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The ACS-NIR spectrometer on board the Trace Gas Orbiter (TGO) is currently used to probe the atmosphere of Mars. It is, however, capable of measuring the near-infrared solar spectrum in the 0.7-1.7 µm domain with high spectral resolution when pointed at the Sun and its line of sight is above the atmosphere of Mars i.e. with its Solar Occultation mode. Specific observations were therefore made during 10 months in order to construct the solar spectrum in this spectral domain. The observations consist in recording all the diffraction orders of ACS-NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). We will first present how we have treated each order of diffraction to improve the solar spectrum on the 0.7-1.7 µm band by considering for this purpose off-center images attached to certain AOTF frequencies. This method makes it possible to avoid contamination between the successive diffraction orders but also to increase the detection of the solar lines at the ends of each order where the intensity is low due to the Blaze function. We will then show the final version of the solar spectrum that we obtain. It will be compared to the reference spectrum, which is that of Toon. We will finish by showing and discuss some results revealing new solar lines appearing in certain diffraction orders of the ACS-NIR spectrometer and which are not present in the corresponding parts of the reference solar spectrum.

The Atmospheric Chemistry Suite (ACS) on the ExoMars Trace Gas Orbiter (TGO) took its first science observation in April 2018, right before the onset of the Mars Year (MY) 34 global dust storm. One of the main objectives of the TGO mission is to search for as-yet undetected trace gases that can tell us about contemporary volcanism on Mars, or its present and past habitability. In the data collected those first months, heavily impacted by dust activity, the first novel trace gas was discovered: hydrogen chloride. MY 37 has just begun and we have recently finished observing our third full dusty season on Mars with TGO and ACS (around perihelion, spring and summer in the southern hemisphere). HCl in the atmosphere of Mars is a seasonal phenomenon, having appeared coincidentally with the start of dust activity in each MY. HCl was thought to be an indication of contemporary volcanism, but its widespread distribution across both hemispheres and recurring seasonality are suggestive of a photochemical source. Here, we present the climatology of HCl after three Martian perihelion periods, as well as a comparison with other parameters measured with ACS, such as water, temperature, and aerosols. From coincident measurements made with the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO), we can also compare the climatology of HCl with those of dust and water ice. HCl is strongly correlated to water vapour, which is itself correlated to atmospheric temperatures. While HCl only appears in the presence of suspended dust aerosols, the measured abundances of these two quantities are poorly correlated. The disappearance of HCl towards the autumnal equinox may be related to changes in temperature. The cooling atmosphere removes water vapour from the gas phase, necessary for formation of HCl, and promotes ice formation, which HCl may adhere to. We will show the evolution of HCl abundance over three Martian years in both hemispheres, and show how they fit into the seasonality of Martian dust, the water cycle, and ice formation, and discuss the possible mechanisms of its formation and destruction.

The atmosphere of Mars is enriched in heavy isotopes with respect to Earth as a result of the escape of the atmosphere to space over billions of years. Estimating the enrichment in heavy isotopes due to atmospheric escape requires a rigorous understanding of all atmospheric processes that contribute to the evolution of isotopic ratios between the lower and upper atmosphere, where escape processes take place. Using a combination of vertical profiles from the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) with the predictions of a photochemical model, we provide evidence for a new process of photochemistry-induced fractionation that depletes the heavy isotopes of C and O in CO. Accounting for this new source of fractionation, we find that only a fraction of ≈12% of the atmosphere needs to have been lost to space through CO photochemical escape to explain the enrichment of 13C/12C in CO2 measured by the Curiosity Rover.

We develop a 1-D atmospheric photochemistry model for Mars to interpret hydrogen chloride (HCl) profile measurements collected by the ACS MIR spectrometer aboard the ExoMars Trace Gas Orbiter (TGO) in Mars Year (MY) 34. We include a gas-phase chlorine chemistry scheme and study 1) surface chemistry, 2) hydrolysis, 3) photolysis, and 4) hydration and photolysis of dust grains as possible sources of gas-phase chlorine chemistry. Heterogeneous uptake of chlorine species onto water ice and minerals in Martian dust are loss processes common to all mechanisms. We drive the 1-D model using TGO profile measurements of aerosols and water vapour. We find that mechanism four can reproduce observed HCl profile tendencies during MY34. It reproduces the HCl cut-off at high southern latitudes (

Carbon monoxide is a non-condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long-term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls = 163° of MY34 to the end of MY35. We obtained a mean CO mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls = 100–200° in high southern latitudes with a layer of 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid-latitudes southern summer atmosphere amounts to 700–750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.

1. Introduction Thermal infrared datasets from planetary bodies have the potential to provide key insights for the understanding of a planet's climate evolution and history: valuable information about composition, temperature and state of the atmosphere can be retrieved from thermal spectral data.In this work, we present the application of a methodology based on Principal Component Analysis (PCA) and Target Transformation (TT) Factor Analysis (FA) techniques to thermal infrared data of the ExoMars2016 Trace Gas Orbiter (TGO) with the main goal to retrieve the different particulate atmospheric contributions present in the spectra and separate them from the surface contributions. 2. Instrument and dataset TGO has a suite of spectroscopic instruments for the investigation of the Red Planet in the infrared spectral range known as Atmospheric Chemistry Suite (ACS) [1] consisting of three spectrometers observing Mars in solar occultation, nadir and limb geometry in the Near-InfraRed (NIR), Mid-InfraRed (MIR) and Thermal InfraRed (TIR) spectral channels. Among them, the ACS thermal-infrared channel (TIRVIM) covers the spectral range between 1.7-17 μm with apodized resolution varying from 0.2 to 1.3 cm−1 [1]. TIRVIM has similar capabilities to IRIS (Mariner 9), TES, and PFS in several aspects, but has some advantages: - higher spectral resolution; - better noise equivalent radiance (from 0.08 mW/m2/sr-1/cm-1); - dense spatial coverage (TGO has a circular orbit of 400 km) [1].Martian TIRVIM spectra acquired along the orbits that cross Elysium Planitia, landing site of the NASA’s Insight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) lander, were here analyzed. The selected data orbits span from Ls=150°C to Ls=210°C, covering part of the Martian summer and of the Martian autumn going through the autumn equinox in the northern hemisphere of the planet. Corresponding Local Times cover Martian days and nights. 3. Methodology To analyze the ExoMars TIRVIM data and retrieve and characterize the number of varying atmospheric components present in these data and their spectral shape, a methodology was applied based on a combination of PCA and TT – FA techniques [2, 3]. These techniques are demonstrated to be able to extract the composition of laboratory samples [4, 5]; to extract the principal varying components from a big spectral dataset in the thermal infrared [3], and then to identify them as components of the atmosphere and separate their contribution from surface emission [6]. This methodology, previously successfully adopted for the analysis of TES and PFS data [2, 7], was, here, applied for the first time to the analysis of the new and at higher spectral resolution TIRVIM dataset. Considering that the PCA works best when the different components have significant variation in the data, we selected data where these variations are more likely to occur: the analyzed data cover diurnal as well as seasonal variations. A list of PFS and TES atmospheric end-members extracted from [3] and [2] were used as initial guess for the TT, to interpret the abstract eigenvectors retrieved from the PCA and attribute them a physically coherent meaning. 4. Results and future developments Spectral shapes of atmospheric dust and water ice aerosols were recovered from the analyzed TIRVIM data. Comparisons with results previously obtained on PFS and TES data (Figure 1) validate the methodology used and show that it is capable of determining the number of independently variable components and recovering the spectral endmembers present in the TIRVIM data. Figure 1. Comparison between the general TES/PFS extracted endmembers [2, 3] (upper panel) and the endmembers extracted from the TIRVIM data in this work (bottom panel). Grey area: wavelength cut due to high instrumental noise. Deconvolution of the original spectra using the recovered spectral end-members allowed us to obtain the concentration coefficients of the dust and water ice end-members and retrieved abundances maps. The results obtained are promising in terms of identification and separation of the atmospheric components and have strong impact on the identification of surface contributions in the spectra. We are further investigating this point with the main goal of showing the potential of this method to extract valuable information from TIRVIM data not only on the atmosphere, but also on the surface of the Red Planet. Derived surface emissivity products are being analyzed by linear deconvolution analysis with the laboratory spectral database of Martian analogues measured at the Planetary Spectroscopy Laboratory (PSL) of DLR, in Berlin [5]. Acknowledgments ExoMars is a space mission of ESA and Roscosmos. The ACS experiment is led by IKI, the Space Research Institute in Moscow, assisted by LATMOS in France. The science operations of ACS are funded by Roscosmos and ESA. References [1] Korablev et al. (2018) Space Science Review, 214:7. [2] D’Amore M. et al. (2013) Icarus, 226, 1294-1303. [3] Bandfield J.L., Christensen P.R., Smith M.D. (2000) JGR, 105 (E4), 9573-9587. [4] Smith, M.O., Adams, J.B., Johnson, P.E. (1985) JGR, 90, 797-804. [5] Alemanno G., Maturilli A., D’Amore M., Helbert J. (2021) Icarus, 368. [6] Smith M. D., Bandfield J. L. and Christensen P.R. (2000) JGR, 85 (E4), 9589-9607. [7] Maturilli, A., Helbert, J., D’Amore, M. (2009) EPSC2009, Abstract #EPSC2009-106.

