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Baroclinic waves has been observed in the Martian atmosphere, and their periods have been calculated by means of spectral analysis of the pressure and wind velocity data, provided by the InSight mission for the time periods of the spring and winter seasons. For the spring time, the pressure spectrum shows a peak at the frequency of 0.246 (4 sol). For the winter time, the spectra of the pressure and wind velocity show a dominant peak at the frequency of ∼0.15 (6.7 sol), and a smaller peak at the frequency of ∼0.435 (2.3 sol). The results of this study are consistent with those determined in previous studies from spectral analysis of the Viking 2 data, and it is expected that a further analysis of the results determined in this study will provide a valuable insight into the seasonal changes of the atmospheric structure and circulation, which may be useful to fit General Circulation Models of the Martian atmosphere. However, a general understanding of Martian baroclinic phenomena only will be achieved with a combination of data from multiple Martian midlatitude stations in future missions. • Baroclinic waves has been observed in the Martian atmosphere. • Their periods have been calculated by means of spectral analysis from InSight data. • For the spring time, the pressure spectrum shows a peak at the frequency of 0.246 (4 sol). • For the winter time, the spectra of the pressure and wind velocity show two peaks: ∼0.15 (6.7 sol) and ∼ 0.435 (2.3 sol). • The results of this study may be useful to fit General Circulation Models of the Martian atmosphere. [ABSTRACT FROM AUTHOR]

We apply a novel method to retrieve water vapor vertical distributions in the Martian atmosphere to spectra collected by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES). The new method exploits differences in sensitivity of the daytime and nighttime TES nadir spectra to water vapor content in different parts of the lower atmosphere (0–40 km). Based on the test retrievals from simulated environments, vertical profiles of water vapor can be retrieved with accuracy ∼20–40 % depending on the season and altitude above surface. Retrievals from observations collected by the MGS TES between L s = 135° in MY24 and L s = 75° in MY27 enable exploration of the seasonal and interannual variabilities of the vertical distribution of water vapor. Retrieved vapor distributions and their seasonal variability are generally consistent with those predicted by a numerical circulation model. Vapor is concentrated near the surface in the northern polar region and the mid-latitudes during the northern summer (L s = 90°). During the southern summer (L s = 270°) vapor extends higher in the atmosphere in the southern polar region, reflecting both sublimation of the seasonal surface ice and atmospheric transport by the upper branch of the southern Hadley cell. During both equinoxes (L s = 0° and L s = 180°) vapor is found in the lower part of the tropical atmosphere. At the same time the retrieved vertical distributions exhibit notable interannual variability. Following the global dust storm of MY25, vapor is confined to lower altitudes in the southern polar region during the southern summer. During the southern spring in MY26 (L s = 180°-270°) vapor extends higher in the atmosphere and mixing ratios increase faster with height at the equator and in the northern tropics. These examples suggest that Martian atmosphere may experience interannual changes in the atmospheric transport, some of which could be in response to global dust storm events. • MGS TES nadir spectra are used to retrieve vertical extent of water vapor on Mars. • Observed vertical distributions are consistent with GCM modeling. • Vapor is concentrated near surface in the northern extra-tropics during summer. • Vapor extends high in the atmosphere over southern polar region during summer. • Vertical extent of vapor in the south is reduced following the dust storm of MY25. [ABSTRACT FROM AUTHOR]

Mars South Polar Residual Cap (SPRC) persists throughout the Southern hemisphere's warmest months. During the colder seasons, carbon dioxide (CO 2) accumulates as ice on the surface of the planet, forming a seasonal cap during fall and winter and sublimating throughout spring. The seasonal cap and SPRC both exhibit unexpected reflectance properties that have not been explained. To better understand the Martian surface CO 2 ice and how it affects Mars' geomorphology, atmosphere, and climate, it is imperative to corroborate laboratory experiments with orbital observations. This study uses a custom-designed goniometer for making reflectance measurements within the MARs Volatile and Ice evolutioN (MARVIN) environmental chamber, in which we can measure the Bidirectional Reflectance Distribution Function (BRDF) of different phases of CO 2 under Mars polar conditions for the first time. This has the potential to explain present-day observations from orbit and enhance our understanding of the evolution of ice on Mars. • First laboratory measurements of CO 2 ice BRDF under simulated Mars polar conditions. • Translucent and frosty CO 2 ice morphologies show distinct reflectance properties. • Translucent ice exhibits specular reflection with multiple axes, while frosty ice is diffuse. • Initial deposition temperature influences CO 2 ice texture and optical properties. • Results provide new insights to interpret Mars' "Cryptic Terrain" observations. [ABSTRACT FROM AUTHOR]

The ever-changing transparency of the Martian atmosphere hinders the determination of absolute surface colour from spacecraft images. While individual high-resolution images from low orbit reveal numerous colour details of the geology, the colour variation between images caused by scattering off atmospheric dust can easily be of greater magnitude. The construction of contiguous large-scale mosaics has thus required a strategy to suppress the influence of scattering, often a form of high-pass filtering, which limits their ability to convey colour variation information over distances greater than the dimensions of single images. Here we use a dedicated high altitude observation campaign with the Mars Express High Resolution Stereo Camera (HRSC) (Neukum and Jaumann, 2004; Jaumann et al., 2007), applying a novel iterative method to construct a globally self-consistent colour model. We apply the model to colour-reference a high-altitude mosaic incorporating long-range colour variation information. Using only the relative colour information internal to individual images, the influence of absolute image to image colour changes caused by scattering is minimised, while the model enables colour variations across image boundaries to be self-consistently reconstructed. The resulting mosaic shows a level of colour detail comparable to single images, while maintaining continuity of colour features over much greater distances, thereby increasing the utility of HRSC colour images in the tracing and analysis of martian surface structures. • Use iterative method with high-altitude images to determine global brightness models in five colours.s • Derive global high-altitude image mosaics at 2 km/pix for each of the HRSC colour filters. • Demonstrate use of high-altitude mosaic to reference full-resolution colour mosaic products. [ABSTRACT FROM AUTHOR]

Retrieving the optical depth of the Martian clouds (τ cld) is a powerful way to monitor their spatial and temporal evolution. However, such retrievals from nadir imagery rely on several assumptions, including the vertical structure of the clouds in the atmosphere. Here we compare the results of cloud optical depth retrievals at 320 nm from the Emirates eXploration Imager (EXI) onboard the Emirates Mars Mission (EMM) "Hope" orbiter performed using a basic uniform cloud profile used in previous studies and using derived cloud profiles obtained from near-simultaneous Solar Occultation observations in the 3.1–3.4 μ m spectral range from the Middle-Infrared channel of the Atmospheric Chemistry Suite (ACS) instrument onboard the ESA Trace Gas Orbiter (TGO). We show that the latitudinal dependence of the cloud vertical profiles can have a strong impact on the nadir retrievals; neglecting it can lead to a significant underestimation of τ cld in the polar regions (up to 25 % to 50 %, depending on the vertical distribution of the dust in the atmosphere) and to a lesser extent, to an overestimation of τ cld around the equator. We also discuss the impact of a vertically-dependent particle size profile, as previous studies have shown the presence of very small water ice particles at the top of the clouds. From this analysis, we provide recommendations for the improvement of water ice cloud parameterization in radiative transfer algorithms in nadir atmospheric retrievals. • Simultaneous Solar Occultation and nadir observations of water ice clouds. • Strong impact of the latitudinal variation of the cloud profiles on the retrievals. • Cloud optical depths can be underestimated if not accounting for size profiles. [ABSTRACT FROM AUTHOR]

The wind interacts with the surface of Mars, forming aeolian landforms. The orientation of these landforms informs us about the formative wind directions. The observations of the orientation of aeolian landforms on Meridiani Planum between Mars years 26 and 38 indicated that there is a seasonality of formative wind directions in this region. The studied landforms were shaped by a predominant SE wind during summer, while during winter several wind directions played a role in their formation. Throughout the Mars year, the most formative were the NW winds. The presence of dark wind streaks oriented toward the west during winter indicates that E winds occur in this season on Meridiani Planum, as previously predicted by numerical simulations. It was also found that aeolian deflation led to complete erosion of smaller dunes, and that the relatively strong deflation was responsible for the scarcity of fine-grained ripples on Meridiani Planum. In this region, fine-grained ripples were found only in a few locations, and they were mainly small bedforms with wavelengths up to several centimeters. • In situ and orbital data from a period between Mars years 26 and 38 were analyzed. • There is a seasonality of aeolian landforms on Meridiani Planum. • The most formative winds blow from NW, but in summer SE winds dominate. • Numerically predicted modern E winds occur mainly in winter. • Deflation led to complete erosion of three dome dunes in the studied period. [ABSTRACT FROM AUTHOR]

Recent observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Express (MEX) spacecraft have suggested that pressure pulses originating from upstream proton cyclotron waves (PCWs) can "ring" the Martian magnetopause at the same frequency and drive magnetosonic waves in the upper ionosphere of Mars, thereby transporting energy from the solar wind into the ionosphere. However, the limitation of single-spacecraft measurements prevents simultaneous observations of the driver and response of this "ringing" process of the Martian magnetosphere. Here we utilize two-point measurements from MAVEN and MEX to characterize the ringing probability at which upstream PCWs drive compressional fluctuations in the ionospheric magnetic field. We develop an algorithm to identify PCW-driven magnetosonic waves in the upper ionosphere of Mars from the two-point magnetic field data. The derived ringing probability is higher on the dayside, outside strong crustal magnetic fields, and under high solar wind density conditions. We also show that the median power of dayside ionospheric magnetic field fluctuations is enhanced by a factor of ∼ 2 at corresponding frequencies in the presence of upstream PCWs compared to the median power in the absence of upstream PCWs. These results demonstrate the prevalence of energy deposits into the dayside Martian ionosphere from the solar wind mediated by the PCW-driven ringing of the magnetosphere. Future studies, possibly with new multi-point observations, should address the detailed processes of wave propagation and energy transport through the system and the long-term impact of this chain of processes on the planetary ion heating in the ionosphere and atmospheric loss from Mars. • We utilize two-point measurements by Mars Express and MAVEN. • The probability of upstream waves driving ionospheric waves is derived. • The probability is higher on the dayside and outside strong crustal fields. • The probability is higher under high solar wind density conditions. • Ionospheric magnetic perturbations are 2x enhanced in the presence of upstream waves. [ABSTRACT FROM AUTHOR]

