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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]

Carbon monoxide is a non‐condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars 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 mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, 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 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend 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. Plain Language Summary: Carbon monoxide (CO) is a product of the photodissociation of the main component of the Martian atmosphere, CO2 (96%). During the polar night on Mars, the temperature drops so low that ∼30% of the global atmospheric carbon dioxide freezes out, condensed into snowflakes. They settle to the ground to form a seasonal polar cap, which sublimates in spring. Unlike CO2, CO is not a condensable species, so its relative abundance increases when CO2 and the atmospheric pressure drops, forming a prominent seasonal cycle. This behavior of CO was well documented by spacecraft measurements of column abundances, while its vertical distribution remained poorly known until recently. Still, it helps to separate the impacts of atmospheric dynamics and chemistry, addressing the problem of the Martian atmosphere's stability. We present the first climatology of the CO vertical distribution lasting more than one Martian year based on solar occultations by the ExoMars Trace Gas Orbiter. We report a previously unobserved CO layer near the surface in high southern latitudes in late winter and spring and document the difference between two Martian years caused by the global dust storm. These observations give new challenges to General Circulation Models for understanding the modern atmosphere of Mars. Key Points: The first climatology of CO vertical profiles is collected for the period Ls = 163° of Martian Year 34 to the end of Martian Year 35The mean CO volume mixing ratio is ∼960 ppmv at 0–35 km and 45°S–45°NStrong enrichment of CO, >3,000 ppmv, is detected at 10–20 km at Ls = 100–200 in high southern latitudes and above 50 km at equinoxes near Poles [ABSTRACT FROM AUTHOR]

We assimilate atmospheric temperature profiles and column dust optical depth observations from the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite thermal infrared channel (TIRVIM) into the Mars Planetary Climate Model. The assimilation period is Mars Year 34 Ls = 182.3°–211.4°, covering the onset and peak of the 2018 global dust storm. We assimilated observations using the Local Ensemble Transform Kalman Filter with 36 ensemble members and adaptive inflation; our nominal configuration assimilated TIRVIM temperature profiles to update temperature and dust profiles, followed by dust column optical depths to update the total column dust abundance. The observation operator for temperature used the averaging kernels and prior profile from the TIRVIM retrievals. We verified our analyses against in‐sample TIRVIM observations and independent Mars Climate Sounder (MCS) temperature and dust density‐scaled opacity profiles. When dust observations were assimilated, the root‐mean‐square temperature error verified against MCS fell by 50% during the onset period of the storm, compared with assimilating temperature alone. At the peak of the storm the analysis reproduced the location and magnitude of the peak in the nighttime MCS dust distribution, along with the surface pressure diurnal cycle measured by Curiosity with a bias of less than 10 Pa. The analysis winds showed that, at the peak of the storm, the meridional circulation strengthened, a 125 m s−1 asymmetry developed in the midlatitude zonal jets, the diurnal tide weakened near the equator and strengthened to 10–15 K at midlatitudes, and the semi‐diurnal tide strengthened almost everywhere, particularly in the equatorial lower atmosphere. Plain Language Summary: ExoMars Trace Gas Orbiter has been in low orbit around Mars since early 2018. During 2018 a large amount of dust was lifted from Mars' surface into the atmosphere. This eventually obscured most of the planet, forming a global dust storm. These storms have a large effect on the temperature and weather in Mars' atmosphere. We used observations from the spacecraft's thermal infrared instrument. This measures how much light comes from the atmosphere and surface at wavelengths where heat is emitted, and can be used to work out the atmospheric temperature and dust content at different times of day. We combined these observations with a numerical model of Mars' atmosphere, considering uncertainties in the measured atmospheric properties and the modeled atmosphere. We compared our results with temperature and dust observations made by Mars Climate Sounder on board Mars Reconnaissance Orbiter. Including dust observations was better than just using temperature observations. While the dust storm was raging, north‐south wind speeds increased, the strong eastward wind in the northern hemisphere became much stronger than in the southern hemisphere, and differences between day and night became larger. Our results mean we can better understand aspects of Mars' atmosphere throughout the daily cycle. Key Points: We assimilate temperature and dust observations from ExoMars Trace Gas Orbiter‐Atmospheric Chemistry Suite‐TIRVIM during the MY34 global dust storm (GDS)The analysis verifies well against independent Mars Climate Sounder temperature and dust profiles, and Curiosity pressure measurementsAt the peak of the GDS the winds strengthen, the diurnal tide migrates poleward, and the semi‐diurnal tide strengthens [ABSTRACT FROM AUTHOR]