The D/H ratio measured in the Martian atmosphere gives an estimation of the planet’s water escape rate. We present here a global circulation model including the water isotope HDO and the main physical processes of fractionation. Modeling the D/H cycle accompanies the recent measurements from the ACS spectrometer onboard the orbiter TGO. The comparison between the model outputs and the TGO/ACS observations reveal some discrepancies, in particular during the second half of the martian year, when the escape is supposed to be the most efficient. The present work aims at studying these main differences. Introduction The high value of the Martian D/H ratio, derived from the HDO/H2O abundance ratio, in comparison to that of the Earth’s oceans[1],[2],[3],[4], is an indicator of the large escape of water from Mars over time. Apart from the mass difference between both isotopes, the differential escape of H and D comes from the preferential photolysis of H2O over HDO[5] and the preferential condensation of HDO over H2O [6],[7],[8]. The NOMAD and ACS instruments onboard ExoMars Trace Gas Orbiter (TGO) have recently provided unprecedented observations of vertical variations in HDO abundance and D/H ratio in the martian atmosphere [9],[10],[11],[12], motivating model development [13],[14]. In particular, TGO data cover the second half (“dusty season”) of martian year (MY) 34, which includes a Global Dust Storm (GDS) (Ls 180°-230°) and a regional dust storm (Ls 315°-330°). These dust events have been proven to be of particular importance in the hydrogen escape inventory [12],[15],[16],[17],[18]. HDO modeling The D/H ratio measured in the Martian atmosphere gives an estimation of the planet’s water escape rate. We present here a global circulation model including the water isotope HDO and the main physical processes of fractionation. Modeling the D/H cycle accompanies the recent measurements from the ACS spectrometer onboard the orbiter TGO. The comparison between the model outputs and the TGO/ACS observations reveal some discrepancies, in particular during the second half of the martian year, when the escape is supposed to be the most efficient. The present work aims at studying these main differences. Based on Montmessin et al. 2005[19], Rossi et al. 2021[13] have introduced the HDO cycle in the Mars Planetary Climate Model (Mars PCM), formely known as the LMD Mars Global Climate Model[20]. Vals et al. 2022[21] have recently adapted these implementations to the complete representation of the water ice clouds, including the radiative effect of clouds[22] and microphysics, the latter allowing the modeling of supersaturation[23]. The effect of kinetics in the fractionation by condensation has also been included, as has the photodissociation of HDO and the photochemical reactions of the deuterated species [24],[25],[26]. Rossi et al. 2022[27] have used the improved model to compare with TGO/ACS/MIR retrievals of D/H profiles[28] and revealed some discrepancies, which account for the challenge of reproducing the D/H cycle, depending on key components of the Martian atmosphere such as the water and dust cycles. Preliminary results By comparing the simulations and the observations, we particularly notice an overestimation by the model of water vapor above ∼60 km occuring near the GDS in MY34, and an underestimation near perihelion in the southern hemisphere (see Figure 1). The difference in water vapor vertical distribution has a direct impact on the D/H profiles (see Figures 2 and 3). In this work, we want to analyse these differences to eventually propose some model improvements. Further comparisons and results for MY34 and 35 will be shown at the conference. Figure 1: Water vapor volume mixing ratio in ppmv as measured by TGO/ACS/NIR[16] (black) and as computed by the model (red) at different altitude ranges. Figure 2: Water vapor volume mixing ratio, temperature and saturation as measured by TGO/ACS/NIR[16], and D/H profiles as measured by TGO/ACS/MIR[28] (black) and as computed by the model (red) during MY34 between solar longitudes 235°, 245° and latitudes -25°N, 25°N. Figure 3: Same layout as Figure 2 between solar longitudes 270°, 300° and latitudes -60°N, 45° N. References [1] Owen et al.: Science, 240 (4860), 1767-1770, 1988 [2] Encrenaz et al.: A&A, 612 , A112, 2018. [3] Krasnopolsky: Icarus, 257 , 377 - 386, 2015 [4] Villanueva et al.: Science, 348 (6231), 218–221, 2015. [5] Cheng et al.: Geophysical Research Letters, 26 (24), 3657-3660, 1999 [6] Krasnopolsky: Icarus, (2), 597-602, 2000. [7] Fouchet et al.: Icarus, 144 , 114-123, 2000. [8] Bertaux et al.: Journal of Geophysical Research: Planets, 106 (E12), 32879-32884, 2001. [9] Vandaele et al.: Space Science Reviews, 214 (5), 80, 2018. [10] Vandaele et al.: Nature, 568 (7753), 521-525, 2019. [11] Korablev et al.: Space Science Reviews, 214 , 7, 2018. [12] Korablev et al.: Nature, 568 (7753),517-520, 2019. [13] Rossi et al.: grl , 48 (7), e90962, 2021. [14] Daerden et al.: Journal of Geophysical Research (Planets), 127 (2), e07079, 2022. [15] Aoki et al.: Journal of Geophysical Research (Planets), 124 (12), 3482-3497, 2019. [16] Fedorova et al.: Science, 367 (6475), 297-300, 2020. [17] Neary et al.: Geophysical Research Letters, 47 (7), e84354, 2020. [18] Chaffin et al.: Nature Astronomy, 5 , 1036-1042, 2021. [19] Montmessin et al.: Journal of Geophysical Research, 2005. [20] Forget et al.: JGR, 104:24,155–24,176, 1999. [21] Vals et al.: Journal of Geophysical Research (Planets), under revision, 2022. [22] Madeleine et al. : Geophys. Res. Lett., 39 , 23202, 2012. [23] Navarro et al.: Journal of Geophysical Research (Planets), 2014. [24] Lefèvre et al.: Journal of Geophysical Research (Planets), 126 (4), 2021. [25] Cheng et al.: Journal of Chemical Physics, 120 (1), 224-229, 2004. [26] Chung et al.: Nuclear Instruments and Methods in Physics Research A, 467 (2002), 1572-1576, 2001. [27] Rossi et al.: Journal of Geophysical Research (Planets), under revision, 2022. [28] Alday et al.: Nature Astronomy, 5, 943-950, 2021.

Summary The mid-infrared channel of the Atmospheric Chemistry Suite (ACS) [1] onboard ESA’s Trace Gas Orbiter (TGO) has performed a sensitive search of methane in the Martian atmosphere using the solar occultation technique for more than two Mars years (four terrestrial years). The first two reports [2,3] have concluded on the global absence of methane detection above a mean level of 50 then 20 parts-per-trillion-per-volume (pptv), contrasting with the 0.4 to 45 ppbv concentrations reported from Earth-ground-observations, Mars orbiters, and from Mars’ surface by the Curiosity rover [5,6,7,8,9]. Here we present an update of the ACS methane search compiling all the ACS data collected up to summer 2022. Introduction For several decades, methane on Mars has been the subject of a continuous and intense search for it may resonate with an ongoing geophysical or biogenic activity on a planet that has long been perceived as inert. After a succession of failed attempts and positive detections between 2004 and 2019 [4,5,6,7,8,9,21,22], TGO started its trace gas detection mission in April 2018 and has continued since then. With its two separate state-of-the-art infrared spectrometers specifically conceived to achieve outstanding detection performances [1,10] for the ultra-sensitive search of a list of trace gases, including methane, TGO was aimed at confirming the presence of methane at Mars, or alternatively to establish the smallest upper limits possible. The Atmospheric Chemistry Suite (ACS) developed by IKI with the support of CNES is one of the spectrometers of TGO. Together with NOMAD, they offer a complementary access to the entire wavelength range from UV to thermal IR. They are also both observing the mid-IR range and therefore provide a redundant and thus reliable exploration of the presence of a variety of trace gases. Among the greatest achievements of these two experiments, we shall mention the joint observation of HDO and H2O [11,12], the discovery of HCl [13,14,15], the close and joint monitoring of water and ozone [16,17,18], as well as the first CO profiles [19]. Method The mid-infrared range, in particular the region around the fundamental 3.3 μm band of the C-H bond, provides the most sensitive way to detect methane and higher-order hydrocarbons (Fig. 1). With ACS, this spectral region has been explored around Mars during the last four years and has revealed an absence of methane above a limit of 20 pptv (Fig. 2), while NOMAD similarly established such limit to 60 pptv [20]. The technique of solar occultation is known to be more sensitive than nadir since the Sun is a source at Mars that is ~600 000 times brighter that Mars itself. While the ubiquitous atmospheric dust prevents observing the near-surface region, the high-resolving power (30 000) and its high Signal-to-Noise Ratio (~10 000 at low level of dust) compensate for this limitation. We attempt to detect CH4 using the R2 region which features the most intense individual lines of the band system. This region was backed up by parallel search in three other regions to have confirmation in case of a positive detection. Results With two Martian years of survey, the dataset consists of more than 1 000 individual altitude profiles. Since [3], improvements in the data processing have been implemented. This 4-year dataset will be presented along with its implications in regard to other observational studies and model attempts to explain reconcile the various observations. [1] Korablev, O., et al. 2018. Space Sci. Rev. [2] Korablev, O., et al. 2019. Nature [3] Montmessin, F., et al. 2021. Astron. & Astrophys. [4] Lellouch, E., et al. 2000. Astron. & Astrophys. [5] Formisano, V., et al., 2004. Science [6] Krasnopolsky, V. et al., 2004. Icarus [7] Mumma, M. et al., 2009. Science [8] Webster, C.R., et al., 2015, Science [9] Webster, C.R., et al., 2018, Science [10] Vandaele, A.C., et al. 2018. Space Sci. Rev. [11] Villanueva, G. et al., 2020. Sci. Adv. [12] Alday, J. et al., 2021. Nat. Ast. [13] Korablev, O., et al., 2021. Sci. Adv. [14] Aoki, S., et al., 2021. Geophys. Res. Let. [15] Olsen, K. et al., 2021. Astron. & Astrophys. [16] Fedorova et al., A., et al., 2020, Science [17] Patel, M., et al., 2021, J. of Geophys. Res. [18] Belyaev, D., et al, 2021., Geophys. Res. Let. [19] Olsen, K. et al., 2021. Nat. Geo. [20] Knutsen, E., et al., 2021, Icarus.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made., HDO and the D/H ratio are essential to understand Mars past and present climate, in particular with regard to the evolution through ages of the Martian water cycle. We present here new modeling developments of the HDO cycle with the Laboratoire de Météorologie Dynamique Mars Global Climate Model (GCM). The present study aims at exploring the behavior of the D/H ratio cycle and its sensitivity to the modeling of water ice clouds and the formulation of the fractionation by condensation. Our GCM simulations are compared with observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter (TGO), and reveal that the model quite well reproduces the temperature and water vapor fields, which offers a good basis for representing the D/H ratio cycle. The comparison also emphasizes the importance of modeling the state of supersaturation, resulting from the microphysical processes of water ice clouds, to correctly account for the water vapor and the D/H ratio of the middle-to-upper atmosphere. This work comes jointly with a detailed comparison of the measured D/H profiles by TGO/ACS and the model outputs, conducted in the companion paper of Rossi et al. (2022, https://doi.org/10.1029/2022JE007201) (this issue). © 2022. The Authors., ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. M.V. acknowledges support from the DIM ACAV labelled by the Ile-de-France region in support for the research (Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie). L.R. acknowledges support from CNES. F. G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

Introduction: Between March 2018 and December 2019, millions of spectra of Mars' surface and atmosphere thermal emission have been recorded in nadir geometry by TIRVIM/ACS, a thermal infrared spectrometer onboard the ExoMars Trace Gas Orbiter (TGO) [7]. From this wealth of data, we characterized in detail the thermal structure of the Martian lower atmosphere at a great variety of local times. Here we focus on the study of migrating thermal tides derived from TIRVIM observations, including during the Global Dust Event of MY34. Observations and Methods: We have developed an algorithm coupling line-by-line radiative transfer and optimal estimation theory to retrieve the surface temperature, the vertical profiles of the temperature between a few km and 50-55 km (2–3 Pa), and the integrated optical depth of dust and water ice clouds [3]. The retrieved temperature profiles have a vertical resolution of 10 km in the lower atmosphere and a coarser resolution of 15-20 km in the range 2–20 Pa. They were validated against thousands of co-located observations from the Mars Climate Sounder, acquired in limb viewing geometry [3]. Thermal structure and tides: Thermal tides are planetary-scale oscillations resulting from the diurnal solar forcing of the thin Martian atmosphere. Migrating tides describe sun-synchronous modes propagating westward with the sun. The diurnal mode has one maximum and one minimum per day and a zonal wavenumber of one; the semi-diurnal mode has two maxima and two minima per day and a zonal wavenumber of two; etc. One of the main advantage of TGO’s orbit is that the local time of TIRVIM nadir observations drifts by 13 minutes earlier each sol. After 54 sols, or 25-35° of Ls, a full coverage of the daily cycle is achieved, allowing to separate between diurnal and seasonal temperature variations. This is clearly an asset to study thermal tides compared to previous studies, mostly based on 2-4 AM and 2-4 PM temperatures obtained from sun-synchronous orbiters [1,4,6,9]. An example of the zonally-averaged temperature retrieved from TIRVIM at 30 Pa in the period March 13 - April 28, 2018 (Ls~150° of MY34) is shown below, as a function of latitude and local time. At the equator, the temperature is found maximum near 3~AM and minimum near 7~PM. This feature of warm nighttime temperatures at this altitude is well known and is a manifestation of the diurnal thermal tide [8]. The fact that these two temperature extrema are not separated by 12 hours is a first hint that the thermal field is also influenced by a semi-diurnal tide. Figure 1: Zonally-averaged temperature retrieved from TIRVIM/ACS at 30 Pa, with latitude and local time. This figure gathers 45 sols of data around Ls~150°, MY34. To study migrating tides qualitatively, we consider the zonally-averaged temperature in a fixed local time reference frame, in which other types of wave signatures (stationary waves, non-migrating tides,…) are averaged out. Following [6], we decompose the observed zonally-averaged temperature into a daily-averaged temperature plus sinusoidal functions at diurnal, semi-diurnal and ter-diurnal frequencies. We obtain the amplitude and phase of the migrating tides at each pressure level and 10°-wide latitudinal bin (see below an example of fits). An amplitude of 4K is found for the diurnal mode near the equator, which decreases to 2K near 20°, and increases to 6K near 50°N. Very similar results are found at Ls~90° for MY35 [2]. We employ the same methodology to derive tides characteristics simulated in the LMD GCM. Temperature profiles from the model are extracted at the same locations and times as TIRVIM observations and are smoothed vertically using the appropriate averaging kernel matrix. The modeled tides exhibit very similar characteristics as those observed, except for a 1 to 2 hours phase shift; temperature extrema occurring at later local times in the model. Figure 2: Fit (purple) to TIRVIM equatorial temperatures at 50 Pa (black stars, upper part) as a function of local time. It combines a daily average temperature (horizontal dashed line), diurnal and semi-diurnal signals, as labeled. The bottom part shows the same for GCM outputs (shifted by -15K). One caveat is that even small seasonal variations over a martian month can hamper our analysis when characterizing the semi-diurnal tide [2]. To overcome this issue, we de-trend the data for seasonal variations using MCS observations. The derived amplitude of the semi-diurnal tide near the equator at Ls=150° ranges from 2K (at 100 Pa) to 6K (at 2 Pa), confirming that this tide mode is significant even in non-dusty seasons [6]. Impact of the MY34 GDE: A Global Dust Event started locally on 2 June 2018 and became planet-encircling on 21 June [5]. Compared to pre-storm conditions, we find that the diurnal tide amplitude strongly increases, reaching 32K at 50-60°S and 20K at 50-60°N at 50 Pa, while it remains unchanged (~4K) near the equator. The semi-diurnal mode increases slightly (6-8K at 30-50 Pa, equator), while the ter-diurnal mode is also detected (amplitude of 5K near 50 Pa, equator). GCM simulations run with the MY34 dust scenario agree very well with TIRVIM observations, both in terms of thermal structure and tide characteristics. Bibliography: [1] Banfield et al., JGR Vol. 105, 2000. [2] Fan et al., GRL Vol. 49, 2022. [3] Guerlet et al., JGR Vol. 127, 2022. [4] Guzewich et al., JGR Vol. 117, 2012. [5] Kass et al., GRL Vol. 47, 2020. [6] Kleinbohl et al., GRL Vol. 40, 2013. [7] Korablev et al., SSR Vol. 214, 2018. [8] Lee et al., JGR Vol. 114, 2009. [9] Wilson et al., GRL Vol. 27, 2000.