We use 62 electron density profiles collected by the Radio Occultation Science Experiment (ROSE), on the Mars Atmosphere and Volatile EvolutioN (MAVEN), when Mars was hit by CIRs and ICMEs close to aphelion (April 2021) and during two dust storms (June/July 2022) to examine the response of the Martian ionosphere to solar events and to solar events hitting during dust storms. We do so through three proxies – variation in total electron content between 80 and 300 km altitude, peak density, and peak altitude – of the aforementioned 62 ROSE electron density profiles, relative to a characterisation of the ionosphere through solar minimum leading to solar maximum, specific to local time sector and season, presented in the companion manuscript by Segale et al. (2024). We observe an increased Total Electron Content (TEC) between 80 and 300 km altitude up to ≃ 2. 5 × 1 0 15 m−2 in April 2021 and up to ≃ 5 × 1 0 15 m−2 in June/July 2022 compared to the baseline photochemically produced ionosphere. This increase in TEC corresponds mainly to increases in the solar energetic particles flux (detected by MAVEN Solar Energetic Particle, SEP) and electron fluxes (detected by the MAVEN Solar Wind Electron Analyzer, SWEA). In addition to solar events, in June/July 2022, an A storm and a B storm were occurring and merging on the surface of Mars. We observe a raise in peak altitude in general lower than expected during dust storms, possibly due to high values of solar wind dynamic pressure (derived from MAVEN Solar Wind Ion Analyzer, SWIA). From 31 ROSE profiles collected in this time period that showed both the M2 and M1 layer, we observe that, on average, M1 and M2 peak altitudes raise the same amount, suggesting that the thermosphere might loft as a unit during dust storms. During this time period, several proton aurora events of variable brightness were detected with MAVEN Imaging Ultraviolet Spectrograph (IUVS), underlining the complex and multifaceted impact of dust activity and extreme solar activity on the Martian ionosphere. • Increase in TEC up to 2. 5 × 1 0 15 m−2 between 80 and 300 km following an ICME/CIR impact. • Increase in TEC up to 5 × 1 0 15 m−2 between 80 and 300 km following ICME/CIR impacts during dust storms. • During dust storms, the peak altitudes of the M1 and M2 layers loft of the same amount. [ABSTRACT FROM AUTHOR]

Mars's atmosphere shares many of the same types of oscillations exhibited in Earth's climate, like the Semiannual Oscillation and Annular Modes, but using two Martian reanalysis datasets derived from Mars Climate Sounder observations, we find that Mars fails to generate a Quasi-Biennial Oscillation (QBO). The lack of a QBO may stem from the absence of a stratosphere on Mars — as the background flow is critical for setting the altitude that vertically propagating waves deposit momentum — or the extreme latitudinal variation of the overturning circulation disrupts waves sources. The lack of a QBO on Mars may enable unique comparisons between Earth's and Mars's atmospheres. • Mars seems to lack a Quasi-Biennial Oscillation (QBO) in two reanalyses. • The lack of a QBO may stem from the absence of a stratosphere on Mars or variation of the overturning circulation. • The lack of a QBO on Mars enables unique comparisons between Earth's and Mars's atmospheres. [ABSTRACT FROM AUTHOR]

NASA's InSight lander monitored the Martian atmosphere while conducting its primarily geophysical investigation. Atmospheric imaging was used to study dust and ice at the site for over two Mars years in 2018–2022. An optical depth record, including dust and ice, was derived from systematic sky imaging in the mornings (for the first part of the mission) and evenings. Optical depths ranged from 0.5 to 1.9 but were typically under 1. Dust storms were seen at expected times in late northern autumn and early winter, including one shortly after landing, along with one late summer storm in January 2022. The optical depth record closely matched that of Curiosity, 600 km to the south, except for the expected additional water ice content during the aphelion cloud belt (ACB, spring to early summer). In addition to ice hazes, the ACB included discrete clouds, whose motion was tracked to show northeasterly to southeasterly daytime winds. While InSight recorded many meteorological vortices, no dust devils were seen, requiring that dust-devil occurrence was <10−3 times as common as during Spirit rover dust devil seasons. • We describe the derivation of InSight's record of optical depth from sky images. • InSight required new techniques compared to previous Martian optical depth records. • InSight had similar optical depths to nearby Curiosity, with more ice and clouds. • Cloud motion was characterized during the aphelion cloud belt season. • An extensive search revealed no dust devils despite the abundant vortices. [ABSTRACT FROM AUTHOR]

We performed retrievals of dust and water–ice optical depths at select times spanning Mars years (MYs) 33 through 36. We used data taken by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, whose precessing orbit allowed IUVS to image the diurnal evolution of water–ice clouds at local times throughout this MY range where they have not been studied. We created a radiative-transfer retrieval algorithm and used it to simultaneously fit the dust and water–ice optical depth of each usable spectrum in IUVS apoapse data for the majority of the MAVEN mission. We compared our results to other datasets to ensure our retrievals produced plausible values and discuss potential reasons for discrepancies. We obtained global climate model (GCM) simulations from two state-of-the-art models and compared our results to both sets of simulations. We show that there are significant differences between our water–ice retrievals and both sets of simulations, both in the spatial cloud patterns and column-integrated optical depths. We discuss the different approaches both models used for simulating these clouds and the implications for accurately modeling these clouds in future GCM work. • We investigated the diurnal evolution of water–ice clouds across several Mars years. • Global climate models struggle to reproduce the clouds patterns we observe. • Supersaturation and rapid nucleation are necessary to accurately model these clouds. • These insights may further improve global climate model predictions. [ABSTRACT FROM AUTHOR]

Global climate models (GCM) predict snow accumulation/glaciation on the Hellas basin eastern rim, where Amazonian glacial landforms are observed. We explore GCM results for Late Noachian-Early Hesperian higher atmospheric pressure/higher obliquity/faint young sun conditions, assess accumulation rates, and use these as input to an ice sheet model. We assess ice sheet growth, thickness, basal temperatures, basal melting water volumes, flow/sliding-ice velocities and the fate of basin floor glacial ice/meltwater. We conclude that significant volumes of glacial ice (with shear-induced basal melting) could have flowed down the basin wall and onto the basin floor, covering the majority with glacial ice. Late Noachian water budgets of up to 1000 m global equivalent layer (GEL) and Hellas floor ice sequestration are implied. Subsequent emplacement of Hesperian volcanic plains superposed on the ice may have caused melting (contact/deferred) and deformation (loading-induced flow/diapirism), potentially accounting for several observed basin floor geologic units. • We use climate and ice sheet models to assess predicted Late Noachian glaciation at Hellas. • Ice accumulates on the eastern rim, flows downslope on basin wall; slope promotes shear heating, melting. • Predicted glacial system extends across basin floor, stabilizes, ablates; sequestered by debris cover. [ABSTRACT FROM AUTHOR]

Tides are one of the important reasons for the spatiotemporal variation of Mars' upper atmosphere. The study of atmospheric tides is crucial for gaining insights into the planet's climate, material cycling dynamics, and its potential habitability. This investigation utilizes the data observed by the Neutral Gas and Ion Mass Spectrometer (NGIMS) to examine atmospheric tides in the Martian upper atmosphere, with a particular emphasis on the influence of Martian dust storms. It is the first time to find that zonal wave-number 4 (WN4) is an important longitudinal structure of the Martian upper atmospheric density. The results indicate the latitude dependence of the longitudinal structure. There is WN4 structure at lower latitudes and WN3 structure at middle-high latitudes. The dust storm possibly can cause the WN4 in the longitudinal structure for CO 2 and N 2. However, O and He show differences in the east-west direction during the 2018 dust storm and complex structure during the 2016 dust storm. This suggests that different neutral species respond differentially to the dust storm. • WN4 is an important structure of the Martian upper atmospheric density. • The longitude structure has a latitude dependence, with WN4 at low latitudes and WN3 at mid-latitudes. • The dust storm possibly can enhance the WN4 peak near 320° and cause phase changes in the longitude structure. • The longitudinal structure of neutral species is different during the dust storm period. [ABSTRACT FROM AUTHOR]

This study investigated gravity waves in the Martian polar lower atmosphere using Mars Global Surveyor (MGS) radio occultation data from February 1998 to March 1999. In addition to the conventional geometrical optics method, Full Spectrum Inversion (FSI) was used to achieve a vertical resolution of ∼300 m in terms of the wavelength. In the polar night in the southern high latitude, wavelike structures with vertical wavelengths ranging from ∼1 km to several km extend from the surface to high altitudes. The positive static stability near the surface and the high terrain in the southern high latitude suggest the waves were topographically generated. The near-surface wind speed was estimated to be a few m s−1. In the midnight sun in the northern high latitude, waves show smaller amplitudes up to 10–15 km altitudes and attenuation above, which is attributable to reduced background stability. Near-neutral thin layers were ubiquitously observed in both groups, indicating the generation of turbulent layers by saturated gravity waves. It was shown that high-resolution radio occultations with radio holographic analysis offer a valuable tool for understanding atmosphere-solid planet interactions. • Gravity waves in Mars' polar atmosphere were studied using MGS radio occultation data • A radio holographic method, FSI, was used to achieve a high vertical resolution • Short-vertical wavelength waves and near-neutral thin layers were frequently observed • Orographic generation and saturation of gravity waves were suggested • Surface wind speed was estimated to be a few m s−1 from the observed wave structure [ABSTRACT FROM AUTHOR]

