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Odd-hydrogen (HO |$_\mathrm{x}$|⁠) species have a crucial role in regulating the chemistry of the atmosphere of Mars and are important to understand some of the most fundamental aspects regarding its atmospheric composition such as the long-term stability of CO |$_2$|⁠. Despite the key role of these species for our understanding of the Martian photochemistry, there is little observational evidence constraining their abundances. In this study, we use infrared solar occultation observations from the Atmospheric Chemistry Suite aboard the ExoMars Trace Gas Orbiter to search for spectral signatures of HO |$_2$| in the atmosphere of Mars. In our analysis of the data, we retrieve vertical profiles of pressure, temperature, and water vapour mixing ratio, but are unable to confidently detect the presence of HO |$_2$| features in the spectra. We report upper limits of 15 ppbv (5 |$\sigma$|⁠), which represents an order of magnitude improvement with respect to previous investigations. Comparing the derived upper limits with the expectations from 3-dimensional Global Climate Models, we find that approximately an order of magnitude improvement in the instrument sensitivity would be required to detect this molecule and/or constrain the models. [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]

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 ExoMars Trace Gas Orbiter (TGO) is a joint ESA-Roscosmos mission to Mars that waslaunched in March 2016. The nominal science phase with regular measurement could onlystart 2 years later, that is in late spring 2018. The Atmospheric Chemistry Suite (ACS) is a setof three spectrometers (NIR, MIR, and TIRVIM), capable of sounding Mars’ atmosphere innadir and solar occultation modes. The design of the middle infrared (MIR) channel wasoptimized to overachieve the primary science goal of the TGO mission by accomplishing themost sensitive measurements ever of the trace gases present in the Martian atmosphere. Thehigh aperture of the spectrometer, the ultra-high spectral resolution, and a fast onboard frameprocessing ensure quality meeting the highest standards. Besides methane survey, ACSMIR is capable of investigating many other minor species: CH2O, HCl, HO2, andothers. Preliminary results and status of this work will be presented in the talk. [ABSTRACT FROM AUTHOR]

The molecular oxygen is the minor constituent of the Martian atmosphere with the meanmixing ratio of (1.56 ± 0.06) 10−3. As it is a long-lived incondensable species(with the lifetime ∼60 years) the O2 mixing ratio should have latitudinal variationsinduced by condensation and sublimation of CO2 from the polar caps that result inenrichment and depletion and seasonal variations following the total CO2 amount inthe atmosphere. The O2 column-averaged mixing ratio was provided by severalground-based observations as well as by Herschel orbiting observatory. Now the highprecision measurements of the O2 mean surface mixing ratio were obtained by thequadrupole mass spectrometer as a part of Mars Science Laboratory at the Curiosityrover. The Atmospheric Chemistry Suite (ACS) is a set of three spectrometers (-NIR, -MIR, and-TIRVIM) intended to observe Mars atmosphere onboard the ESA-Roscosmos ExoMars2016 Trace Gas Orbiter (TGO) mission. The near infrared channel (NIR) is a compactspectrometer operating in the range of 0.7–1.7 μm with a resolving power of λ/Δλ ∼25,000. It is designed to operate in nadir and in solar occultation modes. A verticalprofiling of the O2 density based on 0.76 μm band is a unique feature of the ACSNIR science in occultation. No other instrument on a Mars orbiting platform beingsensitive to O2 from 10 to 60 km altitude range. Here we present the first results ofthe O2 density retrievals from the ACS/NIR solar occultations for the first yearof observations and the comparison with the LMD General Circulation model. [ABSTRACT FROM AUTHOR]

The Atmospheric Chemistry Suite (ACS) on board ESA-Roscosmos ExoMars Trace GasOrbiter started routine science operations in March 2018. The mid-infrared channel (MIR),operating in the 2.2-4.4 μm range, allows the detection of trace gases in the Martianatmosphere, as well as the simultaneous profiling of several species in solar occultationobservations. We report retrievals in the 2.51 – 2.64 μm spectral region which allow simultaneousmeasurement, for the first time, of vertical profiles of H216O, H218O, H217O and HD16O.The spectral region also includes CO2 spectral lines which allow temperature and pressureretrieval through the assumption of hydrostatic equilibrium. Monitoring of this spectral region with ACS-MIR will provide a better understanding ofthe isotopic fractionation of water vapour, its local and seasonal variability, and itsfractionation in condensation/evaporation processes. [ABSTRACT FROM AUTHOR]