Evaluation of the capability of ExoMars-TGO NOMAD infrared nadir channel for water ice clouds detection on Mars

The Nadir and Occultation for Mars Discovery (NOMAD) is one of the four instruments on board the 2016 ExoMars Trace Gas Orbiter. The instrument is a suite of three spectrometers, mainly designed to study minor atmospheric species at high spectral resolution (A.C. Vandaele et al., 2015; E. Neefs et al., 2015). Nevertheless, Oliva et al. (2022) demonstrated the capability of NOMAD infrared nadir channel to investigate surface ice composition in the 2.3 – 2.6 μm wavelength range. Ice signatures have been also observed at mid/equatorial latitudes suggesting, after analysis, the first detection of CO₂ ice clouds through the study of the narrow 2.35 μm absorption band.

In this work, we also use observations of the NOMAD infrared LNO channel in order to evaluate its capability to detect H₂O ice clouds. We present a technique taking advantage of the 2.7 µm strong ice absorption band. For this study, we select LNO spectral orders 167, 168, 169 and combine them to derive a spectral index for H₂O ice detection, namely the Frost and Clouds Index (FCI). The acquisition of data during Mars Year 34 and 35 (March 2018 to February 2021) allows us to construct seasonal maps for H₂O ice clouds. The results appear in agreement with previous studies focused on Mars Express SPICAM/UV and OMEGA data analysis (Willame et al., 2017; Olsen et al., 2021). FCI is sensitive to the Polar Hood clouds, although the full structure is not detected. Moreover, detections in the Aphelion Cloud Belt (ACB) are limited. This is consistent with previous OMEGA spectrometer observations (Olsen et al, 2021) showing different physical properties between the two main Martian atmospheric structures and making the ACB less detectable in the infrared. We hence derive the LNO channel sensitivity limit for these clouds detection.

Acknowledgements

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 (The 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), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-BI00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 and ST/S00145X/1  and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). 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). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency. SR thanks BELSPO for the FED-tWIN funding (Prf-2019-077 - RT-MOLEXO).

References

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Willame, Y., et al., 2017. Retrieving cloud, dust and ozone abundances in the martian atmosphere using spicam/uv nadir spectra.Planetary and SpaceScience, 142:9–25.

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