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has provided a unique opportunity to study the variability of the Martian upper atmosphere and ionosphere, as well as their interaction with the solar wind under various conditions. This study characterizes the large-scale spatial-temporal evolution of the dayside ionosphere as observed by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard MAVEN. NGIMS measures the composition and structure of both the ions and neutrals in the upper ionosphere and thermosphere at altitudes below ∼500 km. Thanks to the precessing orbit of MAVEN, NGIMS has accumulated millions of ion and neutral measurements over a wide range of spatial and temporal parameters. In this study, we present what we have learned from the NGIMS measurements during the first 3.5 Mars years (2015–2021), including the spatial and compositional variation of the median dayside ionosphere, the observed ionospheric peak layer at high solar zenith angles (SZAs), which complements the observations by previous missions at lower SZAs, and the variability of the topside ionosphere. • Median ionospheric states derived from the NGIMS measurements agree with the density and composition estimated by models. • This first NGIMS in situ SZA variation of the ionospheric density complements observations by other instruments on Mars. • These NGIMS measurements show how the topside ionosphere varies with the solar cycle and seasons. [ABSTRACT FROM AUTHOR]

Launched on August 12, 2005, the Mars Reconnaissance Orbiter (MRO) entered Mars orbit on March 10, 2006. Following a period of aerobraking, MRO completed its entry into its primary science orbit in September 2006, initiating a program of systematic observations of Mars that still continues. Five providers, including the Italian Space Agency, provided 6 instruments for flight, observing the surface, atmosphere, and subsurface of Mars with greater spatial resolution and systematic coverage than ever before. Two investigations utilized the spacecraft accelerometers and tracking of the orbiter via the Deep Space Network to study upper atmosphere densities and the gravity field of the planet. The data acquired by MRO have revealed a dynamic planet whose change from an ancient wetter climate to the drier climate of today was a complex transition and not a simple "drying out". Furthermore, that climate continues to change even today. The diversity of the early habitable environments, the ice ages recorded in the polar cap layering and subsurface ice deposits, the repeating patterns of dust storms, and the revelation of new features at the limit of resolution are all part of the scientific return from MRO during nearly a decade of Mars years. The story of that mission, of the evolution of its capabilities, and its contributions to our current understanding of Mars are the subject of nearly two dozen papers in the Icarus special issue, MRO: Sixteen Years at Mars. Three papers describe in more detail the evolution of instrument operations and data products over the mission; several papers describe new analysis techniques for the radar, including construction of 3-dimensional views, and for the atmospheric sounder, enabling better retrievals in a dusty lower atmosphere. Other papers report on recent research including, but not limited to, dune movement, the roles of water and carbon dioxide ice in surface change, and attempts to understand the formation and fading of the enigmatic recurring slope lineae. This paper describes general aspects of the MRO spacecraft, payload, and mission as context for the special issue papers; it also summarizes scientific results and mission support events on a mission phase by mission phase basis to give a time history of discovery and effort. • Diverse potentially habitable environments have been identified from orbit. • Evidence found for climate change throughout Mars history, including today. • Recurring slope lineae were discovered; origins are still being investigated. • Internal layering of the Mars polar caps appears linked to orbital cycles. • Mid-latitude, near-surface water ice is exposed in cliffs and new impact craters. [ABSTRACT FROM AUTHOR]

Limb sounding of thermal emission in the infrared wavelength range is a powerful technique for measuring temperature and aerosols in the martian atmosphere. However, the long optical path may provide challenges to limb retrievals in high aerosol conditions. These can be mitigated by considering limb measurements in the far infrared, where opacities of most aerosols are lower than in the mid-infrared. We present analyses of radiative properties of Mars dust and water ice aerosols at far infrared wavelengths based on measurements by the Mars Climate Sounder (MCS) in limb geometry at mid- and far infrared wavelengths. For dust aerosols, derived far infrared radiative properties show a homogeneous behavior that is consistent with particle sizes in the order of 1 μm effective radius. Far infrared radiative properties for water ice aerosols exhibit a larger variability in local time and region, leading to significant differences between the aphelion cloud belt and the north polar hood, with the resulting parameters suggesting particle sizes around 3 μm or larger. Using the derived parameters, we develop a method for retrieving aerosol profiles from MCS limb measurements that combines information from mid- and far infrared spectroscopic channels. The use of far infrared channels enables aerosol profile retrievals from limb measurements that typically reach about a scale height deeper into the atmosphere than would be possible using mid-infrared channels only. The extended vertical range of the aerosol profiles allows the derivation of aerosol column optical depths through vertical integration, with dust column derivations in global or large-scale regional dust storms also being available by extrapolating dust profiles below the lowest retrievable altitude of a limb measurement. The quantification of aerosol columns allows us to retrieve surface brightness temperatures from MCS on-planet viewing measurements that are corrected for atmospheric contributions. We show that differences between surface brightness and top-of-the-atmosphere temperatures are typically within 20 K, with surface brightness temperatures generally being warmer (colder) than top-of-the-atmosphere temperatures at daytime (nighttime), except at high latitudes. [Display omitted] • We derive radiative properties of Mars dust and water ice in the far infrared from Mars Climate Sounder limb measurements. • Use of combined mid- and far infrared measurements enables aerosol profile retrievals of extended vertical range. • The extended vertical range of aerosol profiles allows the derivation of aerosol columns through vertical integration. [ABSTRACT FROM AUTHOR]

As tracers of the major volatile cycles of MarsCO 2 , H 2 O, and dustclouds are important for understanding the circulation of the martian atmosphere and hence martian climate. We present the spatial and seasonal distribution of laterally-confined clouds in the middle atmosphere of Mars during one Mars Year as identified in limb radiance measurements by the Mars Climate Sounder. Cloud identifications were made by citizen scientists through the "Cloudspotting on Mars" citizen science project, hosted on the citizen science platform Zooniverse. A method to aggregate the crowdsourced data using a novel clustering algorithm is developed. The derived cloud catalog is presented and the seasonal and spatial distribution of clouds is discussed in terms of key populations. • The Mars Climate Sounder serendipitously observes aerosol layers in limb observations. • The Cloudspotting on Mars citizen science project enabled identification of clouds throughout Mars Year 29. • A customized clustering algorithm optimizes the aggregation of citizen-scientist annotations. • Maps of clouds show effects of thermal tides and dust on cloud formation. [ABSTRACT FROM AUTHOR]

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) collected hyperspectral images of the Martian surface and atmosphere from September 27, 2006, through May 7, 2022. Over that time, nearly twenty scientific investigations were completed, most of which arose as a result of the findings from previous investigations. Two review papers published in 2009 (Murchie et al., 2009a, b) described the initial two-year investigation during MRO's Primary Science Phase, its key findings, and the CRISM data products that were developed and released to the community through that time. Here we describe the conduct and evolution of the CRISM investigation since then, which includes MRO's Extended Science Phase and first five Extended Missions. We document the physical changes in the instrument as it aged, including capabilities that were lost as well as new modes of operation not initially envisioned; the new science questions that were investigated and their key findings; anatomy of the extensive collection of data products that have been released to the Planetary Data System; the "final" radiometric calibration; high-order derived products produced from high-resolution targeted observations and global mapping campaigns; and data processing and analysis tools which have been developed and released by the CRISM team. • CRISM acquired Mars orbital measurement from Sep. 2007 through May 2022. • The orbital science investigations and constituent observation types are described. • Final radiometric calibration is described. • Characteristics and processing of derived data products is described. [ABSTRACT FROM AUTHOR]

New analyses of thermal infrared spectra obtained from the Emirates Mars Infrared Spectrometer (EMIRS) allow for retrieval of water-ice cloud effective radius, r eff,ice , in the lower atmosphere of Mars across spatial, seasonal, and daytime diurnal scales. Using a one Martian-year observational dataset the mean retrieved r eff,ice is 3.1 μm (standard deviation, σ = 1.5 μm) for an assumed effective variance, ν eff,ice = 0.1, and with most individual values between 1 and 10 μm. Sensitivity analyses show the uncertainty associated with r eff,ice is largely inversely proportional to water-ice cloud optical depth, τ ice. As a result, our retrieval is generally applicable to water-ice clouds with τ ice of at least ∼0.02 to 0.08 at 825 cm−1. Seasonal results show mean r eff,ice values of 4.3 μm (σ = 1.9 μm) during aphelion season (Ls = 40°–140°), associated with larger particle sizes in the aphelion cloud belt (ACB), and 2.5 μm (σ = 0.8 μm) during perihelion season (Ls = 225°–360°). Diurnal analyses between roughly 06:00 and 20:00 local true solar time indicate the largest variability occurs in ACB clouds, with effective radii tending to be smaller during midday (zonal-mean r eff,ice of 4–5 μm) as compared to the mornings and afternoons (zonal-mean r eff,ice of 6–7 μm). Relatively large spatial gradients in r eff,ice within the ACB are observed, including a seasonal mean r eff,ice of ∼8 μm in the Tharsis region and regions in the southern mid-latitudes with seasonal mean r eff,ice of ∼2 μm. Spatial variability in r eff,ice tends to be smaller throughout the rest of the year, though with some similar patterns. This includes several time periods between regional dust storms when a low-latitude band of water-ice clouds formed in the afternoons near Tharsis, which grew in both optical depth and effective radius to maxima in the early evenings. Lastly, changes in r eff,ice are found to be generally positively correlated with changes in τ ice and negatively correlated with estimated cloud height, though r eff,ice distributions also show this to be a more complex relationship than simple averages might suggest. • Mars water-ice cloud particle effective radius is retrieved from EMIRS observations. • Individual retrieved effective radius values range from 1 to 10 μm. • Mean water-ice cloud effective radius is 3.1 μm for all results. • Variability is investigated across spatial, diurnal, and seasonal scales. [ABSTRACT FROM AUTHOR]