The D/H ratio and its implications on the atmospheric escape, make it an essential observable to study the current and past inventory of water on Mars. With the arrival of the Trace Gas Orbiter around Mars, new measurements of the D/H ratio are now available and require tools to interpret the observations and understand the HDO cycle. We here present simulations of an updated version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model which includes HDO and in particular the fractionation processes it undergoes. We compare our model simulations with the HDO observations in solar occultation from the Atmospheric Chemistry Suite mid-infrared channel on board the Trace Gas Orbiter (Alday et al., 2021; https://doi.org/10.5281/ZENODO.5100448). The model successfully reproduces the general trends of the D/H ratio, indicating that the main physical processes are captured by theory. A consistent simulation of condensation processes is found to be key in the representation of the D/H ratio. Improvements in the representation of clouds and on the water cycle will help improving the representation of the HDO cycle and better help extrapolate back in times the conditions of water escape on Mars. © 2022. American Geophysical Union. All Rights Reserved., L.R. acknowledges support from CNES. F.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). M.V. acknowledges support from the DIM ACAV labelled by the Ile-de-France region in support for the research (“Domaine d’Intérêt Majeur, Astrophysique et Conditions d’Apparition de la Vie).”

The detection of sulphur species in the Martian atmosphere would be a strong indicator of volcanic outgassing from the surface of Mars. We wish to establish the presence of SO2, H2S, or OCS in the Martian atmosphere or determine upper limits on their concentration in the absence of a detection. We perform a comprehensive analysis of solar occultation data from the mid-infrared channel of the Atmospheric Chemistry Suite instrument, on board the ExoMars Trace Gas Orbiter, obtained during Martian years 34 and 35. For the most optimal sensitivity conditions, we determine 1-sigma upper limits of SO2 at 20 ppbv, H2S at 15 ppbv, and OCS at 0.4 ppbv; the last value is lower than any previous upper limits imposed on OCS in the literature. We find no evidence of any of these species above a 3-sigma confidence threshold. We therefore infer that passive volcanic outgassing of SO2 must be below 2 ktons/day., 13 pages, 12 figures, received for production in A&A on the 19th of November 2021

On Mars, condensation is the major factor constraining the vertical distribution of water vapor. Recent measurements of water and temperature profiles showed that water can be strongly supersaturated at and above the level where clouds form during the aphelion and perihelion seasons. Since 2018, the near-infrared spectrometer (NIR) of the Atmospheric Chemistry Suite onboard the Trace Gas Orbiter has measured H2O and temperature profiles using solar occultation in the infrared from below 10 to 100 km of altitude. Here, we provide the first long-term monitoring of the water saturation state. The survey spans 2 Martian years from Ls = 163° of MY34 to Ls = 170° of MY36. We found that water is often supersaturated above aerosol layers. In the aphelion season, the water mixing ratio above 40 km in the mid-to-high latitudes was below 3 ppmv and yet is found to be supersaturated. Around the perihelion, water is also supersaturated above 60 km with a mixing ratio of 30–50 ppmv. Stronger saturation is observed during the dusty season in MY35 compared to what was observed in MY34 during the Global Dust Storm and around the perihelion. Saturation varied between the evening and morning terminators in response to temperature modulation imparted by thermal tides. Although water vapor is more abundant in the evening, colder morning temperatures induce a daily peak of saturation. This data set establishes a new paradigm for water vapor on Mars, revealing that supersaturation is nearly ubiquitous, particularly during the dust season, thereby promoting water escape on an annual average.

Thermal tides in the Martian atmosphere are analyzed using temperature profiles retrieved from nadir observations obtained by the TIRVIM Fourier-spectrometer, part of the Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The data is selected near the northern summer solstice at solar longitude (LS) 75{\deg}-105{\deg} of Martian Year (MY) 35. The observations have a full local time coverage, which enables analyses of daily temperature anomalies. The observed zonal mean temperature is lower by 4-6K at ~100Pa, but higher towards the summer pole, compared to the LMD Mars General Circulation Model (GCM). Wave mode decomposition shows dominant diurnal tide and important semi-diurnal tide and diurnal Kelvin wave, with maximal amplitudes of 5K, 3K, and 2.5K, respectively, from tens to hundreds of Pa. The results generally agree well with the LMD Mars GCM, but with noticeable earlier phases of diurnal (~1h) and semi-diurnal (~3h) tides., Comment: 7 pages, 4 figures, accepted by GRL

The Trace Gas Orbiter (TGO) was launched in 2016 to probe the atmosphere of Mars. It has on board the Atmospheric Chemistry Suite (ACS), which is a set of observation instruments in the infrared domain, in particular, the NIR (Near Infrared) and the MIR (Mid-Infrared) photometers. These photometers detect the atmospheric components of Mars when they are pointed at the Sun and the Line Of Sight (LOS) crosses the atmosphere. The solar spectrum is directly measured when the LOS is above the atmosphere of Mars and serves as a spectral reference for calibration. We consider here particular observations made with the NIR photometer to construct the solar spectrum allowed by its spectral range, i.e. 0.7 to 1.7 nm. This is motivated by the need to have a solar spectrum with high spectral resolution for NIR calibrations but also useful for models (Sun, climate…) or for other remote sensing experiments (Earth or other planets). We have acquired all the diffraction orders of the NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). They were then successively imaged on the CCD and series of its useful part were recorded for each order. We will present how we process this data to extract the solar spectrum, in particular how we calculate the flat field useful for image correction. We will then present how to overcome the contamination of successive spectral orders using a geometric method. We will then show how to correct the order intensity variations to obtain the solar spectrum. We will end by discussing our results, in particular by comparing them with other existing spectra on the same band.

The Atmospheric Chemistry Suite (ACS) MIR channel onboard the ESA-Roscosmos Trace Gas Orbiter (TGO) (Korablev et al., 2018, 2019) probes the Martian atmosphere in the 2.3 – 4.2 µm spectral range using the Solar Occultation technique. ACS-MIR has now provided infrared observations of the Martian atmosphere over more than one and a half regular Martian Year since the end of the 2018/MY34 Global Dust Storm (GDS).We analyzed this ACS-MIR dataset to detect the presence of water ice particles in the Martian atmosphere and retrieve their size from the 3 μm atmospheric absorption signature. Each observation results in a vertical profile of ice particle size within the cloud layer, with a vertical resolution of a few kilometers. The temporal and spatial sampling provided by the 2-hour period of TGO’s orbit allows us to observe the seasonal and latitudinal trends of the water ice clouds, with variations of about 20 to 40 km of the cloud’s altitude.The method was first applied solely to the 2018/MY34 GDS year (Stcherbinine et al., 2020). This first study notably revealed the presence of small-grained clouds at very high altitudes (above 100 km) at the onset of the MY34 GDS, along with the presence of large water ice particles (reff > 1.5 µm) up to 65 km during the storm.Data acquired during MY35, where no GDS occurred, provides a reference to be compared with the observations obtained during the MY34 GDS. We observe that the maximum altitude of the water ice clouds increases by about 10 km during the GDS compared to a nominal year, which suggests that the GDS significantly impacts water ice cloud distribution.

Measuring vertical variations in the deuterium to hydrogen ratio with altitude is essential in understanding the processes that lead to the escape of water vapour from the Martian atmosphere. We retrieve vertical profiles of HDO and H2O from the ACS instrument, monitoring seasonal changes particularly above the water condensation level. We discuss these results in relation to previously observed seasonal variations in D/H together with the expected variations from theoretical models. Introduction The ratio of deuterium to hydrogen (D/H) is a sensitive tracer of the rate of escape of water from the Martian atmosphere over its history. Hydrogen preferentially escapes from the atmosphere over its heavier isotope (e.g. [1]), and so the greater the amount of historical escape of water vapour, the larger the average D/H ratio. On Mars, this value is measured to be around 4-6 times that of terrestrial distilled ocean water [2,3], showing that the early atmosphere of Mars contained significantly more water than it does today. In addition, a number of processes in the lower and middle atmosphere can cause relative changes in the concentrations of semi-heavy water (HDO) with respect to water vapour, notably due to differences in rates of cloud deposition (e.g. [4,5]) and photolysis (e.g. [6,7]). We therefore wish to look at spatial and temporal changes in the vertical profile of D/H, particularly above the level of water condensation, in order to better characterise the processes that influence the escape of water vapour from the lower atmosphere into space. Method The mid-infrared channel of the Atmospheric Chemistry Suite Instrument (ACS MIR, [8]) on board the ExoMars Trace Gas Orbiter obtains transmission spectra of the Martian atmosphere in solar occultation geometry, which is sensitive to trace gases at very low abundance and at high vertical resolution. We make use of observations in grating position 11, which is sensitive to a wavenumber regime in which a large number of resolvable HDO lines are present that in the best cases provide sensitivity to HDO abundance up to around 70 km. These are then inverted using the RISOTTO radiative transfer and retrieval pipeline [9,10] to give vertical profiles of HDO volume mixing ratio. Concurrent vertical profiles of H2O are obtained using the near-infrared (NIR) channel of the same instrument [11], and then the D/H ratio computed assuming that H2O and HDO are the main carriers of the two isotopes of hydrogen. Accurate quality control of the data is performed using probability-sparse Non-negative Matrix Factorisation (psNMF [12,13]). Results and Perspective We report seasonal changes in the fractionation of D/H in the middle atmosphere throughout the temporal range of the data starting from the autumn equinox in MY34 to the end of MY 35. These will be discussed in relation to the findings of seasonal changes in the vertical profiles of D/H reported by the NOMAD instrument [14], and the results interpreted in relation to predictions from Global Climate Models (GCMs) of HDO ([5,15,16]). Fig. 1: Retrieved vertical profiles of (left) water vapour from (right) D/H from the near- and mid-infrared channels respectively of the ACS instrument. References [1] Krasnopolsky, V. A., Mumma, M. J., & Randall Gladstone, G. 1998, Science, 280, 1576 [2] Owen, T., Maillard, J. P., de Bergh, C., et al. 1988, Science, 240, 1767-1770 [3] Webster, C. R.; Mahaffy, P. R.; Flesch, G. J., et al. 2013, Science, 341, 260-263 [4] Fouchet, T. & Lellouch, E. 2000, Icarus, 144, 114-123 [5] Montmessin, F., Fouchet, T. & Forget, F. 2005, J. Geophys. Res. Plan., 110 [6] Yung, Y. L., Wen, J.-S., Pinto, J. P., et al. 1988, Icarus 76, 146-159 [7] Alday, J., Trokhimovskiy, A., Irwin, P. G. J., et al. 2021, Nat. As. 5, 943-950 [8] Korablev, O., Montmessin, F., Trokhimovskiy, A., et al. 2018, Space Sci. Rev.,214,7 [9] Braude, A. S., Ferron, S., & Montmessin, F. 2021, J. Quant. Spec. Rad. Transf.,274,107848 [10] Braude, A. S., Montmessin, F., Olsen, K. S., et al. 2022, A&A,658,A86 [11] Fedorova, A. A., Montmessin, F., Korablev, O., et al. 2020, Science 367, 297-300 [12] Hinrich, J.L. & Mørup, M. 2018, Latent Variable Analysis and Signal Separation. LVA/ICA 2018. Lecture Notes in Computer Science(), 10891, 488–498 [13] Schmidt, F.; Mermy, G. C.; Erwin, J., et al. 2021, JQSRT, 259, 107361 [14] Villanueva, G. L.; Liuzzi, G.; Crismani, M. M. J., et al. 2021, Sci. Adv. 7, eabc8843 [15] Rossi, L., Vals, M., Montmessin, F., et al., J. Geophys. Res. Plan., in review. [16] Vals, M., Rossi, L., Montmessin, F., et al. 2022, J. Geophys. Res. Plan., in press.

