The NOMAD ("Nadir and Occultation for MArs Discovery") spectrometer suite on board the ExoMars Trace Gas Orbiter (TGO) has been designed to investigate the comThis paper is dedicated to the memory of M. Allen, V. Formisano, and J. McConnell. position 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.

International audienceThe ACS package for ExoMars Trace Gas Orbiter is a part of Russian contribution to ExoMars ESA-Roscosmos mission. On the Orbiter it complements NOMAD investigation and is intended to recover in much extent the science lost with the cancellation of NASA MATMOS and EMCS infrared sounders. ACS includes three separate spectrometers, sharing common mechanical, electrical, and thermal interfaces. NIR is a versatile spectrometer for the spectral range of 0.7-1.6 μm with resolving power of ~20000. It is conceived on the principle of RUSALKA/ISS or SOIR/Venus Express experiments combining an echelle spectrometer and an AOTF (Acousto-Optical Tuneable Filter) for order selection. Up to 8 diffraction orders, each 10-20 nm wide can be measured in one sequence record. NIR will be operated principally in nadir, but also in solar occultations, and possibly on the limb. MIR is a high-resolution echelle instrument exclusively dedicated to solar occultation measurements in the range of 2.2-4.4 μm targeting the resolving power of 50000. The order separation is done by means of a steerable grating cross-disperser, allowing instantaneous coverage of up to 300-nm range of the spectrum for one or two records per second. MIR is dedicated to sensitive measurements of trace gases, approaching MATMOS detection thresholds for many species. TIRVIM is a 2- inch double pendulum Fourier-transform spectrometer for the spectral range of 1.7-17 μm with apodized resolution varying from 0.2 to 1.6 cm-1. TIRVIM is primarily dedicated to monitoring of atmospheric temperature and aerosol state in nadir, and would contribute in solar occultation to detection/reducing of upper limits of some components absorbing beyond 4 μm, complementing MIR and NOMAD. Additionally, TIRVIM targets the methane mapping in nadir, using separate detector optimized for 3.3-μm range. The concept of the instrument and in more detail the optical design and the expected parameters of its three parts, channel by channel are described. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

The echelle spectrometer TIMM-2 is the instrument developed for the unsuccessful Russian mission Phobos-Grunt. The instrument was dedicated to solar occultation studies of the Martian atmosphere by measuring the amount of methane, by sensitive measuring of other minor constituents, and by profiling the D/H ratio and the aerosol structure. The spectral range of the instrument is 2300-4100 nm, the spectral resolving power λ/Δλ exceeds 25,000, and the field of view is 1.5×21 arc min. The spectra are measured in narrow spectral intervals, corresponding to discreet diffraction orders. One measurement cycle includes several spectral intervals. To study the vertical profiles of aerosol, the instrument incorporates four photometers in the UV to near-IR spectral range. The mass of the instrument is 2800 g, and its power consumption is 12 W. One complete flight model remains available after the Phobos-Grunt launch. We discuss the science objectives of the occultation experiment for the case of Mars, the implementation of the instrument, and the results of ground calibrations.

International audienceThe atmospheric chemistry suite (ACS) package is a part of the Russian contribution to the ExoMars ESA-Roscosmos mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. The near-infrared (NIR) channel is a versatile spectrometer for the spectral range of 0.7-1.6 μm with a resolving power of ∼20,000. The instrument employs the principle of an echelle spectrometer with an acousto-optical tunable filter (AOTF) as a preselector. NIR will be operated in nadir, in solar occultations, and possibly on the limb. Scientific targets of NIR are the measurements of water vapor, aerosols, and dayside or nightside airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the range of 2.2-4.4 μm targeting the resolving power of 50,000. MIR is dedicated to sensitive measurements of trace gases. The thermal infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer for the spectral range of 1.7-17 μm with apodized resolution varying from 0.2 to 1.6 cm−1. TIRVIM is primarily dedicated to the monitoring of atmospheric temperatures and aerosol states in nadir. The present paper describes the concept of the instrument, and in more detail, the optical design and the expected parameters of its three parts channel by channel

