is controlled by the atmospheric dynamics. The observations made by the Solar Occultation (SO) spectrometer onboard TGO ExoMars space-craft presents with opportunity for systematic mapping of CO density profiles in the atmosphere for the first time.

Retrieval Methodology: SO is one of the spectrometers in the NOMAD suite onboard Exo Mars Trace Gas Orbiter designed for the observations of the trace species active in the spectral range 2.3 – 4.3 μm [2]. The spectrometer is built using diffraction grating in Littrow configuration. It uses Acousto Optical Tunable Filter (AOTF) to select one wavelength range corresponding to a single diffraction order. The selected order passes through a set of parabolic mirrors before getting incident on the grating. The diffracted light is then guided through the same mirrors to the detector of the spectrometer. The spectrometer is operated usually to record diffraction orders from 110 to 191, but in regular operations, up to six different diffraction orders can be selected. In this wide range of diffraction orders, CO sounding is suitable only in the diffraction orders 186 – 191 where the CO absorption lines are strong and well separated from each other.

The diffraction orders 186 – 191, however, are not measured simultaneously. Orders 186, 189, and 190 are the most commonly used so far, and therefore, a better geographical coverage was obtained from them compared to the other orders. Our aim is to obtain a similar retrieval performance for these orders using the state-of-the art retrieval method developed at IAA/CSIC. The transmission spectra for every order suffer from residual calibration effects like spectral shift and bending which are corrected using a cleaning methodology described in [3,4]. We use line-by-line radiative transfer model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm)[5] as a forward model, which was adapted for Mars and to the NOMAD instrument characteristics, in conjunction with an interactive solver (RCP) to retrieve CO from the cleaned spectra [6]. This follows a similar work for the diffraction order 190 and is applied to the first year of TGO observation [1].

 

Acknowledgement: The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). MALV was funded by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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). US investigators were supported by the National Aeronautics and Space Administration.

 

References

[1]A. Modak, et al. Retrieval of martian atmospheric CO vertical profiles from the first year of NOMAD/TGO solar occultation observations. JGR-submitted , 2022.

[2]A. C. Vandaele, et al. NOMAD, an integrated suite of three spectrometers for the exoMars trace gas mission: Technical description, science objectives and expected performance. Space Science Reviews , 214(5):1–47, 2018.

[3]M.-A. López-Valverde, et al, Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements. JGR-submitted , 2022.

[4]A. Brines, et al. Water vapor vertical distribution on Mars during the perihelion season of MY34 and MY35 from ExoMars TGO/NOMAD solar occultation measurements. JGR-submitted, 2022.

[5]Stiller, G. P. The Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA), 2000.

[6]Á. A. Jurado Navarro et al. Retrieval of CO2 and collisional parameters from the mipas spectra in the earth atmosphere. 2016.