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Detecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid‐infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl abundance from below detection limits to 1–3 ppbv in both hemispheres coincident with the start of dust activity, followed by very sudden and rapid loss at the southern autumnal equinox. In this study, we have investigated the relationship between HCl and atmospheric dust by making comparisons in the vertical distribution of gases measured with ACS and aerosols measured co‐located with the Mars Climate Sounder (MCS). This study includes HCl, water vapor, and ozone measured using ACS MIR, water vapor and temperature measured with the near infrared channel of ACS, and temperature, dust opacity, and water ice opacity measured with MCS. In part 1, we show that dust loading has a strong impact in temperature, which controls the abundance of water ice and water vapor, and that HCl is very closely linked to water activity. In part 2, we investigate the quantitative correlations between each quantity and discuss the possible source and sinks of HCl, their likelihood given the correlations, and any issues arising from them. Plain Language Summary: After four full Martian years in orbit since 2018, the ExoMars Trace Gas Orbiter (TGO) has observed three Martian dusty seasons, which occur when it is spring and summer in the southern hemisphere. The first, starting in summer 2018, featured a global dust storm (GDS) after which we made the first detection of hydrogen chloride (HCl) in the Martian atmosphere using the Atmospheric Chemistry Suite (ACS) instrument. Finding this gas was a priority of ExoMars because its presence may indicate that the planet is volcanically active. Since then, we have observed two more dusty periods without a GDS and observed the reappearance of HCl each time. Here, we present the climatology of HCl in both hemispheres over these three dusty periods (in Mars years 34, 35, and 36) and investigate their relationships with temperature and water vapor measured by ACS, and with airborne dust and water ice measured with the Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO). In this paper, we examine how the vertical structure of each quantity changes over time. We show that there is a close relationship between HCl and H2O, and that both are controlled by temperature, driven by dust loading. Key Points: Comparison of trace gas abundance and aerosol extinction between Atmospheric Chemistry Suite on ExoMars TGO and Mars Climate Sounder on Mars Reconnaissance OrbiterReveal the seasonal changes in hydrogen chloride on MarsShow that dust affects atmospheric temperature, which controls water vapor abundance and ice extinction, and thus impacts HCl abundance [ABSTRACT FROM AUTHOR]

The detection of hydrogen chloride (HCl) in the atmosphere of Mars was among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Its discovery using the Atmospheric Chemistry Suite mid‐infrared channel (ACS MIR) showed a distinct seasonality and possible link to dust activity. This paper is part 2 of a study investigating the link between HCl and aerosols by comparing gas measurements made with TGO to dust and water ice opacities measured with the Mars Climate Sounder (MCS). In part 1, we showed, and compared, the seasonal evolution of vertical profiles of HCl, water vapor, temperature, dust opacity, and water ice opacity over the dusty periods around perihelion (solar longitudes 180°–360°) across Mars years 34–36. In part 2, we investigated the quantitative correlations in the vertical distribution between each quantity, as well as ozone. We show that there is a strong positive correlation between HCl and water vapor, which is expected due to fast photochemical production rates for HCl when reacting with water vapor photolysis products. We also show a strong positive correlation between water vapor and temperature, but are unable to show any correlation between temperature and HCl. There are weak correlations between the opacities of dust and water ice, and dust and water vapor, but only very low correlations between dust and HCl. We close with a discussion of possible sources and sinks and that interactions between HCl and water ice are the most likely for both, given the inter‐comparison. Plain Language Summary: The ExoMars Trace Gas Orbiter (TGO) has observed three Martian dusty seasons (summer in the southern hemisphere). In mid‐2018, we made the first detection of hydrogen chloride (HCl) in the Martian atmosphere using the Atmospheric Chemistry Suite (ACS). Finding this gas, among others, was a priority of ExoMars because its primary source may be volcanic. Since then, we have observed two more dusty periods with the reappearance of HCl each time. In part 1, we presented the climatology of HCl over these three dusty periods (in Mars years 34, 35, and 36) and investigate their