The loss of water from Mars to space is thought to result from the transport of water to the upper atmosphere, where it is dissociated to hydrogen and escapes the planet. Recent observations have suggested large, rapid seasonal intrusions of water into the upper atmosphere, boosting the hydrogen abundance. We use the Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter to characterize the water distribution by altitude. Water profiles during the 2018–2019 southern spring and summer stormy seasons show that high-altitude water is preferentially supplied close to perihelion, and supersaturation occurs even when clouds are present. This implies that the potential for water to escape from Mars is higher than previously thought.

The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 µm-the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7-1.6 µm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2-4.4 µm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7-17 µm with apodized resolution varying from 0.2 to 1.3 cm −1 . TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.

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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.

The current Martian atmosphere is about five times more enriched in deuterium than Earth's, providing a direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective role in influencing the relative proportion of atomic D and H populations has remained speculative. Here we report HDO and H 2 O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H 2 O is a more important factor in determining the isotopic composition of the dissociation products.

The Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) European Space Agency‐Roscosmos mission began science operations in March 2018. ACS Mid‐InfraRed (MIR) channel notably provides solar occultation observations of the Martian atmosphere in the 2.3‐ to 4.2‐ normalμ m spectral range. Here, we use these observations to characterize water ice clouds before and during the MY 34 Global Dust Storm (GDS). We developed a method to detect water ice clouds with mean particle size ≤ 2  normalμ m and applied it to observations gathered between Ls=165∘ and Ls=243∘. We observe a shift in water ice cloud maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small‐sized ( reff≤0.3.3emnormalμ m) water ice clouds are more frequent during MY 34 compared to non‐GDS years at the same season. Particle size frequently decreases with altitude, both locally within a given profile and globally in the whole data set. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of kilometers for certain sizes. We notably notice some large water ice particles ( reff≥1.5.3emnormalμ m) at surprisingly high altitudes during the GDS (50–70 km). These results suggest that GDS can significantly impact the formation and properties of high‐altitude water ice clouds as compared to the usual perihelion dust activity.

Dust and water ice aerosols play a major role in the energy budget and circulation of the atmosphere of Mars and its climate. Dust affects the thermal structure and the dynamics of the atmosphere. Water ice clouds play an important part in the water cycle by altering the global transport of water vapor (Montmessin et al., 2004; Richardson & Wilson, 2002), by influencing the Martian climate radiatively (Madeleine et al., 2012; Wilson et al., 2008), and by changing the dust distribution through the scavenging of dust aerosols' particles due to the condensation of ice (Navarro et al., 2014; Pearl et al. 2001). Information on the aerosols physical properties is needed for a better understanding and modeling of Martian climate (