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The Perseverance rover is carrying out an original acoustic experiment on Mars: the SuperCam microphone records the spherical acoustic waves generated by laser sparks at distances from 2 m to more than 8 m. These N-shaped acoustic waves scatter from the multiple local heterogeneities of the turbulent atmosphere. Therefore, large and random fluctuations of sound travel time and intensity develop as the waves cross the medium. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) are studied within the mathematical formalism of the propagation of spherical acoustic waves through thermal turbulence to infer statistical properties of the Mars atmospheric temperature fluctuation field. The comparison with the theory is made by simplifying assumptions that do not include wind fluctuations and diffraction effects. Two Earth years (about one Martian year) of observations acquired during the maximum convective period (10:00–14:00 Mars local time) show a good agreement between the dataset and the formalism: the travel time variance diverges from the linear Chernov solution exactly where the density of occurrence of the first caustic reaches its maximum. Moreover, on average, waves travel faster than the mean speed of sound due to a fast path effect, which is also observed on Earth. To account for the distribution of turbulent eddies, several power spectra are tested and the best match to observation is obtained with a generalized von Karman spectrum with a shallower slope than the Kolmogorov cascade, ϕ (k) ∝ (1 + k 2 L 2) − 4 / 3 . It is associated with an outer scale of turbulence, L, of 11 cm at 2 m above the surface and a standard deviation of 6 K over 9 s for the temperature. These near-surface atmospheric properties are consistent with a weak to moderate wave scattering regime around noon with little saturation. Overall, this study presents an innovative and promising methodology to probe the near-surface atmospheric turbulence on Mars. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The rate of water loss from Mars depends on hydrogen, the main product of the H2O photodissociation escaping from the upper atmosphere. The ability of water to reach high altitudes and to be a direct source of atomic hydrogen is limited by cloud formation, which holds water vapor in the lower atmosphere. This process is regulated by temperature and pressure. Condensation starts when the temperature is cold enough and condensation nuclei (CN) (e.g., small dust particles) are available. If CN are lacking, or their sizes are too small, or the temperature drops too fast, then water vapor becomes supersaturated. Recent studies have shown that Martian water vapor is often supersaturated. Here, we present the first seasonal cycle of the saturation state from simultaneous measurements of water and temperature during two Martian years by Atmospheric Chemistry Suite on the Trace Gas Orbiter. Our results show that supersaturation is typical on Mars. It occurs above clouds both in the aphelion and perihelion seasons as well as in the lower polar atmosphere. We demonstrate supersaturation to be an important factor facilitating the escape of water. Unlike the Earth, water easily penetrates the upper atmosphere and reaches photodissociation altitudes. Key Points: Long‐term observations of H2O saturation state in the atmosphere of Mars show that supersaturation is nearly ubiquitous above aerosol layersDuring the dusty season, the supersaturation is stronger in MY35 compared to MY34More water was found in the evening terminator and stronger supersaturation in the morning terminator [ABSTRACT FROM AUTHOR]

The middle infrared (MIR) channel of the atmospheric chemistry suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter ESA‐Roscosmos mission has performed Solar occultation measurements of the Martian atmosphere in the 2.3–4.2 μm 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, https://doi.org/10.1029/2019je006300) to monitor the properties (effective radii, extinction, and altitude) of the Martian water ice clouds over the first two Martian years covered by ACS‐MIR. The observations encompass the period Ls = 163° in MY 34 to Ls = 181° in MY 36. We determine that the typical altitude of the clouds varies by 20–40 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 20–40 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 the 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, 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. Plain Language Summary: We use data from the middle infrared channel of the atmospheric chemistry suite instrument onboard the ExoMars Trace Gas Orbiter mission, which has been probing the Martian atmosphere for two Martian years (MY), to study the properties of the water ice clouds in the Martian atmosphere. We observe that the clouds' altitude increases by 20–40 km during summer and that they are also observed 20–40 km higher around the equator compared to the polar regions. This highlight the spatial and temporal diversity of the Martian water ice clouds, along with the large amplitude of the annual variations of the atmospheric structure. In addition, we contrast observations acquired during the global dust storm (GDS) event of Summer 2018 (MY 34) with data from the following MY that we use as a reference to understand the effects of the GDS on the clouds, which reveals that the presence of the GDS increases the altitude of the clouds by 10–20 km. Finally, we compare our results to the predictions of the Mars planetary climate model which shows a reasonable agreement overall, although the model tends to predict lower altitudes for the top of the clouds. Key Points: Cloud altitude varies by 20–40 km between winter and summer and between polar and midlatitudesThe global dust storm in MY 34 increases the altitude of the clouds by 10–20 km compared to the same season in MY 35Clouds are predicted to be lifted at lower altitudes in the Mars planetary climate model compared to atmospheric chemistry suite detections [ABSTRACT FROM AUTHOR]

