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The Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has been used to observe the Mg+ in the upper atmosphere of Mars since 2015. These observations reveal significant diurnal, seasonal and latitudinal variations. For example, a factor of two change in Mg+ occurs during the day at the equator and Mg+ above 100 km decreases across the globe during the Southern Hemisphere summertime. Possible causes include the influence of photo-chemistry, atmospheric dynamics and the injection rate of meteoric material. To delineate these sources of variability, a 3-dimensional model of the Martian Mg has been developed. The model is based on the Laboratoire de Météorologie Dynamique (LMD) Mars global circulation model (GCM), which is capable of simulating the seasonal cycles in the Martian atmosphere arising from changes in heating rates from orbital geometry and the distribution of dust in the lower atmosphere. The model solves for the dynamics and primary constituents of the atmosphere from the surface to the thermosphere. For this study, the chemistry has been augmented with neutral and ion chemistry for meteoric metals within a CO2 atmosphere, incorporating 7 neutral and 8 ionized Mg-containing species and an additional 42 neutral and ion-molecule reactions. The atmospheric input of meteoric material is specified from seasonal and latitudinal fits to new estimates of the deposition of the ablated metals in the atmosphere. Here we present the first detailed intercomparison of the MAVEN IUV Mg+ observations with simulated Mg+ from the LMD Mars GCM. Model variability is consistent with observations and indicates tropical variability is caused by a combination of photochemistry and vertical transport by atmospheric tides. Comparisons of simulations performed with variable and fixed meteoric input show that the high latitude variations are caused by both seasonal variation in ablation rates and the residual circulation. Finally, we show how Mg+ varies in relation to neutral Mg and the remaining Mg-containing species to determine how Mg+ may be used as a proxy for total Mg variability.

Since the discovery of atmospheric Mg+ at Mars in 2015 by the Mars Atmosphere and Volatile Evolution (MAVEN) mission, there have been almost continuous observations of this meteoric ion layer in a variety of seasons, local times, and latitudes. Here we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand composite maps of a metallic ion species. These maps demonstrate that Mg+ appears in almost all conditions when illuminated, with peak values varying between 100 and 500 cm-3, dependent on season and local time. There exists significant latitudinal variation within a given season, indicating that Mg+ is not simply an inert tracer, but instead may be influenced by the meteoric input distribution and/or atmospheric dynamics and chemistry. Geographic maps of latitude and longitude indicate that Mg+ may be influenced by atmospheric tides, and there is no apparent correlation with remnant crustal magnetic fields. This work also presents counter-intuitive results, such as a reduction of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols.

It is evident that a variety of metals are deposited in the Earth’s mesosphere and lower thermosphere (MLT, ~70-120 km) through meteoric ablation when the cosmic dust particles enter the atmosphere at high entry velocity (11-72 km/s). However, it is still unclear how much and accuarate cosmic dust enters the atmosphere of Mars (though the estimation of global dust input would be a few tons per sol) and what is the difference comparing to Earth’s atmosphere (which has a 1-2 order global input range from different estimations). We have developed global atmospheric meteoric models of Na, Fe, K, Mg, Ni, Ca, Al, Si, P, S etc) into the Whole Atmosphere Community Climate Model (WACCM) and its vertical extensions to 600 km (WACCM-X) from US National Center for Atmospheric Research (NCAR, termed WACCM-metals), which simulate well the metal layers compared with the available lidar/rocket/satellite measurements. New observations of some metals for the Martian atmosphere (i.e., Mg+ observations from IUVS (Imaging UV Spectrometer) and Mg+, Na+ and Fe+ from NGIMS (Neutral Gas Ion Mass Spectrometer)) instruments on NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft are available from 2014. Therefore, we have incorporated the chemistry of three metals (Mg, Na and Fe) in the Laboratoire de Météorologie Dynamique (LMD) Mars global circulation model (termed as LMD-Mars-Metals), following similar work we have done for the Earth’s atmosphere. The model has been developed by combining three components: the state-of-the-art LMD-Mars model covering the whole atmosphere from the surface to the upper thermosphere (up to ~ 2 x10-8 Pa or 240 km), a description of the neutral and ion-molecule chemistry of Mg, Fe and Na in the Martian atmosphere (where the high CO2 abundance produces a rather different chemistry from the terrestrial atmosphere), and a treatment of injection of the metals into the atmosphere from the ablation of cosmic dust particles. The LMD-Mars model contains a detailed treatment of atmospheric physics, dynamics and chemistry from the lower atmosphere to the ionosphere. The model also includes molecular diffusion and considers the chemistry of the C, O, H and N families and major photochemical ion species in the upper atmosphere, as well as improved treatments of the day-to-day variability of the UV solar flux and 15 mm CO2 cooling under non-local thermodynamic equilibrium conditions. We have incorporated the chemistries of Mg, Fe and Na into LMD-Mars because these metals have different chemistries which control the characteristic features of their ionized and neutral layers in the Martian atmosphere. The Mg chemistry adds 7 neutral and 8 ionized Mg-containing species, connected by 42 neutral and ion-molecule reactions. The corresponding Fe chemistry has 39 reactions with 14 Fe-containing species. Na chemistry adds 7 neutral and only 2 ionized Na-containing species, with 32 reactions. The injection rate of these metals as a function of latitude, solar longitude at different pressure levels is pre-calculated from the Leeds Chemical Ablation Model (CABMOD) combined with an astronomical model which predicts the dust from Jupiter Family and Long Period comets, as well as the asteroid belt, in the inner solar system. The model has been evaluated against by Mg+, Na+ and Fe+ observations from IUVS and NGIMS measurements. The comparison of these metal layers between Earth’s and Mar’s atmospheres will be discussed, which allows us to understand the meteor astronomy, chemistry and transport processes that control the different metal layers in the upper atmosphere on different planets.

Interplanetary dust particles, liberated from the surfaces of comets and asteroids, are ubiquitous in interplanetary space within the solar system. These particles travel at orbital velocities and ablate upon entry into planetary atmospheres, where they are the sole explanation for high altitude atmospheric metal layers. On Earth, such layers inform us of the dynamics of the upper atmosphere, and we use the abundance of relative species to investigate the origin of these particles from various potential sources (Jupiter family comets, asteroids, etc.). Since the discovery of atmospheric Mg+ at Mars in 2015, there have been almost continuous observations of this layer in a variety of seasons, local times, and latitudes. Here we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand average maps of metal ions species. Such maps can be compared to current and future modeling efforts, which attempt to track mesospheric transport, chemistry and interplanetary dust particle sources. This work confirms some previous model predictions and observations, such as the relatively long lifetime of Mg+, but also presents counter-intuitive results, such as a paucity of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols. Previous discrepancies between model predictions and metal ion observations led to the development of a novel nucleation scheme for mesospheric clouds, and we revisit these ideas on a global and seasonally varying scale. Overall, this represents the broadest investigation of meteoric metal ions, summarizes the first order behavior and outlines new model challenges for the future.

