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Ozone simulated in Mars Global Climate Models (MGCMs) is used to assess the underlying chemistry occurring in the atmosphere. Currently, ozone total column abundance (TCA) is under‐predicted in MGCMs by up to 120%, implying missing or inaccurate chemistry in models. Heterogeneous reactions of hydroxyl radicals (HOX) have been offered as an explanation for some of this bias, because they cause ozone to increase at locations where it's currently under‐predicted. We use four simulations to compare modeled ozone TCA with observations from the UVIS spectrometer aboard the ExoMars Trace Gas Orbiter to improve the representation of heterogeneous processes and their impact on ozone. We use a gas‐phase only run, a dissociative scheme, an adsorbed HOX retention scheme, and a hybrid scheme that combines the dissociative mechanism with the retention of HOX on water ice. We find retention of HOX is dependent on water ice sublimation, and ozone abundance increases when water ice persists for longer periods (1–20 sols). Over time, the loss of HOX causes a depletion in H2O2 concentration (HOX reservoir), and thus allows ozone concentration to increase. When adsorbed HOX are desorbed and dissociate into other by‐products, HOX are not immediately available to destroy ozone. This results in larger ozone concentrations than if desorbed HOX are released directly back into their gaseous states. When using the hybrid scheme, ozone TCA is increased up to 50% where the ozone deficit is greatest, demonstrating the best agreement with observations, and implying that HOX radicals are both retained when adsorbed and dissociate. Plain Language Summary: Ozone is a trace gas in the martian atmosphere, sensitive to chemical and light‐induced reactions. This makes it ideal for assessing the underlying reactions for chemical species called hydroxyl radicals, which destroy ozone and would otherwise be too reactive and shortlived to be measured directly. There has been an under‐prediction of ozone in global climate models which implies missing or inaccurate reactions. Heterogeneous reactions (a reaction which includes two or more phases) have been suggested to fill the deficit in modeled ozone. This involves hydroxyl radicals adsorbing onto water ice, which causes ozone to increase. We look at different simulations to understand hydroxyl radical chemistry on the surface of water ice by comparing to observations from the ExoMars Trace Gas Orbiter. We run four climate model simulations to test different theories as to what is occurring at the surface. There are two theories (a) reactions happen at the surface which alter hydroxyl radicals into other chemicals, and (b) hydroxyl radicals stay adsorbed onto water ice for as long as the ice persists. When we combine these theories, we have a more accurate prediction of ozone, which we can then use to infer what happens at the surface. Key Points: Heterogeneous reactions can have a major impact on martian ozone abundance at locations where water ice clouds persist for several solsLifetime of adsorbed hydroxyl radicals depends on water ice sublimation and longer lifetimes result in a greater abundance of ozoneA combination of longer lifetimes and dissociation of adsorbed HOX suppresses gaseous HOX and improves simulated ozone total column abundance [ABSTRACT FROM AUTHOR]

Since the discovery of atmospheric Mg+ on Mars in 2015 by the Mars Atmosphere and Volatile Evolution mission, there have been almost continuous observations of this meteoric ion layer in a variety of seasons, local times, and latitudes. Here, we present the most comprehensive set of observations of the persistent metal ion layer at Mars, constructing the first grand composite maps from pooled medians of subsamples of a metallic ion species. These maps demonstrate that Mg+ appears in almost all conditions when illuminated, with peak density values varying between 100 and 500 cm−3, dependent on season and local time. There exists significant latitudinal variation within a given season, indicating that Mg+ is not simply an inert tracer, but may instead be influenced by the meteoric input distribution and/or atmospheric dynamics and chemistry. Geographic maps of Mg+ density as a function of latitude and longitude indicate the influence of atmospheric tides, and there is no apparent correlation with remnant crustal magnetic fields. This work also presents counter‐intuitive results, such as a reduction of Mg+ ions in the northern hemisphere during Northern Winter in an apparent correlation with dust aerosols. Plain Language Summary: Metallic atoms in a planet's atmosphere are present when interplanetary dust particles burn up, releasing atomic species not typically found in the lower atmosphere. The discovery of a high altitude metallic layer on Mars in 2015 has led to continued monitoring in a variety of seasons across the entire planet. These results demonstrate that this magnesium ion (Mg+) layer appears throughout the year, with variations in peak abundances and layer heights, due to interactions with the background atmosphere. These variations track the dynamics of the middle atmosphere, providing insight into global climate patterns and may inform our understanding of seasonal deposition of interplanetary dust particles and their sources. This first‐order analysis supports future modeling efforts and provides model challenges to be understood, both of which can be explored in detail with time varying full planet climate modeling. Key Points: Eight Earth years of Mars Atmosphere and Volatile Evolution/Imaging Ultraviolet Spectrograph observations show that Mars' persistent meteoric metal ion layer is more dynamic than initially assumedMg+ layer peak altitude, abundance, and top and bottom side slopes vary significantly over the observed time periodThe relative absence of northern hemispheric Mg+ during southern summer is surprising and may be related to lower atmospheric dust loading [ABSTRACT FROM AUTHOR]