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). Plain Language Summary: The D/H ratio observed in a planetary atmosphere is traditionally used as a proxy to estimate the initial water reservoir of the planet. We present here an improved global circulation model including HDO, the main isotope of water on Mars. The updated model takes into account the details of the formation of clouds and their radiative effect. It also includes the effect of photochemistry on HDO and deuterated species. We find that supersaturation is key to the representation of the D/H cycle by making the hygropause more porous, allowing more HDO in the upper atmosphere. It also reduces the efficiency of the isotopic fractionation occurring at condensation. We present here comparisons between observations by the Atmospheric Chemistry Suite spectrometer onboard the orbiter TGO. While the model is globally in agreement with the observations, the inability of the model to reproduce the observed vertical distribution of dust, especially during the Global Dust Storm, causes discrepancies in the representation of water vapor. The model reveals the importance of representing the state of supersaturation to correctly account for the water vapor amount reaching the top of the atmosphere and estimate the D/H ratio at escape. Key Points: The HDO cycle has been implemented in the last version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model including microphysics and radiative effect of water ice cloudsKinetics effect is now included in HDO fractionation during condensation and proves to be significantSupersaturation, the presence of which is regulated by cloud processes, alters significantly the relative abundances of HDO and H2O [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Understanding how the Martian climate affects the isotopic ratio of water is key to understand the history of water on the planet. We use a general circulation model to simulate the cycle of HDO, an isotope of water, in the atmosphere of Mars. We compare our model results with spacecraft observations from the Atmospheric Chemistry Suite spectrometer on board the Trace Gas Orbiter, currently in orbit around Mars. Our model provides a good qualitative agreement with the observations. We find that the condensation of water vapor into ice is a critical process for determining the isotopic ratio of the vapor phase. Key Points: We simulated the HDO cycle on Mars using a 3D Global Climate ModelGCM simulations were compared with solar occultations from Atmospheric Chemistry Suite on board the Trace Gas OrbiterA good representation of the condensation processes is key to reproduce the observed D/H ratio [ABSTRACT FROM AUTHOR]

With the utilization of a novel synergistic approach, we constrain the vertical distribution of water vapor on Mars with measurements from nadir‐pointing instruments. Water vapor column abundances were retrieved simultaneously with PFS (sensing the thermal infrared range) and SPICAM (sensing the near‐infrared range) on Mars Express, yielding distinct yet complementary sensitivity to different parts of the atmospheric column. We show that by exploiting a spectral synergy retrieval approach, we obtain more accurate water vapor column abundances compared to when only one instrument is used, providing a new and highly robust reference climatology from Mars Express. We present a composite global data set covering all seasons and latitudes, assembled from colocated observations sampled from seven Martian years. The synergy also offers a way to study the vertical partitioning of water, which has remained out of the scope of nadir observations made by single instruments covering a single spectral interval. Special attention is given to the north polar region, with extra focus on the sublimation of the seasonal polar cap during the late spring and summer seasons. Column abundances from the Mars Climate Database were found to be significantly higher than synergistically retrieved values, especially in the summer Northern Hemisphere. Deviations between synergy and model in both magnitude and meridional variation of the vertical confinement were also discovered, suggesting that certain aspects of the transport and dynamics of water vapor are not fully captured by current models. Plain Language Summary: Water vapor plays an important role in the weather and climate on Mars, even though little of it remains today. The behavior of water vapor has been studied for decades, yet how water vapor varies with altitude, especially close to the surface, remains an open question. In this study, we use measurements from two instruments on the Mars Express satellite to learn about the near‐surface water vapor. By combining measurements from the SPICAM and PFS spectrometers, a composite full‐year climatology is assembled. We measure the total amount of water vapor with great accuracy, and also obtain information about the vertical distribution. The north polar cap is studied in detail during early summer, when part of the polar ice cap sublimates into water vapor and is transported south. The results are compared to model data from the Mars Climate Database, and significant differences between the observations and the model are identified. The total water content is found to be smaller than model estimates, while observations indicate that more water than expected is confined near the surface. This suggests that some aspects of the atmospheric transport processes are not currently fully understood. Key Points: Using a spectral synergy retrieval method on nadir observations from SPICAM and PFS to obtain a highly accurate water vapor climatologyThe synergy method is sensitive to the vertical distribution of H2O, and can distinguish near‐surface water from the rest of the columnDiscrepancies in meridional and seasonal behavior of vertical confinement are revealed between the synergy and the Mars Climate Database [ABSTRACT FROM AUTHOR]