The middle infrared (MIR) channel of the Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) ESA-Roscosmos mission has performed Solar occultation measurements of the Martian atmosphere in the 2.3-4.2 $\mu$m spectral range since March 2018, which now covers two Martian Years (MY). We use the methodology previously developed for the study of the MY 34 Global Dust Storm (GDS) (Stcherbinine et al., 2020) to monitor the properties (effective radii, extinction, altitude) of the Martian water ice clouds over the first two Martian years covered by ACS-MIR. The observations encompass the period $L_s$ = 163{\deg} in MY 34 to $L_s$ = 181{\deg} in MY 36. We determine that the typical altitude of the clouds varies by 20 to 40 km between the summer and winter, with a maximum extension up to 80 km during summer in the midlatitudes. Similarly, we also note that for a limited temporal range, the altitude of the clouds also varies by 20 to 40 km between the polar regions and the midlatitudes. We also compare observations acquired during the MY 34 GDS to observations from the same period in MY 35, using that latter as a reference to characterize the effects of this GDS on the clouds' properties. In addition, we compare our retrievals with the predictions of the Mars Planetary Climate Model (PCM), which shows a reasonable agreement overall for the altitude of the clouds, although the model usually predicts lower altitudes for the top of the clouds.

The geological record provides convincing evidence for very recent volcanic activity on Mars (e.g. Horvath et al. 2021, Icarus 365, 114499). In this talk we present the results of Braude et al. (2022, A&A 658, A86), in which a comprehensive analysis was performed of solar occultation observations from the ACS MIR instrument from MY 34 to 35, in search of three tracers of residual volcanic outgassing from the surface of Mars: SO2, H2­S and OCS. We found no statistically significant detections of either of these three molecules, instead imposing upper limits of 20 ppbv of SO2, 15 ppbv of H2S, and 0.4 ppbv of OCS. We thereby estimated that volcanic outgassing from the surface of Mars must be below 2 ktons day­-1. Introduction Several pieces of geological evidence indicate the presence of residual volcanic activity in the Tharsis and Elysium regions dating back only a few million [1] or even hundreds of thousands [2] of years, with the InSight Lander recently confirming present-day seismic activity in the Cerberus Fossae region [3]. On Earth, volcanic emission is dominated by carbon dioxide, water vapour and sulphur species, notably sulphur dioxide (SO­2) but with smaller amounts of hydrogen sulphide (H2S) and the occasional detection of carbonyl sulphide (OCS). Previous ground-based observations of sulphur species on Mars, however, have not resulted in any confirmed detections, instead imposing lowest disc-integrated upper limits of 0.3 ppbv of SO2 [4,5], 1.5 ppbv of H2S [6] and 1.1 ppbv of OCS [7]. In this talk we summarise the results of [8], in which we used solar occultation measurements from the mid-infrared channel of the Atmospheric Chemistry Suite (ACS MIR, [9]) instrument from the start of the ACS science phase in MY 34 to the end of MY 35, in search of SO2, H2S and OCS in Mars’ lower atmosphere. Fig. 1: Retrieved vertical profiles of SO2 upper limit values from each of the 190 processed solar occultations between Ls = 165 - 280° of MY 34 and Ls = 140 - 350° of MY 35. Yellower colours indicate an increased density of retrieved values. (Reproduced with permission from Fig. 4 of [8]) Method We made use of three sets of ACS MIR data each obtained using different diffraction grating positions that are sensitive to different wavenumber ranges. 190 individual occultations carried out using grating position 9 were analysed, covering a time period of Ls = 165 - 280° of MY 34 and Ls = 140 - 350° of MY 35. This grating position covers the strongest SO2 absorption features in the wavenumber range covered by ACS MIR. Using the RISOTTO radiative transfer and retrieval code [10], we performed retrievals of this data within a wavenumber range of 2481 and 2492 cm-1, which is sensitive to a series of overlapping SO2 and CO2 absorption lines, and imposed upper limits by deriving the volume mixing ratio (VMR) of SO2 that would result in a change in goodness of fit (χ2/n) to the observed spectra above a given threshold. Equivalently, we searched for H2S in two spectral windows between 3827 and 3833 cm-1 and around a single absorption line at 3839.2 cm-1 covered by grating position 5, of which we analysed 86 occultations between Ls = 164 - 218° and Ls = 315 - 354° of MY 34. Finally, we searched for OCS in two spectral windows between 2908 - 2912 and 2923 - 2927 cm-1, corresponding to the P and R branches respectively of an OCS absorption band in a wavenumber region that is mostly devoid of other gas absorption lines. This region is covered by grating position 11, for which there is almost complete coverage between Ls = 163° of MY 34 and Ls = 355° of MY 35. Upper limits for these two molecules were imposed by averaging retrieved vertical VMR profiles from adjacent rows on the MIR detector. Results and Perspective We found no significant detections (>3σ) of any of these three molecules, instead deriving our best upper limits of 20 ppbv of SO2, 15 ppbv of H2­S and 0.4 ppbv of OCS. The latter value is the lowest reported upper limit of OCS in the Martian atmosphere to date, as opposed to the previous value of 1.1 ppbv reported by [7]. However, SO2 is expected to dominate in concentration relative to H2S and OCS in volcanic emission. We estimate that during the period of observation, no more than 750 ktons of SO2 could be present in the entire Martian atmosphere, implying surface outgassing of SO2 below 2 ktons/day. Future targeted measurements of sulphur species in regions associated with historic volcanism (e.g. the Tharsis region, Elysium Planitia) could shed greater light on the probability of residual present-day volcanism. References [1] Neukum, G., Jaumann, R., Hoffmann, H., et al. 2004, Nature, 432, 971 [2] Horvath, D. G., Moitra, P., Hamilton, C.W., Craddock, R. A, & Andrews-Hanna,J. C. 2021, Icarus, 365, 114499 [3] Giardini, D., Lognonné, P., Banerdt, W. B., et al. 2020, Nat. Geosci.,13,205 [4] Encrenaz, T., Greathouse, T. K., Richter, M. J., et al. 2011, A&A,530,A37 [5] Krasnopolsky, V. A. 2012, Icarus,217,144 [6] Khayat, A. S., Villanueva, G. L., Mumma, M. J., & Tokunaga, A. T. 2015, Icarus,253,130 [7] Khayat, A., Villanueva, G. L., Mumma, M. J., & Tokunaga, A. T. 2017, Icarus,296,1 [8] Braude, A. S., Montmessin, F., Olsen, K. S., et al. 2022, A&A,658,A86 [9] Korablev, O., Montmessin, F., Trokhimovskiy, A., et al. 2018, Space Sci. Rev.,214,7 [10] Braude, A. S., Ferron, S., & Montmessin, F. 2021, J. Quant. Spec. Rad. Transf.,274,107848

Introduction Vertical profiles of CO2 and temperature with good vertical resolution are key measurements to characterize the Martian atmosphere, although difficult to obtain from remote observations [1]. For the first time these vertical profiles can be routinely obtained with a solar occultation technique by the instruments NOMAD and ACS on board the Exomars Trace Gas Orbiter [2,3]. A state-of-the-art retrieval scheme designed to derive atmospheric profiles of CO2 and temperature from the bottom to the top of the Martian atmosphere [4] is adapted to solar occulation and applied to exploit the operational sounding of these two instruments. The final goal of this on-going work is to characterize the Martian thermal structure from the troposphere up to the thermosphere with unprecedented vertical resolution and also to cross-validate both TGO instruments as best as possible, with a single retrieval code and entirely consistent data analysis approaches. Retrieval approach This work is focussed on the solar occultation channels NOMAD-SO and ACS-MIR, in routine operations since April 2018. To exploit these unique datasets, it is of paramount importance to examine the performance of the two instruments and to cross-validate their retrieval results as accurately as possible. For this purpose we apply a flexible and well tested Earth atmosphere retrieval scheme [5,6,7], to both of them, after adaptation to Mars atmospheric conditions [4] and the necessary accomodation of these channels characteristics [8]. The retrievals use calibrated transmittance spectra to tackle three targets, CO2 density, temperature, and dust loading, in a simultaneous global-fit inversion, with updated hydrostatic equilibrium in every iteration, including contaminant species like H2O, and after a pre-processing/data cleaning analysis which is also similar in both instruments. A first error analysis is performed for both instruments with the help of synthetic retrievals and a series of sensitivity tests performed with the same inversion scheme and similar treatment of the key error terms (measurement noise and systematics). Comparison of results We will present data obtained in the 2.7 µm region, dominated by a well known ro-vibrational band of CO2, and sampled by NOMAD-SO in a mixture of diffraction orders that are used routinely in the operational sounding in the vertical. Similarly, we used 3 consecutive orders in one of the ACS-MIR difraction positions, which contain a sufficient number of CO2 lines in the same 2.7 µm band with the capability to sample the whole atmosphere, up to about 180 km, in a single vertical scan. For both instruments the sounding of the lowest troposphere is limited by the amount of atmospheric dust, which is also retrieved simultaneously with CO2 and temperature. We will compare the vertical profiles obtained in a small sample of profiles from each instrument which span different seasons, latitudes and atmopheric dust loadings, during the first year of TGO operations. The comparisons take into account that the two instruments' individual solar occulation scans are non-coincident in time and space. Comparions are also peformed with results from similar efforts by other groups in the NOMAD and ACS teams [9]. Two important applications of the obtained retrievals are : (i) to supply the most appropriate inputs to the retrieval of other atmospheric species from the same instruments and the same scans, without the need to assume a prior or first guesses from global circulation models (GCM), see companion contributions to this conference [10,11], and (ii) to validate predictions from these GCMs, and therefore, to help to improve them, particularly at high altitudes and at the terminator, where these datasets are particularly valuable [1]. Acknowledgements 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. Thanks are extensive to all members of the NOMAD Science Team and the ACS Science Team. References [1] Lopez-Valverde et al., Space Sci Rev, 214, 29 (2018) [2] Vandaele et al., Space Science Reviews 214, 5, 2018 [3] Korablev et al., Space. Sci. Rev. 214, 7 (2018). [5] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411 (2009). [4] Jimenez-Monferrer et al., Icarus, 353, 113830 (2020), doi.org/10.1016/j.icarus.2020.113830. [6] Stiller et al., JQSRT, 72, 249–280 (2002) [7] von Clarmann et al., J. Geophys. Res. 108, 4746 (2003) [8] Lopez-Valverde et al., EPSC Abstracts, Vol. 14, EPSC2020-924 (2020), doi.org/10.5194/epsc2020-924 [9] Trompet et al., "Update on CO2 and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO", contribution to this conference, EPSC 2021. [10] Brines et al., "Martian water vapor vertical profiles with data from solar occultation measurements by NOMAD onboard TGO/ExoMars", contribution to this conference, EPSC 2021. [11] Modak et al., "Retrieval of Martian CO vertical profiles from NOMAD solar occultation measurements", contribution to this conference, EPSC 2021.