relationships with temperature and water vapor measured by ACS, and with airborne dust and water ice measured with the Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO). In this paper, part 2, we quantify the correlations between HCl, water vapor, ozone, dust, temperature, and water ice. We show that HCl is closely correlated to water vapor, which is, in turn, correlated to temperature. There is only a very low correlation between HCl and dust or water ice, which both impact water vapor. Possible mechanisms for seasonal HCl production and destruction are discussed, concluding that the freeze‐thaw cycle of atmospheric water is most likely. Key Points: Comparison of trace gases and aerosol opacity between ACS on ExoMars TGO and MCS on MROLow correlations between HCl and the opacities of dust or water ice, but very strong correlation with water vapor and the formation of icesHCl source and sink possibilities are discussed, concluding that frost activity may be the most likely possibility for both [ABSTRACT FROM AUTHOR]

Here we present water vapor vertical profiles observed with the ExoMars Trace Gas Orbiter/Nadir and Occultation for MArs Discovery instrument during the perihelion and Southern summer solstice season (LS = 240°–300°) in three consecutive Martian Years 34, 35, and 36. We show the detailed latitudinal distribution of H2O at tangent altitudes from 10 to 120 km, revealing a vertical plume at 60°S–50°S injecting H2O upward, reaching abundance of about 50 ppmv at 100 km. We have observed this event repeatedly in the three Martian years analyzed, appearing at LS = 260°–280° and showing inter‐annual variations in the magnitude and timing due to long term effects of the Martian Year 34 Global Dust Storm. We provide a rough estimate of projected hydrogen escape of 3.2 × 109 cm−2 s−1 associated to these plumes, adding further evidence of the key role played by the perihelion season in the long term evolution of the planet's climate. Plain Language Summary: Studying the vertical distribution of the Martian atmosphere is crucial to understand what happened to the water presumably present in larger abundance on ancient Mars. We have analyzed the vertical profiles of three Martian Years during the Southern summer, revealing a strong vertical transport of water vapor to the upper atmosphere. This seasonal phenomenon seems to be repeated annually, although with variations in the location and time of the year. Our estimation of the associated upward hydrogen flux represents an important loss which could have contributed to the escape of water to space for at least the period in which Mars had its present orbital inclination. Key Points: Latitudinal distributions of water vapor up to 120 km are analyzed in detail using Nadir and Occultation for MArs Discovery (NOMAD) observations with an improved retrieval schemeWater vapor injection during the perihelion localized around 50°–60°S in three consecutive Martian yearsMartian year 34 Global Dust Storm may have affected the driving mechanisms of the plume, delaying its appearance and reducing its magnitude [ABSTRACT FROM AUTHOR]

BepiColombo, the ESA/JAXA joint mission performed its first flyby of Mercury on 1 October 2021 and its second on 23 June 2022. PHEBUS observed the exosphere of Mercury during these flybys notably with its visible channels c404 (centered on the potassium emission line at 404 nm) and c422 (centered on the calcium emission line at 422 nm). The c422 signal shows not only an enhancement of calcium (Ca) near the dawn region but also a very extensive Ca exosphere on the morning side beyond 10,000 km. The e‐folding distance deduced from our Ca profiles (2,500–2,800 km) is in agreement with the value reported by MESSENGER at similar true anomaly angles. We use a Chamberlain model to determine the temperature and density at the exobase. Using the morning side low‐altitude data, we derived a high temperature at the exobase (>50,000 K), in agreement with the MESSENGER results. We also report a day/night asymmetry in the Ca exosphere that could indicate that the source of Ca is predominantly on the dayside or be the consequence of a shift of the main source of Ca away from the dawn region. The c404 channel detected additional species at low altitudes on the morning side during both flybys. Comparison with previous studies is inconclusive and further analysis will be necessary to identify the species. Nevertheless, we can note that the e‐folding distance deduced from our profile is relatively small (135 km) and that the Chamberlain model applied to our profiles seems to indicate a temperature at the exobase <3,000 K. Plain Language Summary: BepiColombo, the ESA/JAXA joint mission on its way to Mercury, has already flown over the planet twice in October 2021 and June 2022. PHEBUS (Probing of Hermean Exosphere By Ultraviolet Spectroscopy), the UV spectrometer onboard the