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- Title
The Ultraviolet Martian Dayglow Observed With NOMAD/UVIS on ExoMars Trace Gas Orbiter.
- Authors
Soret, L.; Gérard, J.‐C.; Hubert, B.; Vandaele, A. C.; Thomas, I. R.; Ristic, B.; Willame, Y.; Schneider, N.; Jain, S.; Gupta, S.; Mason, J. P.; Patel, M. R.
- Abstract
The Trace Gas Orbiter has been orbiting Mars since 2016 with the Nadir and Occultation for MArs Discovery (NOMAD) UltraViolet and Visible Spectrometer (UVIS) instrument on board. Focusing on limb observations recorded in the ultraviolet (UV) part of the NOMAD/UVIS spectra, we describe here the CO Cameron bands, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ UV doublet and Fox‐Duffendach‐Barker (FDB) bands and [OI] UV emissions. Averaged limb profiles are presented, showing that the strongest brightness and the highest peak altitudes are reached near perihelion. Ratios between the UV emissions are also estimated and compared with previous observations from Mariner and Mars Express. NOMAD/UVIS is the first instrument able to simultaneously acquire data both in the UV and the visible in the Mars atmosphere so that the oxygen green line at 557.7 nm and its UV counterpart at 297.2 nm, both originating from the same O(1S) upper state level, may be directly compared. A mean ratio of 15.8 is derived, in close agreement with ab initio calculations. The spectral composition of the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB system that has not been observed entirely since the Mariner missions in the 1970s is analyzed. According to the spectral composition of the FDB bands, we show that this emission is produced at ∼70% by photoionization of CO2 (which populates the shorter wavelengths of the spectrum) and ∼30% by resonance scattering of solar radiation (which populates longer wavelengths). No evidence of a change with altitude in the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB spectral composition is observed in the NOMAD/UVIS spectra. Plain Language Summary: Dayglow is an atmospheric emission occurring on the illuminated side of a planet. This phenomenon occurs when atmospheric atoms or molecules interact with incoming solar radiation that produce excited atoms and molecules. When these atoms and molecules de‐excite radiatively, they release photons at specific wavelengths and altitudes, generating airglow. Several space missions since the 1970s observed the Mars ultraviolet (UV) dayglow. The Nadir and Occultation for MArs Discovery (NOMAD)/UltraViolet and Visible Spectrometer (UVIS) instrument, currently orbiting Mars since 2016, is providing new insights on this phenomenon. In this study, we describe the results of the observations of the Martian UV dayglow by NOMAD/UVIS. We show that the strongest emissions occur at higher altitudes, when the planet is closest to the Sun. We also estimate the intensity ratios between the emissions and compare our results with previous observations from Mariner and Mars Express. The CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ emission between 300 and 400 nm is also detected for the first time since the Mariner missions in the 1970s. The production mechanisms can be deduced by analyzing the spectral distribution of this emission. We conclude that photoionization of CO2 represents ∼70% of its production, while resonance scattering of solar radiation accounts for ∼30%. This proportion was predicted by models, but this is the first time that observations validate these simulations. This statement is true at all the observed altitudes. Key Points: The Mars ultraviolet (UV) dayglow emissions from CO Cameron, CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ ultraviolet doublet, [OI] UV and CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ Fox‐Duffendach‐Barker (FDB) are analyzed using Nadir and Occultation for MArs Discovery/UltraViolet and Visible Spectrometer limb spectraThe controversial 557.7/297.2 nm intensity ratio derived from those simultaneous measurements is highly reliably determinedThe full wavelength range of the CO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ FDB band system is detected for the first time since Mariner
- Subjects
AIRGLOW; TRACE gases; MARTIAN atmosphere; AB-initio calculations; OCCULTATIONS (Astronomy); SOLAR radiation; ULTRAVIOLET spectrometers
- Publication
Journal of Geophysical Research. Planets, 2023, Vol 128, Issue 5, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2023JE007762