MARS OXYGEN GREEN LINE DAYGLOW FROM NOMAD/UVIS AND MODEL COMPARISON

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer onboard the ExoMars Trace Gas Orbiter performs limb observations of the dayside of the Mars atmosphere in both the visible and the ultraviolet domains since April 2019. The recently discovered visible emissions of the oxygen green line at 557.7 nm has here been investigated. The variations of the limb profile of this emission are studied over seasons. These average limb profiles are compared to photochemical model simulations with MAVEN/EUVM solar flux and the LMD GCM as inputs of the model. The global shape of the profile and the intensities are generally well reproduced. However, the peak altitude can sometimes be underestimated by the model and needs an adjustment of the CO2 density to reproduce the observations. We also compare the variations of the green line intensities over some individual UVIS limb tracking observations (observations of the atmosphere at a quasi-constant altitude) to model simulations and demonstrate a very good agreement. Finally, we show that the intensity and altitude of the lower emission peak are correlated with the solar Ly-α flux as expected from the theory of its production

Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD

sotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible

Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements

We present vertical profiles of temperature and density from solar occultation (SO) observations by the “Nadir and Occultation for Mars Discovery” (NOMAD) spectrometer on board the Trace Gas Orbiter (TGO) during its first operational year, which covered the second half of Mars Year 34. We used calibrated transmittance spectra in 380 scans, and apply an in-house pre-processing to clean data systematics. Temperature and CO2 profiles up to about 90 km, with consistent hydrostatic adjustment, are obtained, after adapting an Earth-tested retrieval scheme to Mars conditions. Both pre-processing and retrieval are discussed to illustrate their performance and robustness. Our results reveal the large impact of the MY34 Global Dust Storm (GDS), which warmed the atmosphere at all altitudes. The large GDS aerosols opacity limited the sounding of tropospheric layers. The retrieved temperatures agree well with global climate models (GCM) at tropospheric altitudes, but NOMAD mesospheric temperatures are wavier and globally colder by 10 K in the perihelion season, particularly during the GDS and its decay phase. We observe a warm layer around 80 km during the Southern Spring, especially in the Northern Hemisphere morning terminator, associated to large thermal tides, significantly stronger than in the GCM. Cold mesospheric pockets, close to CO2 condensation temperatures, are more frequently observed than in the GCM. NOMAD CO2 densities show oscillations upon a seasonal trend that track well the latitudinal variations expected. Results uncertainties and suggestions to improve future data re-analysis are briefly discussed

No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today. A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations. These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere, which-given methane's lifetime of several centuries-predicts an even, well mixed distribution of methane. Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections. We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally

Martian dust storm impact on atmospheric H₂O and D/H observed by ExoMars Trace Gas Orbiter

Global dust storms on Mars are rare but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere, primarily owing to solar heating of the dust. In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars. Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes, as well as a decrease in the water column at low latitudes. Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H2O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals. The observed changes in H2O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere

Water Vapor Vertical Profiles on Mars in Dust Storms Observed by TGO/NOMAD

It has been suggested that dust storms efficiently transport water vapor from the near‐surface to the middle atmosphere on Mars. Knowledge of the water vapor vertical profile during dust storms is important to understand water escape. During Martian Year 34, two dust storms occurred on Mars: a global dust storm (June to mid‐September 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162–260° and Ls = 298–345°) from the solar occultation measurements by Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). We show a significant increase of water vapor abundance in the middle atmosphere (40–100 km) during the global dust storm. The water enhancement rapidly occurs following the onset of the storm (Ls~190°) and has a peak at the most active period (Ls~200°). Water vapor reaches very high altitudes (up to 100 km) with a volume mixing ratio of ~50 ppm. The water vapor abundance in the middle atmosphere shows high values consistently at 60°S‐60°N at the growth phase of the dust storm (Ls = 195°–220°), and peaks at latitudes greater than 60°S at the decay phase (Ls = 220°–260°). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single pole‐to‐pole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322–327°), in particular at latitudes greater than 60°S

Science objectives of the NOMAD spectrometer on ExoMars Trace Gas Orbiter

The "Nadir and Occultation for MArs Discovery" (NOMAD) instrument on ESA and NASA's ExoMars Trace Gas Orbiter (EMTGO) will conduct a spectroscopic survey of the Martian atmosphere in the infrared (IR) and UV/visible spectral regions, both in solar occultation and nadir looking modes (see: Vandaele, A.C., same session). In the IR wavelength domain, the spectral resolution (~0.15 cm-1) surpasses those of previous surveys of Mars by more than an order of magnitude, this channel's heritage derives from the SOIR instrument with proven success on ESA's Venus Express mission [1]. An additional light-weight channel UVIS extends the survey to UV and visible wavelengths with a 1-2 nm resolution. NOMAD will search for active geology, volcanism and life by looking for the atmospheric markers of these processes on Mars, confining the source regions, and providing crucial information on the nature of the processes involved. NOMAD will also extend the survey of major climatology cycles of Mars