spacecraft, observed Mercury's exosphere during the flybys at the closest parts of approach to the planet with its two visible channels notably. Mercury is surrounded by a tenuous collisionless atmosphere (exosphere) that contains a variety of species. We report the detection of exospheric calcium with an enhancement near the dawn region. We show that the calcium exosphere is much extended on the morning side, beyond 10,000 km. The observations also reveal a day/night asymmetry of the calcium exosphere. Furthermore, we report the detection of additional species. However, the flyby data alone do not allow us to formally identify the species. Key Points: PHEBUS observed Mercury's extended exosphere of calcium on the morning side beyond ∼10,000 kmThe observed calcium profiles may be explained by the 2D effects of altitude and local timeAdditional species were detected, potentially manganese and potassium [ABSTRACT FROM AUTHOR]

On 9 and 10 October 2021, Probing the Hermean Exosphere by Ultraviolet Spectroscopy aboard BepiColombo observed the disk‐integrated reflected light from the Sun by Mercury in the spectral range 90–160 nm at a phase angle of 70°. These observations are used to derive the EUV reflectance of the surface of Mercury in this wavelength range for the first time since the Mariner 10 observations in 1974–1975. Our observations are in agreement with a surface of Mercury darker by a factor ∼0.5–0.6 compared to the Moon in this wavelength range, with an average reflectance of 0.3% at a phase angle of 70°. This lower reflectance of Mercury could be due to the lower abundance of FeO. The derived reflectance near 160 nm is close to the derived reflectance at 220 nm by MESSENGER/MASCS at a similar phase angle. Possible spectral variations measured between 110 and 140 nm could be attributed to SiO2 glass, but the signal to noise ratio is low and other observations, possible at several periods during the rest of the cruise, are needed to confirm this last result. Plain Language Summary: Due to the lack of atmosphere, the solar UV radiation directly impacts the surface of Mercury. A fraction of this radiation is reflected into space in all directions. This reflected fraction varies with wavelength and can be analyzed to constrain the composition and texture of the surface of Mercury. Probing the Hermean Exosphere by Ultraviolet Spectroscopy aboard BepiColombo observed the reflected radiation between 90 and 160 nm for the first time since the Mariner 10 observations in 1974–1975. In this study, we measure the fraction of UV radiation reflected at different wavelengths. We compare our results with past observations from Mariner 10. We explain part of the observed spectral variations as possible due to the presence of SiO2 glass at the surface of Mercury and an albedo lower than the Moon due to a lower abundance of FeO. Key Points: We derive the reflectance of Mercury's surface between 90 and 160 nm for the first time since Mariner 10The lower albedo of Mercury compared to the Moon could be due to a lower FeO concentration but other opaque species may be neededSome spectral variations could possibly be due to SiO2 [ABSTRACT FROM AUTHOR]

Temperature and density in the upper Martian atmosphere, above ∼100 km, are key diagnostic parameters to study processes of the species' escape, investigate the impact of solar activity, model the atmospheric circulation, and plan spacecraft descent or aerobraking maneuvers. In this paper, we report vertical profiling of carbon dioxide (CO2) density and temperature from the Atmospheric Chemistry Suite (ACS) solar occultations onboard the ExoMars Trace Gas Orbiter. A strong CO2 absorption band near 2.7 μm observed by the middle infrared spectrometric channel (ACS MIR) allows the retrieval of the atmospheric thermal structure in an unprecedentedly large altitude range, from 20 to 180 km. We present the latitudinal and seasonal climatology of the thermal structure for 1.5 Martian years (MYs), from the middle of MY 34 to the end of MY 35. The results show the variability of distinct atmospheric layers, such as a mesopause (derived from 70 to 145 km) and homopause, changing from 90 to 100 km at aphelion to 120–130 km at perihelion. Some short‐term homopause fluctuations are also observed depending on the dust activity. Plain Language Summary: We report vertical distributions of the density and temperature in the Martian atmosphere in the altitude range from 20 to 180 km. This broad interval of heights embraces regions of the troposphere (<50 km), the mesosphere (50–100 km), and the thermosphere (>100 km). Knowledge of thermal structure in the middle and upper atmosphere (above 50 km) is unique for the Martian climate modeling, studying the atmospheric escape and the impact of solar activity, as well as planning spacecraft maneuvers. Our data are based on remote measurements of the carbon dioxide (CO2) absorption in