Thermal tides in Martian atmosphere from ACS/TIRVIM Siteng Fan1 (sfan@lmd.ipsl.fr), Sandrine Guerlet1, Francois Forget1, Antoine Bierjon1, Ehouarn Millour1, Nikolay Ignatiev2, Alexey Shakun2, Alexey Grigoriev2, Alexander Trokhimovskiy2, Franck Montmessin3, Oleg Korablev2 1: LMD/IPSL, Sorbonne Université, PSL Research University, École Normale Supérieure, École Polytechnique, CNRS, Paris, France 2: Space Research Institute (IKI), Moscow, Russia 3: LATMOS/IPSL, Guyancourt, France Abstract Atmospheric thermal tides are planetary-scale temperature oscillations driven by diurnal solar forcing. On Mars, they dominate the temperature variations and winds throughout the thin atmosphere due to its low heat capacity. Given the short timescales of tide modes, which are typically equal or shorter than a Martian day, analyses of these tides require observations with large spatial coverage and high local time resolution. Observations from the TIRVIM Fourier-spectrometer, part of the Atmospheric Chemistry Suite (ACS) onboard ExoMars Trace Gas Orbiter (TGO), provide data meeting such requirement. Here we report the spatial- and temporal-dependent temperature profiles of Martian atmosphere retrieved from TIRVIM nadir data. Amplitudes and phases of thermal tide modes are obtained, and their influence on the thermal structure of Martian atmosphere is estimated using numerical simulations, The LMD Mars Global Circulation Model (GCM). Introduction Diurnal thermal forcing by solar radiation excites temperature oscillations in Martian atmosphere through direct sunlight absorption and heat exchange with Mars surface (Gierasch & Goody 1968). Influenced by surface topography, the varying solar forcing produces harmonic responses in the Martian atmosphere, thermal tides (Zurek 1976). Some modes of the tides can vertically propagate to upper altitudes with amplitudes growing exponentially with decreasing air density, which therefore influences the atmospheric circulation. The existence of such temperature and the related pressure variations has been observed by many Mars landers and orbiters (e.g., Hess et al., 1977, Banfield et al. 2000, Lee et al. 2009, Kleinböhl et al. 2013, Forbes et al. 2020), but observations are still lacking for comprehensive studies as large spatial coverage and high temporal resolution are required to study such planetary-scale diurnal/sub-diurnal variations. Methodology and Results We analyze the thermal tides using temperature structures retrieved from TIRVIM nadir data. TIRVIM is a thermal infrared Fourier-spectrometer, as part of the ACS instrument onboard the ESA-Roscosmos mission ExoMars TGO (Korablev et al., 2018, Svedhem et al. 2020). TGO is on a non-solar-synchronous orbit with a period of 2 hours, so observations obtained at nadir viewing geometry could cover a wide range of local time due to the short orbital period and orbit drifting (Capderou & Forget, 2004, Figure 1). A full diurnal cycle can be sampled between 74°N and 74°S within ~55 solar days (sols), which is equivalent to ~30° in solar longitude (Ls). Binning data within this range results in data covering most local time in a specific Martian month with little seasonal variations. TIRVIM operated from April 2018 to December 2019, nearly one Martian year (MY) from MY34 Ls134° to MY35 Ls 115°, and provides ~1.5 million thermal infrared nadir-viewing spectra with decent data quality. Despite some degeneracy, we retrieved temperature profiles together with dust and water ice from these spectra (Guerlet et al. 2018). Each profile covers a vertical range from ~2-3km above Mars surface to ~50-55km (~1-2Pa) with a vertical resolution of ~1 scale height (10km). Full coverage of local time is obtained for the first time on sub-seasonal scales (Figure 1), which makes it possible to evaluate thermal tides in Martian atmosphere with details and resolve the harmonic aliasing introduced by solar-synchronous observations, e.g., observations at 3am and 3pm local time by the Mars Climate Sounder (MCS, Lee et al. 2009). A preliminary result of the thermal structure on the Martian southern hemisphere near MY35 spring equinox (Ls=90°) is shown in Figure 2a. We binned the data between Ls 75° to 105° so that most of the local time is covered. No dust storms are identified during this period. The selected bin size of latitude is 3° and that of the local time is 1 hour. Temperature profiles are also zonal averages, so only migrating tides are shown. The diurnal temperature difference structure shows diurnal and semi-diurnal cycles at lower and middle atmosphere, respectively, with “apparent” amplitudes of ~7K. Due to the degeneracy introduced by vertically distributed sensitivity response functions of TIRVIM thermal infrared spectra, the retrieved temperature profiles are vertically smoothed (Guerlet et al. 2018). Suggested by the LMD Mars GCM (Forget et al. 1999), amplitudes of these tides are 2-3 times larger (Figure 2b), as smoothing them with the same response functions gives similar diurnal temperature difference structure (Figure 2c). A priori in the temperature profile retrieval makes a difference in extracting the diurnal temperature anomaly at low pressure level due to small weights in the convolution kernels (Figure 2b and 2c), which requires further investigation with GCMs. Figure 1. Spatial-temporal coverage of ACS/TIRVIM observations. Data from 20 orbits are shown for the purpose of illustration, with colors denoting the local time. Figure 2. (a) Diurnal temperature anomaly derived using ACS/TIRVIM observations. The data is binned between MY35 Ls 75° and 105°, and between 10.5°S and 13.5°S in latitude. (b) Result of the LMD Mars GCM. The model result has the same spatial-temporal sampling weight as observations. (c) Same as (b), but vertically convolved with the averaging kernels of TIRVIM. References Banfield, D., Conrath, B., Pearl, J. C., et al. (2000). JGR, 105, 9521. Capderou, M., & Forget, F. (2004). P&SS, 52, 789. Forbes, J. M., Zhang, X., Forget, F., et al. (2020). JGR: Space Physics, 125, e28140. Forget, F., Hourdin, F., Fournier, R., et al. (1999). JGR, 104, 24155. Gierasch, P., & Goody, R. (1968). P&SS, 16, 615. Guerlet, S., Ignatiev, N., Fouchet, T., et al. (2018). EPSC, 2018-223. Hess, S. L., Henry, R. M., Leovy, C. B., et al. (1977). JGR, 82, 4559. KleinböHl, A., John Wilson, R., Kass, D., et al. (2013). GRL, 40, 1952. Korablev, O., Montmessin, F., Trokhimovskiy, A., et al. (2018). SSW, 214, 7. Lee, C., Lawson, W. G., Richardson, M. I., et al. (2009). JGR: Planets, 114, E03005. Svedhem, H., Korablev, O., Mitrofanov, I., et al. (2020). EPSC, 2020-802. Zurek, R. W. (1976). JAS, 33, 321.

HDO and the D/H ratio are important in order to understand Mars past and present climate, in particular regarding the evolution through ages of the Martian water cycle. We present simulations of the HDO cycle with the LMD Mars GCM and compare the retrieved cycle with observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter (TGO). This work is a companion study of Vals et al. presented in session TP12, which presents the evolutions of the model since Rossi et al, (2021), and explores the behaviour of the D/H ratio cycle when taking into account the microphysics and radiative effect of clouds, as well as the effect of kinetics on fractionation during condensation. Introduction Mars is known to have had a significant liquid water reservoir on the surface and the D/H ratio derived from the HDO/H2O abundance ratio is a sensitive tool to constrain the primordial abundance of the water reservoir on Mars and its evolution with time. The current ratio is at least five times that of the Vienna Standard Mean Ocean Water (SMOW) (Owen et al. 1988 , Encrenaz et al. 2018, Krasnopolsky 2015, Villanueva et al. 2015). H and D atoms in the upper atmosphere come from H2O and HDO, their sole precursor in the lower atmosphere. The lower mass of H over D atoms and the fact that H2O is preferentially photolysed over HDO (Cheng et al. 1999) explain the differential escape of these two elements. Also, the heavier isotope, HDO, has a lower vapor pressure than H2O, which results in an enrichment of the deuterated isotope in the solid phase of water. This effect is known as the Vapor Pressure Isotope Effect (VPIE) and can reduce the D/H ratio above the condensation level to values as low as 10% of the D/H ratio near the surface (Bertaux et al. 2001, Fouchet et al. 2000). Fractionation should affect the amount of HDO depending on latitude, longitude, altitude and season (Montmessin et al. 2005, Rossi et al. 2021). Modeling HDO We present here the results from our (re)integration of the HDO cycle into the LMD Mars GCM, taking into account HDO in its vapour and ice phases in the atmosphere, and as surface ice. In Rossi et al. (2021), we presented first results with a dust annual scenario mimicking the dust seasonal and spatial evolution observed during Martian Year 34 (MY34) and including the occurrence of the Global Dust Storm (GDS). The simulations illustrated the effect of the dust on the temperature field, and therefore on the circulation and the cloud formation. Since clouds are forming at higher altitude (if at all), the deuterium is less constrained vertically and can extend to higher altitudes. A similar effect being observed for water vapour (Fedorova et al., 2020). In Rossi et al. (2021) we used a simplified cloud formation scheme, without condensation nuclei, and didn't include the radiative effect of clouds. Figure 1: Zonally averaged meridional profiles of the D/H ratio in the vapour phase, as predicted by the updated GCM for MY34. Each subplot shows the beginning of each month. Here we present results of a new version of the model, this time including the microphysics of cloud formation, the radiative effect of clouds but also the effect of kinetics on the fractionation factor (see companion abstract of Margaux Vals in TP12). As shown in Fig 1, the effect of the dust storm of MY34 is still quite visible, with a D/H ratio relatively constant in higher parts of the atmosphere (up to 1 Pa). With this updated model, we are able to make comparisons with the HDO profiles observed by ACS onboard TGO. Preliminary results In order to be comparable with the solar occultations from ACS, the GCM output is interpolated at a solar zenith angle of 90° and at the coordinates (latitude, longitude, local time and solar longitude) of each solar occultation. In particular, we compare the model with ACS observations presented in Alday et al. (2021). Figure 2 presents some early results of such comparison. Figure 2: Profiles of the D/H ratio from the GCM (in orange) and from ACS data (in blue). Profiles are averaged over bins of solar longitude (covering the end of MY34 and the beginning of MY35) and latitude. Error bars indicate the standard deviation within the bin. The altitude is given in kilometers above the areoid. The comparison shows a good agreement between the GCM profiles and the observations for the D/H ratio in several bins in latitude and solar longitude. This indicates that the model is capable of representing the process of isotopic fractionation and its effect on the D/H ratio. A more detailed comparison between the observations and the GCM, in particular with respect to the distribution of water vapour is ongoing. Acknowledgements ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. Science support in IKI is funded by Federal agency of science organizations (FANO). L.R. acknowledges support from CNES and from the Excellence Laboratory "Exploration Spatiale des Environnements Planétaires (ESEP)".

Hydrogen chloride (HCl) was recently discovered in the atmosphere of Mars by two spectrometers onboard the ExoMars Trace Gas Orbiter. The reported detection made in Martian Year 34 was transient, present several months after the global dust storm during the southern summer season. Here, we present the full data set of vertically resolved HCl detections obtained by the NOMAD instrument, which covers also Martian year 35. We show that the particular increase of HCl abundances in the southern summer season is annually repeated, and that the formation of HCl is independent from a global dust storm event. We also find that the vertical distribution of HCl is strikingly similar to that of water vapor, which suggests that the uptake by water ice clouds plays an important role. The observed rapid decrease of HCl abundances at the end of the southern summer would require a strong sink independent of photochemical loss. © 2021. American Geophysical Union., ExoMars is a space mission of the European Space Agency 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). This project acknowledges funding by the Belgian Science Policy Office, with the financial and contractual coordination by the European Space Agency Prodex Office (PEA 4000103401 and 4000121493), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1 ST/S00145X/1 ST/S00145X/1 and ST/T002069/1, and Italian Space Agency through grant 2018-2-HH.0. This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars." 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 Astrofisica de Andalucia (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique-FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and Belgian Science Policy Office BrainBe SCOOP and MICROBE Projects. S. A. is "Charge de Recherches" at the F.R.S.-FNRS. U.S. investigators were supported by the National Aeronautics and Space Administration. We acknowledge support from the Institut National des Sciences de l'Univers" (INSU), the "Centre National de la Recherche Scientifique" (CNRS) and "Centre National d'Etudes Spatiales" (CNES) through the "Programme National de Planetologie".