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., We present observations of terminator CO2 ice clouds events in three groups: Equatorial dawn, Equatorial dusk (both between 20°S and 20°N) and Southern midlatitudes at dawn (45°S and 55°S east of Hellas Basin) with ESA ExoMars Trace Gas Orbiter's Nadir and Occultation for MArs Discovery instrument. CO2 ice abundance is retrieved simultaneously with water ice, dust, and particle sizes, and rotational temperature and CO2 column profiles in 16 of 26 cases. Small particles (, 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). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 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/R001405/1 and ST/S00145X/1 and Italian Space Agency through grant 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 NASA's Mars Program Office under WBS 604796, “Participation in the TGO/NOMAD Investigation of Trace Gases on Mars” and by NASA's SEEC initiative under Grant Number NNX17AH81A, “Remote sensing of Planetary Atmospheres in the Solar System and Beyond”. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004052. U.S. investigators were supported by the National Aeronautics and Space Administration.

The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies respectively determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in‐situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring non‐globally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bowshock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers’ wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN (MAVEN) Radio Occultation Science Experiment (ROSE) in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed.

Here we report a global model of meteoric metals including Mg, Fe and Na in the Laboratoire de Météorologie Dynamique (LMD) Mars global circulation model (termed as LMD-Mars-Metals), following on similar work as we have done for the Earth’s atmosphere. The model has been developed by combining three components: the state-of-the-art LMD-Mars model covering the whole atmosphere from the surface to the upper thermosphere (up to ~ 2 x10-8 Pa or 240 km), a description of the neutral and ion-molecule chemistry of Mg, Fe and Na in the Martian atmosphere (where the high CO2 abundance produces a rather different chemistry from the terrestrial atmosphere), and a treatment of injection of the metals into the atmosphere as a result of the ablation of cosmic dust particles. The LMD-Mars model contains a detailed treatment of atmospheric physics, dynamics and chemistry from the lower atmosphere to the ionosphere. The model also includes molecular diffusion and considers the chemistry of the C, O, H and N families and major photochemical ion species in the upper atmosphere, as well as improved treatments of the day-to-day variability of the UV solar flux and 15 mm CO2 cooling under non-local thermodynamic equilibrium conditions. So far, we have incorporated the chemistries of Mg, Fe and Na into LMD-Mars because these metals have different chemistries which control the characteristic features of their ionized and neutral layers in the Martian atmosphere. The Mg chemistry has 4 neutral and 6 ionized Mg-containing species, connected by 25 neutral and ion-molecule reactions. The corresponding Fe chemistry has 39 reactions with 14 Fe-containing species. Na chemistry has 7 neutral and only 2 ionized Na-containing species, with 32 reactions. The injection rate of these metals as a function of height is pre-calculated from the Leeds Chemical Ablation Model (CABMOD) combined with an astronomical model which predicts the dust from Jupiter Family and Long Period comets, as well as the asteroid belt, in the inner solar system. The LMD-Mars-Metals model has been run for several full Martian years under different surface dust scenarios to investigate the impact of high atmospheric dust loadings on the modelled metal layers. The model has been evaluated against Mg+ observations from IUVS (Imaging UV Spectrometer) and NGIMS (Neutral Gas Ion Mass Spectrometer) instruments on NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. We have also carried out other sensitivity experiments with different seasonality/altitude/latitudinal varying of Meteoric Input Function (MIF) of these metals in the model. These sensitivity results will be discussed.

Observations of the vertical distribution of water vapour provide a unique snapshot of the vertical transport processes that contribute to the global martian hydrological cycle. While previous datasets have largely been seasonally and spatially sparse, vertical profiles of water retrieved from the Nadir and Occultation for MArs Discovery (NOMAD) and Atmospheric Chemistry Suite (ACS) instruments on the ExoMars Trace Gas Orbiter (TGO) provide the most complete dataset so far. These data are now capable of providing robust constraints on the 4-D distribution of water, especially when also combined with retrievals of additional atmospheric properties (e.g. temperature profiles, dust column) that exert an influence on the evolving global water distribution. A key limitation though is the fact that observations of water profiles are still relatively limited in coverage, in the global sense, and the vertical distribution of water at latitudes and times not regularly probed by NOMAD and ACS remains poorly understood. To address this, we have created a global reference climatology of water vertical distribution for Mars Year (MY) 34 through a multi-spacecraft data assimilation combining several retrieval datasets with a Mars Global Circulation Model. Retrievals of dust column and temperature profiles from Mars Climate Sounder on the Mars Reconnaissance Orbiter and water vapour and temperature profiles from multiple instruments on the ExoMars TGO during the primary science phase covering the latter half of MY34 are combined through assimilation to create one unified physically consistent global dataset. The vertical water vapour distribution is investigated globally. During the initial coverage of TGO observation that covers the dusty season in MY34, northern polar latitudes are largely absent of water vapour below 20 km with variations in abundance above this altitude throughout the dusty season linked to transport from mid-latitudes during a global dust storm, perihelion season and the intense MY34 C storm. The atmosphere is in a supersaturated state above 60 km for most of the time period investigated, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. A key benefit of the data assimilation technique is that constraints on dynamical transport imposed by the assimilated water vapour and temperature profiles leads to improvements in the simulated water ice distribution even though it is not altered directly by the assimilation process. The climatology created, which will become publicly available for wider use by the martian scientific community, has also been independently validated against water vapour profiles from the SPICAM instrument.

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".

Following the recent detection of HCl in the atmosphere of Mars by ExoMars/Trace Gas Orbiter, we present here the first measurement of the 37Cl/35Cl isotopic ratio in the Martian atmosphere using a set of Nadir Occultation for MArs Discovery (NOMAD) observations. We determine an isotopic anomaly of −6 ± 78‰ compared to Earth standard, consistent with the −51‰–−1‰ measured on Mars’ surface by Curiosity. The measured isotopic ratio is also consistent with surface measurements, and suggests that Cl reservoirs may have undergone limited processing since formation in the Solar Nebula. The examination of possible sources and sinks of HCl shows only limited pathways to short-term efficient Cl fractionation and many plausible reservoirs of “light” Cl. © 2021. American Geophysical Union., 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). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 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 and ST/S00145X/1 and Italian Space Agency through grant 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 Astrofisica de Andalucia (SEV-2017-0709), and the CBK PAN team from the EXOMHYDR project, carried out within the TEAM program of the Foundation for Polish Science cofinanced by the European Union under the European Regional Development Fund (TEAM/2016-3/20). This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars" and by NASA's SEEC initiative under Grant Number NNX17AH81A, "Remote sensing of Planetary Atmospheres in the Solar System and Beyond." U.S. investigators were supported by the National Aeronautics and Space Administration. S. Viscardy acknowledges support from the Belgian Fonds de la Recherche Scientifique-FNRS under grant numbers 30442502 (ET_HOME) and Belgian Science Policy Office BrainBe MICROBE Projects. S. Aoki is "Charge de Recherches" at the F.R.S-FNRS. F. Schmidt acknowledges support from the "Institut National des Sciences de l'Univers" (INSU), the "Center National de la Recherche Scientifique" (CNRS) and "Center National d'Etudes Spatiales" (CNES) through the "Program National de Planetologie". S. Robert thanks BELSPO for the FED-tWIN funding (Prf-2019-077-RT-MOLEXO).