the atmosphere at very high altitude resolution of one kilometer. Sensing this major component, 95% of the total density, in the infrared wavelength range allows us to derive the atmospheric temperature as well. We observe the climatology of different atmospheric layers depending on latitude and seasons for 1.5 Martian years, from May 2018 to January 2021. For example, we reveal extremely high variability of the coldest layer, mesopause, from 70 km in the southern winter to 150 km in the southern summer. Key Points: Seasonal variability of the Martian atmospheric thermal structure at altitudes 20–180 km is reported from CO2 infrared spectroscopyThe mesopause altitude rises from 70 to 90 km in the high‐winter latitudes to 130–150 km in the summer season for both hemispheresThe homopause altitude varies from 90 km at aphelion to 130 km at perihelion in the Martian years 34 and 35, and it depends on dust activity [ABSTRACT FROM AUTHOR]

The mid‐infrared channel of the Atmospheric Chemistry Suite (ACS MIR) onboard the ExoMars Trace Gas Orbiter is capable of observing the infrared absorption of ozone (O3) in the atmosphere of Mars. During solar occulations, the 003←000 band (3,000‐3,060 cm−1) is observed with spectral sampling of ∼0.045 cm−1. Around the equinoxes in both hemispheres and over the southern winters, we regularly observe around 200–500 ppbv of O3 below 30 km. The warm southern summers, near perihelion, produce enough atmospheric moisture that O3 is not detectable at all, and observations are rare even at high northern latitudes. During the northern summers, water vapor is restricted to below 10 km, and an O3 layer (100–300 ppbv) is visible between 20 and 30 km. At this same time, the aphelion cloud belt forms, condensing water vapor and allowing O3 to build up between 30 and 40 km. A comparison to vertical profiles of water vapor and temperature in each season reveals that water vapor abundance is controlled by atmospheric temperature, and H2O and O3 are anti‐correlated as expected. When the atmosphere cools, over time or over altitude, water vapor condenses (observed as a reduction in its mixing ratio) and the production of odd hydrogen species is reduced, which allows O3 to build up. Conversely, warmer temperatures lead to water vapor enhancements and ozone loss. The LMD Mars Global Climate Model is able to reproduce vertical structure and seasonal changes of temperature, H2O, and O3 that we observe. However, the observed O3 abundance is larger by factors between 2 and 6, indicating important differences in the rate of odd‐hydrogen photochemistry. Plain Language Summary: Ozone on Mars is part of the so‐called odd‐oxygen family of reactive, oxidizing gases. It is part of many chemical cycles that help convert one type of gas into another, facilitating the transfer of carbon or hydrogen. Odd‐oxygen is crucial to linking the cycles of water vapor and carbon dioxide, or the destruction of trace gases, such as methane. With the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter, we are able to study the vertical structure of ozone in the Martian atmosphere and make direct comparisons between it and water vapor and temperature. We have observed ozone abundances several times larger than predicted, suggesting that the oxidizing power of the Martian atmosphere is stronger or faster than expected. We have also observed and measured the relationship between these products: temperature controls the abundance of water vapor, and when the atmosphere cools and water condenses, ozone is able to build up. It is the by‐products of when water vapor breaks down in sunlight that remove odd‐oxygen from the atmosphere. Key Points: Observations of the vertical distribution of ozone on Mars over 3 yearsDirect comparison of water vapor, ozone, and temperature, revealing trends and correlationsOzone is observed in higher abundances than photochemical models predict [ABSTRACT FROM AUTHOR]

Water escape on Mars has recently undergone a paradigm shift with the discovery of unexpected seasonal variations in the population of hydrogen atoms in the exosphere where thermal escape occurs and results in water lost to space. This discovery led to the hypothesis that, contradicting the accepted pathway, atomic hydrogen in the exosphere was not only produced by molecular hydrogen but mostly by high altitude water vapor. Enhanced presence of water at high altitude during southern spring and summer, due to atmospheric warming and intensified transport, favors production of H through photon‐induced ion chemistry of water molecules and thus appears to be the main cause of the observed seasonal variability in escaping hydrogen. This hypothesis is supported by the observation of large concentrations of water vapor between 50 and 150 km during the southern summer solstice and global dust events. Using a simplified yet representative air parcel transport model, we