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

SPICAM-IR, an AOTF spectrometer onboard Mars Express spacecraft with a resolving power of 2,000 covering the spectral range 1-1.7 μm has been observing Mars since 2004. In this wavelength range, it is possible to distinguish between CO2 and H2O ices and measure their band depths. We obtained about 200,000 high-fidelity spectra of CO2 ice in different seasons and locations over the Martian polar caps. The spectra have been associated with slab ice, fine-grained ice, permanent caps, and dark and dirty ice at the cryptic region of the south polar cap. Also, we observed more than 200,000 water ice spectra, specifically its broad feature around 1.5 μm. Water ice is present at the surface or in the atmosphere resulting in a variety of different band depths, often in combination with the CO2 ice. We mapped the equivalent width of 1.43 μm CO2 ice band and the depth of 1.5 μm water ice band, which are the proxies for grain size, and followed their seasonal evolution. From the maps, we produced the edge (outer crocus line) of the CO2 south and north caps for nine Martian Years. The cap edges evolve similarly through all years and are in good agreement with previous OMEGA/Mars Express observations. We also discuss the impact of the global dust storms on the cap edges. Lastly, we interpret some of the water ice observations as water ice clouds in the aphelion cloud belt and the polar hoods.

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 dataset covering all seasons and latitudes, assembled from co-located 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. Deviances 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.

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.

On Mars, saturation 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 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 km 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 the Ls=180° of MY36. We found that water is often supersaturated above aerosol layers. In the aphelion season, water mixing ratio above 40 km in the mid-to-high latitudes was below 3 ppmv and yet is found to be supersaturated. Around 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 perihelion. Saturation varied between 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 dataset 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.

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 (3000-3060 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 vapour is restricted to below 10 km, and an O3 layer (100-300 ppbv) is visible between 20-30 km. At this same time, the aphelion cloud belt forms, condensing water vapour and allowing O3 to build up between 30-40 km. A comparison to vertical profiles of water vapour and temperature in each season reveals that water vapour 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 vapour 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 vapour 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 a factor of 2-6, indicating important differences in the rate of odd-hydrogen photochemistry.

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 photolysis ionized 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 km 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, 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.

Carbon monoxide is a non-condensable species of the Martian atmosphere produced by the photolysis of CO2. Its mixing ratio responds to the condensation and sublimation of CO2; from the polar caps, resulting in seasonal variations of the CO abundance. Since 2018, all three spectrometers of the Atmospheric Chemistry Suite (ACS) onboard the Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long-term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls=163; of MY34 to the end of MY35. We obtained a mean CO volume mixing ratio of ~960 ppm at latitudes from 45S to 45N, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls=100-200; in high southern latitudes with a layer of 3000-4000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of mixing ratio above 50 km to 3000–4000 ppmv explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. The general circulation chemical model tends to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid-latitudes southern summer atmosphere amounts to 700-750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.

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 LMD Mars GCM. The present study aims at exploring the behaviour 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, 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 modelling the effect 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 (this issue).