Mars almost certainly had a considerable amount of water in its past. Recent observations reveal that during southern summer, when the atmosphere is warmer and dustier, water vapor can reach high altitudes without condensing, leading to water loss to space. Here, by combining infrared, visible, and ultraviolet data from multiple Mars orbiters, we identify a new pathway for water loss, observed for the first time to our knowledge during the opposite season. Our findings show that a strong, localized, and short-lived dust storm in Martian Year 37 (August 2023) drove considerable vertical transport of water vapor in the northern summer season. Just days after the storm, enhanced water vapor concentrations were observed at altitudes over 40 km across northern high latitudes, followed by an increase in escaping hydrogen detected at the exobase. These results suggest that water loss on Mars can be triggered by strong local dust storms at any time of year. Observations compiled from several Mars observation missions suggest a significant but short-lived dust storm during the Northern hemisphere summer of Mars Year 37 drove substantial vertical transport of water vapor into the upper atmosphere.

We investigate supersaturation in the Martian southern polar night using rederived vertical profiles from Mars Global Surveyor (MGS) radio occultation (RO) measurements. Supersaturation, which affects not only local condensation but also the polar and global atmospheric dynamics, occurs when temperatures fall below the saturation point. Conventional MGS-RO retrievals suffer from uncertainties in atmospheric composition, as they do not account for depletion due to its condensation onto the seasonal polar ice caps, and from the use of fixed or empirically chosen uppermost temperatures. To address these limitations, we apply a revised retrieval scheme that incorporates a latitude- and season-dependent estimate of the mixing ratio based on Mars Odyssey Gamma Ray Spectrometer argon data, along with a temperature climatology derived from Mars Climate Sounder (MCS) observations on board the Mars Reconnaissance Orbiter. We find that the original RO data underestimated supersaturation occurrence by 10%-20 % inside, and overestimated it by a similar fraction outside, the polar night. The updated profiles reveal that supersaturation commonly occurs up to 15 km altitude and S latitude, with seasonal and vertical variations peaking before the southern winter solstice. A comparison with MCS-derived surface ice temperatures shows general consistency, although notable vertical gradients and near-surface cold layers indicate complex boundary-layer thermodynamics. These results demonstrate that reducing key assumptions in RO retrievals yields improved constraints on Martian polar processes, providing new insights into the spatial and temporal behavior of supersaturation during southern winter.

Traveling waves in the Martian atmospheres play a crucial role in determining the weather and climate, particularly at mid-to-high latitudes. Previous observations have shown that these waves become prominent from early autumn to late winter in the northern hemisphere, influencing the dust cycle. However, their impact on the transport of dust and water ice clouds has not been studied quantitatively. Investigating the interaction between traveling waves and transport of such substances provides deeper insights into the climatology of Mars. In this study, we utilize data taken by the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter. Observations reveal eastward-propagating waves during the northern autumn and winter, identified as Rossby waves. The results show that waves with a zonal wavenumber of 1 become prominent during this period and in this region. Moreover, there is a correlation among the periodic variations in temperature, dust, and water ice. The amplitudes of the temperature, dust, and water ice variations are roughly consistent with each other, suggesting that the variations are all driven by the meridional advection associated with the traveling waves. These findings suggest that traveling waves play a significant role in the transport of dust and water ice clouds on Mars.

We investigate supersaturation in the Martian southern polar night using rederived vertical profiles from Mars Global Surveyor (MGS) radio occultation (RO) measurements. Supersaturation, which affects not only local condensation but also the polar and global atmospheric dynamics, occurs when temperatures fall below the saturation point. Conventional MGS‐RO retrievals suffer from uncertainties in atmospheric composition, as they do not account for depletion due to its condensation onto the seasonal polar ice caps, and from the use of fixed or empirically chosen uppermost temperatures. To address these limitations, we apply a revised retrieval scheme that incorporates a latitude‐ and season‐dependent estimate of the mixing ratio based on Mars Odyssey Gamma Ray Spectrometer argon data, along with a temperature climatology derived from Mars Climate Sounder (MCS) observations on board the Mars Reconnaissance Orbiter. We find that the original RO data underestimated supersaturation occurrence by 10%–20 % inside, and overestimated it by a similar fraction outside, the polar night. The updated profiles reveal that supersaturation commonly occurs up to 15 km altitude and S latitude, with seasonal and vertical variations peaking before the southern winter solstice. A comparison with MCS‐derived surface ice temperatures shows general consistency, although notable vertical gradients and near‐surface cold layers indicate complex boundary‐layer thermodynamics. These results demonstrate that reducing key assumptions in RO retrievals yields improved constraints on Martian polar processes, providing new insights into the spatial and temporal behavior of supersaturation during southern winter.