Isotopic ratios in atmospheric CO2 are shaped by various processes throughout Mars' history, and can help understand what the atmosphere of early Mars was like to sustain liquid water on its surface. In this study, we monitor the O and C isotopic composition of CO2 between 70 and 130 km for more than half a Martian year using solar occultation observations by the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter. We find the vertical trends of the isotopic ratios to be consistent with the expectations from diffusive separation above the homopause, with average values below this altitude being consistent with Earth‐like fractionation (δ13C = −3 ± 37‰; δ18O = −29 ± 38‰; and δ17O = −11 ± 41‰). Using these measurements, we estimate that at least 20%–40% of primordial C on Mars has escaped to space throughout history. The total amount of C lost from the atmosphere is likely to be well in excess of this lower limit, due to carbonate formation and further sink processes. In addition, we propose a photochemical transfer of light O from H2O to CO2 to explain the larger enrichment in the O18/O16 ${}^{18}\mathrm{O}/{}^{16}\mathrm{O}$ ratio in H2O than in CO2. Plain Language Summary: There is ample evidence suggesting that liquid water was abundant on the surface of Mars in the past. However, climatic conditions on early Mars must have been very different from the ones we observe today to sustain liquid water on its surface. The ratios of the heavy and light isotopes in different species provide a very useful tool to estimate the early climate of Mars. In this study, we monitor the isotopic ratios of carbon dioxide in the atmosphere of Mars to provide more accurate estimates of these. With our measurements, and in context with previous studies, we estimate that at least 20%–40% of the carbon reservoir has been lost to space throughout Martian history. This, together with the sequestration of atmospheric C on the surface in the form of minerals, is consistent with the idea that the atmosphere of early Mars was denser than the one we observe today. Key Points: Isotopic ratios in CO2 are observed to be consistent with telluric standards and to fractionate by diffusive separation above the homopauseAt least 20%–40% of the C reservoir has escaped to space throughout Martian historyThe higher 18O/16O ratio in H2O than in CO2 may be explained by a photochemical transfer of lighter O from H2O to CO2 [ABSTRACT FROM AUTHOR]

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 CO2 absorption 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. Plain Language Summary: Gravity waves (GWs) of lower atmospheric origin continuously disturb the Martian atmosphere. While propagating upward, their amplitudes grow and eventually GWs break up or dissipate. The deposited momentum and energy are the major mechanisms driving the circulation in the thermosphere above 100 km. Since spatial scales of GWs are relatively small, they are difficult to measure. Atmospheric Chemistry Suite (ACS) instrument on board the ExoMars Trace Gas Orbiter allows for extracting altitude profiles of density and temperature from the troposphere to the thermosphere (20–160 km) with high vertical resolution, around 2 km. The instrument measures the solar spectrum occulted by the atmosphere with the carbon dioxide absorption in the middle infrared wavelength range. The observations provide latitudinal and seasonal coverage of the GW activity and its characterization on Mars. Our results allow for the first observational validation of model predictions, quantifying dynamical effects of GWs and constraining Martian general circulation models. Key Points: Observations of gravity waves from the Atmospheric Chemistry Suite instrument on board ExoMars Trace Gas Orbiter are presentedGlobal distribution of the observed wave activity, potential energy, momentum fluxes and wave drag agrees well with model predictionsWe found no correlation between wave amplitudes and buoyancy frequency, an extension of previously observed anticorrelation with temperature [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: We report regular events of high abundances of the water vapor (H2O) in the upper atmosphere of Mars (100–120 km). So far, any water enrichment has not been revealed by remote sensing at such high altitudes. Higher than 80 km, solar light breaks water vapor molecules into H and O atoms, which may reach the exosphere and escape the planet. When Mars is closer to the Sun (the perihelion season), the atmosphere's circulation intensifies, causing increased dust activity with global dust storms (GDS), occurring every 3–4 Mars years. We observed during the second halves of Martian years 34 and 35 (2018–2020), including one GDS and two perihelion seasons. We report that the maximum water relative abundance reaches 10–50 parts per million in volume (ppmv) at 100–120 km during the GDS and every perihelion season. These high values indicate that the Martian atmosphere above 100 km regularly hosts large amounts of water, facilitating the long‐term escape of water from the planet. Key Points: Water abundances are reported in a previously unexplored altitude range: from 100 to 120 kmThe observed GDS (MY34) and two perihelion seasons (MY34, 35) reveal the H2O content around 10–50 parts per million by volume at 100–120 kmContributions of the MY34 GDS and perihelion periods into the projected hydrogen escape from Mars are nearly equivalent [ABSTRACT FROM AUTHOR]