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 Laboratoire de Météorologie Dynamique Mars Global Climate Model (GCM). The present study aims at exploring the behavior 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 (TGO), 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 modeling the state 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, https://doi.org/10.1029/2022JE007201) (this issue). Plain Language Summary: The D/H ratio observed in a planetary atmosphere is traditionally used as a proxy to estimate the initial water reservoir of the planet. We present here an improved global circulation model including HDO, the main isotope of water on Mars. The updated model takes into account the details of the formation of clouds and their radiative effect. It also includes the effect of photochemistry on HDO and deuterated species. We find that supersaturation is key to the representation of the D/H cycle by making the hygropause more porous, allowing more HDO in the upper atmosphere. It also reduces the efficiency of the isotopic fractionation occurring at condensation. We present here comparisons between observations by the Atmospheric Chemistry Suite spectrometer onboard the orbiter TGO. While the model is globally in agreement with the observations, the inability of the model to reproduce the observed vertical distribution of dust, especially during the Global Dust Storm, causes discrepancies in the representation of water vapor. The model reveals the importance of representing the state of supersaturation to correctly account for the water vapor amount reaching the top of the atmosphere and estimate the D/H ratio at escape. Key Points: The HDO cycle has been implemented in the last version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model including microphysics and radiative effect of water ice cloudsKinetics effect is now included in HDO fractionation during condensation and proves to be significantSupersaturation, the presence of which is regulated by cloud processes, alters significantly the relative abundances of HDO and H2O [ABSTRACT FROM AUTHOR]

The D/H ratio and its implications on the atmospheric escape, make it an essential observable to study the current and past inventory of water on Mars. With the arrival of the Trace Gas Orbiter around Mars, new measurements of the D/H ratio are now available and require tools to interpret the observations and understand the HDO cycle. We here present simulations of an updated version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model which includes HDO and in particular the fractionation processes it undergoes. We compare our model simulations with the HDO observations in solar occultation from the Atmospheric Chemistry Suite mid‐infrared channel on board the Trace Gas Orbiter (Alday et al., 2021; https://doi.org/10.5281/ZENODO.5100448). The model successfully reproduces the general trends of the D/H ratio, indicating that the main physical processes are captured by theory. A consistent simulation of condensation processes is found to be key in the representation of the D/H ratio. Improvements in the representation of clouds and on the water cycle will help improving the representation of the HDO cycle and better help extrapolate back in times the conditions of water escape on Mars. Plain Language Summary: Understanding how the Martian climate affects the isotopic ratio of water is key to understand the history of water on the planet. We use a general circulation model to simulate the cycle of HDO, an isotope of water, in the atmosphere of Mars. We compare our model results with spacecraft observations from the Atmospheric Chemistry Suite spectrometer on board the Trace Gas Orbiter, currently in orbit around Mars. Our model provides a good qualitative agreement with the observations. We find that the condensation of water vapor into ice is a critical process for determining the isotopic ratio of the vapor phase. Key Points: We simulated the HDO cycle on Mars using a 3D Global Climate ModelGCM simulations were compared with solar occultations from Atmospheric Chemistry Suite on board the Trace Gas OrbiterA good representation of the condensation processes is key to reproduce the observed D/H ratio [ABSTRACT FROM AUTHOR]

Ultraviolet spectral imaging has been a powerful tool to investigate the cloud top of Venus, allowing for measurement of several minor gases (especially SO 2 , SO, O 3), of cloud top aerosol's microphysical properties and of atmospheric dynamics through tracking of the unevenly distributed UV absorber. After a brief review of recent UV instruments that orbited around Venus, we present the results of a state-of-the-art radiative transfer model from Marcq et al. (2020) to derive the spectral resolution and Signal-to-Noise ratio (SNR) required to derive abundances of these gases, retrieve optical properties of the aerosols beyond our current knowledge. This leads us to propose a two-channel UV hyperspectral push-broom imager called VeSUV (standing for Venusian Spectroscopy in UV) whose technical characteristics will improve on existing measurements by a factor of at least 2, and which is well suited to the integration into the payload of future low Venus orbit platforms such as the proposed EnVision mission to ESA M5 call. [ABSTRACT FROM AUTHOR]