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

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

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

Calibration of NOMAD on ESA's ExoMars Trace Gas Orbiter: Part 1 – The Solar Occultation channel

Nadir and Occultation for MArs Discovery (NOMAD) is a 3-channel spectrometer suite that is currently orbiting Mars onboard ESA's ExoMars Trace Gas Orbiter, measuring the composition of the Martian atmosphere in unprecedented detail. Of the three channels, two operate in the infrared: the Solar Occultation (SO) channel observes gas species in the 2.2–4.3 μm spectral region in solar occultation mode, while the Limb, Nadir and Occultation (LNO) channel observes in the 2.2–3.8 μm spectral region and can operate in limb-, nadir- and solar occultation-pointing modes. The Ultraviolet–VISible (UVIS) channel operates in the UV–visible region, from 200 to 650 nm. Both infrared channels have a spectral resolution typically an order of magnitude better than previous instruments orbiting Mars, to measure molecular absorption lines and therefore determine the abundances of constituents of the Martian atmosphere and the processes that govern their distribution and transport. To maximise the full potential of the instrument, a wide range of calibration measurements were made prior to launch and continue to be made in-flight. This work, part 1, addresses the aspects of the SO channel calibration that are not covered elsewhere, namely: the SO channel ground calibration setup, boresight pointing vector determination, detector characterisation, detector illumination pattern and saturation levels, and an investigation of the instrument line shape. An accompanying paper, part 2, addresses similar aspects for LNO, the other infrared channel in NOMAD (Thomas et al., 2021, this issue)

Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission

The NOMAD spectrometer suite on the ExoMars Trace Gas Orbiter will map the composition and distribution of Mars׳ atmospheric trace species in unprecedented detail, fulfilling many of the scientific objectives of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument is a combination of three channels, covering a spectral range from the UV to the IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and how these objectives have influenced the design of the channels. We also discuss the expected performance of the instrument in terms of coverage and detection sensitivity

Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission

peer reviewedThe NOMAD spectrometer suite on the ExoMars Trace Gas Orbiter will map the composition and distribution of Mars’atmospheric trace species in unprecedented detail, fulfilling many of the scientific objectives of the joint ESA-Roscosmos ExoMars Trace Gas Orbiter mission. The instrument is a combination of three channels, covering a spectral range from the UV to the IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and how these objectives have influenced the design of the channels. We also discuss the expected performance of the instrument in terms of coverage and detection sensitivity

NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance

International audienceThe NOMAD (“Nadir and Occultation for MArs Discovery”) spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the composition of Mars’ atmosphere, with a particular focus on trace gases, clouds and dust. The detection sensitivity for trace gases is considerably improved compared to previous Mars missions, compliant with the science objectives of the TGO mission. This will allow for a major leap in our knowledge and understanding of the Martian atmospheric composition and the related physical and chemical processes. The instrument is a combination of three spectrometers, covering a spectral range from the UV to the mid-IR, and can perform solar occultation, nadir and limb observations. In this paper, we present the science objectives of the instrument and explain the technical principles of the three spectrometers. We also discuss the expected performance of the instrument in terms of spatial and temporal coverage and detection sensitivity