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]

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]

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]

The persistent two‐peaked vertical structure of the Martian ionosphere is created by extreme and far ultraviolet radiation whose energies, respectively, determine their ionization altitude. A third low‐altitude transient layer (previously referred to as M3 or Mm) has been observed by radio occultation techniques and attributed to meteor ablation. However, recent remote sensing and in situ observations disfavor a meteoric origin. Here we propose an alternative hypothesis for these apparent layers associated with impact ionization from penetrating solar wind ions, previously observed as proton aurora. Localized ionization, occurring nonglobally at a given altitude range, breaks the symmetry assumed by the radio occultation technique, and creates electron layers apparently lower in the ionosphere than their true altitude. This may occur when the upstream bow shock is altered by a radial interplanetary magnetic field configuration, which allows the solar wind to penetrate directly into the thermosphere. This localized ionization hypothesis provides an explanation for apparent layers' wide variation in heights and their transient behavior. Moreover, this hypothesis is testable with new observations by the Mars Atmospheric and Volatile EvolutioN Radio Occultation Science Experiment in future Mars years. This hypothesis has implications for the ionospheres of Venus and Titan, where similar transient layers have been observed. Plain Language Summary: The ionosphere of a planet couples the neutral atmosphere to the exosphere and solar wind. Created by solar ionizing radiation, the ionosphere on the dayside of planet is expected to be hemispherically symmetric. When Mars Express discovered transient low‐altitude ionospheric layers in 2005, they were initially predicted to be due to meteor ablation. However, recent observations indicate that they are due to a geometric observational effect related to local, not hemispheric, ionization. Observations from three instruments on Mars Atmospheric and Volatile EvolutioN are used concurrently to understand this phenomenon. These transient layers have been observed at Venus and Titan as well, and this hypothesis indicates novel physics for those ionospheric systems. Key Points: Low‐altitude transient ionospheric layers may be explained by a geometric observational effect from localized, not hemispheric, ionizationA radio occultation of a transient ionospheric layer was obtained with observations of variable proton aurora and penetrating solar windTransient layers observed at Mars, Venus, and Titan are consistent with the presence of solar wind impact ionization at those planets [ABSTRACT FROM AUTHOR]

On 19 October 2014, comet C/2013 A1 (Siding Spring) had a close encounter with Mars and deposited cometary dust particles into the Martian atmosphere. We report a comprehensive analysis of the resulting meteor shower and its perturbation on Mars' atmosphere and ionosphere. Using Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph observations of ablated meteoric metallic species, we show this shower lasted less than 3 hr and was therefore limited to one hemisphere. Meteoric ablation occurred in a narrow altitude layer, with Mg+, Mg, Fe+, and Fe deposited between about 105 and 120 km, consistent with comet Siding Spring's relative velocity of 56 km/s. We find that 82 ± 25 t of dust was deposited, improving previous measurements and a thousand times larger than model expectations. With regular observations over two Mars days, we show that horizontal winds globally redistribute this material and also suggest new vertical transport mechanisms for metallic ions. Such transport is inconsistent with diffusion and may be related to electrodynamic processes. The rapid loss of neutral species and presence of ions at high altitudes indicate that our understanding of existing Martian meteoric chemistry modeling and ionospheric dynamics is incomplete. Plain Language Summary: A close encounter of Mars with Comet Siding Spring resulted in the largest meteor shower in modern history, depositing orders of magnitudes more dust than a typical meteor shower. This unique event creates the opportunity to observe metallic species not normally observed, and their resulting chemistry and dynamics. This work constrains the duration of the meteor shower and revises the deposited cometary dust mass to 82,000 kg, more than the initial study and in excess of model expectations. Key Points: Comet Siding Spring's meteor shower allows for the only remote sensing observations of Mg, Fe+, and Fe in the Martian atmosphereWe constrain the duration of the meteor shower to less than 3 hr, and the total meteoric fluence to 82,000 kgThe distribution of these metallic ions defy expectation, prompting further study with 3‐D global circulations models [ABSTRACT FROM AUTHOR]