International audience; Carbon monoxide (CO) is the main product of CO2 photolysis in the Martian atmosphere. Production of CO is balanced by its loss reaction with OH, which recycles CO into CO2. CO is therefore a sensitive tracer of the OH-catalysed chemistry that contributes to the stability of CO2 in the atmosphere of Mars. To date, CO has been measured only in terms of vertically integrated column abundances, and the upper atmosphere, where CO is produced, is largely unconstrained by observations. Here we report verti- cal profiles of CO from 10 to 120 km, and from a broad range of latitudes, inferred from the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter. At solar longitudes 164–190°, we observe an equatorial CO mixing ratio of ~1,000 ppmv (10–80 km), increasing towards the polar regions to more than 3,000 ppmv under the influence of downward transport of CO from the upper atmosphere, providing a view of the Hadley cell circulation at Mars’s equinox. Observations also cover the 2018 global dust storm, during which we observe a prominent depletion in the CO mixing ratio up to 100 km. This is indicative of increased CO oxidation in a context of unusually large high-altitude water vapour, boosting OH abundance.

International audience; We present the first water vapor profiles encompassing the upper mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes where remote sensing has been able to observe water to date. Our results are based on solar occultation measurements by Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The observed wavelength range around 2.7 μm possesses strong CO2 and H2O absorption lines allowing sensitive temperature and density retrievals. We report a maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km during the global dust storm (GDS) of Martian Year (MY) 34 and around southern summer solstice of MY 34 and 35. During other seasons water remains persistently below ∼2 ppmv. We claim that contributions of the MY34 GDS and perihelion periods into the projected hydrogen escape from Mars are nearly equivalent.

Carbon dioxide is the major constituent of the Martian atmosphere. Its seasonal cycle plays an important role in atmospheric dynamics and climate. Formation of the polar CO2 frost deposits results in up to 30% of atmospheric pressure variations as well as in dramatic change in surface reflectance and emissivity. Another case of carbon dioxide condensation is formation of a CO2 clouds that are still poorly studied, despite the fact that they have been observed by a number of instruments [1−6] on the orbit of Mars.In this work, we will present first results of CO2 clouds observations from a combination of thermal-infrared (1.7−17 µm) and near-infrared (0.7-1.6 µm) spectra measured by TIRVIM and NIR instruments onboard the ExoMars Trace Gas Orbiter (TGO) in solar occultation geometry. These instruments are part of the Atmospheric Chemistry Suite (ACS), a set of three spectrometers (NIR, MIR, and TIRVIM) that is conducting scientific measurements on the orbit of Mars since the spring of 2018 [7].This work was funded by Russian Science Foundation, grant number 20-42-09035.References[1] Montmessin et al. (2006). Subvisible CO2 ice clouds detected in the mesosphere of Mars. Icarus, 183, 403–410. https://doi.org/10.1016/j.icarus.2006.03.015[2] Montmessin et al. (2007). Hyperspectral imaging of convective CO2 ice clouds in the equatorial mesosphere of Mars. Journal of Geophysical Research, 112, E11S90. https://doi.org/10.1029/2007JE002944[3] Määttänen et al. (2010). Mapping the mesospheric CO2 clouds on Mars: MEx/OMEGA and MEx/HRSC observations and challenges for atmospheric models. Icarus, 209, 452–469. https://doi.org/10.1016/j.icarus.2010.05.017[4] McConnochie et al. (2010). THEMIS-VIS observations of clouds in the Martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology. Icarus, 210, 545–565. https://doi.org/10.1016/j.icarus.2010.07.021[5] Vincendon et al. (2011). New near-IR observations of mesospheric CO2 and H2O clouds on Mars. Journal of Geophysical Research, 116, E00J02. https://doi.org/10.1029/2011JE003827[6] Jiang et al., (2019). Detection of Mesospheric CO 2 Ice Clouds on Mars in Southern Summer. Geophysical Research Letters, 46(14), 7962–7971. https://doi.org/10.1029/2019GL082029[7] Korablev et al., (2018). The Atmospheric Chemistry Suite (ACS) of three spectrometers for the ExoMars 2016 Trace Gas Orbiter. Space Sci. Rev. 214, 7. doi:10.1007/s11214-017-0437-6

The molecular oxygen (O2) and carbon oxide (CO) are minor constituents of the Martian atmosphere with the annual mean mixing ratio of (1560 ± 60 ppm) and (673 ± 2.6 ppm), respectively (Krasnopolsky, 2017). Both are non-condensable species and their latitudinal variations are induced by condensation and sublimation of CO2 from the polar caps that result in enrichment and depletion and seasonal variations are following the total CO2 amount in the atmosphere.The Atmospheric Chemistry Suite (ACS) is a set of three spectrometers (-NIR, -MIR, and -TIRVIM) intended to observe Mars atmosphere onboard the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission (Korablev et al., 2018). The near infrared channel (NIR) is a compact spectrometer operating in the range of 0.7–1.7 µm with a resolving power of λ/Δλ ~ 25,000. It is designed to operate in nadir and in solar occultation modes. The simultaneous vertical profiling of the O2 and CO density at altitudes of 10-60 km based on 0.76 µm and 1.57 µm bands, respectively, is a unique feature of the ACS NIR science in occultation. In this work we present the seasonal and latitudional distribution of the O2 and CO mixing ratios obtained for period of 2018-2020 (MY34 and35) and the comparison with the LMD General Circulation model. We report the averaged mixing ratio for CO of ~950 ppm and for O2 of~1800 ppm at low altitudes (~20 km). Also, we detected extremely enriched CO layer at 10-15 km in the southern polar region at Ls=100-200° both for MY34 and MY35.

The current Martian atmosphere is about five times more enriched in deuterium than Earth's, providing a direct testimony that Mars hosted several times more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. The transport of water to the upper atmosphere, from whence it may escape to space, has been assumed to be controlled by water condensation and photolysis, both of which affect the isotope composition of the escaping hydrogen. Their respective role in influencing the relative proportion of escaping D and H atom populations in the upper atmosphere has remained speculative. Here we report HDO and HO profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal that the ultraviolet dissociation of water not only governs the production of atomic hydrogen, prevailing over the ion chemistry mechanism, but also dominates the production of H relative to D atoms, disrupting the old paradigm of atmospheric condensation being the main process differentiating D and H in the upper atmosphere.

International audience; The paper presents observations of gravity wave-induced temperature disturbances in the Martian atmosphere obtained with the mid-infrared (MIR) spectrometer, a channel of the Atmospheric Chemistry Suite instrument on board the Trace Gas Orbiter (ACS/TGO). Solar occultation measurements of a CO2absorption band at 2.7 µm were used for retrieving density and temperature profiles between heights of 20 and 160 km with vertical resolution sufficient for deriving small-scale structures associated with gravity waves. Several techniques for distinguishing disturbances from the background temperature have been explored and compared. Instantaneous temperature profiles, amplitudes of wave packets and potential energy have been determined. Horizontal momentum fluxes and associated wave drag have been estimated. The analyzed data set of 144 profiles encompasses the measurements made over the second half of Martian Year 34, from the Solar longitude 165° through 355°. We observe enhanced gravity wave dissipation/breaking in the mesopause region of 100-130 km. Our analysis shows no direct correlation between the wave amplitude and Brunt-Väisälä frequency. It may indicate that convective instability may not be the main mechanism limiting gravity wave growth in the middle atmosphere of Mars.

Phosphine (PH3) is proposed to be a possible biomarker in planetary atmospheres and has been claimed to have been observed in the atmosphere of Venus, sparking interest in the habitability of Venus’s atmosphere. Observations of another biomarker, methane (CH4), have been reported several times in the atmosphere of Mars, hinting at the possibility of a past or present biosphere. The Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter has a spectral range that includes several absorption lines of PH3 with line strengths comparable to previously observed CH4 lines. The signature of PH3 was not observed in the 192 observations made over a full Martian year of observations, and here we report upper limits of 0.1–0.6 ppbv.

International audience; The D/H ratio is commonly used to investigate the history of water on Mars, yet the mechanisms controlling present‐day HDO behavior are poorly understood. Significant variations of the D/H ratio were first predicted on the basis of a 3D global climate model, which were later confirmed by ground‐based observations. This behaviour, consisting of lower HDO/H2O ratios in the colder regions of Mars, is related to the isotopic fractionation occurring at condensation. We leverage this previous effort and present an updated implementation, using the modern version of the model, that remains in agreement with the older version. We explore the impact of the Global Dust Storm that occurred during Martian Year 34 on HDO. Our simulations indicate that HDO is on average 40% more abundant at 100 km during the MY34 GDS year than during a regular year, with likely large consequences for the escape flux of water that year.

Full list of authors: Chaffin, M. S.; Kass, D. M.; Aoki, S.; Fedorova, A. A.; Deighan, J.; Connour, K.; Heavens, N. G.; Kleinbohl, A.; Jain, S. K.; Chaufray, J. -Y.; Mayyasi, M.; Clarke, J. T.; Stewart, A. I. F.; Evans, J. S.; Stevens, M. H.; McClintock, W. E.; Crismani, M. M. J.; Holsclaw, G. M.; Lefevre, F.; Lo, D. Y.; Montmessin, F.; Schneider, N. M.; Jakosky, B.; Villanueva, G.; Liuzzi, G.; Daerden, F.; Thomas, I. R.; Lopez-Moreno, J. -J.; Patel, M. R.; Bellucci, G.; Ristic, B.; Erwin, J. T.; Vandaele, A. C.; Trokhimovskiy, A.; Korablev, O. I., Mars has lost most of its initial water to space as atomic hydrogen and oxygen. Spacecraft measurements have determined that present-day hydrogen escape undergoes large variations with season that are inconsistent with long-standing explanations. The cause is incompletely understood, with likely contributions from seasonal changes in atmospheric circulation, dust activity and solar extreme ultraviolet input. Although some modelling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. Here we present synoptic measurements of dust, temperature, ice, water and hydrogen on Mars during a regional dust event, demonstrating that individual dust events can boost planetary H loss by a factor of five to ten. This regional storm occurred in the declining phase of the known seasonal trend, establishing that dust forcing can override this trend to drive enhanced escape. Because similar regional storms occur in most Mars years, these storms may be responsible for a large fraction of Martian water loss and represent an important driver of Mars atmospheric evolution. © 2021, The Author(s), under exclusive licence to Springer Nature Limited., This research was supported by NASA through the MAVEN and MRO projects. IUVS data products were produced using the RMACC Summit supercomputer, which is supported by the National Science Foundation (award nos. ACI-1532235 and ACI-1532236), by the University of Colorado Boulder and by Colorado State University. The Summit supercomputer is a joint effort of the University of Colorado Boulder and Colorado State University. M.M.J.C. is supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, which is administered by the Universities Space Research Association under contract with NASA. A.K. acknowledges support from the NASA Mars Data Analysis Program (80NM0018F0719). Work at the Jet Propulsion Laboratory, California Institute of Technology, is performed under contract with NASA. ExoMars is a space mission of the European Space Agency and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), assisted by co-principal investigator teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). For this project we acknowledge funding by the Belgian Science Policy Office, with financial and contractual coordination by the European Space Agency Prodex Office (PEA 4000103401 and 4000121493); by the Spanish MICINN through its Plan Nacional; by European funds under grant nos. PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER); by the United Kingdom Space Agency through grant nos. ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/S00145X/1; and by the Italian Space Agency through grant no. 2018-2-HH.0. 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). This work was supported by the Belgian Fonds de la Recherche Scientifique-FNRS under grant nos. 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and by the Belgian Science Policy Office BrainBe SCOOP Project. S.A. is ‘Chargé de Recherches’ at the F.R.S.-FNRS. NOMAD’s United States investigators are supported by NASA. Science operations of ACS on TGO are funded by Roscosmos and the ESA. IKI affiliates acknowledge support from the Ministry of Science and Higher Education of the Russian Federation. F.M. acknowledges funding from the CNES and ANR (PRCI, CE31 AAPG2019-MCUBE project).

International audience; The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective roles in influencing the proportions of atomic D and H populations have remained speculative. Here we report HDO and H2O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H2O is a more important factor in determining the isotopic composition of the dissociation products.