The deuterium to hydrogen (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 behavior, 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 (GDS) that occurred during Martian year 34 (MY34) 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. Plain Language Summary: HDO, the semi‐heavy isotope of water, when compared to water, is a good indicator of how much water has been escaping from the atmosphere of Mars over the ages. Ultimately, it can be used to estimate the past reservoir of water available on Mars in its early youth. Because HDO has a slightly higher molecular mass compared to H2O, condensation induces an enrichment of HDO in the ice phase compared with the vapor phase. This subsequently causes spatial and temporal variations of the deuterium to hydrogen ratio. We use a global circulation model to simulate the HDO cycle in the atmosphere. Our model is an upgrade of the previous model presented in Montmessin et al. (2005, Journal of Geophysical Research, 110(E3), E10004. doi:10.1029/2004JE002357). We then simulate the effect of the global dust storm that affected Mars during the summer of 2018, and show that it should have had a strong impact on the vertical distribution of HDO, allowing it to reach higher altitudes. Such simulations are intended to be compared with observations from the Trace Gas Orbiter, currently in orbit around Mars. Key Points: We reimplemented the HDO cycle in a more recent version of the Laboratoire de Météorologie Dynamique Mars global climate modelWe reproduce the observed gradient of the deuterium to hydrogen ratio between cold and warm regionsThe global dust storm of Martian year 34 has a strong effect on the vertical distribution of HDO [ABSTRACT FROM AUTHOR]

We report a new set of stellar occultation measurements for nightside temperature profiles made by the Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph that provide evidence for a recurring layer of warm air between 70 and 90 km altitudes in the nightside mesosphere of Mars during Ls = 0–180° in Martian Year 33–34. The nightside profiles reveal a recurring peak of atmospheric temperature around 80 km over the equator to the middle latitudes in the northern hemisphere. The predictions of the Mars Climate Database have a warm layer with much smaller amplitudes. The observed peak amplitudes are larger than those predicted by the model by up to 90 K. Wavenumber‐3 structures are seen in the warm layer that are potentially signatures of thermal tides or stationary planetary waves, with amplitudes two times larger than predicted. Plain Language Summary: The Mars middle atmosphere is an intermediate region with rich and complex dynamics influenced by the underlying lower atmosphere and the overlying upper atmosphere. We report a new set of stellar occultation measurements made by the MAVEN/IUVS that provide evidence for a warm layer between 70 and 90 km altitude in the nightside mesosphere of Mars during Ls = 0–180° in Marian Year 33–34. Key Points: MAVEN/IUVS revealed a warm layer between 70 and 90 km altitude in the nightside on Mars during Ls = 0–180° in Martian Year 33–34The observed peak temperature amplitudes of nightside profiles are higher than those predicted by the model by up to 90 KLongitudinal wavenumber‐3 structures are seen in the warm layer, two times larger amplitudes than predicted [ABSTRACT FROM AUTHOR]

