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Observation of the Mars O2 visible nightglow by the NOMAD spectrometer onboard the Trace Gas Orbiter


On Mars, atomic oxygen controls the carbon dioxide radiative cooling of the upper atmosphere and the presence of an ozone layer near the poles. To remotely probe meridional transport of O atoms from the summer to the winter hemisphere and the descending flow in the winter polar regions, the O2 Herzberg II atmospheric emission could be used as a proxy. This emission is quite weak on Earth’s nightside, but it is prominent in the Venus night airglow, and it has not previously been observed on Mars. Here we report the limb detection of the O2 Herzberg II visible bands in the Mars nightglow with the NOMAD ultraviolet–visible spectrometer onboard the European Space Agency’s Trace Gas Orbiter. The emission layer reaches up to hundreds of kilorayleighs in the limb viewing geometry. It is mainly located between 40 km and 60 km at high latitudes during the winter season, consistent with three-body recombination of oxygen atoms. This O2 nightglow should be observable from a Martian orbiter as well as from the Martian surface with the naked eye under clear sky conditions. These observations pave the way to future global observations of the Martian atmospheric circulation with simpler lower-cost instrumentation.

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Fig. 1: Average high-latitude nightglow spectrum.
Fig. 2: Distribution of detections of the O2 Herzberg II nightglow.
Fig. 3: Altitude distribution of the nightside limb observations.
Fig. 4: Model simulations of the limb brightness distribution of the O2 Herzberg II bands.

Data availability

The NOMAD-UVIS spectra can be downloaded from ESA’s SA archives at!Table%20View/NOMAD=instrument (select UVIS from the list of instruments and ‘Level 3 Calibrated’ from the processing level). Observed limb intensities and model calculations supporting Fig. 4 are available from BIRA-IASB repository at or from the corresponding author upon reasonable request.


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B.H. is research associate of the Belgian Fund for Scientific Research (FNRS). ExoMars is a space mission of ESA and Roscosmos. The NOMAD experiment is led by the IASB-BIRA, assisted by co-principal-investigator teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (The Open University). This project acknowledges funding from BELSPO, with the financial and contractual coordination by the ESA Prodex Office (PEA grant numbers 4000140863, 4000121493 and 4000129683). M.A.L.-V. was supported by grant number PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU) and CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. We also acknowledge support from the UK Space Agency through grant numbers ST/V002295/1, ST/V005332/1, ST/Y000234/1 and ST/X006549/1’. We thank the ESA TGO team and its project scientists H. Svedhem and C. Wilson for supporting these observations.

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Authors and Affiliations



J.-C.G. and L.S. conceived the study and wrote the paper. Y.W., C.D., I.R.T. and J.P.M. calibrated the UVIS data and prepared the datasets. Observation planning was managed by J.-C.G., L.S., B.R., and M.R.P. A.C.V. is the NOMAD principal investigator, M.R.P. is NOMAD co-principal investigator. All authors contributed to discussion and comments on the paper.

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Correspondence to J.-C. Gérard.

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Gérard, JC., Soret, L., Thomas, I.R. et al. Observation of the Mars O2 visible nightglow by the NOMAD spectrometer onboard the Trace Gas Orbiter. Nat Astron (2023).

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