International audience; Ozone (O3) in the atmosphere of Mars is produced following the photolysis of CO2 and is readily destroyed by the hydrogen radicals (HOx) released by the photolysis and oxidation of water vapor. As a result, an anti‐correlation between ozone and water vapor is expected. We describe here the O3‐H2O relationship derived from four Martian years of simultaneous observations by the SPICAM spectrometer onboard the Mars Express spacecraft. A distinct anti‐correlation is found at high latitudes, where the O3 column varies roughly with the ‐0.6 power of the H2O column. The O3 and H2O columns are uncorrelated at low latitudes. To evaluate our quantitative understanding of the Martian photochemistry, the observed O3‐H2O relationship is then compared to that predicted by a global climate model with photochemistry. For identical model and observed abundances of H2O, the model underpredicts observed ozone by about a factor of two relative to SPICAM when using the currently recommended gas‐phase chemistry. Sensitivity studies employing low‐temperature CO2 absorption cross‐sections, or adjusted kinetics rates, do not solve this bias. Taking into account potential heterogeneous processes of HOx loss on clouds leads to a significant improvement, but only at high northern latitudes. More broadly, the modeled ozone deficits suggest that the HOx‐catalyzed photochemistry is too efficient in our simulations. This problem is consistent with the long‐standing underestimation of CO in Mars photochemical models, and may be related to similar difficulties in modeling O3 and HOx in the Earth’s upper stratosphere and mesosphere.

Introduction The southern spring and summer on Mars in the 34th Martian Year (MY34, Ls 180o-360o) was decorated by two dust storms: the global one (GDS) at Ls 190o-240o, and the regional one at Ls 320o-330o. Just before the onset of the GDS, the nominal ExoMars Trace Gas Orbiter mission began (April 2018) with an onboard solar occultation experiment by the spectrometric Atmospheric Chemistry Suite (ACS) [1]. The first ACS-NIR retrievals of the temperature and water mixing ratio revealed an increase the H2O abundance up to altitudes of 80-100 km not only in the stormy events but also at the perihelion in the southern hemisphere (around Ls~270o) [2]. Moreover, as it was shown, the water supersaturation occurs even in presence of the ice clouds, while the stormy warm atmosphere rises the hygropause level [2, 3]. This phenomenon increases a capability of water escape from Mars. Simultaneous investigation from the NOMAD/TGO solar occultations demonstrated analogue results for the H2O enhancement up to 80 km in the stormy seasons, but without the saturation estimations [4]. In this paper, we present results from more sensitive solar occultation measurements in the 2.65-2.7 μm spectral range by the middle infrared channel of ACS (ACS-MIR). The channel allows mesospheric and thermospheric retrievals of temperature and CO2 density in parallel with the mesospheric water abundance. Our results confirm the ACS-NIR and NOMAD conclusions concerning the H2O vapour behaviour in the second half of the MY34. We report the most humid mesosphere occurred in the perihelion season where the water enrichment was established about 5-10 ppbv at altitudes of 110-120 km. We also present the temperature and CO2 density seasonal distribution covering the troposphere, the mesosphere and the thermosphere of Mars. Measurements The ACS-MIR channel is a cross-dispersion echelle spectrometer dedicated to solar occultation measurements in the 2.3–4.3 μm wavelength range [1]. The instrumental resolving power λ/Δλ reaches ~30 000, while the altitude resolution is 1-2 km and the signal/noise ratio is about ~1000. Each occultation session covers a spectral interval with one or a few CO2 absorption bands appropriate for the atmospheric density and temperature retrievals. In this work, we perform data analysis in the 2.65-2.7 μm spectral range, which occupies echelle diffraction orders from 221 to 224, observed simultaneously. This spectral region hosts strong CO2 and H2O absorption bands that are sensitive to detections of CO2 density up to 180-190 km and H2O density up to 120-130 km. For the MY34 the MIR dataset in the considered spectral range is made of ~100 occultation profiles in the Northern hemisphere and of ~90 in the Southern one encompassing seasonal period from Ls 165o to 355o. Retrieval concept The temperature and densities retrieval scheme consists of several iterations with a fitting of a forwardly modelled transmission spectrum to a measured one at each observed altitude. The model includes line-by-line calculations CO2 and H2O absorption cross sections depending on specified temperature and pressure. The fitting procedure is based on Jacobian matrix containing transmission derivatives on free parameters: temperature, CO2 density and H2O density. The clue contribution to an optimal estimation comes from the molecular cross sections derivatives on temperature; they differ significantly from line to line in the considered spectral range. Thanks to that, an independent and simultaneous retrieval of temperature and density is possible. Once the altitude profiles of these parameters are estimated at the first iteration, we calculate the atmospheric pressure profile assuming the hydrostatic equilibrium. This assumption was successfully tested by other vertical profile retrievals from the ACS data [1, 5]. The uncertainty of the retrievals is determined by the transmission errors and the Jacobian matrix for each of free parameters. This estimation allows establishing maximal altitudes with a positive detection of CO2 and H2O densities. Results ACS-MIR solar occultations in the 2.65-2.7 μm spectral range provide us with unprecedented capability to profile CO2 density and temperature from 20 to 180 km, covering the troposphere, the mesosphere and the thermosphere of Mars. The homopause varies around ~130 km and CO2 mixing ratio decreases from 96% to 20-40% at 180 km due to photolysis and molecular diffusion. In parallel with the same dataset, we report very high altitude water abundance, at 110-120 km, that occurred in the southern spring and summer including the GDS and the perihelion in the 34th Martian Year. However, our measurements in the northern hemisphere revealed such a high H2O enrichment only at the stormy events. Comparison of our climatology with some improvements in the Global Circulation Model for the MY34 is in progress. Acknowledgements The retrievals of density/temperature profiles in IKI are funded by the RSF grant #20-42-09035. References [1] Korablev O. et al., 2018. The Atmospheric Chemistry Suite (ACS) of three spectrometers for the ExoMars 2016 Trace Gas Orbiter. Space Sci. Rev., 214:7. DOI 10.1007/s11214-017-0437-6. [2] Fedorova A. et al., 2020. Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season. Science, eaay9522. DOI: 10.1126/science.aay9522. [3] Fedorova A. et al., 2018. Water vapor in the middle atmosphere of Mars during the 2007 global dust storm. Icarus 300: 440-457. [4] Aoki S. et al., 2019. Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD. JGR Planets, 124, 3482-3497. https://doi.org/10.1029/2019JE006109. [5] Alday J. et al., 2019. Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter. A&A, 630, A91. DOI: 10.1051/0004-6361/201936234.

The ExoMars Trace Gas Orbiter (TGO) mission had started regular measurements in 2018. Primary goal of the mission is to quantify trace gases that could indicate geologic or biogenic activity on Mars (Vago et al., 2015). Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) is a high resolution cross-dispersion spectrometer operating in solar occultation mode (Korablev et al., 2018). It was designed to make the most sensitive measurements of the atmosphere to date. During each occultation up to 20 diffraction orders are simultaneously recorded at different tangent altitudes. In 2020 ACS MIR reports the discovery of the gaseous hydrogen chloride (HCl). Absorption features are present in several consecutive diffraction orders, withal both isotopes H37Cl and H35Cl are clearly observed. HCl was observed by ACS simultaneously in both hemispheres after the main phase of the global dust storm. Though the formation mechanism is not fully clear, we believe that the presence of HCl is associated with the lifted dust and chlorine component in it. On Earth, in general, the chlorine isotope variations in nature are relatively small, ranging from ~-2 to +2 ‰. However, large variations are observed, e.g. in extraterrestrial materials and volcanic gases, due to kinetic fractionation. On Mars Farley et al. (2016) reported a range from -1 ‰ to −51 ‰ (5% reduction) for the δ37Cl in the samples drilled in the Gale Crater. ACS observations demonstrate enrichment of the 37Cl up to +250 ‰ on average in the atmospheric gaseous. In principle, most atmospheric elements on Mars have heavy isotope enrichments due to preferential loss of the light isotope to space (e.g. Vandaele et al., 2019). Early hydrodynamic escape during intense extreme ultraviolet radiation followed by prolonged atmospheric ‘erosion’ explains the heavy isotope enrichment. Chlorine loss, as HCl, would raise the δ37Cl value of the residual materials, involved in the dust-atmospheric exchange cycle. References Farleya K.A., Martina P., Archer P.D. , et al.: Light and variable 37Cl/35Cl ratios in rocks from Gale Crater, Mars: Possible signature of perchlorate, Earth and Planetary Science Letters 438:14-24, DOI: 10.1016/j.epsl.2015.12.013, 2016. Korablev, O., Montmessin, F., Trokhimovskiy, et al..: The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter, Space. Sci. Rev., 214(1), 7, doi:10.1007/s11214-017-0437-6, 2018. Vago, J., Witasse, O., Svedhem, et al.: ESA ExoMars program: The next step in exploring Mars, Sol. Syst. Res., 49(7), 518–528, doi:10.1134/S0038094615070199, 2015. Vandaele, A. C., Korablev, O., Daerden, F. et al.: Martian dust storm impact on atmospheric H2O and D/H observed by ExoMars Trace Gas Orbiter, Nature, 568, 521–525, doi:10.1038/s41586-019-1097-3, 2019.

We present simultaneous retrievals of vertical profiles of CO2 and temperature obtained from a small sample of solar occultation scans by the NOMAD-SO [1] and ACS-MIR [2] instruments on board the ExoMars Trace Gas Orbiter (TGO). The orbits or scans selected from each instrument's observations are sufficiently close in time and space so that the atmospheric variability plays a minor role and a meaningful comparison of the performance of both instrument channels is possible. This is an on-going work devoted to a proper validation of both instruments. We will present a small selection of retrievals than can be used for a critical analysis of the results obtained and as a first step into the mutual validation of NOMAD-SO and ACS-MIR.1. Instruments and observationsOur work is focussed in the NOMAD-SO and the ACS-MIR channels, those specifically devoted to operational observations in solar occultation. Several groups in the NOMAD and ACS teams are performing retrievals of species abundances from these signals and some are reported in this conference [3,4]. We focus on the retrieval of CO2 and temperature in the 2.7 µm range, the strongest IR bands by this gas in the spectra region coverd by SO and ACS [5]. The spectra correspond to calibrated transmittances derived from SO difraction orders 164 and 165 and from MIR position 4 spectra. About a dozen strong CO2 lines are observed in these spectra up to very high tangent altitudes, around 170-180 km. The present study targets at both solar occultation signals, NOMAD-SO and ACS-MIR, but unfortunately, due to some unexpected miss-alignments among the different channels in each instrument and between NOMAD and ACS, an entirely simultaneous observation of the solar disc as it emerges or hides behind the atmosphere is not possible with both SO and MIR channels. Instead this work study the inversions from the two channels in nearly coincident solar occultations, with small changes in latitude, longitude, solar longitude and time.2. Retrieval Scheme We have performed the present retrievals using an inversion code previously used in the Earth upper atmosphere for routine operations of the MIPAS instrument on board the Envisat mission [6]. The inversion scheme, which has been recently adapted to Mars for limb sounding in emission from the OMEGA spectrometer on board the Mars Express spacecraft [7], is adapted here for occultation observations. At the core of the inversion scheme is the forward model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm [8] ) and the inversion processor (RCP), jointly developed by the Institute for Meteorology and Climate Research (IMK) and the IAA [9].As mentioned above we used here CO2 calibrated transmittances obtained from routine processing by the PI teams. But before injection into the inversion algorithm, a pre-processing of every orbit or scan is performed in order to correct every spectrum of spurious effects. These are essentially two: residual spectral shifts and artificial bending in single spectra. The magnitude of these two effects changes from order to order and between SO and ACS. In spite of these differences, the pre-processing is conceptually similar to both signals, and is actually performed with the same Python code. Further, the ILS associated to each of the signals is best reproduced by a double gaussian, whose detailed description (parameters describing their FWHM and peak ratios, for example) requires also a precise determination by the user, as apparently it slightly varies with time and observing conditions.It is specially useful to obtain both density and temperature information from a single scan, and this is the purpose of the present study. The simultaneous retrieval from a given scan actually derives a CO2 density profile, and after an iterative process, a temperature profile assuming hydrostatic equilibrium. An adjustment for the pointing is also intruduced, and the aerosol extinction and possible interference from species like H2O are also considered during the inversion to obtain the best fits.3. Results We will discuss the fist comparisons of CO2 and temperatures from NOMAD-SO and ACS-MIR, using a common pre-processing and inversion code. The retrievals perform in a similar manner, as in both cases the number of lines, the ro-vibrational bands and the spectral region are similar. Small differences in noise and spectral resolution have some impact but overall both channels permit sounding the Mars atmosphere up to the upper thermosphere with just one single scan. Averaging kernels indicate effective vertical resolutions around 4-6 km in the middle atmosphere (50-100 km), both in CO2 and temperature. The inversion is affected by heavy dust loading and in general degrades below about 30 km and above 150 km due to saturation and noise, respectively. AcknowledgementsThe 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). MALV was funded 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. References[1] Vandaele et al., Space Science Reviews 214 (5), 2018 [2] Korablev et al., Space. Sci. Rev. 214, 7 (2018). [3] Loic et al., Update on CO2 and temperature profiles retrievals from NOMAD-SO on board ExoMars TGO, contribution to this conference, 2020. [4] Shohei et al., Water vapor vertical profiles on Mars: Results from the first full Mars Year of TGO/NOMAD science operations, contribution to this conference, 2020. [5] Lopez-Valverde et al., Space Sci Rev (2018) 214:29 [6] Funke, B., et al. , Atmos. Chem. Phys., 9(7), 2387–2411, 2009.Funke et al., ACP, 9, 2387–2411, 2009. [7] Jimenez-Monferrer, Icarus in press, 2020 [8] Stiller et al., JQSRT, 72, 249–280, 2002 [9] von Clarmann et al., J. Geophys. Res. 108, 4746, 2003 How to cite: Lopez-Valverde, M. A., Funke, B., Hill, B., Gonzalez-Galindo, F., Aoki, S., Trompet, L., Thomas, I., Villanueva, G., Erwin, J., Olsen, K., Belyaev, D., Trokhimovskiy, A., Alday Parejo, J., Lopez-Puertas, M., Lopez-Moreno, J. J., Montmessin, F., Patel, M., Bellucci, G., Vandele, A. C., and Korablev, O.: CO2 and temperature retrievals in the Mars atmosphere from solar occultation by NOMAD-SO and ACS-MIR: performance and cross validation., Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-924, 2020