Mars ultra-violet dayglow from NOMAD/UVIS

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer onboard the ExoMars Trace Gas Orbiter performs limb observations of the dayside of the Mars atmosphere in both the visible and the ultraviolet domains since April 2019. The recently discovered visible emissions of the oxygen green line (557.7 nm) and the oxygen red line doublet (630.0 and 636.4 nm) have already been investigated by Soret et al. (2022). The variation of their peak brightness and altitudes have been studied over seasons and compared to photochemical model simulations. In this work, we present the ultraviolet dayglow counterpart of this study. The [OI] 297.2 nm emission that, like the oxygen green line, originates from the O(1S) state is analyzed. The [OI] 297.2 nm has already been extensively studied with previous missions but it is the first time that both the 297.2 and 557.7 nm can be observed simultaneously by the same instrument. The correlation of those emissions is shown and the ratio I(557.7 nm)/I(297.2 nm) is established with NOMAD/UVIS concurrent measurements. The CO Cameron bands, that have not been studied from the UVIS observations so far, are also presented. Finally, the CO2+ Fox-Duffendack-Barker (FDB) system, that has never been observed on Mars since Mariner, is also investigated. We present averaged limb profiles of all these ultraviolet emissions, their seasonal variations and ratios of their brightness. NOMAD/UVIS spectrometer is the first instrument that allows studying these UV emissions simultaneously that can, thus, be directly compared

Ultraviolet dayglow emissions and ratios in the Mars atmosphere

The Imaging UltraViolet Spectrograph (IUVS) (McClintock et al., 2015) onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft performs limb observations of the Mars atmosphere in the dayside. Spectra have been acquired in the ultraviolet domain, between and 110 and 340 nm, since September 2014.Additionally, the UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer (Patel et al., 2017; Vandaele et al., 2018) onboard the ExoMars Trace Gas Orbiter has performed the same type of observations both in the middle ultraviolet and the visible domains (200 - 650 nm), since April 2019. It is the first instrument that allows studying UV and visible emissions simultaneously that can, thus, be directly compared.The visible emissions of the oxygen green line (557.7 nm, Gérard et al., 2020) and the oxygen red line doublet (630.0-636.4 nm, Gérard et al., 2021) have already been investigated by Soret et al. (2022), using the NOMAD/UVIS data. The variation of their peak brightness and altitudes have been studied over seasons and compared to photochemical model simulations.In this work, we present the ultraviolet dayglow counterpart of the visible dayglow and compare the results of both the NOMAD/UVIS and MAVEN/IUVS instruments.The [OI] 297.2 nm emission that, like the oxygen green line, originates from the O(1S) state is analyzed. The [OI] 297.2 nm has already been extensively studied with the IUVS instrument (Gkouvelis et al., 2018) but it is the first time that both the 297.2 and 557.7 nm can be observed simultaneously, with NOMAD/UVIS. The correlation of those emissions is shown and the ratio I(557.7 nm)/I(297.2 nm) is established with NOMAD/UVIS concurrent measurements.The CO2+ UVD and CO Cameron bands, that have been studied with the MAVEN/IUVS instrument are also presented and compared to UVIS observations. Finally, the CO2+ Fox-Duffendack-Barker (FDB) system, that has not been observed on Mars since Mariner, is also investigated. We present averaged limb profiles of all these ultraviolet emissions, their seasonal variations and ratios of their brightness. References: Gérard, J. C. et al. (2020), Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations. Nature Astronomy, 4(11), 1049-1052Gérard, J. C. et al. (2021), First observation of the oxygen 630 nm emission in the Martian dayglow. Geophysical Research Letters, 48(8), e2020GL092334, https://doi.org/10.1029/2020GL092334Gkouvelis, L., et al. (2018), The O(1S) 297.2-nm dayglow emission: a tracer of CO2 density variations in the Martian lower thermosphere. Journal of Geophysical Research: Planets, 123(12), 3119-3132McClintock, W. E. et al. (2015), The Imaging Ultraviolet Spectrograph (IUVS) for the MAVEN mission. Space Science Reviews, 195(1-4), 75-124. https://doi.org/10.1007/s11214-014-0098-7Patel, M. R. et al. (2017), NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 2—design, manufacturing, and testing of the ultraviolet and visible channel. Applied optics, 56(10), 2771-2782. https://doi.org/10.1364/AO.56.002771Soret, L. et al. (2022), The Mars oxygen visible dayglow: a Martian year of NOMAD/UVIS observations, Journal of Geophysical ResearchVandaele, A. C. et al. (2018), NOMAD, an integrated suite of spectrometers for the ExoMars Trace Gas Mission: Technical description, science objectives, and expected performance. Space Science Reviews, 214(5). https://doi.org/10.1007/s11214-018-0517-