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The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5 degrees and 148 degrees ). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H(2)O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO(3), aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.

The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface.

The vertical profiles of water vapor and its semi‐heavy hydrogen isotope HDO provided by instruments on ExoMars Trace Gas Orbiter constitute a unique new data set to understand the Martian water cycle including its isotopic composition. As water vapor undergoes hydrogen isotopic fractionation upon deposition (but not sublimation), the D/H isotopic ratio in water is a tracer of phase transitions, and a key quantity to understand the long‐term history of water on Mars. Here, we present 3D global simulations of D/H in water vapor and compare them to the vertically resolved observations of D/H and water ice clouds taken by NOMAD during the second half of Mars year 34. D/H is predicted to be constant with height up to the main cloud level, above which it drops because of strong fractionation, explaining the upper cut‐off in the NOMAD observations when HDO drops below detectability. During the global and regional dust storms of 2018/2019, we find that HDO ascends with H2O, and that the D/H ratio is constant and detectable up to larger heights. The simulations are within the provided observational uncertainties over wide ranges in season, latitude and height. Our work provides evidence that the variability of the D/H ratio in the lower and middle atmosphere of Mars is controlled by fractionation on water ice clouds, and thus modulated by diurnally and seasonally varying cloud formation. We find no evidence of other processes or reservoirs that would have a significant impact on the D/H ratio in water vapor. Plain Language Summary: The isotopic composition of atmospheric water on Mars provides insights about the phase transitions in the water cycle, because the light (H2O) and semi‐heavy (HDO) form of water vapor deposit at different saturation pressures. Knowing the isotopic composition of atmospheric water also provides insights on the long‐term evolution of water on Mars. The NOMAD instrument on ExoMars Trace Gas Orbiter provided the first vertical profiles of the D/H ratio in water vapor on Mars. The data set shows a large variability with season and latitude. To understand this behavior, a general circulation model is required. We provide detailed simulations of the D/H ratio in Martian water vapor and compare them with the NOMAD observations. The model predicts that the D/H ratio is constant in the lower atmosphere and decreases across a layer of strong cloud formation that varies with season. During the global dust storm of 2018, this cloud layer was severely lifted. The simulations compare well to the observations, both out and in the dust storm, and explain their upper cut‐off by this predicted decrease in D/H. No other processes than cloud formation, nor special surface ice reservoirs with strongly different D/H values, were needed to reproduce the observations. Key Points: Hydrogen fractionation by clouds in Mars water vapor is simulated and evaluated with NOMAD D/H observations in and out of dust stormsThe model D/H ratio is constant and drops when clouds form, explaining the upper cut‐off in the NOMAD profiles as HDO becomes undetectableNOMAD water ice observations provide evidence that fractionation by clouds is the main factor controlling the HDO distribution on Mars [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]

Abstract: We update the results of cloud imaging sequences from the Mars Science Laboratory (MSL) rover Curiosity to complete two Mars years of observations (LS=160° of Mars year (MY) 31 to LS=160° of MY 33). Relatively good seasonal coverage is achieved within the study period, with just over 500 observations obtained, averaging one observation every 2–3 sols. Cloud opacity measurements are made using differential photometry and a simplified radiative transfer method. These opacity measurements are used to assess the interannual variability of the aphelion cloud belt (ACB) for MY 32 and 33. Upon accounting for a statistical bias in the data set, the variation is found to be <30% within uncertainty. Diurnal variation of the ACB is also able to be examined in MY 33 owing to an increased number of early morning observations in this year. Although a gap in data around local noon prevents a complete assessment, we find that cloud opacity is moderately increased in the morning hours (07:00–09:00) compared to the late afternoon (15:00–17:00). [ABSTRACT FROM AUTHOR]