show that in addition to the formation of atomic hydrogen from water photolysis above 80 km, a major fraction of the exospheric hydrogen is formed at altitudes as low as 60 km and is then directly advected to the upper atmosphere. Comparing the injection modes of a variety of events (global dust storm, perihelion periods, and regional storm), we conclude that southern spring/summer controls H production and further ascent into the upper atmosphere on the long term with direct implication for water escape. Plain Language Summary: Numerous lines of evidence suggest that Mars' water inventory was much larger in the past than it is today. The loss of this inventory has been driven by the formation of hydrated minerals on the surface, as well as by the escape of water to space. The first part of the escape process comprises the formation of H atoms, which may escape the planet once they reach the uppermost layers of the atmosphere. Here, we investigate one mechanism by which the H atoms may reach these high altitudes: the breakdown of water molecules by solar ultraviolet photons in the middle atmosphere (60–70 km above the surface), and the posterior ascent of the newly formed H atoms to the upper altitudes. We use a model that reveals that this process is the dominant contributor of atomic H to the upper atmosphere during periods of strong atmospheric circulation. In particular, we find that this mechanism is most efficient during the spring/summer season in the Southern Hemisphere, when Mars is closest to the Sun. Given that this season occurs every Martian year, our calculations suggest that this process has been the dominant contributor to water escape in the long term. Key Points: We decipher hydrogen production and migration to Mars' upper atmosphere using a box model for a variety of elevated water vapor casesH atoms formed between 60 and 80 km supply a dominant fraction of hydrogen to the upper atmosphereOur results suggest that perihelion climate has a key role in the hydrogen transfer to the upper atmosphere overall [ABSTRACT FROM AUTHOR]

The nighttime ozone and sulfur dioxide distributions were analyzed using the entire Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV/Venus Express stellar occultation data set. After the discovery of an ozone layer at 100 km in the mesosphere reported by Montmessin, Bertaux, et al. (2011, https://doi.org/10.1016/j.icarus.2011.08.010), 132 other detections were made during the entire 8 years long observing period of the SPICAV UV instrument. In the rare detections, the peak abundances of O3 accumulating in the mesosphere are observed with densities from 107 to 108 molecules⋅cm−3 at 85–110 km. The ozone layer is estimated to vary from 1 to 30 ppbv at 85–95 km while at 95–105 km the volume mixing ratio is expected within an interval from 6 to 120 ppbv. Below 93 km, a puzzling decrease of mixing ratio is observed towards midnight at 30°N. Our work also provides an improved sequel to the analysis of the sulfur dioxide survey previously made in the upper mesosphere by Belyaev, Evdokimova, et al. (2017, https://doi.org/10.1016/j.icarus.2017.05.002). On average, the SO2 content is found to remain constant throughout the vertical profile at a value of around 135 ± 21 ppbv between 85 and 100 km. Rapid and large variations prevent to conclude firmly on any time or space pattern of SO2. Plain Language Summary: Observations of light attenuation while bright ultraviolet stars were rising or setting behind Venus permitted us to characterize the quantity of carbon dioxide (the main atmospheric component), sulfur oxide, and ozone between 85 and 110 km of altitude at night. They were performed by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus (SPICAV) UV instrument on board the ESA Venus Express mission in 2006–2014. For the first time, this data set has been entirely analyzed with the benefit of recent calibration's improvements enabled more faithful restitution of gases' abundances. For ozone, discovered on Venus in 2011, thanks to SPICAV, we have been able to detect maximum concentrations and to estimate its nightside characterization. At midlatitudes, a ratio between ozone and carbon dioxide densities, or O3 mixing ratio, decreases below 93 km at midnight. This puzzling result is tentatively explained by chemical compounds transported by winds from the dayside hemisphere to the nightside and which are known to quickly react with ozone and destroy it. Sulfur dioxide has been found to vary considerably on a short time scale (days) but, on average, the SO2 mixing ratio is constant within 85–100 km. Key Points: Ozone density is within 107 to 108 cm−3 in the 85–110 km altitude range from 2006 to 2015The altitude profile of the sulfur dioxide volume mixing ratio is vertically uniform around 135 ppbv between 85 and 100 kmTemporal variations of ozone and sulfur dioxide are analyzed [ABSTRACT FROM AUTHOR]