International audience; Interplanetary dust is a scientifically important constituent of the Solar System that consists of material shed by asteroids, comets, and other airless bodies. As used here, the term “dust” includes interplanetary dust particles and micrometeoroids. Dust has been studied by missions such as Mariner, Pioneer, and Voyager in both interplanetary space and in the vicinity of most of the planets.To date, however, no dedicated interplanetary dust instrument has yet been employed for detailed analysis of the dust environment of Mars. Partial data on dust flux has been provided by the 1965 Mariner IV flyby, the MAVEN orbiter, and other missions, but a complete understanding of interplanetary dust abundance, composition, debris hazard, annual flux variation, and origins is lacking. These data are critical for understanding the effects of dust upon the martian system, including the carbonaceous input into the regolith of Mars and its moons, the chemical input into the martian atmosphere, potential effects upon remote sensing data, the hypothesized existence of a Phobos dust ring, and possible annual variations from meteor shower infall. These effects have direct ramifications for interpretation of Mars/Phobos/Deimos mission science and analysis of returned samples from those worlds. To remediate this shortfall, the authors recommend that a dedicated interplanetary dust analysis instrument should be included in the instrument package for an upcoming martian orbiter in the near term. Such an interplanetary dust analysis instrument should collect data over a time period of several martian years in order to generate a statistically robust data set on interplanetary dust concentration and flux over a wide range of mass, and to discern temporal variation over multiple martian years.

Wind is the process that connects Mars’ climate system. Measurements of Mars atmospheric winds from orbit would dramatically advance our understanding of Mars and help prepare for human exploration. Multiple instruments in development will be ready for flight in the next decade. We urge the Decadal Survey to make these measurements a priority.

Mars Polar Science is an integrated, compelling system that serves as a nearby analogue to numerous other planets, supports human exploration, and habitability. Mars possesses the closest and most easily accessible layered ice deposits outside of Earth, and accessing those layers to read the climate record would be a triumph for planetary science.

Methane (CH4) on Mars has attracted a great deal of attention since it was first detected in January 2003. As methane is considered a potential marker for past/present biological or geological activity, any possible detection would require evidence with strong statistical significance. Ethane (C2H6) and ethylene (C2H4) are also relevant chemical species as their shorter lifetimes in the Martian atmosphere make them excellent tracers for recent and ongoing releases. If detected, a CH4/C2Hn ratio could aid in constraining the potential source of organic production. Here we present the results of an extensive search for hydrocarbons in the Martian atmosphere in 240,000 solar occultation measurements performed by the ExoMars Trace Gas Orbiter/NOMAD instrument from April 2018 to April 2019. The observations are global, covering all longitudes and latitudes from 85°N to 85°S, and sampled from 6 to 100 km altitude with a typical vertical resolution of 2 km. There were no statistically significant detections of organics and new stringent upper limits for global ethane and ethylene were set at 0.1 ppbv and 0.7 ppbv, respectively. No global background level of methane was observed, obtaining an upper limit of 0.06 ppbv, in agreement with early results from ExoMars (Korablev et al., 2019). Dedicated searches for localized plumes at more than 2000 locations provided no positive detections, implying that if methane were released in strong and rapid events, the process would have to be sporadic. © 2020 Elsevier Inc. All rights reserved., The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAACSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) 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 and ST/S00145X/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique -FNRS under grant number 30442502 (ET_HOME). 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). Canadian investigators were supported by the Canadian Space Agency. This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars". MC is supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association (USRA) under contract with NASA. S. A. is "Charge de Recherches" at the F.R.S.-FNRS. We would also like to thank Dr. Sara Faggi for valuable advice, expertise and continuous support during this study.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited., Isotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible. © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science., This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401 and 4000121493), by the Spanish MICINN through its Plan Nacional, by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), and by the Spanish Science Ministry Centro de Excelencia Severo Ochoa Program under grant SEV-2017-0709, as well as by the U.K. Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1, and ST/S00145X/1 and the 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” and by NASA’s SEEC initiative under grant number NNX17AH81A, “Remote sensing of planetary atmospheres in the solar system and beyond.” M.J.C. was supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association (USRA) under contract with NASA. S.A. is postdoctoral researcher of the Belgian Fund for Scientific Research (FNRS).

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).

The NOMAD instrument suite on the ESA-Roskosmos ExoMars Trace Gas Orbiter (TGO) observes the physical and chemical composition of the Martian atmosphere with highly resolved vertical profiles and nadir sounding in the IR and UV-vis domains. Vertically resolved profiles of, amongst other species, water vapor, HDO, ozone, CO, CO2, oxygen airglow, dust and clouds were obtained for more than one Martian year [1-5]. During its first year of operations, NOMAD witnessed the 2018 Global Dust Storm (GDS) during its onset, peak and decline. The redistribution of water vapor to high altitudes and latitudes observed during the GDS was explained using the GEM-Mars General Circulation Model (GCM) [6-8]. The GCM was driven by the dust optical depths for Mars Year 34 provided by [9]. The photolysis products of water vapor are a major driver for the atmospheric chemistry on Mars. As water vapor is redistributed over the atmosphere, it is expected to have considerable impact on many other species. GEM-Mars contains routines for atmospheric chemistry and here we present some results of the simulated impact of the GDS on atmospheric chemistry and on several of the observed species. GEM-Mars now also includes the simulation of HDO and the fractionation of water vapor upon cloud formation. The simulations will be compared with the vertical profiles of the D/H ratio obtained from NOMAD observations. The impact of the GDS on D/H can be estimated from these simulations. References [1] Vandaele, A. C. et al. (2019), Nature, 568, 7753, 521-525, doi: 10.1038/s41586-019-1097-3.[2] Aoki, S. et al. (2019), J. Geophys. Res.: Planets, 124, 3482–3497. https://doi.org/10.1029/2019JE006109[3] Gérard et al. (2020), Nature Astronomy, https://doi.org/10.1038/s41550-020-1123-2[4] Villanueva et al., submitted.[5] Korablev et al., 2020, in rev.[6] Neary, L. and F. Daerden (2018), Icarus, 300, 458–476, https://doi.org/10.1016/j.icarus.2017.09.028[7] Daerden, F. et al. (2019), Icarus, 326, 197-224, doi: 10.1016/j.icarus.2019.02.030.[8] Neary, L. et al. (2020), Geophys. Res. Lett., 47, e2019GL084354. https://doi.org/10.1029/2019GL084354[9] Montabone, L. et al. (2019), J. Geophys. Res.: Planets. doi: 10.1029/2019JE006111. 2.11.0.0 BIRA-IASB NOMAD team (continued): S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1). BIRA-IASB NOMAD team (continued): S. Robert (1), L. Trompet (1), A. Mahieux (1), C. Depiesse (1), E. Neefs (1) and B. Ristic (1). How to cite: Daerden, F., Neary, L., Villanueva, G., Aoki, S., Viscardy, S., Thomas, I., Vandaele, A. C., Liuzzi, G., Crismani, M., Khayat, A., Smith, M. D., Clancy, R. T., Wolff, M. J., Sandor, B. J., Whiteway, J. A., Mumma, M. J., Erwin, J., Willame, Y., and Piccialli, A. and the BIRA-IASB NOMAD team (continued): Simulating Mars D/H and atmospheric chemistry during the 2018 Global Dust Storm and comparing with NOMAD observations , Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-371, 2020