HDO and the D/H ratio are important in order to understand Mars past and present climate, in particular with regards to the evolution through ages of the Martian water cycle. We present here new modeling efforts aimed at rejuvenating the representation of HDO in the LMD Mars GCM, with the perspective of comparison with the new observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter. Introduction Mars is known to have had a significant liquid water reservoir on the surface and the D/H ratio derived from the HDO/H2O abundance ratio is a sensitive tool to constrain the primordial abundance of the water reservoir on Mars and its evolution with time. The current ratio is at least five times that of the Vienna Standard Mean Ocean Water (SMOW) (Owen et al. 1988 , Encrenaz et al. 2018, Krasnopolsky 2015, Villanueva et al. 2015). H and D atoms in the upper atmosphere are coming from H2O and HDO, their sole precursor in the lower atmosphere. The lower mass of H over D atoms and the fact that H2O is preferentially photolysed over HDO (Cheng et al. 1999) explain the differential escape of these two elements. Also, the heavier isotope, HDO, has a lower vapor pressure than H2O, which results in an enrichment of the deuterated isotope in the solid phase of water. This effect is known as the Vapor Pressure Isotope Effect (VPIE) and can reduce the D/H ratio above the condensation level to values as low as 10% of the D/H ratio near the surface (Bertaux et al. 2001, Fouchet et al. 2000). Fractionation should affect the amount of HDO depending on latitude, longitude, altitude and season. Modeling HDO In particular, a previous model (Montmessin et al., 2005) has predicted that an isotopic gradient should exist between the cold regions where condensation depletes the atmosphere in HDO and the warmer, condensation-free, regions where D/H is found to be maximum. This leads to a latitudinal gradient of D/H (with variations greater than a factor of 5) between the warm and moist summer hemisphere and the cold and dry winter hemisphere. This gradient was later confirmed by several Earth observing assets (Krasnopolsky 2015, Villanueva et al. 2015, Encrenaz et al. 2018, Aoki et al. 2019). Yet some observations (Villanueva et al., 2015, Khayat et al., 2019) have revealed longitudinal variations of D/H ratios which have remained explained and are not reproduced by any model. It is therefore essential to model the HDO cycle and the associated processes of fractionation in a 3D general climate model (GCM). With the Trace Gas Orbiter (TGO) mission now in its mission phase, very strong and precise constraints will be soon available to evaluate the GCM prediction capability. We present here the results from our (re)integration of the HDO cycle into the LMD Mars GCM, taking into account HDO in its vapour and ice phases in the atmosphere, and as surface ice. The model presented here uses a simplified cloud formation scheme, without condensation nuclei. The updated model is in good agreement with the results of Montmessin et al. (2005), showing the latitudinal gradient between cold and warm regions and its seasonal variation (see fig 1) Figure 1 : Latitudinal and seasonal variation of the zonally averaged D/H ratio in the vapor phase, as predicted by the model. The present study gives a 3D climate model projection of the behavior of HDO in the context of a dust annual scenario mimicking the dust seasonal and spatial evolution observed during Martian Year 34 (MY34) and including the occurrence of the Global Dust Storm (GDS). The simulations reported here emphasize the contrast in HDO behavior between a perturbed year such as MY34 and a more traditional MY where the seasonal evolution of dust obeys a recurrent and more quiet, so-called "climatological" scenario. The simulations (see fig. 2) show that in the climatological scenario the cloud formation, and the subsequent fractionation processes, trap HDO within the ice particles, depleting the vapour phase above the condensation level (the so-called "deuteropause"). Figure 2 : Zonally averaged meridional profiles of temperature, HDO ice mixing ratio and D/H ratio of vapour for (left) the climatology scenario and (right) the MY34 scenario, at Ls = 210, during the GDS. By comparison, during the GDS, the dust has a strong effect on the temperature field, and therefore on the circulation and the cloud formation. Since clouds are forming at higher altitude (if at all), the deuterium is less constrained vertically and can extend to higher altitudes. A similar effect being observed for water vapour (Fedorova et al., 2020). Early observations with the ACS and NOMAD instruments, on board the ExoMars Trace Gas Orbiter, indicate that HDO vertical distribution is indeed affected by the GDS, in qualitative agreement with our model (Vandaele et al., 2019). We will here present the results of these comparative simulations and discuss the differences and similarities with the observations from ACS and NOMAD. Acknowledgements ExoMars is the space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. Science support in IKI is funded by Federal agency of science organizations (FANO). L.R. acknowledges support from CNES and from the Excellence Laboratory "Exploration Spatiale des Environnements Planétaires (ESEP)".

The ExoMars Trace Gas Orbiter (TGO) mission was sent to Mars in 2016 to make the most sensitive measurements of the atmosphere to date and to hunt for any trace gases diagnostic of active geologic or biogenic processes (Vago et al., 2015). After the first full Mars year of observations, we are able to report the first such discovery: gaseous hydrogen chloride (HCl) has been detected by the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR). ACS science operations commenced at a crucial time for Mars observations. The first solar occultation spectra were recorded on solar longitude (Ls) 163 (late April 2018) at a time of changing seasons on Mars. The northern hemisphere was entering winter, while the southern hemisphere was beginning to warm towards summer season. Around Ls 190, we witnessed a global dust storm of unprecedented severity, with dust remaining in the atmosphere through Ls 250 (Montabone et al., 2020). HCl was observed by ACS simultaneously in both hemispheres after the main phase of the global dust storm. It remained detectable to ACS MIR for several months, while the impact of the dust was still being felt in the atmosphere, before vanishing. The dust lofted and mixed into the atmosphere caused the atmosphere to warm resulting in an expansion of the lowest layers, increasing the water vapour content, elevating the hygropause to 50-60 km, and enhancing meridional Hadley cell circulation (Fedorova et al., 2020). Fig. 1 shows a model of the absorption spectrum contributions of the Martian atmosphere as seen by ACS MIR. The HCl branches, shown in the lower panel, partially overlap with the region of interest used to investigate methane (CH4). So far, TGO instruments have found no evidence of the absorption signature of methane, but in its place ACS MIR has made two surprising discoveries: we have identified a previously unknown CO2 absorption-rotation band (Trokhimovskiy et al., 2020); and we were able to resolve the spectral signature of ozone at low altitudes in the north polar region at the start of norther winter (Olsen et al., 2020a). Methods ACS MIR is a cross-dispersion spectrometer operating in solar occultation mode (Korablev et al., 2018). This geometry provides high signal-to-noise ratios, excellent sensitivity to the vertical structure of the atmosphere, and a very long optical path, amplifying trace gas absorption. The instrument consists of a primary echelle diffraction grating to disperse infrared radiation, followed by a secondary, steerable diffraction grating to separate diffraction orders. The secondary grating position changes the instantaneous spectral range. The full coverage is 2300-4500 cm-1, and the spectral resolution achieved is 0.040-0.045 cm-1 in the region of interest for HCl. The spectral range shown in Fig. 1 covers two secondary grating positions, labelled 11 and 12. Fig. 1. Modelled gas absorption contributions as seen in the lower atmsophere by ACS MIR during solar occultation. The top panel shows contributions from major species: CO2, H2O, and HDO. The bottom panel shows contributions from HCl at 1.5 ppbv, ozone at 140 ppbv, and methane at 1 ppbv. HCl and O3 signatures at these mixing ratios have been observed, while CH4 has not. Spectral fitting was done using the JPL Gas Fitting Software Suite (GFIT) (Irion et al., 2002; e.g., Sen et al., 1996). GFIT computes volume mixing ratio scaling factors for each spectral window and at each altitude. A retrieved profile of gas abundance is derived by inverting the matrices of optical paths and the estimated column abundances. Temperature and pressure profiles are retrieved from coincident observations made with the ACS near infrared channel (ACS NIR) (Fedorova et al., 2020; Vandaele et al., 2019). A full description of the retrieval method can be found in (Olsen et al., 2020b). References Fedorova, A. A., et al.: Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season, Science, 367(6475), 297–300, doi:10.1126/science.aay9522, 2020. Irion, F. W., et al.: Atmospheric Trace Molecule Spectroscopy (ATMOS) Experiment Version 3 data retrievals, Appl. Opt., 41, 6968–6979, doi:10.1364/AO.41.006968, 2002. Korablev, O., et al.: The Atmospheric Chemistry Suite (ACS) of Three Spectrometers for the ExoMars 2016 Trace Gas Orbiter, Space. Sci. Rev., 214(1), 7, doi:10.1007/s11214-017-0437-6, 2018. Montabone, L., Spiga, A., Kass, D. M., Kleinböhl, A., Forget, F. and Millour, E.: Martian Year 34 Column Dust Climatology from Mars Climate Sounder Observations: Reconstructed Maps and Model Simulations, J. Geophys. Res., 2020. Olsen, K. S., et al.: First detection of ozone in the mid-infrared at Mars: implications for methane detection, Astron. Astrophys. in press, doi:10.1051/0004-6361/202038125, 2020a. Olsen, K. S., et al.: The vertical structure of CO in the Martian atmosphere as observed by ACS on ExoMars TGO, Nat. Geosci. submitted, 2020b. Sen, B., Toon, G. C., Blavier, J.-F., Fleming, E. L. and Jackman, C. H.: Balloon-borne observations of midlatitude fluorine abundance, J. Geophys. Res., 101, 9045–9054, doi:10.1029/96JD00227, 1996. Trokhimovskiy, A., Perevalov, V., Korablev, O., Fedorova, A. F., Olsen, K. . S., Bertaux, J.-L., Patrakeev, A., Shakun, A. and Montmessin, F.: First observation of the magnetic dipole CO₂ main isotopologue absorption band at 3.3 μm in the atmosphere of Mars by ACS, Astron. Astrophys. in press, doi:10.1051/0004-6361/202038134, 2020. Vago, J., Witasse, O., Svedhem, H., Baglioni, P., Haldemann, A., Gianfiglio, G., Blancquaert, T., McCoy, D. and de Groot, R.: ESA ExoMars program: The next step in exploring Mars, Sol. Syst. Res., 49(7), 518–528, doi:10.1134/S0038094615070199, 2015. Vandaele, A. C., et al., the NOMAD Science Team and ACS Science Team: Martian dust storm impact on atmospheric H2O and D/H observed by ExoMars Trace Gas Orbiter, Nature, 568, 521–525, doi:10.1038/s41586-019-1097-3, 2019.