The evolution and history of water on Mars plays a key role in the assessment of the habitability of the planet across time. There is abundant geomorphological evidence suggesting that Mars had a wetter past (Bibring et al., 2006;Carr & Head, 2003), yet the duration and extent of this more humid past remains a topic of substantial debate. For instance, the large deltas, basins, and valleys on Mars are suggestive of large bodies of water that were stable over relatively long periods of time. Some estimates suggest past volumes of water in excess of a 500 m deep Global Equivalent Layer (GEL; Carr & Head, 2003), which is many times larger than the current estimates of labile water on Mars (∼30 m, Lasue et al., 2013).The large enrichments of D/H measured in atmospheric water suggest that a large fraction, beyond 80%, of this water was lost over time (Jakosky, 2021;Villanueva et al., 2015), and Ar and O isotopic ratios measured with MAVEN (Jakosky et al., 2017) and TGO (Alday, Wilson, et al., 2021) indicate that Mars has lost a large fraction of its atmosphere. Because Mars is less massive than Earth, the neutral escape of volatiles is easier on Mars, considering the similar equilibrium temperatures of the two planets, although Mars is obviously colder. Recent results indicate that most of this escape occurred via neutral and nonionized processes (Brain et al., 2015), in which temperature and its variability across geological times were key factors defining the state of the Martian atmosphere. Recent results from dust storms suggest that dust storms can greatly heat the atmosphere, leading to the upward transport and more readily escape of water (

The Open University's repository of research publications and other research outputs Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements

We present water vapor vertical distributions on Mars retrieved from 3.5 years of solar occultation measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter, which reveal a strong contrast between aphelion and perihelion water climates. In equinox periods, most of water vapor is confined into the low‐middle latitudes. In aphelion periods, water vapor sublimated from the northern polar cap is confined into very low altitudes—water vapor mixing ratios observed at the 0–5 km lower boundary of measurement decrease by an order of magnitude at the approximate altitudes of 15 and 30 km for the latitudes higher than 50°N and 30–50°N, respectively. The vertical confinement of water vapor at northern middle latitudes around aphelion is more pronounced in the morning terminators than evening, perhaps controlled by the diurnal cycle of cloud formation. Water vapor is also observed over the low latitude regions in the aphelion southern hemisphere (0–30°S) mostly below 10–20 km, which suggests north‐south transport of water still occurs. In perihelion periods, water vapor sublimated from the southern polar cap directly reaches high altitudes (>80 km) over high southern latitudes, suggesting more effective transport by the meridional circulation without condensation. We show that heating during perihelion, sporadic global dust storms, and regional dust storms occurring annually around 330° of solar longitude (LS) are the main events to supply water vapor to the upper atmosphere above 70 km.

The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument has been scanning the Martian atmosphere for almost 2 Martian years. In this work, we present a subset of the NOMAD SO data measured at the mesosphere at the terminator. From the data set, we investigated 968 vertical profiles of carbon dioxide density and temperature covering the Martian Year (MY) 35 as well as MY 36 up to a solar longitude (Ls) of 135° and altitudes around 60–100 km. While carbon dioxide density profiles are directly retrieved from the spectral signature in the spectra, temperature profiles are more challenging to retrieve as unlike density profiles, temperature profiles can present some spurious features if the regularization is not correctly managed. Comparing seven regularization methods, we found that the expected error estimation method provides the best regularization parameters. The vertical resolution of the profiles is on average 1.6 km. Numerous warm layers and cold pockets appear in this data set. The warm layers are found in the Northern hemisphere at dawn and dusk as well as in the Southern hemisphere at dawn. Strong warm layers are present in more than 13.5% of the profiles. The Southern hemisphere at dusk does not present any warm layer between Ls 50° and 150°. The height and latitudinal distribution of those warm layers were similar in MY 35 and MY 36 during the first half of the year (Ls = 0°–135°).

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We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, LS = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around LS = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed.

CO is produced by the photodissociation of CO 2 and recycled to CO 2 by the catalytic cycle involving HOx in the Martian atmosphere (e.g., McElroy & Donahue, 1972). The photochemical lifetime of CO is ∼6 years in the lower atmosphere (Krasnopolsky, 2007). The previous nadir observations revealed latitudinal and seasonal distributions of CO in the lower atmosphere, which indicate CO 2 condensation/sublimation in the polar caps and dynamics (Encrenaz et al., 2006;Smith et al., 2009Smith et al., , 2021. In the middle and upper atmosphere (>∼50 km), the photochemical lifetime of CO becomes much longer due to the decrease in HOx species density. Thus, the characteristic times of production and eddy diffusion of CO are shorter than the photochemical lifetime of CO in

The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument scans the Martian atmosphere since 21 April 2018. In this work, we present a subset of the NOMAD SO data measured at the mesosphere. We focused on a spectral range that started to be recorded in Martian year (MY) 35. A total of 968 vertical profiles of carbon dioxide density and temperature covering MY 35 and the beginning of MY 36 were investigated until 135° of solar longitude. We compared 47 profiles with co‐located profiles of the Mars Climate Sounder onboard the Mars Reconnaissance Orbiter. Most profiles show a good agreement as SO temperatures are only 1.8 K higher, but some biases lead to an average absolute difference of 7.4°K. The SO data set is also compared with simulations from the Global Environmental Multiscale‐Mars general circulation model. Both data sets are in good agreement except for the presence of a cold layer in the winter hemisphere and a warm layer at dawn in the Northern hemisphere for solar longitudes between 240° and 360°. Five profiles contain temperatures lower than the limit for CO2 condensation. Strong warm layers were found in 13.5% of the profiles. They are present mainly at dawn and in the winter hemisphere, while the Northern dusks appear featureless. The data set mainly covers high latitudes around 60° and we derived some non‐migrating tides. In the Southern winter hemisphere, we derived apparent zonal wavenumber‐1 (WN‐1) and WN‐3 tidal components with a maximum amplitude of 10% and 5% at 63 km, respectively.