The NOMAD-SO channel [1, 2] is an infrared spectrometer working in the 2.2 to 4.3 µm spectral range (2325-4545 cm-1). The instrument is composed of an echelle grating coupled to an Acousto-Optical Tunable Filter for the diffraction order selection [3]. NOMAD started to perform solar occultation measurement on April 21, 2018. As TGO is on a quasi-circular orbit at around 400 km of altitude, it performs one orbit every two hours. During a solar occultation measurement, SO scans six diffraction orders each second. These diffraction orders are recorded on four bins leading to a vertical sampling below 1 km. The calibration of the SO channel is described in [4] and is being refined.NOMAD-SO regularly scans different diffraction orders containing CO2 lines to allow CO2 retrievals from low to high altitudes. For each solar occultation measurement, we derive a slant column profile of CO2 using ASIMUT-ALVL [4]. ASIMUT is a radiative transfer program developed at BIRA-IASB and based on the Optimal Estimation Method [5]. The GEM-Mars GCM provides the a priori profiles of CO2 local density, pressure and temperature. We then apply Tikhonov linear regularization on the slant column to derive a smoothed local density. We finally apply the hydrostatic equilibrium equation and the ideal gas law to derive the temperature profiles [6-8]. That derived temperature profile serves then in a new loop where we perform again the previous steps until the profiles converge [8]. Several comparisons are ongoing with joint or co-located measurements from MAVEN-EUVM, Maven-NGIMS, and TGO-ACS-NIR as well as with GCM derived profiles from GEM-Mars and LMD-MGCM. We derived the NOMAD-SO CO2 and temperature profiles for MY34 with solar longitudes (Ls) extending from 298° to 326°. That time range contains the regional dust storm of MY34 that started at Ls 317°. We will present the updated CO2 and temperature profiles from NOMAD-SO measurementsAcknowledgementsExoMars 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). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grant ST/R005761/1 and Italian Space Agency through grant 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 numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. US investigators were supported by the National Aeronautics and Space Administration. References[1] Vandaele, A.C., et al.: Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission, Planet. Space Sci., Vol. 119, pp 233-249, 2015.[2] Neefs, E., et al.: NOMAD spectrometer on the ExoMars trace gas orbiter mission: part1 – design, manufacturing and testing of the infrared channels, Appl. Opt., Vol. 54(28), pp 8494-8520, 2015.[3] Thomas, I.R., et al.: Optical and radiometric models of the NOMAD instrument part II: the infrared channels – SO and LNO, Opt. Express, Vol. 24(4), pp 3790-3805, 2016.[4] Liuzzi, G. et al.: Methane on Mars: New insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration. Icarus. 321, 2018.[4] Vandaele, A.C., Kruglanski, M., De Mazière, M., Modeling and retrieval of atmospheric spectra using ASIMUT. Proceedings of the First Atmospheric Science Conference, ESRIN, Frascati, Italy, 2006.[5] Rodgers, C.D., Inverse method for atmospheric sounding: theory and practice. Hackensack, N.J. (Ed.), World Scientific University of Oxford, Oxford, 2000.[6] Gröller, H., et al.: MAVEN/IUVS stellar occultation measurements of Mars atmospheric structure and composition. Journal of Geophysical Research: Planets, 123, 1449–1483, 2018.[7] Snowden D., et al. :The thermal structure of titan's upper atmosphere, I: temperature profiles from Cassini INMS observations, Icarus, Vol. 226, pp. 552-582, 2013.[8] Mahieux, A. et al.: Densities and temperatures in the Venus mesosphere and lower thermosphere retrieved from SOIR on board Venus Express: Carbon dioxide measurements at the Venus terminator, J. Geophys. Res., Vol 117, E07001, 2012. How to cite: Trompet, L., Vandaele, A. C., Aoki, S., Erwin, J., Thomas, I., Villanueva, G., Liuzzi, G., Crismani, M., Lopez-Valverde, M. A., Hill, B., Piccialli, A., Daerden, F., Ristic, B., Lopez-Moreno, J. J., Bellucci, G., and Patel, M.: Update on CO2 and temperature profiles from NOMAD-SO on board ExoMars TGO, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-641, 2020

Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO) started the science measurements on 21 April, 2018. We present results on the retrievals of water vapor vertical profiles in the Martian atmosphere from the first Mars year measurements of the TGO/NOMAD.NOMAD is a spectrometer operating in the spectral ranges between 0.2 and 4.3 μm onboard ExoMars TGO. NOMAD has 3 spectral channels: a solar occultation channel (SO – Solar Occultation; 2.3–4.3 μm), a second infrared channel capable of nadir, solar occultation, and limb sounding (LNO – Limb Nadir and solar Occultation; 2.3–3.8 μm), and an ultraviolet/visible channel (UVIS – Ultraviolet and Visible Spectrometer, 200–650 nm). The infrared channels (SO and LNO) have high spectral resolution (λ/dλ~10,000–20,000) provided by an echelle grating used in combination with an Acousto Optic Tunable Filter (AOTF) which selects diffraction orders. The concept of the infrared channels are derived from the Solar Occultation in the IR (SOIR) instrument onboard Venus Express (VEx). The sampling rate for the solar occultation measurement is 1 second, which provides better vertical sampling step (~1 km) with higher resolution (~2 km) from the surface to 200 km. Thanks to the instantaneous change of the observing diffraction orders achieved by the AOTF, the SO channel is able to measure five or six different diffraction orders per second in solar occultation mode. In this study, we analyze the solar occultation measurements at diffraction order 134 (3011-3035 cm-1), order 136 (3056-3080 cm-1) and 168 (3775-3805 cm-1) acquired by the SO channel in order to investigate H2O vertical profiles.Knowledge of the water vapor vertical distribution is important to understand the water cycle and escape processes. Solar occultation measurements by the two spectrometers onboard TGO - NOMAD and Atmospheric Chemistry Suite (ACS) - allow us to monitor daily the water vapor vertical profiles through one whole Martian Year and obtain a latitudinal map for every ~20° of Ls. In 2018, for the first time after 2007, a global dust storm occurred on Mars. It lasted for more than two months (from June to August). Moreover, following the global dust storm, a regional dust storm occurred in January 2019. TGO began its science operations on 21 April 2018. NOMAD observations therefore fully cover the period before/during/after the global and regional dust storms and offer a unique opportunity to study the trace gases distributions during such events. We have analyzed those datasets and found a significant increase of water vapor abundance in the middle atmosphere (40-100 km) during the global dust storm from June to mid-September 2018 and the regional dust storm in January 2019. In particular, water vapor reaches very high altitudes, at least 100 km, during the global dust storm (Aoki et al., 2019, Journal of Geophysical Research, Volume124, Issue12, Pages 3482-3497, doi:10.1029/2019JE006109). A GCM simulation explained that dust storm related increases of atmospheric temperatures suppress the hygropause, hence reducing ice cloud formation and so allowing water vapor to extend into the middle atmosphere (Neary et al., 2020, Geophysical Research Letters, 47, e2019GL084354., doi: 10.1029/2019GL084354). The current study presents the results obtained when considering the extended dataset, which covers a full Martian year. The extended dataset includes the recent aphelion season that involves interesting phenomena such as sublimation of water vapor from the northern polar cap and formation of the equatorial cloud belt, and is known as a key period to understand the large north-south hemispheric asymmetries of Mars water vapor. Yet, until now, only few papers reported the water vapor vertical distribution during the aphelion season. The extended dataset also includes the period when the global dust storm occurred the year before; this will allow us to compare the water vapor distributions under global dust storm conditions with those found during non-global dust storm years. In the presentation, we will discuss the H2O vertical profiles as well as the aerosols vertical distribution retrieved from the first full Martian year measurements of the TGO/NOMAD. How to cite: Aoki, S., Vandaele, A., Daerden, F., Villanueva, G., Thomas, I., Erwin, J., Trompet, L., Robert, S., Neary, L., Viscardy, S., Piccialli, A., Liuzzi, G., Crismani, M., Clancy, T., Smith, M., Ristic, B., Lopez-Valverde, M.-A., Patel, M., Bellucci, G., and Lopez-Moreno, J.-J.: Water vapor vertical profiles on Mars: Results from the first full Mars Year of TGO/NOMAD science operations, Europlanet Science Congress 2020, online, 21 September–9 Oct 2020, EPSC2020-392, 2020