Martian regolith holds promise as a potential construction material for future settlements on the planet's surface. However, the planet's predominantly CO 2 -rich atmosphere poses challenges that may impact the mechanical properties of regolith-based concrete materials. In this study, we explore the compressive and flexural characteristics of Martian regolith simulant concrete mixtures incorporating sulphur, processed under CO 2 -rich conditions. The results reveal that regolith simulant concrete mixtures containing 70 wt% sulphur exhibit a maximum flexural strength of 5.95 ± 0.25 MPa. Meanwhile, the highest compressive strength of 3.66 ± 0.25 MPa and elastic modulus of 9.57 ± 0.32 GPa are achieved with regolith simulant mixtures comprising 60 wt% sulphur. These findings offer crucial insights for the advancement of construction materials tailored for future Martian settlements. • Processing and mechanical behavior of sulfur-based Martian regolith concrete reported. • Sulfur-based Martian regolith concrete prepared under CO2-rich conditions to simulate Mars atmosphere. • Maximum flexural strength of 5.95 ± 0.25 MPa for 70 wt.% sulfur concrete mixtures. • Maximum compressive strength of 3.66 ± MPa and elastic modulus of 9.57 ± 0.32 GPa from 60 wt.% sulfur mixtures. [ABSTRACT FROM AUTHOR]

We present here vertically resolved measurements of the carbon isotopic composition of CO 2 in the Martian atmosphere between the surface and 50 km of altitude. The results are based on data taken by the ExoMars Trace Gas Orbiter with the NOMAD instrument and have been aggregated to derive an average vertical profile of 13C/12C. We find no seasonal or spatial trends with variabilities beyond the sensitivity of the measurements. For the analysis, we developed a method that allows us to estimate whether the observed variability of isotopic measurements is beyond their intrinsic accuracy, by exploiting ab-initio spectroscopy and the radiometric noise of the instrument. Applying this method to our data, we find that atmospheric CO 2 is depleted in 13C compared to the Earth standard by 30‰ to 45‰, in line with previous ground-based measurements values of the atmosphere and in contrast with the average value obtained by Curiosity at the surface (46 ± 4‰). These differences in isotopic signatures of CO 2 as measured across the atmosphere and near-surface pose new questions when inferring the evolution and history of carbon on Mars, and suggest that processes such as a strong atmosphere-surface interactions may be fractionating the carbon reservoirs on the planet. • The 13C/12C isotopic ratio in CO 2 is derived in the lower atmosphere of Mars with NOMAD data. • There is indication that CO 2 at 20–35 km of altitude is depleted in 13C compared to Earth Standard. • A quantitative method to characterize retrieval uncertainty with spectroscopy is presented. • Surface-atmosphere processes should be investigated to explain the discrepancies between values from NOMAD and Curiosity. [ABSTRACT FROM AUTHOR]

Dust aerosols in the Martian atmosphere have important impacts on temperatures and atmospheric dynamics, with the most intense effects occurring during global and regional dust storms. Measurements by the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor (MGS) and by the Mars Climate Sounder (MCS) on board Mars Recoinnasance Orbiter (MRO) provide an opportunity to study the interannual variability of these dust storms during 12 Martian years. Late northern winter large regional storms (type C storms) show the highest variability, which does not appear to be driven by global dust storms (GDSs). An analysis of recent intense type C storms (MY 34, 35 and 36, with particular focus on MY 34) allows the characterization of latitudinal, longitudinal and altitudinal dust transport. This study is important to better understand the interannual variability of regional dust storms on Mars and the potential effect of GDS, as well as dust transport during late northern winter regional storms. • Temperature and dust analysis during 12 Martian Years from TES and MCS instruments. • Interannual variability in intensity and duration of regional type C storms • Dust transport during intense regional type C storms, with focus on that of MY 34. [ABSTRACT FROM AUTHOR]

In 2020, hydrogen chloride (HCl) in the gas phase was discovered in the atmosphere of Mars with the Atmospheric Chemistry Suite (ACS) onboard the Trace Gas Orbiter (TGO) mission (Korablev et al., 2021). Its volume mixing ratio (VMR) shows a seasonal increase of up to 5 ppbv during the perihelion season, followed by a sudden drop to undetectable levels, contradicting modelling estimates of the HCl lifetime of several months. In the Earth's stratosphere, heterogeneous uptake of HCl onto water ice is known to be a major sink for this species. This reaction is now also considered when modelling HCl abundances in the Martian atmosphere. In this work, we use simultaneous measurements of water ice and HCl obtained by the ACS instrument to find particular structures in the vertical profiles as detached gas layers at ice-free altitudes ("ice-holes"). From these particular examples we conclude that the heterogeneous uptake of HCl onto water ice operates on Mars and is a fast mechanism regulating the HCl abundance in the atmosphere of Mars. • Simultaneous vertical profiles of water ice and HCl analysed over 2.5 Martian years. • HCl gas layers at ice-free altitudes reveal local anticorrelation of HCl vs water ice. • Heterogeneous uptake of HCl onto water ice rapidly regulates HCl abundance. [ABSTRACT FROM AUTHOR]

Numerous planetary bodies express ices on their surfaces that cannot be replicated in conditions found on Earth. Robotic observations of these materials raise questions that require experimental measurements made in analogue conditions to be answered. Several labs on Earth are capable of recreating some of the conditions found on Mars, but they may be missing some capabilities that are important for the accurate recreation of ices, such as a cold sky, that influence ice behavior. To address this need, we have created the MARs Volatile and Ice evolutioN (MARVIN) Chamber at York University. MARVIN permits careful control of environmental properties with sufficient space to bring equipment into the chamber to make measurements that other labs cannot make. We also have the capacity for upgrades that can replicate conditions on colder planets with lower pressure. It is impossible to explain some of the observations that robotic probes have made at other planets using knowledge just from natural terrestrial environments or planetary analogue environments on Earth. Many questions must be answered by performing experiments in conditions similar to those found on those planetary bodies. Various labs can approximate some conditions of certain target environments on the Martian surface; however, replicating each of them simultaneously is beyond the reach for many. To address this need, we have created the MARs Volatile and Ice evolutioN (MARVIN) Chamber at York University. Inside of MARVIN, we can better approximate the conditions at the poles of Mars during winter and fit equipment for making measurements that are impossible in smaller chambers. Further, we have capacity to upgrade and eventually replicate conditions on other planets farther from the sun than Mars. • We designed an environmental chamber to simulate winter conditions of the polar regions of Mars. • Our chamber has capabilities that no other lab in the world combines, including a cold sky and an internal goniometer. • We can create carbon dioxide ice for experimental measurements and have plans to upgrade for forming other types of ice. [ABSTRACT FROM AUTHOR]

As the solar luminosity continues to increase over the next 6 billion years, the Martian surface will heat up to above the melting temperature of water ice. Water ice currently in the polar caps and ground ice will melt, potentially repopulating crater lakes or a very small ocean, with the abundance possibly enhanced due to diffusion upward of water currently locked in the deep crust. CO 2 adsorbed in the regolith and locked up in the south-polar ice cap will diffuse into the atmosphere, providing up to 50 mbar total pressure; this will further increase the temperature via greenhouse warming. There are two end-member scenarios for the climate under these conditions: (i) Surface water could drive an atmospheric water cycle somewhat analogous to present-day Earth's, which would create a global clement climate conducive to the existence of widespread surface life. (ii) The widespread presence of liquid water could cause CO 2 to form carbonate minerals and H 2 O to hydrate surface/subsurface minerals (or allow H released from water to escape to space), and CO 2 and H 2 O could be lost to space, leaving Mars with a dry environment. The climate could end up between these end-members and also could vary with time. This uncertain climate end state informs our view that exoplanet evolution and habitability can be driven by endogenous processes associated with a terrestrial planet as well as by the ability of EUV and a stellar wind from the host star to strip away atmosphere. • Mars will continue to warm as the Sun heats up throughout its lifetime. • Mars will reach the melting temperature of water ice, and both CO2 and H2O will be mobilized at the surface. • Mars could end up with a climate conducive to supporting life. • Results show the importance of processes intrinsic to a planet in determining the climate and habitability of an exoplanet. [ABSTRACT FROM AUTHOR]

Mainly designed to study minor atmospheric species in the Martian atmosphere, the Nadir and Occultation for MArs Discovery (NOMAD) instrument suite onboard the 2016 ExoMars Trace Gas Orbiter (TGO) can also be exploited for surface ice detection. In this work, we investigate the nadir observations of the NOMAD infrared channel from the Martian Years 34 to 36 (Mars 2018 to December 2022), especially for CO 2 ice detection. Based on Oliva et al. (2022) , we present an updated method taking advantage of the 2.7 μm absorption band for surface ice detection by selecting the diffraction orders 190, 169, 168 and 167. We focus the analysis on the Southern polar cap and define its boundaries during its sublimation phase in MY34–36. Globally, seasonal changes seem repeatable for MY34–36. Moreover, we show the potential of the 2.29 μm absorption band for surface CO 2 ice identification through the diffraction order 193. We define a pseudo-band depth as a good proxy for CO 2 ice detection. Following a semi-qualitative approach, we attempt to reproduce such spectra by using the Planetary Spectrum Generator (PSG) model in order to estimate CO 2 ice equivalent grain size. For the selected periods, the estimations are in the order of centimetres, which is in agreement with previous studies using spectral observations of OMEGA, CRISM and TES instruments. [ABSTRACT FROM AUTHOR]