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<p><span>One of the goals of the ESA and Roscosmos Exomars Trace Gas Orbiter (TGO) is the exploitation of the two solar occultation instruments NOMAD </span><span>[1] </span><span>and ACS </span><span>[2] </span><span>to characterize the thermal structure of the Martian atmosphere with unprecendented vertical resolution, and from the ground up to the thermosphere. </span><span>Specifically for the upper atmosphere this is a unique opportunity [3] and we present </span><span>here </span><span>an on-going effort to retrieve CO2 abundance and temperature profiles simultaneously from each of these instruments and to combine them for a </span><span>mutual validation and for obtaining </span><span>a</span> <span>most complete mapping from the TGO orbit. </span></p><p><span>W</span><span>e developed a retrieval suite common for both instruments, comprising: (a) a cleaning/pre-processing module to build vertical profiles of calibrated transmittances which computes and correct for residual calibration and instrumental effects like spectral shifts, bending of the continuum and variations in the instrument line shape; (b) a state-of-the-art </span><span>retrieval scheme designed </span><span>originally for Earth atmospheric remote sensing </span><span>[4,5,6] </span><span>and applied to Mars </span><span>[</span><span>7</span><span>]</span><span>, in order to </span><span>derive </span><span>simultaneous density and temperature profiles allowing for hydrostatic adjustments during the internal iteration.</span></p><p>Recently the first application of this retrieval scheme focused on measurements by the NOMAD SO channel at altitudes below 100 km, and for the first year of TGO operations, from April 2018 to March 2019 (second half or “perihelion” season of MY34), and revealed very interesting results [8]. The thermal structure is strongly affected by the MY34 global dust storm at all altitudes, a cold mesosphere (in comparison to global climate models) was found during the post-GDS period, and wavy structures at mesospheric altitudes in the morning terminator seem to reveal very strong thermal tides at low-mid latitudes.</p><p><span>In this presentation we also focus on the Martian troposphere and mesosphere and build upon the above mentioned work during the 1</span><sup><span>st</span></sup><span> year of TGO by extending the study to a full Mars year and adding retrievals from ACS MIR channel. </span><span>Both NOMAD/SO and ACS/MIR channels observe the strong CO2 ro-vibrational band at 2.7 micron in the same spectral region with some differences in spectral resolution and noise level, in addition to very differnet instrument characteristics, which are included in our retrieval approach. </span> <span>In obth cases we use calibrated atmospheric transmittances </span><span>to </span><span>tackle </span><span>three </span><span>targets, CO2 densit</span><span>y, </span><span>temperature,</span><span> and dust loading,</span><span> in a simultaneous </span><span>global-fit </span><span>inversion, </span><span>in</span><span>c</span><span>luding contaminant species like H2O. </span><span>The contamination by aerosol </span><span>can severely</span><span> limit the ability to sound low </span><span>tangent </span><span>altitudes </span><span>with both instruments</span><span>, </span><span>when</span><span> the gas absorption lines </span><span>become </span><span>hidden </span><span>within</span> <span>the</span><span> aerosol continuum. </span><span>But also these measurements permit a good characterization of aerosol properties [9]. </span><span>Propagation of measurement noise, tunning of regularization, and computation of averaging kernels are performed with the same code and the comparison of the retrieval performance is a first step towards a common validation between these two instruments, </span><span>considering that a complete correlation is not possible since </span><span>the two </span><span>instruments' individual solar occulation scans </span><span>are non-coincident in time and space. </span><span>Our results will be discussed and compared to paralell retrieval</span><span> efforts by other </span><span>teams</span> <span>within</span><span> the NOMAD and ACS </span><span>consortia</span> <span>using the same datasets </span><span>[</span><span>10, 11, </span><span>12</span><span>]. </span><span>We will also compare them with specific runs of the LMD-Mars GCM (</span><span>see also a companion presentation in this conference on this specific topic [1</span><span>3</span><span>]</span><span>). </span><span>One of the important applications of our inversion is to supply true-field and a.priori inputs for the inversion of trace species from collocated NOMAD spectra, as we are doing for water vapour [1</span><span>4</span><span>] and carbon monoxide [1</span><span>5</span><span>]. </span></p><p><strong>Acknowledgments</strong></p><p>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) and funding by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). 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 (Open University). US investigators were supported by the National Aeronautics and Space Administration.</p><p><strong>References</strong></p><p>[1] Vandaele et al., Space Science Reviews 214, 5, 2018</p><p>[2] Korablev et al., Space. Sci. Rev. 214, 7 (2018).</p><p>[3] Lopez-Valverde et al., Space Sci Rev, 214, 29 (2018)</p><p>[4] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411 (2009).</p><p>[5] Stiller et al., JQSRT, 72, 249–280 (2002)</p><p>[6] von Clarmann et al., J. Geophys. Res. 108, 4746 (2003)</p><p>[7] Jimenez-Monferrer et al., Icarus, 353, 113830 (2020), doi.org/10.1016/j.icarus.2020.113830.</p><p>[8] Lopez-Valverde et al., JGR-Planets (submitted, 2022)</p><p>[9] Stolzenbach et al., JGR-Planets (submitted, 2022) and Stolzenbach et al., EPSC 2022 (this conference)</p><p>[10] Trompet et al., JGR-Planets (submitted, 2022)</p><p>[11] Belyaev et al., GRL</p><p>[12] Belyaev et al., JGR-Planets (submitted, 2022)</p><p>[13] Gonzalez-Galindo et al., EPSC 2022 (this conference)</p><p>[14] Brines et al., JGR-Planets (submitted, 2022) and Brines et al., EPSC 2022 (this conference)</p><p>[15] Modak et al., JGR-Planets (submitted, 2022) and Modak et al., EPSC 2022 (this conference)</p>

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