International audience; The close approach of comet C/2013 A1 (Siding Spring) to Mars in 2014 October provided a unique opportunity to observe a dynamically new Oort cloud comet with potential for interaction with a planet's atmosphere. The water-originating hydrogen coma of the comet extended to over 20 million km from the nucleus. Determining the properties of this coma contributes to characterizing the comet's water content and production rate. The present study analyzes a unique data set of high spectral resolution UV observations of comet C/2013 A1 Siding Spring measured by the Mars Atmosphere and Volatile Evolution spacecraft. The Siding Spring observations capture Lyα emissions from the Martian corona, the interplanetary medium, as well as the cometary H and D reservoirs. The isolated cometary spectra are analyzed to reveal a velocity distribution of H atoms that are consistent with model estimates of H2O photodissociated H emissions and of OH photodissociated H emissions, Doppler shifted from the main comet H emission line center by 18 km s−1 and 8 km s−1, respectively. The variations in comet H brightness with distance from the nucleus are used to constrain cometary water production to a rate of 0.5 × 1028 molecules s−1 at a time when Siding Spring was at 1.5 au, pre-perihelion.

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter (TGO) measured a large increase in water vapor at altitudes in the range of 40‐100 km during the 2018 global dust storm on Mars. Using a three‐dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere the temperature increases and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70‐100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high‐altitude atomic hydrogen following the high‐altitude water vapor increase by a few days.

International audience; We report the detection of bright aurora spanning Mars’ nightside during the space weather event occurring in September 2017. The phenomenon was similar to diffuse aurora detected previously at Mars, but 25 times brighter and detectable over the entire visible nightside. The observations were made with the Imaging UltraViolet Spectrograph (IUVS), a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft orbiting Mars. Images show that the emission was brightest around the limb of the planet, with a fairly uniform faint glow against the disk itself. Spectra identified four molecular emissions associated with aurora, and limb scans show the emission originated from an altitude of ~60 km in the atmosphere. Both are consistent with very high energy particle precipitation. The auroral brightening peaked around 13 September, when the flux of solar energetic electrons and protons both peaked. During the declining phase of the event, faint but statistically significant auroral emissions briefly appeared against the disk of the planet in the form of narrow wisps and small patches. These features are approximately aligned with predicted open field lines in the region of strong crustal magnetic fields in Mars’ southern hemisphere.

International audience; We report observations of Martian mesospheric ice clouds and thermospheric scale heights by the Imaging Ultraviolet Spectrograph on NASA's Mars Atmosphere and Volatile Evolution mission. The clouds are observed between 6 AM and 8 AM local time using mid-UV limb observations between 60 and 80 km tangent altitude where ice particles that scatter sunlight can appear as detached layers near the equator. The equatorial longitudinal distribution shows populations of clouds near -110° E and -10° E as well as a population near 90° E, which does not have a clear precedent. The cloud populations indicate a wave 3 pattern near 70 km, which is confirmed by independent mesospheric temperature observations. Scale heights 100 km above the clouds derived from concurrent IUVS observations also reveal a wave 3 longitudinal structure, suggesting that the temperature oscillations enabling the formation of mesospheric clouds couple to the upper atmosphere.

Observations of water ice clouds and aerosols on Mars can provide important insights into the complexity of the water cycle. Recent observations have indicated an important link between dust activity and the water cycle, as intense dust activity can significantly raise the hygropause, and subsequently increase the escape of water after dissociation in the upper atmosphere. Here present observations from Nadir and Occultation for MArs Discovery/Trace Gas Orbiter that investigate the variation of water ice clouds in the perihelion season of Mars year 34 (April 2018-2019), their diurnal and seasonal behavior, and the vertical structure and microphysical properties of water ice and dust. These observations reveal the recurrent presence of a layer of mesospheric water ice clouds subsequent to the 2018 global dust storm. We show that this layer rose from 45 to 80 km in altitude on a time scale of days from heating in the lower atmosphere due to the storm. In addition, we demonstrate that there is a strong dawn-dusk asymmetry in water ice abundance, related to nighttime nucleation and subsequent daytime sublimation. Water ice particle sizes are retrieved consistently and exhibit sharp vertical gradients (from 0.1 to 4.0 mu m), as well as mesospheric differences between the global dust storm (, 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). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 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), and by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1, and ST/S00145X/1 and Italian Space Agency through grant 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 Astrofisica de Andalucia (SEV-2017-0709). This work was supported by NASA's Mars Program Office under WBS 604796, "Participation in the TGO/NOMAD Investigation of Trace Gases on Mars" and by NASA's SEEC initiative under grant NNX17AH81A, "Remote sensing of Planetary Atmospheres in the Solar System and Beyond." M.C. is supported by the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Universities Space Research Association (USRA) under contract with NASA. The retrieval package used in this study is the Planetary Spectrum Generator, free and available online at https://psg.gsfc.nasa.gov/helpatm.php#retrieval, at the PSG GitHub site: https://github.com/nasapsg/retrievalOE, and on Zenodo (Liuzzi, 2020a). The database with the retrieved values is available of the PSG Exomars server at https://psg.gsfc.nasa.gov/apps/exomars.php and on Zenodo (Liuzzi, 2020b). The data used in this analysis are available at https://nomad.aeronomie.be/index.php/data.