Vertical distribution of water vapor in the Martian lower atmosphere (0–40 km) is an important characteristic of the Martian water cycle. It reflects a complex interplay between various processes shaping the contemporary water cycle, including atmospheric transport, formation of water ice clouds, sublimation of surface frost, and possible interaction with the subsurface. Despite the growing dataset of water vapor observations, there is yet no comprehensive climatology of the vertical distribution of vapor. Limb and solar occultation observations that are typically used to retrieve vapor vertical distribution are much less numerous than nadir observations and are available for limited seasons and locations. We propose a new methodology to retrieve vertical distribution of water vapor in the Martian lower atmosphere using only nadir data. The methodology is based on the observation that infrared spectra collected over same location during different parts of day are sensitive to water vapor in different parts of the atmosphere. We validate our new methodology by applying it to infrared spectra simulated for realistic Mars environments created with Mars General Circulation Model (GCM). These tests show that vertical profiles of vapor can be recovered with accuracy ∼20–40%, which is comparable to accuracy of limb and occultation observations. Importantly, the new retrieval algorithm can constrain near-surface (0–10 km) vapor abundances that cannot be observed by limb and occultation techniques. Application of this new methodology to a large dataset on Martian nadir spectra, such as the one collected by the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) during its mission in 1999–2004, can result in the development of the detailed climatology of vapor vertical distribution on Mars. As an initial step toward this goal, we apply the new retrieval methodology to a subset of nadir spectra collected by MGS TES during one Martian year. The retrieved vapor profiles are qualitatively consistent with vapor distributions predicted by the GCM for different seasons. Differences between retrieved and simulated profiles can be interpreted as reflecting wetter Martian atmosphere and weaker atmospheric transport, during some seasons. Retrieved vapor distribution during southern summer suggests disruption of atmospheric vapor transport into southern extra-tropics following a global dust storm. • New method to constrain vertical distribution of water vapor on Mars is presented. • Vapor distribution is constrained using combined day and night nadir observations. • Synthetic data and MY25 MGS TES spectra are used to test the new method. • Retrieved vapor distributions in lower atmosphere are similar to predicted ones. [ABSTRACT FROM AUTHOR]

The martian polar atmosphere is characterised by the presence of polar vortices, regions of cold and isolated air over the winter poles common to planetary atmospheres. The polar vortices have important meteorological effects via their interactions with atmospheric tracers and the broader circulation. In turn, they have been shown to be affected by large atmospheric dust loadings such as from dust storms. We make use of an extensive reanalysis dataset of Mars' weather and climate (Open access to Mars Assimilated Remote Soundings; OpenMARS) to investigate the seasonal and interannual behaviour of Mars' northern polar vortex over eight recent martian years, Mars Years (MY) 28–35 (2006–2021). We find that the northern polar vortex shows a high degree of interannual repeatability in its structure and evolution, with the key exception of during the presence of large (regional and global scale) dust storms. Such storms cause significant perturbations to the northern polar vortex, compressing it towards the pole and reducing its radius. However, not all dust storms have equal impacts. We find that the seasonal timing of large dust storms appears to be the key factor determining their impacts on the northern polar vortex. Storms occurring closer to southern summer solstice have greater impacts than those occurring closer to equinox. We propose that this seasonal dependence is due to the structure of the background meridional circulation, which is at its greatest strength and latitudinal extension around southern summer solstice. The enhancement of this existing circulation by dust-induced heating allows greater impacts if the circulation is already stronger and more latitudinally extended, and impacts are likewise lesser if the storm occurs during a period with a weaker and less latitudinally extended circulation. • North polar vortex is very interannually repeatable, except during large dust storms. • Regional dust storms compress vortex poleward, reduce radius, shift centre off-pole. • Dust storm seasonal timing has greater impact on vortex disruption than dust opacity. • Storms nearer northern winter solstice have greater effects on the northern vortex. • Timing matters via dust enhancement of seasonal cross-equatorial Hadley circulation. [ABSTRACT FROM AUTHOR]

The hydration of the martian regolith at decimeter depth scales varies regionally, suggestive of geologically sustained processes, independent of diurnal soil-atmosphere exchange. A combination of factors, including residual hydration from ancient processes in the hydrosphere/cryosphere, seasonal exchange between the crust and atmosphere in the martian critical zone, or structural water in minerals or salts sourced via hydrothermal processes can contribute to such lateral changes in bulk regolith hydration. Here, we consider the theoretical extent of regolith hydration from explosive volcanic eruptions and subsequent hydration. While magmatic degassing has previously been considered as a contributing factor to regolith hydration, the relationship between hydrated regolith and the depositional footprint of volcanic eruption columns and associated atmospheric dispersal remains poorly known. We run simulations of eruptions in an ancient atmosphere over a martian year to account for seasonality and produce theoretical ash deposition maps which are then input into a geographic weighted regression model to predict hydrated regolith. We find that ash deposited via explosive volcanism can help explain certain patterns of hydration enrichment in the martian regolith, including those near Apollinaris Patera Arabia Terra, and Sabea Terra. While the global patterns of regolith hydration cannot be explained by one process alone, we found that hydrated ash likely contributed to hydrated regolith at a commensurable level to other leading hydration mechanisms. The surface of Mars is blanketed by a pervasive layer of regolith, a mixture of unconsolidated sediment of multiple grainsizes that range from very fine dust to larger pebbles. This regolith contains significant amounts of water that are stored in the chemical bonds that make up the individual regolith grains. The amount of water bound in the regolith is spatially inconsistent throughout the planet; some areas contain more water, while others contain less. Several different geologic processes have likely contributed to these inconsistencies, including volcanic processes. While gentle volcanic outgassing has already been considered, here we specifically looked at how explosive volcanism and the associated spreading of volcanic ash in the martian atmosphere may have contributed to hydrated regolith patterns. By using multiple numerical simulations, we found that explosive volcanism likely contributed to certain patterns of hydration in the regolith along with other geologic processes. • There is a pathway between explosive volcanism and hydrated regolith. • Ancient explosive volcanism contributed to hydrated regolith patterns. • Areas near Apollinaris Patera, Arabia Terra, and Terra Sabaea show best fit between deposited ash and regolith hydration. [ABSTRACT FROM AUTHOR]

The atmospheric dust cycle on Mars plays a dominant role in the planetary radiative balance, atmospheric photochemistry escape, and redistribution of materials on the surface. Although this planetary dust cycle has been extensively modelled and characterized with both orbital and in situ observations, to date little is known about the total mass of dust that is circulated, the actual dust lifting and settling rates, and the main dust sources and sinks. Using orbital global and seasonal measurements of atmospheric dust opacity, a data reduction methodology that can describe the annual dust redistribution cycle on a planetary scale with 95% accuracy is presented. The method was applied to the 9.3 μm infrared observations of the Thermal Emission Spectrometer (TES) aboard the Mars Global Surveyor (MGS) during two full Martian Years (MY) 25 and 26, and partly MY 24 and MY 27, disregarding the global dust storm that occurred in MY 25. By comparison with terrestrial observations, a mass-to-extinction conversion factor of 1.9 ± 0.3 g m−2 is proposed, assuming a dust density of 2.6 g cm−3. The analysis shows an estimation of 400 1012 g of dust transported globally in the atmosphere for a typical Mars year, which is comparable to the minimum total annual mass of dust transported on Earth. The methodology proposed here is based on remote sensing and cannot disentangle completely local surface lifting and sedimentation rates from dust advection. However, this analysis provides upper bounds which can be compared with in-situ observations. The analysis of the dust sedimentation cycle suggests that the annual cycle might produce a dust layer of about 50–100 μm on the surface of some particular zones, as Valle Marineris or Meridiani Planum. This estimation agrees with in-situ observations of rovers on Mars. The potential dust sources are mainly located from latitudes of 20°S to 60°S. Our results find the 70% of the sources previously identified by the existing planetary circulation models. This kind of large-scale analysis can be applied to other remote sensing observations to refine these calculations and study the annual and geographical variability of the dust-mass transport on Mars. • About 400 Tg of dust is globally transported into the atmosphere of Mars every year. • Atmospheric dust settling produces a dust layer of about 50–100 μm on the surface. • Representative Earth-based mass to extinction conversion factor of 1.9 ± 0.3 g−2. [ABSTRACT FROM AUTHOR]

• Eris' eccentric orbit results in 2 atmospheric regimes: a global atm near perihelion and a local atm near aphelion. • A coupled thermal-transport numerical model to simulate thermal and volatile evolution in the LCSA regime is presented. • Volatile transport by local, collisional, sublimation atms (LCSA), even at solar distances of 100 AU, can be significant. • Uniform collapse of a global, nitrogen atmosphere likely cannot explain Eris' anomalous albedo in the present epoch. • Changes of Eris' albedo or color from nitrogen transport may be observable. Kuiper belt object (KBO) Eris is exceptionally bright with a greater visible geometric albedo than any other known KBO. Its infrared reflectance spectrum is dominated by methane, which should form tholins that darken the surface on timescales much shorter than the age of the Solar System. Thus one or more ongoing processes probably maintain its brightness. Eris is predicted to have a primarily nitrogen atmosphere that is in vapor pressure equilibrium with nitrogen-ice and is collisional (not ballistic). Eris's eccentric orbit is expected to result in two atmospheric regimes: (1) a period near perihelion when the atmosphere is global (analogous to the atmospheres of Mars, Triton, and Pluto) and (2) a period near aphelion when only a local atmosphere exists near the warmest region (analogous to the atmosphere of Io). A numerical model developed to simulate Eris's thermal and volatile evolution in the local atmosphere regime is presented. The model conserves energy, mass, and momentum while maintaining vapor pressure equilibrium. It is adaptable to other local, collisional, sublimation atmospheres, which in addition to Io and Eris, may occur on several volatile-bearing KBOs. The model was applied for a limiting case where Eris is fixed at aphelion and has an initial nitrogen-ice mass everywhere equal to the precipitable column of nitrogen in Pluto's atmosphere during the New Horizons encounter (the resultant mass if the Pluto atmosphere collapsed uniformly onto the surface). The model results indicate that (1) transport of nitrogen in the local, collisional, sublimation atmosphere regime is significant, (2) changes of Eris's albedo or color from nitrogen transport may be observable, and (3) uniform collapse of a global, nitrogen atmosphere likely cannot explain Eris's anomalous albedo in the present epoch. Seasonal volatile transport remains a plausible hypothesis to explain Eris's anomalous albedo and geologic processes that renew Pluto's brightest surfaces, such as convection and glaciation, may also be operating on Eris. [ABSTRACT FROM AUTHOR]

