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To understand the evolving martian water cycle, a global perspective of the combined vertical and horizontal distribution of water is needed in relation to supersaturation and water loss and how it varies spatially and temporally. The global vertical water vapor distribution is investigated through an analysis that unifies water, temperature and dust retrievals from several instruments on multiple spacecraft throughout Mars Year (MY) 34 with a global circulation model. During the dusty season of MY 34, northern polar latitudes are largely absent of water vapor below 20 km with variations above this altitude due to transport from mid‐latitudes during a global dust storm, the downwelling branch of circulation during perihelion season and the intense MY 34 southern summer regional dust storm. Evidence is found of supersaturated water vapor breaking into the northern winter polar vortex. Supersaturation above around 60 km is found for most of the time period, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. Discrete layers of supersaturated water are found across all latitudes. The global dust storm and southern summer regional dust storm forced water vapor at all latitudes in a supersaturated state to 60–90 km where it is more likely to escape from the atmosphere. The reanalysis data set provides a constrained global perspective of the water cycle in which to investigate the horizontal and vertical transport of water throughout the atmosphere, of critical importance to understand how water is exchanged between different reservoirs and escapes the atmosphere. Plain Language Summary: To better understand how water vapor is transported throughout the atmosphere requires a global perspective of the combined vertical and horizontal distribution of water. By combining a simulation of the martian atmosphere with satellite observations of water, temperature and dust from multiple spacecraft we can investigate the water vapor cycle and the physical processes that result in the observed vertical distribution of water vapor. We find that circulation patterns during major dust storms and the southern summer season have a strong influence on the vertical distribution of water allowing it to reach much higher altitudes (than usually found in less dusty conditions or different seasons) and potentially escape the atmosphere. Supersaturation (when relative humidity is beyond 100%) occurs frequently in the martian atmosphere and this study finds that it allows water vapor to break into the northern winter polar vortex, suggesting the winter polar vortex is less of a boundary to transport of chemical species than previously thought. Key Points: Global dust storm and southern summer regional dust storm events pushed water vapor above 60 km across all latitudesThe saturation state of the global atmosphere is investigated in a reanalysis for the first timeSupersaturated water vapor penetrates the northern winter polar vortex [ABSTRACT FROM AUTHOR]

Slightly less than a Martian Year of nominal science (March 2018–January 2020) with the ExoMars Trace Gas Orbiter has furthered the ongoing investigation of dayside water vapor column abundance. These dayside observations span latitudes between 75°S and 75°N, and all longitudes, which can provide global snapshots of the total water column abundances. In addition to tracking the seasonal transport of water vapor between poles, geographic enhancements are noted, particularly in the southern hemisphere, both in Hellas Basin, and in other regions not obviously correlated to topography. We report consistent water vapor climatology with previous spacecraft observations, however, note a difference in total water vapor content is noted. Finally, we are unable to find evidence for substantial diurnal variation in the total dayside water vapor column. Plain Language Summary: This work provides the first look at the ExoMars Trace Gas Orbiter's ability to track atmospheric water vapor on the day side of Mars, through downward looking observations. Water vapor is reported in a series of maps, with respect to geography and season, and find consistent water vapor climatology with precious spacecraft observations. These maps inform our understanding of where Martian water vapor moves throughout the year and where it is concentrated. Key Points: Water vapor can be retrieved robustly from the Nadir and Occultation for MArs Discovery limb and Nadir observatory nadir observations for most dayside conditionsRetrieved water vapor columns demonstrate volatile transport, geographic variations, and lack of evidence for daytime local variationsTrace Gas Orbiter observations continue the legacy of monitoring the Martian water cycle and will contribute to future modeling efforts [ABSTRACT FROM AUTHOR]

Hydrogen chloride (HCl) was recently discovered in the atmosphere of Mars by two spectrometers onboard the ExoMars Trace Gas Orbiter. The reported detection made in Martian Year 34 was transient, present several months after the global dust storm during the southern summer season. Here, we present the full data set of vertically resolved HCl detections obtained by the NOMAD instrument, which covers also Martian year 35. We show that the particular increase of HCl abundances in the southern summer season is annually repeated, and that the formation of HCl is independent from a global dust storm event. We also find that the vertical distribution of HCl is strikingly similar to that of water vapor, which suggests that the uptake by water ice clouds plays an important role. The observed rapid decrease of HCl abundances at the end of the southern summer would require a strong sink independent of photochemical loss. Plain Language Summary A new species, hydrogen chloride (HCl), was recently discovered in the atmosphere Mars. Whereas, this gas plays a key role in the atmospheric chemistry on Earth and Venus, the chlorine cycle on Mars is not understood. HCl was found just after the global dust storm, so a connection between the storm and appearance of HCl was suggested. This study shows that HCl is detected even in a year without a global dust storm, which demonstrates that the formation of HCl is primary independent from global dust storms. This study also finds that the vertical distributions of HCl and water vapor are strikingly similar in shape. As the vertical profile of water vapor is controlled by the formation of water ice clouds, it suggests that HCl is taken up by water ice clouds. Finally, we observe that HCl is rapidly disappearing at the end of the southern summer on Mars. We demonstrate that this fast destruction cannot be explained by the standard photochemistry, which suggests that another (unknown) process plays a key role. [ABSTRACT FROM AUTHOR]

The Nadir and Occultation for MArs Discovery (NOMAD) instrument on board ExoMars Trace Gas Orbiter measured a large increase in water vapor at altitudes in the range of 40–100 km during the 2018 global dust storm on Mars. Using a three‐dimensional general circulation model, we examine the mechanism responsible for the enhancement of water vapor in the upper atmosphere. Experiments with different prescribed vertical profiles of dust show that when more dust is present higher in the atmosphere, the temperature increases, and the amount of water ascending over the tropics is not limited by saturation until reaching heights of 70–100 km. The warmer temperatures allow more water to ascend to the mesosphere. Photochemical simulations show a strong increase in high‐altitude atomic hydrogen following the high‐altitude water vapor increase by a few days. Plain Language Summary: The ExoMars Trace Gas Orbiter (TGO) is currently in orbit around Mars measuring the composition of the atmosphere. TGO was able to make observations before, during, and after a large planet‐encircling dust storm that occurred from June to August 2018. The TGO measurements provide the first opportunity to observe how water vapor is distributed with height in the atmosphere during a global‐scale dust event. It was found that there was a large increase in water vapor very high (40–100 km) in the atmosphere during the dust storm. Using a three‐dimensional numerical model of the Mars atmosphere, we found that when dust from the storm is transported up to levels above ~40 km, it warms the atmosphere due to solar absorption, and this in turn prevents ice clouds from forming at heights of 40–60 km and allows more water vapor to ascend to greater heights in the atmosphere. This is of interest in terms of the planetary evolution, as water molecules at greater heights are more readily dissociated by sunlight and lost from the atmosphere. This is an important factor for understanding the changes that have occurred since the period when surface features on Mars indicate that liquid water was present. Key Points: Transport of water vapor is simulated during the 2018 global dust storm on Mars in the GEM‐Mars General Circulation ModelThe simulation explains NOMAD observations of enhanced high‐altitude water abundances during the global dust stormThe vertical distribution of dust is a key factor for the transport of water vapor through the equatorial hygropause [ABSTRACT FROM AUTHOR]