The Trace Gas Orbiter (TGO) of the ESA-Roscosmos ExoMars mission has ended its trip to Mars, reaching the planet in October 2016. After a year-long aerobraking phase, its scientific mission has begun on April 22nd, 2018 with the execution of the first solar occultation. The primary objective of TGO is to detect, map and locate trace gas sources, possibly revealing a residual geophysical (or even biological) activity on Mars. The instrument of interest here is the infrared spectrometer Atmospheric Chemistry Suite (ACS). ACS covers a wavelength range from 0.7 to 17 μm at very high spectral resolution (λ / Δλ from 5,000 to 50,000). ACS operates in nadir and solar occultation. Its performance and scientific objectives make it complementary to NOMAD, the other spectrometer dedicated to trace gas characterization.A large part of the first months of the ACS observing mission has enabled the sensitive search of gaseous methane over a substantial fraction of the Martian globe. The results from the first occultation up until early September 2018 will be presented. This period incidentally covered the onset, the full development and the demise of the Planetary Encircling Dust Event observed by several other instruments orbiting currently around Mars. Observed conditions proved more favorable than anticipated, and it was possible in a few cases to probe the Martian atmosphere close to the surface (<5 km) where the MSL Curiosity rover has established the existence of a seasonally varying background level of CH4 concentrations within a range comprised between 240 and 650 pptv. [ABSTRACT FROM AUTHOR]

After launching from the martian surface via the Mars Ascent Vehicle (MAV), the MAV and the Orbiting Sample (OS) capsule containing the samples collected on Mars by the Perseverance rover are to be identified by the Narrow Angle Camera (NAC) on the Earth Return Orbiter (ERO) spacecraft in order to determine the exact orbit of the capsule before rendezvous. To ensure detection of the OS, noise and straylight contributions to the NAC must be well characterized. Here, we assess the radiometric environment at Mars likely to be encountered by the NAC—from the surface through the middle atmosphere—using the High Resolution Stereo Camera (HRSC) onboard Mars Express (MEx) and the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter (MRO). The results show that the radiance values in general tend to increase as phase angle increases, as the season progresses from L s = 60° to L s = 230°, and as altitude decreases. We compare HRSC and MCS profiles where observing conditions were similar and find good agreement. At specific latitudes, high-altitude aerosols are present in 1–5% of observations and significantly increase the worst-case radiance contribution above 50 km. We construct envelope profiles from the maximum radiances at 5 km intervals from 0 to 90 km that provide important input for straylight calculations of the NAC and for the validation of models that may be used as input for straylight calculations. [ABSTRACT FROM AUTHOR]

Mars northern polar latitudes are known to harbor an enhanced 3 μ m spectral signature when observed from orbit. This may indicate a greater amount of surface adsorbed or bound water, although it has not yet been possible to easily reconcile orbital observations with ground measurements by Phoenix. Here we re-analyzed OMEGA/Mars Express observations acquired during the Northern summer to further characterize this 3 μ m absorption band increase. We identify the presence of a new specific spectral signature composed of an additional narrow absorption feature centered at 3.03 μ m coupled with an absorption at λ ≥ 3. 8 μ m. This signature is homogeneously distributed over a high-albedo open ring surrounding the circumpolar low-albedo terrains between ∼ 68°N and 76°N and ∼ 0°E and 270°E. This location includes the Phoenix landing site. This feature shows no time variability and can be confidently attributed to a seasonally stable surface component. All together, the stability, spectral shape and absence of significant correlation with other signatures in the 1–2.5 μ m range discard interpretations relying on water ice or easily exchangeable adsorbed water. Sulfates, notably anhydrite, provide interesting comparisons to several sections of the spectrum. Analogies with Earth samples also show that the spectral signature could result from a latitudinal modification of the hydration state and/or grains size of salts contaminants. While the exact full spectral shape cannot be easily reproduced, plausible explanations to this observation seem to involve geologically recent water alteration at high northern latitudes. • Using OMEGA data, we have identified a new spectral signature composed of a narrow 3 μ m band associated with a 4 μ m absorption. • This signature is observed exclusively over an open-ring area surrounding the North polar dunes between 68°N and 76°N. • This observation may indicate a specific physical or chemical alteration potentially involving sulfates in a polar environment. [ABSTRACT FROM AUTHOR]