The move of the Odyssey spacecraft during Mars Years 31 and 32 to an orbit with local time near 7:00 AM and PM has enabled the systematic retrieval of water ice cloud optical depth using THEMIS thermal infrared images at a time of day not accessible from Mars Global Surveyor, Mars Reconnaissance Orbiter, or previous Odyssey observations. Because water ice clouds form by condensation, relatively small changes in atmospheric temperature can cause clouds to form or sublimate quickly, and there can be large changes in water ice cloud optical depth over the course of a day. Retrievals of water ice cloud optical depth using THEMIS observations show significant differences in cloud locations and optical depth as a function of local time and season. Cloud optical depth generally increases from the earliest (14:30) to latest (19:30) observations. During the aphelion season the increase from afternoon to evening is primarily associated with the thickening of existing clouds, while during the equinoctial and perihelion seasons there is a proportionally greater increase associated with the formation of clouds in the evening at locations where clouds were not present during the afternoon. • The local time variation of cloud optical depth is examined from THEMIS observations. • Clouds have higher optical depth during the evening than earlier in the afternoon. • After sunset, clouds expand to locations where clouds were not present earlier. [ABSTRACT FROM AUTHOR]

We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside. • Temperature and density waves have been extracted from in situ spacecraft data in Mars' upper atmosphere. • Waves cause temperatures and densities to vary by about 10% around the background values. • Wave characteristics, especially amplitude, appear to be correlated with temperature. • Wave amplitudes appear to grow with altitude on the nightside and remain constant on the dayside. • There appears to be enhanced wave activity in the northern hemisphere which may be caused by topography. [ABSTRACT FROM AUTHOR]

A Martian semiannual oscillation (SAO), similar to that in the Earth’s tropical stratosphere, is evident in the Mars Analysis Correction Data Assimilation reanalysis dataset (MACDA) version 1.0, not only in the tropics, but also extending to higher latitudes. Unlike on Earth, the Martian SAO is found not always to reverse its zonal wind direction, but only manifests itself as a deceleration of the dominant wind at certain pressure levels and latitudes. Singular System Analysis (SSA) is further applied on the zonal-mean zonal wind in different latitude bands to reveal the characteristics of SAO phenomena at different latitudes. The second pair of principal components (PCs) is usually dominated by a SAO signal, though the SAO signal can be strong enough to manifest itself also in the first pair of PCs. An analysis of terms in the Transformed Eulerian Mean equation (TEM) is applied in the tropics to further elucidate the forcing processes driving the tendency of the zonal-mean zonal wind. The zonal-mean meridional advection is found to correlate strongly with the observed oscillations of zonal-mean zonal wind, and supplies the majority of the westward (retrograde) forcing in the SAO cycle. The forcing due to various non-zonal waves supplies forcing to the zonal-mean zonal wind that is nearly the opposite of the forcing due to meridional advection above ∼3 Pa altitude, but it also partly supports the SAO between 40 Pa and 3 Pa. Some distinctive features occurring during the period of the Mars year (MY) 25 global-scale dust storm (GDS) are also notable in our diagnostic results with substantially stronger values of eastward and westward momentum in the second half of MY 25 and stronger forcing due to vertical advection, transient waves and thermal tides. Mars; Mars, atmosphere; Mars, climate; Atmospheres, dynamics; Atmospheres, structure; Meteorology. • A semi-annual oscillation in mean zonal wind is observed in Mars' atmosphere. • Unlike on Earth, the semi-annual oscillation is also evident at extratropical latitudes. • Meridional advection primarily drives the westward phase of Mars' SAO. • The eastward phase is forced by a combination of non-axisymmetric waves. • Some distinctive activity occurs during the planet-encircling dust storm in MY25. [ABSTRACT FROM AUTHOR]

We investigate the global seasonal variations of near-surface relative humidity and relevant attributes, like temperature and water vapor volume mixing ratio on Mars using calculations from modelled and measurement data. We focus on 2 AM local time snapshots to eliminate daily effects related to differences in insolation, and to be able to compare calculations based on modelling data from the Laboratoire de Météorologie Dynamique Mars General Circulation Model with the observations of Mars Global Surveyor Thermal Emission Spectrometer. We study the seasonal effects by examining four specific dates in the Martian year, the northern spring equinox, summer solstice, autumn equinox, and winter solstice. We identify three specific zones, where the near-surface relative humidity levels are systematically higher than in their vicinity regardless of season. We find that these areas coincide with low thermal inertia features, which control surface temperatures on the planet, and are most likely covered with unconsolidated fine dust with grain sizes smaller than ∼ 40 μ m. By comparing the data of relative humidity, temperature and water vapor volume mixing ratio at three different heights (near-surface, ∼ 4 m and ∼ 23 m above the surface), we demonstrate that the thermal inertia could play an important role in determining near-surface humidity levels. We also notice that during the night the water vapor levels drop at ∼ 4 m above the surface. This, together with the temperature and thermal inertia values, shows that water vapor likely condenses in the near-surface atmosphere and on the ground during the night at the three aforementioned regions. This condensation may be in the form of brines, wettening of the fine grains by adsorption or deliquescence. This study specifies areas of interest on the surface of present day Mars for the proposed condensation, which may be examined by in-situ measurements in the future. • We investigate the global variations of near-surface relative humidity. • We find 3 zones with elevated near-surface relative humidity regardless of season. • The 3 zones coincide with low thermal inertia and high albedo features. • We find that the water vapor levels drop in the night suggesting condensation. [ABSTRACT FROM AUTHOR]

Laboratory experiments show that sand grain saltation in a Mars-like environment can result in the ionization of argon. This suggests that saltation can be a mechanism for the destruction of methane (CH 4) on Mars given the ionization energy of argon is higher than the energy required to ionize methane to a reactive cation. The ionization energy is also higher than the energy to dissociate methane to highly reactive species like CH 3 , CH 2 , and CH. The feasibility of capturing the resulting emission glow on a windy Martian night is discussed. Unlabelled Image • Experiments that mimic the saltation process in a Martian environment show a glow. • The glow is analyzed spectroscopically in the range 400–1000 nm. • Emissions from ionized argon are a major contributor to the glow. • Analysis shows methane can be ionized/dissociated to reactive species in saltation. [ABSTRACT FROM AUTHOR]

We examine 252 Mars Atmosphere and Volatile EvolutioN (MAVEN) passes through the Martian atmosphere for which the Neutral Gas and Ion Mass Spectrometer (NGIMS) altitude-density profiles show perturbations with amplitudes larger than 40% of the background density that persist above the nominal exobase as defined here. The density profiles exhibiting such perturbations are plotted as a function of atmospheric column density rather than altitude. This roughly removes the dependence of the altitude-density profiles on composition, scale height, and local solar time. Such density structures are of interest as they can affect the local heating rate and, possibly, the escape rate. We find that the observed structures dissipate at roughly the same atomic column density and they affect the composition of the exosphere, raising the O/CO 2 ratio as compared with orbits that do not exhibit significant perturbations. [ABSTRACT FROM AUTHOR]

The Compact Reconnaissance Imaging Spectral Mapper (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) obtains pole-to-pole observations (i.e., full MRO orbits) of vertical profiles for visible/near-IR spectra (λ = 0.4–4.0 μm), which are ideally suited to identifying the composition and particle sizes of Mars ice and dust aerosols over 50–100 km altitudes in the Mars mesosphere. Within the coverage limitations of the CRISM limb data set, a distinct compositional dichotomy is found in Mars mesospheric ice aerosols. CO 2 ice clouds appear during the aphelion period of Mars orbit (Solar Longitudes, L s ∼ 0–160°) at low latitudes (∼20S–10N) over specific longitude regions (Meridiani, Valles Marineris) and at typical altitudes of 55–75 km. Apart from faint water ice hazes below 55 km, mesospheric H 2 O ice clouds are primarily restricted to the perihelion orbital range (L s ∼160 – 350°) at northern and southern mid-to-low latitudes with less apparent longitudinal dependences. Mars mesospheric CO 2 clouds are presented in CRISM spectra with a surprisingly large range of particle sizes (cross section weighted radii, R eff = 0.3 to 2.2 μm). The smaller particle sizes (R eff ≤1 μm) appear concentrated near the spatial (latitude and altitude) boundaries of their global occurrences. CRISM spectra of mesospheric CO 2 clouds also show evidence of iridescence, indicating very narrow particle size distributions (effective variance, V eff ∼ 0.03) and so very abrupt CO 2 cloud nucleation. Furthermore, these clouds are sometimes accompanied by altitude coincident peaks in 1.27 μm O 2 dayglow, which indicates very dry, cold regions of formation. Mesospheric water ice clouds generally exhibit small particle sizes (R eff = 0.1–0.3 μm), although larger particle sizes (R eff = 0.4–0.7 μm) appear infrequently. On average, water ice cloud particle sizes decrease with altitude over 50–80 km in the perihelion mesosphere. Water ice mass appears similar in clouds over a large range of observed cloud particle sizes, with particle number densities increasing to ∼10 cm−3 for R eff = 0.2 μm. Near coincident Mars Climate Sounder (MCS) temperature and aerosol profile measurements for a subset of CRISM mesospheric aerosol measurements indicate near saturation (H 2 O and CO 2) conditions for ice clouds and distinct mesospheric temperature increases associated with mesospheric dust loading. Dayside (3 pm) mesospheric CO 2 clouds with larger particle sizes (R eff ≥0.5 μm) scatter surface infrared emission in MCS limb infrared radiances, as well as solar irradiance in the MCS solar band channel. Scattering of surface infrared emission is most strikingly presented in nighttime (3 am) MCS observations at 55–60 km altitudes, indicating extensive mesospheric nighttime CO 2 clouds with considerably larger particle sizes (R eff ∼7 μm). Mesospheric CO 2 ice clouds present cirrus-like waveforms over extensive latitude and longitude regions (10°×10°), as revealed in coincident Mars Color Imager (MARCI) nadir imaging. Solar tides, gravity waves, and the large orbital variation of the extended thermal structure of the Mars atmosphere influence all of these behaviors. Mesospheric dust aerosols appear infrequently over the non-global (planet encircling) dust storm era of the CRISM limb data set (2009–2016), and exhibit smaller particle sizes (R eff = 0.2–0.7 μm) relative to dust in the lower atmosphere. One isolated case of an aphelion (L s = 96°) mesospheric dust layer with large dust particle sizes (R eff ∼2 μm) over Syria Planum may reflect high altitude, non-local transport of dust over elevated regions. • Characterization of dust and Ice aerosol distributions in the Mars mesosphere • Characterization of Mar mesospheric aerosol compositions and particle sizes • An orbital dichotomy in Mars mesospheric ice cloud composition • Large CO 2 ice cloud particles in the nightside Mars mesosphere • Definition of CO 2 ice cloud iridescence in the Mars mesosphere [ABSTRACT FROM AUTHOR]