Abstract: We report the detection of bright aurora spanning Mars' nightside during the space weather event occurring in September 2017. The phenomenon was similar to diffuse aurora detected previously at Mars, but 25 times brighter and detectable over the entire visible nightside. The observations were made with the Imaging UltraViolet Spectrograph, a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft orbiting Mars. Images show that the emission was brightest around the limb of the planet, with a fairly uniform faint glow against the disk itself. Spectra identified four molecular emissions associated with aurora, and limb scans show the emission originated from an altitude of ~60 km in the atmosphere. Both are consistent with very high energy particle precipitation. The auroral brightening peaked around 13 September, when the flux of solar energetic electrons and protons both peaked. During the declining phase of the event, faint but statistically significant auroral emissions briefly appeared against the disk of the planet in the form of narrow wisps and small patches. These features are approximately aligned with predicted open field lines in the region of strong crustal magnetic fields in Mars' southern hemisphere. [ABSTRACT FROM AUTHOR]

Abstract: We report the response of the Martian upper atmosphere to a strong X‐class flare on 10 September 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hr UT, and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of ∼270 K and a maximum value of ∼310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare‐induced heating is limited between low and middle latitudes. During this event IUVS observed an ∼70% increase in the observed brightness of CO 2 + ultraviolet doublet and CO Cameron band emission at 90 km, where high‐energy photons (< 10 nm) deposit most of their energy. [ABSTRACT FROM AUTHOR]

Abstract: Mars has lost a large fraction of its water to space, with the H component of this loss thought to occur mainly as a result of thermal (Jeans) escape from the upper atmosphere. Constraints on H loss have historically been made using hydrogen Lyman alpha (121.6 nm) light scattered in the planet's extended upper atmosphere or corona. Here we employ observations from the Mars Atmosphere and Volatile Evolution (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) to constrain H escape in December 2014 and August 2016, when MAVEN observed the dayside corona at low latitude. To obtain adequate fits and address systematic sources of uncertainty including instrument calibration, we fit in exobase number density and escape rate instead of density and temperature, employing Markov Chain Monte Carlo techniques. This produces better model fits to data than most previous analyses. When we assume a single population of H atoms, we obtain H temperatures inconsistent with expected trends and a shape mismatch between observed and modeled profiles, similar to previous studies. Introducing either a second population of H (at a distinct temperature and density) or adding deuterium to the corona allows for essentially perfect fits. Despite this model ambiguity, derived loss rates for both periods are within a factor of four, 3.3–8.8×108cm−2/s in December 2014 (Ls∼250) and 0.6–2.3×108cm−2/s in August 2016 (Ls∼200). These rates are similar to those found in prior studies and confirm the known seasonal trend—doing so while incorporating the substantial uncertainty in absolute calibration insufficiently explored by previous studies. [ABSTRACT FROM AUTHOR]

Abstract: Recent measurements by the Imaging Ultraviolet Spectrograph (IUVS) instrument on NASA's Mars Atmosphere and Volatile EvolutioN mission show that a persistent layer of Mg+ ions occurs around 90 km in the Martian atmosphere but that neutral Mg atoms are not detectable. These observations can be satisfactorily modeled with a global meteoric ablation rate of 0.06 t sol−1, out of a cosmic dust input of 2.7 ± 1.6 t sol−1. The absence of detectable Mg at 90 km requires that at least 50% of the ablating Mg atoms ionize through hyperthermal collisions with CO2 molecules. Dissociative recombination of MgO+.(CO2)n cluster ions with electrons to produce MgCO3 directly, rather than MgO, also avoids a buildup of Mg to detectable levels. The meteoric injection rate of Mg, Fe, and other metals—constrained by the IUVS measurements—enables the production rate of metal carbonate molecules (principally MgCO3 and FeCO3) to be determined. These molecules have very large electric dipole moments (11.6 and 9.2 Debye, respectively) and thus form clusters with up to six H2O molecules at temperatures below 150 K. These clusters should then coagulate efficiently, building up metal carbonate‐rich ice particles which can act as nucleating particles for the formation of CO2‐ice clouds. Observable mesospheric clouds are predicted to occur between 65 and 80 km at temperatures below 95 K and above 85 km at temperatures about 5 K colder. [ABSTRACT FROM AUTHOR]