Recent observations of the Martian hydrogen corona in the UV H Ly-alpha emission by the Hubble Space Telescope (HST) (Clarke et al., 2014) and the SPICAM UV spectrometer on Mars Express (Chaffin et al., 2014) reported its rapid change by an order of magnitude over a short few months period in 2007 (MY28), which is inconsistent with the existing models. One proposed explanation of the observed increase of the coronal emission is that during the global dust storm water vapor from the lower atmosphere can be transported to higher altitudes, where its photodissociation rate by near-UV sunlight increases, providing an additional source of hydrogen for the upper atmosphere. In this work we study the water vapor vertical distribution in the middle atmosphere of Mars during the 2007 global dust storm based on solar occultation measurements by the SPICAM IR spectrometer onboard the Mars-Express spacecraft. The vertical profiles of H 2 O density and mixing ratio have been obtained for solar longitudes Ls = 255°–300° in MY28. In the Northern hemisphere from Ls = 268° to Ls = 285° the H 2 O density at altitudes of 60–80 km increased by an order of magnitude. During the dust storm the profiles extended up to 80 km, with an H 2 O density exceeding 10 10 molecules/cm 3 (mixing ratio ≥200 ppm). Two maxima of the H 2 O density were detected. The largest H 2 O densities observed at latitudes higher than 60°N, over Ls = 269°–275°, do not directly correlate with the aerosol loading and likely relate to the downwelling branch of the meridional circulation that was intensified during the dust storm, and transported water from the Southern hemisphere to high northern latitudes. The second smaller maximum coincides with the high dust loading at middle northern latitudes. The comparison with geographically close observations in the quiet Mars year MY32, when the H 2 O content in the Northern hemisphere did not exceed 2 × 10 10 molecules/cm 3 and 50 ppm at 60 km, showed that the global dust storm was a unique event. The situation was different in the Southern hemisphere. During the dust storm the water density at 50–80 km increased by a factor of 4–5 with a mixing ratio >100 ppm, well correlated with the aerosol vertical extension. A somewhat weaker increase of the H 2 O density by a factor of 2–3 with a mixing ratio > 100 ppm was also observed during MY32 starting from Ls = 260°, suggesting a seasonal repeatability. The observed amount of water at high altitudes in both hemispheres can produce a large increase in the H escape rate on a timescale of weeks, as was shown in the photochemical modeling by Chaffin et al. (2017). Future modeling would be necessary to separate the seasonal and the dust storm contributions to the hydrogen escape. [ABSTRACT FROM AUTHOR]

The sounds of the Ingenuity Helicopter flying in the Martian atmosphere are among the most notable recordings of the microphone on the SuperCam instrument on the Mars 2020 Perseverance Rover. Distinct acoustic signatures of the helicopter were recorded on the 4th, 5th, 6th, and 8th flights: prior to this, simultaneous microphone and helicopter operations had not been verified in the testbed, and generally since these early flights the helicopter has been too far away for its emissions to be detectable given CO 2 absorption in the Mars atmosphere. The detected signatures are around 84 ​Hz and (occasionally) at 168 ​Hz, at the blade crossing frequency and its first harmonic. Several higher harmonics were prominent in hover tests in short-range recordings in a test chamber on Earth; these are attenuated by CO 2 absorption at the 50m-plus ranges on Mars. Doppler shift of the 84 ​Hz signal can be measured and is consistent with the trajectory measured with Ingenuity's navigation camera and inertial navigation unit, and documented by Perseverance's cameras. A striking feature of the sound recordings is an unanticipated deep modulation of the signals with nulls spaced by around 15–20s, superposed on the simple and expected decline in amplitude with distance. We have evaluated and rejected models of multipath sound interference as requiring implausibly strong near-surface temperature gradients. We find instead that the modulation appears to be the signature of a slight asynchrony between the rotation rates of the two coaxial rotors, such that the blade-crossing azimuth rotates slowly during flight, resulting in a 'lighthouse' sweeping of the radiated sound pattern. Analysis of blade orientations seen in the shadow of the helicopter observed in down-looking navigation images supports this model. • Documents sound recordings of Ingenuity helicopter flights on Mars. • Demonstrates acoustic data on rover are diagnostic of flights e.g. via Doppler effect. • Unexpected drops in loudness with 10–20s period may relate to slightly mismatched rotor speeds. • Helicopter shadow images show loudness drops correspond to blade-crossings aimed at rover. [ABSTRACT FROM AUTHOR]