Hundreds of new impact craters have been observed to form on Mars since spacecraft began imaging that planet. New impact craters produced visible ejecta deposits and many of them also have visible rays, similar to lunar and mercurian craters. However, some of the new martian impact craters have a circular feature of relatively low reflectance that we call a "halo." This feature is distinct from the usual visible ejecta deposits or ray patterns. In this paper we present an observational study of this halo feature and we discuss the results of this study with respect to the nature of the halos: what they are and how they may have formed. To address these questions, we measured diameters of both halos and their central craters. We found a strong correlation between halo diameter and crater diameter, which indicates that the nature of the halos is fundamentally governed by the amount of impact energy available at their formation. Specifically, halo size is controlled by impact energy according to the non-linear relationship D H ∝ E2/3, where D H is the diameter of the halo and E is the impact energy. We also found that certain factors may influence the formation of the halos: a thicker dust layer and lower elevations are both correlated with larger halos. From these correlations we conclude that the local surface characteristics as well as local atmospheric pressure influence the formation of the halos. Our description and analysis of the martian halo features provide a framework upon which specific halo formation mechanisms can be developed and tested in the future. • Some new martian impact craters are surrounded by a dark circular feature: a halo. • Halo diameter strongly correlates with crater diameter. • Halo formation depends on the amount of available impact energy. • Halo formation depends on local surface properties and atmospheric pressure. [ABSTRACT FROM AUTHOR]

Abstract While dynamical models indicate that both cometary and asteroid collisions likely contribute to the population of bodies that impact the inner solar system, the distinction between the craters produced by the two populations remains uncertain. Although cometary impacts larger than 10 km in diameter are rare, dynamical studies cannot confidently eliminate their possible contribution to the total impact population at levels lower than ~10%. Due to the speed and volatile content of comets, their impact should couple more energy with a planetary atmosphere and result in observable signatures. The tenuous atmosphere of Mars makes the planet an ideal location to recognize such signatures. Here we use computational simulations and laboratory experiments to model both asteroid and cometary impacts on Mars. We explore the relationship between such impacts and enigmatic radial streaks that form around certain impact craters due to high-speed, long-duration winds. While lower speed asteroid impacts into thick (500 m) surface ices also can produce intense outward winds, the global distribution of craters with impact-generated wind streaks limits this mode of origin to just a few candidates; the rest are likely cometary. Based on the occurrence and relatively young age of these craters, we conclude that ~1% of the impactors on Mars over the last 2 Ga resulted from Long-Period cometary impacts, which should be about 6 times higher on Earth. Inclusion of Jupiter-Family and Haley-type comets (but not including cometary asteroids) then predicts that ~ 16% of all terrestrial impacts, with important implications for future impact hazards on Earth. Highlights • Certain craters have wind streaks revealed in the night-time IR on Mars. • Streaks extend from pre-existing relief indicating exposure of coarser materials. • Experiments/models reveal that thermal wind streaks require cometary impacts. • Such craters constrain the flux of cometary impacts on Mars. • Extrapolation to Earth indicates a cometary flux up to 16% of the total population. [ABSTRACT FROM AUTHOR]

Abstract General Circulation Models with interactive physical and chemistry processes are the state-of-the-art tools for an integrated view and understanding of the Martian atmosphere and climate system. The GEM-Mars model currently includes 16 tracers for chemical composition and applies a fully online, interactive calculation of the photo- and gas phase chemistry of carbon dioxide (CO 2) and water vapor (H 2 O). These species largely control the chemical composition of the neutral Mars atmosphere through their photolysis products and their subsequent interactions. Water vapor undergoes a complex cycle on Mars as it is transported and interacts with ice reservoirs both at the planet's surface and at water ice clouds, which in turn provide radiative feedbacks. In the photochemical cycles involving CO 2 and H 2 O, the abundances of 5 species have been reported by previous investigations with significant spatio-temporal coverage: CO 2 , H 2 O, CO, O 3 and H 2 O 2. This paper presents the current status of the atmospheric chemistry simulations in GEM-Mars by comparing them to a selection of these observational datasets as well as to oxygen dayglow emission from O 2 (a1∆ g). The results are consistent with previous model-data comparisons and illustrate that the water cycle and the photochemistry are well implemented in the model. In particular, the simulation of the key reservoir species H 2 O 2 provides a good match to the available data. Model-data biases for ozone columns and oxygen airglow are related to the simulated water vapor vertical profile, as these species have important column contributions from vertical layers at the top of the hygropause. Highlights • GEM-Mars is a global Mars atmospheric model with online atmospheric chemistry. • Comparisons to a broad set of observations show good performance. • Good simulation of CO 2 , CO, H 2 O, H 2 O 2 , O 3 and O 2 (a1∆ g) airglow • Model-data bias for O 3 relates to biases in the simulated water vertical profile. • Middle-atmospheric water excess related to excessive heating by water ice clouds [ABSTRACT FROM AUTHOR]

Abstract For in-situ geochemical analysis of the surface of Mars, laser-induced breakdown spectroscopy (LIBS) is a very useful technique and the first extraterrestrial LIBS instrument ChemCam will soon be followed by others. Appropriate normalization of real mission data that is taken under varying experimental and environmental conditions from diverse geologic samples is an ongoing topic. One approach is the scaling to emission line intensities of carbon and oxygen from the CO 2 -dominated low-pressure martian atmosphere as an internal standard. Here, we performed several experiments to examine the emission of carbon and oxygen from a simulated martian atmosphere on simple, mostly mono-elemental samples, and to compare the emission characteristics of the elements of both origins. Differences in laser irradiance were found to have the biggest impact on sample emission lines scaled to C and O emission of the atmosphere. The temporal behavior shown in time-resolved LIBS measurements is dominated by the degree of ionization, but the ratio of the neutral emission from the sample to neutral emission from the atmosphere also varies over time. The effect of different grain sizes was minor in comparison to the high intrinsic variation in the LIBS data. Different samples were found to affect the absolute intensity of atmospheric C(I) more than O(I). Furthermore, the C(I) emission was found to be inseparably superimposed by iron if the latter was present in the target. The results indicate limitations of the general suitability of atmospheric carbon and oxygen emission for normalization purposes of martian LIBS data. Highlights • Emission of atmospheric carbon and oxygen appears in all martian LIBS data. • Atmospheric C and O emission is affected by experimental parameters and the sample. • In particular varying laser irradiance can result in changes of emission line ratios. • Atmospheric O(I) emission is less affected by sample matrix than C(I) emission. • C(I) at 247 nm is inseparably superimposed by iron emission. [ABSTRACT FROM AUTHOR]

Abstract This study is devoted to the general circulation modeling (GCM) of methane transport in the Martian atmosphere. A localized source originating from a near-subsurface methane reservoir is considered in the GCM simulations, which are performed with a modified version of the Weather Research and Forecasting (WRF) model, MarsWRF. The localized strength of a methane source varies with time, based on a 1-D near-subsurface diffusive transport. Time-varying surface release scenarios are also compared with an instantaneous release scenario. After release from the surface, the methane transport is investigated in the GCM as a passive scalar over time scales varying between 15 and 60 days, much shorter than the photochemical lifetime. Different emission scenarios of various duration and source intensity, as well as multiple locations with different elevation and terrain complexity are considered, to reproduce the substantial concentrations of methane (up to 50 ppb) observed in the northern hemisphere of Mars in 2003 (Mumma et al., 2009). Among the scenarios considered, the observations are reproduced best for an emission scenario of 45 sols duration, during which a total amount of about 90,000 metric tons of methane is released. The results reveal that observed Mars methane plumes in the northern hemisphere of Mars by Mumma et al. (2009) can be reproduced using a localized, time-varying methane source, consistent with a near surface methane reservoir. Highlights • A time-varying emission scenario is determined by using a subsurface transport model. • Subsurfaces sources located ∼15–30 m below the surface are considered. • Best agreement with the observations is obtained for the Iapygia region case. • The results indicates an unknown sink, more efficient than the photochemistry. • Considered release rates are less than the Arctic annual methane releases. [ABSTRACT FROM AUTHOR]