International audience; The MAVEN IUVS instrument contains an echelle spectrograph channel designed to measure D and H Ly α emissions from the upper atmosphere of Mars. This channel has successfully recorded both emissions, which are produced by resonant scattering of solar emission, over the course of most of a martian year. The fundamental purpose of these measurements is to understand the physical principles underlying the escape of H and D from the upper atmosphere into space, and thereby to relate present-day measurements of an enhanced HDO/H2O ratio in the bulk atmosphere to the water escape history of Mars. Variations in these emissions independent of the solar flux reflect changes in the density and/or temperature of the species in the upper atmosphere. The MAVEN measurements show that the densities of both H and D vary by an order of magnitude over a martian year, and not always in synch with each other. This discovery has relevance to the processes by which H and D escape into space. One needs to understand the controlling factors to be able to extrapolate back in time to determine the water escape history from Mars at times when the atmosphere was thicker, when the solar flux and solar wind were stronger, etc. Further measurements will be able to identify the specific controlling factors for the large changes in H and D, which likely result in large changes in the escape fluxes of both species.

On 19 October 2014, comet C/2013 A1 (Siding Spring) had a close encounter with Mars and deposited cometary dust particles into the Martian atmosphere. We report a comprehensive analysis of the resulting meteor shower and its perturbation on Mars' atmosphere and ionosphere. Using Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph observations of ablated meteoric metallic species, we show this shower lasted less than 3 hr and was therefore limited to one hemisphere. Meteoric ablation occurred in a narrow altitude layer, with Mg+, Mg, Fe+, and Fe deposited between about 105 and 120 km, consistent with comet Siding Spring's relative velocity of 56 km/s. We find that 82 ± 25 t of dust was deposited, improving previous measurements and a thousand times larger than model expectations. With regular observations over two Mars days, we show that horizontal winds globally redistribute this material and also suggest new vertical transport mechanisms for metallic ions. Such transport is inconsistent with diffusion and may be related to electrodynamic processes. The rapid loss of neutral species and presence of ions at high altitudes indicate that our understanding of existing Martian meteoric chemistry modeling and ionospheric dynamics is incomplete.

Using the Mars Atmospheric and Volatile EvolutioN mission (MAVEN) Imaging Ultraviolet Spectrograph (IUVS), we found periodic longitudinal variations in CO2 density in the Martian atmosphere. The variations exhibit significant structure with longitudinal wave numbers 1, 2, and 3 in an effectively constant local solar time frame, and we attribute this structure to nonmigrating tides. The wave-2 component is dominated by the diurnal eastward moving DE1 tide at the equator and the semidiurnal stationary S0 tide at the midlatitudes. Wave-3 is dominated by the diurnal eastward moving DE2 tide, with possibly the semidiurnal eastward moving SE1 tide causing an amplitude increase at the midlatitudes. Structure in the wave-1 component can be explained by the semidiurnal westward moving SW1 tide.

International audience; First observations of the O I 130.4 nm resonant line performed by the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) are presented in this paper. This emission line is observed during the different orbit phases of MAVEN. The atomic oxygen density and the temperature at 200 km are retrieved from an automatic pipeline using a radiative transfer model for resonant scattering lines for a selection of coronal profiles. These selected profiles are representative of the coronal scans done during the first months of the mission (from November 2014 to January 2015). The derived oxygen density and the temperature near the exobase are in the predicted range by the current thermospheric models of Mars for moderate solar activity, and some diurnal variations are observed. However, the absolute calibration of the instrument significantly limits the accuracy of density and temperature results.

We present direct number density retrievals of carbon dioxide (CO2) and molecular nitrogen (N2) for the upper atmosphere of Mars using limb scan observations during October and November 2014 by the Imaging Ultraviolet Spectrograph on board NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We use retrieved CO2 densities to derive temperature variability between 170 and 220 km. Analysis of the data shows (1) low-mid latitude northern hemisphere CO2 densities at 170 km vary by a factor of about 2.5, (2) on average, the N2/CO2 increases from 0.042 ± 0.017 at 130 km to 0.12 ± 0.06 at 200 km, and (3) the mean upper atmospheric temperature is 324 ± 22 K for local times near 14:00.

International audience; Mars has lost a large fraction of its water to space, with the H component of this loss thought to occur mainly as a result of thermal (Jeans) escape from the upper atmosphere. Constraints on H loss have historically been made using hydrogen Lyman alpha (121.6nm) light scattered in the planet's extended upper atmosphere, or corona. Here we employ observations from the Mars Atmosphere and Volatile Evolution (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) to constrain H escape in Dec 2014 and Aug 2016, when MAVEN observed the dayside corona at low latitude. To obtain adequate fits and address systematic sources of uncertainty including instrument calibration, we fit in exobase number density and escape rate instead of density and temperature, employing Markov Chain Monte Carlo techniques. This produces better model fits to data than most previous analyses. When we assume a single population of H atoms, we obtain H temperatures inconsistent with expected trends and a shape mismatch between observed and modeled profiles, similar to previous studies. Introducing either a second population of H (at a distinct temperature and density) or adding deuterium to the corona allows for essentially perfect fits. Despite this model ambiguity, derived loss rates for both periods are within a factor of four, 3.3 − 8.8 × 108cm−2s−1 in Dec 2014 (Ls∼250) and 0.6 − 2.3 × 108cm−2s−1 in Aug 2016 (Ls∼200). These rates are similar those found in prior studies and confirm the known seasonal trend— doing so while incorporating the substantial uncertainty in absolute calibration insufficiently explored by previous studies.

International audience; In September 2017, an active region of the Sun produced a series of strong flares and a coronal mass ejection that swept past Mars producing enhanced ionization and heating in the upper atmosphere. Emissions from atmospheric hydrogen Lyman‐α were also enhanced at Mars. Temperatures derived from neutral species scale heights were used in conjunction with the H Lyman‐α observations to simulate the effects of this space weather event on martian hydrogen properties in the exosphere. It was found that hydrogen abundance in the upper atmosphere decreased by ~25%, and that the H escape rate increased by a factor of 5, mainly through an increase in upper atmospheric temperature. This significant escape rate variation is comparable to seasonally observed trends but occurred at much shorter timescales. Such solar events would statistically impact extrapolation of martian water loss over time.