After 15 years in orbit Mars Express remains one of ESA's most scientifically productive Solar System missions whose publication record now exceeds 1200 papers. Characterization of the geological processes on a local-to-regional scale by HRSC, OMEGA and partner experiments on NASA spacecraft has allowed constraining land-forming processes in space and time. Recent results suggest episodic geological activity as well as the presence of large bodies of liquid water in several provinces (e.g. Eridania Planum, Terra Chimeria) in the early and middle Amazonian epoch and formation of vast sedimentary plains north of the Hellas basin. Mars Express observations and experimental teams provided essential contribution to the selection of the Mars-2020 landing sites. Recent discovery of subglacial liquid water underneath the Southern polar cap has proven that the mission science potential is still not exhausted. More than a decade-long record of the atmospheric parameters such as temperature, dust loading, water vapor and ozone abundance, water ice and CO2 clouds distribution, collected by SPICAM, PFS, OMEGA, HRSC and VMC together with subsequent modeling have provided key contributions to our understanding of the martian climate. Recent spectroscopic monitoring of the 2018 dust storm revealed dust properties, their spatial and temporal variations and atmospheric circulation.More than 10,000 crossings of the bow shock by Mars Express allowed ASPERA-3 to characterize complex behavior of the magnetic boundary topology as function of the solar EUV flux. Observations of the ion escape during complete solar cycle revealed important dependencies of the atmospheric erosion rate on parameters of the solar wind and EUV flux and established global energy balance between the solar wind and escaping ion flow. The observations showed that ion escape can be responsible for removal of about 10 mbar over the Mars history that implies existence of other more effective escape channels. The structure of the ionosphere sounded by the MARSIS radar and the MaRS radio science experiment was found to be significantly affected by the solar activity, the crustal magnetic field, as well as by the influx of meteorite and cometary dust. MARSIS and ASPERA-3 observations suggest that the sunlit ionosphere over the regions with strong crustal fields is denser and extends to higher altitudes as compared to the regions with no crustal anomalies. Several models of the upper atmosphere and plasma environment are being developed based on and in support of the collected experimental data. The models aim at creating user-friendly data base of plasma parameters similar to the Mars Climate Database that would be of great service to the planetary community.A significant recent achievement was the flawless transition to the "gyroless" attitude control and operations mode on the spacecraft, that would allow mitigating the onboard gyros aging and extending the mission lifetime. In November 2018 ESA's Science Programme Committee (SPC) confirmed the mission operations till the end of 2020 and notionally approved its extension till the end of 2022. The talk will give the Mars Express status, review the recent science highlights, and outline future plans focusing on synergistic science with TGO. [ABSTRACT FROM AUTHOR]

Our Global Circulation Model (GCM) simulates the atmospheric environment of Mars. It is developped at LMD (Laboratoire de Meteorologie Dynamique, Paris, France) in close collaboration with several teams in Europe (LATMOS, France, University of Oxford, The Open University, the Instituto de Astrofisica de Andalucia), and with the support of ESA (European Space Agency) and CNES (French Space Agency). GCM outputs are compiled to build a Mars Climate Database, a freely available tool useful for the scientific and engineering communities. The Mars Climate Database (MCD) has over the years been distributed to more than 350 teams around the world. The latest series of reference simulations have been compiled in version 5.3 of the MCD which was released in 2017.To summarize, MCD v5.3 provides:- Climatologies over a series of synthetic dust scenarios: standard (climatology) year, cold (ie: low dust), warm (ie: dusty atmosphere) and dust storm, all topped by various cases of Extreme UV solar inputs (low, mean or maximum). These scenarios have been derived from home-made, instrument-derived (TES, THEMIS, MCS, MERs), dust climatology of the last 8 Martian years. The MCD also provides simulation outputs (MY24-33) representative of these actual years.- Mean values and statistics of main meteorological variables (atmospheric temperature, density, pressure and winds), as well as surface pressure and temperature, CO2 ice cover, thermal and solar radiative fluxes, dust column opacity and mixing ratio, [H20] vapor and ice columns, concentrations of many species: [CO], [O2], [O], [N2], [H2], [O3],...- A high resolution mode which combines high resolution (32 pixel/degree) MOLA topography records and Viking Lander 1 pressure records with raw lower resolution GCM results to yield, within the restriction of the procedure, high resolution values of atmospheric variables.- The possibility to reconstruct realistic conditions by combining the provided climatology with additional large scale and small scale perturbations schemes.At EGU, we will report on the latest improvements in the Mars Climate Database, with comparisons with available measurements from orbit (e.g.: TES, MCS, TGO) and landers (Viking, Phoenix, Curiosity). [ABSTRACT FROM AUTHOR]