Abstract Slope streaks are gravity-driven dark or light-toned features that form throughout the martian year in high-albedo and low-thermal inertia equatorial regions on Mars. These distinctive features have never been observed in a terrestrial environment although some analogs have been proposed. Based on diverse orbital observations, a number of mechanisms including granular and aqueous flows have been proposed to explain their formation. Using multi-temporal images taken by the Context Camera (CTX) aboard the Mars Reconnaissance Orbiter (MRO), we identified newly formed streaks in multiple martian years and estimated seasonal streak formation rates at intermediate latitudes, as well as at the equator. We found seasonal variations in streak activity in multiple consecutive martian years, including high formation rates during autumn (solar longitude L S ~190°). During this time, slope streak activity exceeds the long-term formation rate multiple times. The highest seasonal formation rate of 0.16% per streak per martian day was observed in the Olympus Mons aureole in autumn (L S ~210°) of Mars year 30. In some sites, slope streak formation rate at slopes of opposite orientation peaks at different seasons. The seasonal variations of the formation rate are inconsistent with sporadic trigger mechanisms and revealed that changing conditions at the slopes affect the formation of the streaks throughout the martian year. Modelled environmental parameters at the streak-bearing slopes indicate a correlation between seasonal streak activity and surface temperature, as well as wind velocity. Seasonal variations of streak activity could be a result of varying intensities of both dry and wet mechanisms or could be explained by the interaction of multiple mechanisms. [ABSTRACT FROM AUTHOR]

Highlights • Photochemical model of Mars atmosphere and ionosphere at 80–300 km. • The model accounts for water, its ions, and variations with solar activity. • The model explains observed variations of hydrogen escape. • MAVEN/NGIMS ion spectra do not rule out high H 2 O abundances. Abstract Recent SPICAM IR solar occultations reveal significant abundances of water vapor up to ≈80 km near Mars perihelion. These abundances are stimulated by dust that heats the atmosphere and precludes condensation of water. This phenomenon correlates with detections of high escape of hydrogen near perihelion using the H Lyman-alpha in the SPICAM UV and HST observations. Here we present a self-consistent photochemical model of the martian neutral and ion composition at 80–300 km that accounts for variations of the atmospheric composition with solar activity and water abundance at 80 km. The model involves vertical transport by eddy, molecular, and ambipolar diffusion and both thermal and nonthermal escape of light species. The model predicts rather stable hydrogen escape of ≈1.9 × 108 cm−2 s−1 at 250 km during the most of the martian year beyond the perihelion period at L S = 200–330°. The reaction between H 2 and CO 2 + remains here the key process that determines the hydrogen escape. Therefore the HST observation of D and the FUSE observation of H 2 at L S = 68 and 160°, respectively, do not need revision. Appearance of water in the thermosphere during the perihelion period results in significant increase in production of hydrogen by photolysis of water and reactions of its ions. Photolysis of H 2 O is the most effective at 160–180 nm and weakly depends on solar activity. Reactions of water ions start chiefly by charge exchange between CO 2 + and H 2 O and end by recombination of H 3 O+. Both H 2 O photolysis and its ion reactions proceed near 100 km, and just a small part of the hydrogen production can escape. The calculated hydrogen escape at 250 km may be approximated by Φ H (cm−2 s−1) = 1.6 × 108 + 1.4 × 107 f H2O (ppm). Here f H2O is the H 2 O mixing ratio at 80 km that is reduced to 150 km by a factor of 5 by the photolysis and ion reactions. Therefore the observed high hydrogen escape up to 109 cm−2 s−1 requires the dayside-mean water abundances up to 60 ppm at 80 km that are within those observed by the SPICAM IR solar occultations. The significant H 2 O abundances do not deplete densities of HCO+ and are compatible with the MAVEN/NGIMS ion composition. The H Lyman-alpha observations reflect mostly the dayside-mean photochemistry, and our one-dimensional model may adequately respond to the problem. [ABSTRACT FROM AUTHOR]

Highlights • A complete spectral in-flight calibration of NOMAD is presented. • A model to compute spectral continuum is produced as a function of instrument properties. • NOMAD sensitivity to atmospheric CH 4 is evaluated with different aerosol loads. • NOMAD sensitivity to CH 4 in Solar Occultation is on sub-ppbv level. • NOMAD will be able to trace seasonal trends for CH 4 with nadir observations. Abstract The Nadir and Occultation for MArs Discovery instrument (NOMAD), onboard the ExoMars Trace Gas Orbiter (TGO) spacecraft was conceived to observe Mars in solar occultation, nadir, and limb geometries, and will be able to produce an outstanding amount of diverse data, mostly focused on properties of the atmosphere. The infrared channels of the instrument operate by combining an echelle grating spectrometer with an Acousto-Optical Tunable Filter (AOTF). Using in-flight data, we characterized the instrument performance and parameterized its calibration. In particular: an accurate frequency calibration was achieved, together with its variability due to thermal effects on the grating. The AOTF properties and transfer function were also quantified, and we developed and tested a realistic method to compute the spectral continuum transmitted through the coupled grating and AOTF system. The calibration results enabled unprecedented insights into the important problem of the sensitivity of NOMAD to methane abundances in the atmosphere. We also deeply characterized its performance under realistic conditions of varying aerosol abundances, diverse albedos and changing illumination conditions as foreseen over the nominal mission. The results show that, in low aerosol conditions, NOMAD single spectrum, 1σ sensitivity to CH 4 is around 0.33 ppbv at 20 km of altitude when performing solar occultations, and better than 1 ppbv below 30 km. In dusty conditions, we show that the sensitivity drops to 0 below 10 km. In Nadir geometry, results demonstrate that NOMAD will be able to produce seasonal maps of CH 4 with a sensitivity around 5 ppbv over most of planet's surface with spatial integration over 5 × 5° bins. Results show also that such numbers can be improved by a factor of ~10 to ~30 by data binning. Overall, our results quantify NOMAD's capability to address the variable aspects of Martian climate. [ABSTRACT FROM AUTHOR]

Highlights • Solar wind interaction with Mars' atmosphere produces Ly-α aurora on the dayside. • Monte Carlo simulations show that most of the auroral line profile is optical thin. • CO 2 absorption partly controls the altitude of the altitude of the emission peak. • A horizontal induced magnetic field decreases the brightness of auroral emission. Abstract Enhancements of the dayside Lyman-α emission by as much as ∼50% have been observed between 120 and 130 km in the lower Martian thermosphere from the Mars Express and MAVEN satellites, usually following solar events such as coronal mass ejections and corotating interaction regions. They have been assumed to be optical signatures of proton aurora related to an increase in the solar wind proton flux hitting Mars' bow shock. We present model simulations of the Lyman-α line profiles at different altitudes. These are partly guided by in situ measurements of the energy spectrum of protons in the magnetosheath region by the SWIA instrument on board the MAVEN spacecraft. We show that the auroral Lyman-α line profile is significantly broader than the hydrogen core of the planetary thermal H atom. Consequently, most of the auroral emission is produced outside the optically thick hydrogen core and creates the observed intensity enhancement. Simulations with incident energetic hydrogen atoms (H ENAs) produce a somewhat broader line profile. Monte Carlo calculations are made separately for incident solar wind protons and H ENAs produced by charge exchange in the hydrogen corona. Absorption by CO 2 along the line of sight significantly affects the intensity distribution in the lower thermosphere. The calculated altitude of the peak emission for both types of incident particles is consistent with the observed characteristics of the proton aurora. We show that the presence of a horizontal induced magnetic field somewhat increases the line width and decreases the altitude of the emission peak as a consequence of the magnetic barrier effect on proton precipitation. The brightness of the Lyman-α emission also drops as a result of increased magnetic shielding of the protons. [ABSTRACT FROM AUTHOR]

Highlights • A column model with adsorption was forced by 113 ChemCam passive-mode local PWC retrievals for more than a martian year, and results were compared to REMS-H observations. • The pre-dawn model results with regolith adsorption are quite similar to REMS-H at all seasons, whereas without adsorption there is too much nighttime moisture. • The night depletion rate is about 0.25 throughout the year. • Daytime comparisons to ChemCam vmr support a seasonally varying vertical distribution of moisture with a relatively dry lower atmosphere. Abstract Local column precipitable water contents (PWC) for more than a martian year from 113 Curiosity ChemCam passive-mode sky scans were used to force a column model with subsurface adsorption. ChemCam volume mixing ratios (vmr) and T, RH and vmr from REMS-H were compared with model results. The REMS-H observations point to decrease of vmr (i.e. depletion of near-surface water vapor) during every evening and night throughout the year. The model's pre-dawn results are quite similar to the REMS-H observations, if adsorption is allowed. The indicated porosity is about 30% and the night depletion ratio about 0.25. If adsorption is not allowed, RH and vmr become excessive during every night at all seasons, leading to ground frost between Ls 82°–146°; frost has not been observed. As brine formation is unlikely along the Curiosity track, adsorption thus appears to be the depleting process. During daytime the ChemCam vmr is in general close to surface values from the Mars Climate Database (MCD) vmr profiles for the Curiosity site when those profiles are scaled to match the ChemCam PWC. Our simulated daytime surface-vmr is in turn close to the ChemCam vmr when moisture is assumed well-mixed to high altitudes, whereas a low moist layer (15 km) leads to overestimates, which are worse during the warm season. Increased TES-like regional PWC also leads to large overestimates of daytime surface-vmr. Hence the crater appears to be drier than the region surrounding Gale and the results support a seasonally varying vertical distribution of moisture with a dry lower atmosphere (by Hadley circulation), as suggested by MCD and other GCM experiments. [ABSTRACT FROM AUTHOR]