International audience; Proton aurorae are a distinct class of auroral phenomena caused by energetic protons precipitating into a planetary atmosphere. The defining observational signature is atomic hydrogen emissions from the precipitating particles after they obtain an electron from the neutral atmospheric gas, a process known as charge exchange. Until now, proton aurorae have been observed at Earth only. Here, we present evidence of auroral activity driven by precipitating protons at Mars, using observations by the MAVEN spacecraft. We observed transient brightening of upper atmospheric hydrogen Lyman-α emission across the Martian dayside correlated with solar wind activity. The driving mechanism is one not found at Earth and originates from energetic neutral atom production by solar wind protons directly interacting with the extended hydrogen corona surrounding Mars. We characterize this new type of Martian aurora and compare the observed emissions with preliminary modelling guided by simultaneous in situ particle measurements. These observations provide insights into how the solar wind can directly deposit energy into the Martian atmosphere as well as all other planetary objects that are surrounded by a substantial neutral corona exposed to the solar wind.

International audience; Observations of the Mars upper atmosphere made from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have been used to determine the loss rates of gas from the upper atmosphere to space for a complete Mars year (16 Nov 2014 – 3 Oct 2016). Loss rates for H and O are sufficient to remove ∼2-3 kg/s to space. By itself, this loss would be significant over the history of the planet. In addition, loss rates would have been greater early in history due to the enhanced solar EUV and more-active Sun. Integrated loss, based on current processes whose escape rates in the past are adjusted according to expected solar evolution, would have been as much as 0.8 bar CO2 or 23 m global equivalent layer of H2O; these losses are likely to be lower limits due to the nature of the extrapolation of loss rates to the earliest times. Combined with the lack of surface or subsurface reservoirs for CO2 that could hold remnants of an early, thick atmosphere, these results suggest that loss of gas to space has been the dominant process responsible for changing the climate of Mars from an early, warmer environment to the cold, dry one that we see today.

International audience; We report the response of the Martian upper atmosphere to a strong X‐class flare on September 10, 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hrs UT and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of ∼270 K and a maximum value of ∼310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare induced heating is limited between low‐ and mid‐latitudes. During this event IUVS observed an ∼70% increase in the observed brightness of CO2+ Ultraviolet Doublet and CO Cameron band emission at 90 km, where high‐energy photons (< 10 nm) deposit most of their energy.

Stiepen, A. et al., We report results from a study of nitric oxide nightglow over the northern hemisphere of Mars during winter, the southern hemisphere during fall equinox, and equatorial latitudes during summer in the northern hemisphere based on observations of the δ and γ bands between 190 and 270 nm by the Imaging UltraViolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) spacecraft. The emission reveals recombination of N and O atoms dissociated on the dayside of Mars and transported to the nightside. We characterize the brightness (from 0.2 to 30 kR) and altitude (from 40 to 115 km) of the NO nightglow layer, as well as its topside scale height (mean of 11 km). We show the possible impact of atmospheric waves forcing longitudinal variability, associated with an increased brightness by a factor of 3 in the 140–200° longitude region in the northern hemisphere winter and in the −102° to −48° longitude region at summer. Such impact to the NO nightglow at Mars was not seen before. Quantitative comparison with calculations of the LMD-MGCM (Laboratoire de Météorologie Dynamique-Mars Global Climate Model) suggests that the model globally reproduces the trends of the NO nightglow emission and its seasonal variation and also indicates large discrepancies (up to a factor 50 fainter in the model) in northern winter at low to middle latitudes. This suggests that the predicted transport is too efficient toward the night winter pole in the thermosphere by ∼20° latitude north. ©2017. American Geophysical Union. All Rights Reserved., A. Stiepen is supported by the Fund for Scientific Research (F.R.S.-FNRS). The MAVEN mission is supported by NASA through the Mars Exploration Program in association with the University of Colorado and NASA's Goddard Space Flight Center. M. Stevens is supported by the NASA MAVEN Participating Scientist program. B. Hubert and J.-C. Gerard acknowledge support from the SCOOP/BRAIN program of the Belgian Federal Government. A. Stiepen also thanks M. Dumont for her help in the finalization of the figures. F.G.-G. is funded by the European Union Horizon 2020 Programme (H2020 Compet-08-2014) under grant agreement UPWARDS-633127.

International audience; Interplanetary dust particles sporadically enter planetary atmospheres at orbital velocities and ablate as collisions occur with ambient gases to produce a persistent layer of metallic atoms (for example, Fe, Mg, Na) in their upper atmospheres. Such layers are well studied at Earth, but have not been directly detected elsewhere in the Solar System. Here we report the detection of a meteoric layer consisting of Mg+ ions near an altitude of 90 km in the Martian atmosphere from ultraviolet remote sensing observations by NASA’s MAVEN spacecraft. We observe temporal variability in the Mg+ layer over the course of a Martian year, moving up and down in altitude seasonally and in response to dust storms, and displaying diurnal fluctuations in density. We also find that most meteor showers do not significantly perturb this layer, which constrains the fluence of eleven observed Martian meteor showers to less than our estimated global dust flux. The persistence and variability of the Mg+ layer are difficult to explain with existing models and reconcile with other transient layers of ions observed in the Martian ionosphere. We suggest that the transient layers are not sourced from the persistent Mg+ layer and thus not derived from meteoric material, but are ambient ions produced by some unknown mechanism.

Saturn's moon Enceladus offers a unique opportunity in the search for life and habitable environments beyond Earth, a key theme of the National Research Council's 2013-2022 Decadal Survey. A plume of water vapor and ice spews from Enceladus's south polar region. Cassini data suggest that this plume, sourced by a liquid reservoir beneath the moon's icy crust, contain organics, salts, and water-rock interaction derivatives. Thus, the ingredients for life as we know it-- liquid water, chemistry, and energy sources-- are available in Enceladus's subsurface ocean. We have only to sample the plumes to investigate this hidden ocean environment. We present a New Frontiers class, solar-powered Enceladus orbiter that would take advantage of this opportunity, Testing the Habitability of Enceladus's Ocean (THEO). Developed by the 2015 Jet Propulsion Laboratory Planetary Science Summer School student participants under the guidance of TeamX, this mission concept includes remote sensing and in situ analyses with a mass spectrometer, a sub-mm radiometer-spectrometer, a camera, and two magnetometers. These instruments were selected to address four key questions for ascertaining the habitability of Enceladus's ocean within the context of the moon's geological activity: (1) How are the plumes and ocean connected? (2) Are the abiotic conditions of the ocean suitable for habitability? (3) How stable is the ocean environment? (4) Is there evidence of biological processes? By taking advantage of the opportunity Enceladus's plumes offer, THEO represents a viable, solar-powered option for exploring a potentially habitable ocean world of the outer solar system., JPL Summer School 2015

International audience; Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

International audience; We report the detection of intense emission from magnesium and iron in Mars' atmosphere caused by a meteor shower following Comet Siding Spring's close encounter with Mars. The observations were made with the Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft orbiting Mars. Ionized magnesium caused the brightest emission from the planet's atmosphere for many hours, resulting from resonant scattering of solar ultraviolet light. Modeling suggests a substantial fluence of low-density dust particles 1–100 µm in size, with the large amount and small size contrary to predictions. The event created a temporary planet-wide ionospheric layer below Mars' main dayside ionosphere. The dramatic meteor shower response at Mars is starkly different from the case at Earth, where a steady state metal layer is always observable but perturbations caused by even the strongest meteor showers are challenging to detect.