We have carried out ground-based seasonal monitoring of hydrogen peroxide on Mars since 2003. This presentation is on the latest set of observations using thermal imaging spectroscopy, with two observations of the planet near opposition, in May 2016 (Ls = 148.5°, diameter = 17") and July 2018 (Ls = 209°, diameter = 23"). Data have been recorded in the 1232 - 1242 cm-1 range (8.1 micron) with the Texas Echelon Cross Echelle Spectrograph (TEXES) mounted at the 3-m Infrared Telescope Facility (IRTF) at Maunakea Observatory in Hawaii. As in the case of our previous analyses, maps of H2O2 have been obtained using line depth ratios of weak transitions of H2O2 divided by a weak CO2 line. The H2O2 map of April 2016 shows a strong dichotomy between the Northern and Southern hemispheres, as predicted by the photochemical model developed in the Mars Climate Database (Forget et al. 1999) and in the Global Environmental Multiscale model (Daerden et al. 2019). The second measurement in July 2018 was taken in the middle of the MY 34 global dust storm. H2O2 was not detected, with a disk-integrated 2-sigma upper limit of 10 ppbv, while both the MCD and the LEM models predicted a value above 20 ppbv that was actually observed by TEXES in 2003 in the absence of dust storm (Encrenaz et al., 2004). This July 2018 depletion is probably the result of the high dust content in the atmosphere at the time of our observations, which led to a decrease of the water vapor column density in the relevant altitude range, as observed by PFS on Mars Express during this period (Giuranna and Wolkenberg, 2019). GCM simulations using the GEM model show that the H2O depletion leads to a drop of H2O2 due to the depletion of HO2 radicals whose self-recombination gives rise to H2O2. Our data provide new constraints to the photochemical modelling of H2O2 in the presence of a high dust content. In parallel, we have reprocessed the whole TEXES dataset of H2O2 measurements using the latest version of the GEISA database (2015). We have recently found that there is a significant difference in the H2O2 line strengths between the 2003 and 2015 versions of GEISA. Therefore, all H2O2 mixing ratios up to 2014 from TEXES measurements have been reduced by a factor of about 1.75. As a consequence, in four cases (Ls around 80°, 100°, 150° and 209°), H2O2 abundances show contradictory values between different Martian years, while, at Ls = 209°, the cause seems to be the increased dust content associated with the global dust storm. The inter-annual variability in the three other cases remains unexplained at this time. [ABSTRACT FROM AUTHOR]

The ExoMars Trace Gas Orbiter (TGO), a collaborative project between the European Space Agency and Roscosmos, was successfully inserted into Mars orbit on 19 October 2016, and reached its final science orbit on 7 April 2018. TGO began taking observations as part of commissioning operations in March 2018.At the Laboratoire de Météorologie Dynamique (LMD) we are responsible for data assimilation of observations from the Atmospheric Chemistry Suite (ACS) thermal infrared instrument (TIRVIM) on board TGO. This instrument measures vertical profiles of temperature as well as dust and water ice integrated content, at various local times, latitudes and seasons.Here we report on assimilation of the first six weeks of TIRVIM observations into the LMD Mars General Circulation Model (GCM), using the Local Ensemble Transform Kalman Filter (LETKF) technique. The LETKF is an ensemble-based assimilation scheme where we typically use 16 ensemble members and multiplicative inflation to adjust the background ensemble error covariance. The assimilation focuses on atmospheric temperatures retrieved from nadir thermal emission spectra taken between 2018 March 13 and 2018 April 28. These have been calibrated and then retrieved using a line-by-line radiative transfer model. The observations cover various local times of day with full coverage in longitude every 7-10 days, over +/- 75 degree latitude, with vertical coverage between 5-45 km altitude.We shall report on the differences between the assimilation and a free-running model, particularly at local times of day that have not been observed prior to TGO. We shall also report on progress combining assimilation of ACS observations with profiles from Mars Climate Sounder on board NASA's Mars Reconnaissance Orbiter, to fill in anticipated gaps in the ACS observational record and to sample the local time of day more fully. [ABSTRACT FROM AUTHOR]