International audience; Observation of the hot oxygen corona at Mars has been an elusive measurement in planetary science. Characterizing this component of the planet's exosphere provides insight into the processes driving loss of oxygen at the current time, which informs understanding of the planet's climatic evolution. The Mars Atmosphere and Volatile EvolutioN (MAVEN) Imaging Ultraviolet Spectrograph (IUVS) instrument is now regularly collecting altitude profiles of the hot oxygen corona as part of its investigation of atmospheric escape from Mars. Observations obtained thus far have been examined and found to display the expected gross structure and variability with EUV forcing anticipated by theory. The quality and quantity of the data set provides valuable constraints for the coronal modeling community.

International audience; The Mars Atmosphere and Volatile Evolution (MAVEN) mission, during the second of its Deep Dip campaigns, made comprehensive measurements of martian thermosphere and ionosphere composition, structure, and variability at altitudes down to ~130 kilometers in the subsolar region. This altitude range contains the diffusively separated upper atmosphere just above the well-mixed atmosphere, the layer of peak extreme ultraviolet heating and primary reservoir for atmospheric escape. In situ measurements of the upper atmosphere reveal previously unmeasured populations of neutral and charged particles, the homopause altitude at approximately 130 kilometers, and an unexpected level of variability both on an orbit-to-orbit basis and within individual orbits. These observations help constrain volatile escape processes controlled by thermosphere and ionosphere structure and variability.

International audience; Loss of water to space via neutral hydrogen escape has been an important process throughout Martian history. Contemporary loss rates can be constrained through observations of the extended neutral hydrogen atmosphere of Mars in scattered sunlight at 121.6 nm. Historically, such observations have been interpreted with coupled density and radiative transfer models, inferring escape fluxes from brightness profiles gathered by flybys, orbiters, and telescope observations. Here we demonstrate that the spherical symmetry assumed by prior analyses cannot reproduce observations by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We present unique observations of the Mars H corona to large radial distances and mapping results from initial MAVEN science at Mars. These observations represent the first detection of three-dimensional structure in the H corona of Mars, with implications for understanding the atmosphere today and the loss of H to space throughout Martian history.

International audience; We used the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiting spacecraft to construct images of the hydrogen coma of comet C/2013 A1 (Siding Spring) days before its close encounter with Mars. We obtain a water production rate of 1.1 ± 0.5 × 1028 molecules/s and determine the total impacting fluence of atoms and molecules corresponding to the photodissociation of water and its daughter species to be 2.4 ± 1.2 × 104 kg. We use these observations to confirm predictions that the mass of delivered hydrogen is comparable to the existing reservoir above 150 km. Furthermore, we reconcile disparity between observations and predictions about the detectability of the hydrogen perturbation and thermospheric response.

International audience; The first campaign of stellar occultations with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board of Mars Atmosphere and Volatile EvolutioN (MAVEN) mission was executed between 24 and 26 March 2015. From this campaign 13 occultations are used to retrieve CO2 and O2 number densities in the altitude range between 100 and 150 km. Observations probe primarily the low-latitude regions on the nightside of the planet, just past the dawn and dusk terminator. Calculation of temperature from the CO2 density profiles reveals that the lower thermosphere is significantly cooler than predicted by the models in the Mars Climate Database. A systematically cold layer with temperatures of 105–120 K is seen in the occultations at a pressure level around 7 × 10−6 Pa.

International audience; We identify molecular nitrogen (N2) emissions in the Martian upper atmosphere using the Imaging Ultraviolet Spectrograph (IUVS) on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We report the first observations of the N2 Lyman-Birge-Hopfield (LBH) bands at Mars and confirm the tentative identification of the N2 Vegard-Kaplan (VK) bands. We retrieve N2 density profiles from the VK limb emissions and compare calculated limb radiances between 90 and 210 km against both observations and predictions from a Mars general circulation model (GCM). Contrary to earlier analyses using other satellite data, we find that N2 abundances exceed GCM results by about a factor of 2 at 130 km but are in agreement at 150 km. The analysis and interpretation are enabled by a linear regression method used to extract components of UV spectra from IUVS limb observations.

International audience; We report a comprehensive study of Mars dayglow observations focusing on upper atmospheric structure and seasonal variability. We analyzed 744 vertical brightness profiles comprised of ∼109,300 spectra obtained with the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite. The dayglow emission spectra show features similar to previous UV measurements at Mars. We find a significant drop in thermospheric scale height and temperature between LS = 218° and LS = 337–352°, attributed primarily to the decrease in solar activity and increase in heliocentric distance. We report the detection of a second, low-altitude peak in the emission profile of OI 297.2 nm, confirmation of the prediction that the absorption of solar Lyman alpha emission is an important energy source there. The inline image UV doublet peak intensity is well correlated with simultaneous observations of solar 17–22 nm irradiance at Mars.

International audience; Planetary auroras reveal the complex interplay between an atmosphere and the surrounding plasma environment. We report the discovery of low-altitude, diffuse auroras spanning much of Mars’ northern hemisphere, coincident with a solar energetic particle outburst. The Imaging Ultraviolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, detected auroral emission in virtually all nightside observations for ~5 days, spanning nearly all geographic longitudes. Emission extended down to ~60 kilometer (km) altitude (1 microbar), deeper than confirmed at any other planet. Solar energetic particles were observed up to 200 kilo­–electron volts; these particles are capable of penetrating down to the 60 km altitude. Given minimal magnetic fields over most of the planet, Mars is likely to exhibit auroras more globally than Earth.

We present a systematic evaluation of the agreement between the observed radii of 90 well-characterized transiting extrasolar giant planets and their corresponding model radii. Our model radii are drawn from previously published calculations of core-less giant planets that have attained their asymptotic radii, and which have been tabulated for a range of planet masses and equilibrium temperatures. (We report a two-dimensional polynomial fitting function that accurately represents the models). As expected, the model radii provide a statistically significant improvement over a null hypothesis that the sizes of giant planets are completely independent of mass and effective temperature. As is well known, however, fiducial models provide an insufficient explanation; the planetary radius anomalies are strongly correlated with planetary equilibrium temperature. We find that the radius anomalies have a best-fit dependence, ${\cal R}\propto T_{\rm eff}^{\alpha}$, with $\alpha=1.4\pm0.6$. Incorporating this relation into the model radii leads to substantially less scatter in the radius correlation. The extra temperature dependence represents an important constraint on theoretical models for Hot Jupiters. Using simple scaling arguments, we find support for the hypothesis of Batygin and Stevenson (2010) that this correlation can be attributed to a planetary heating mechanism that is mediated by magnetohydrodynamic coupling between the planetary magnetic field and near-surface flow that is accompanied by ohmic dissipation at adiabatic depth. Additionally, we find that the temperature dependence is likely too strong to admit kinetic heating as the primary source of anomalous energy generation within the majority of the observed transiting planets., Comment: Astrophysical Journal Letters, In Press