The Mars Reconnaissance Orbiter (MRO) is uniquely qualified to meet objectives required by human missions to Mars. Landing site characterization capabilities include imaging for boulders and other terrain difficult for landing and/or driving, understanding soil properties for future construction, and locating resources such as subsurface ice deposits and caves. During the entry, descent, and landing phase, the Ultra High Frequency (UHF) radio can provide real-time data return, while several instruments can provide weather and atmospheric density information. The high-resolution imager can capture photos of a vehicle mid-descent and after touchdown. These capabilities could prove vital for determining the root cause after an anomaly. Once human presence on Mars is established, the UHF radio can provide positioning information, relay data from equipment placed outside of direct communication with the landing site, and serve as a backup voice communication system during extravehicular activities. Other MRO capabilities include creating stereo maps for extravehicular route planning, dust storm prediction and monitoring, atmospheric density measurements for aerobraking, improvement of spacecraft ephemerides to assist with precisely targeted landings, and finding lost hardware in orbit and on the ground. MRO remains healthy and retains sufficient fuel to operate until 2038.

This study provides comprehensive intercomparisons of dust optical depths derived from Mars Climate Sounder on Mars Reconnaissance Orbiter with measurements from the orbiter instruments THEMIS on Mars Odyssey, CRISM on Mars Reconnaissance Orbiter, EMIRS on the Emirates Mars Mission, and from the surface-based instruments Pancam on the Spirit and Opportunity rovers and SSI on the Phoenix lander. We demonstrate good overall agreement between these dust optical depth datasets, in particular as these measurements were taken at different wavelengths (near IR to thermal IR) and with different measurement geometries (limb, downlooking from orbit, uplooking from the surface). To draw our conclusions, we use direct comparisons of co-located measurements from these instruments, plots of the dust optical depth differences averaged by latitude and Solar longitude, as well as modified Bland-Altman analysis that takes the measurement uncertainties into account. From the Bland-Altman analysis, we find that error estimates provided by the instruments reasonably represent the variability introduced by the two datasets but do not capture offsets. Based on our investigations, we propose potential physical causes such as surface reflectance factor estimation and thermal contrast, for some of the observed differences. We find a linear relation of the differences in dust optical depth between MCS and the other instruments with derived MCS water ice optical depth. Taking this relation into account, together with a constant parameter that accounts for general offsets between the datasets, significantly reduces differences.

The loss of water on Mars to space largely occurs through the decomposition of water vapour after transport to high altitudes with the subsequent escape of atomic hydrogen. However, there are still open questions about the relative importance of water photolysis and ion chemistry as decomposition mechanisms. In addition, the relevance of seasonally recurring compared with impulsive vertical water transport driven by dust storms is not fully understood. Using photochemical modelling based on a synergistic dataset from three Mars orbiters, we show that water photolysis above the main region of cloud formation (above the hygropause) is the dominant source of hydrogen available for escape, significantly exceeding hydrogen production through ion chemistry. We also show that seasonally recurring transport dominates hydrogen escape over impulsive transport, suggesting that dust storms play only a minor role in atmospheric water loss. Modelled hydrogen escape rates show good agreement with available measurements, which demonstrates the importance of the mechanisms investigated here for improving quantitative estimates of long-term water loss on Mars. Utilizing a synergistic dataset from three Mars orbiters, it is shown that water photolysis above the main region of cloud formation in the Martian atmosphere is the dominant source of hydrogen available for atmospheric escape on Mars.

Over the last 16 years, the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter (MRO) has acquired a large body of surface observations at visible and thermal infrared wavelengths. Primary differentiators between the MCS record and other surface datasets include mission duration, regular global coverage, and high emission angles associated with most observations. These data have been analyzed to generate a global median apparent thermal inertia map that smooths out artificial spatial variability (i.e., streaking along orbital ground tracks) common in other maps. At mid and low latitudes, high emission angle observations yield similar nighttime thermal inertia values compared to nadir observations, even if grazing angles should favor vertically rough materials oriented towards MCS (i.e., material poking out of the ground, presumably associated with high thermal inertia rocks and scarps). This result independently confirms that fines control the Martian thermal inertia, not rocks, and also suggests that most rock and bedrock exposures should be characterized by low aspect ratios (i.e., appear platy or flat, regardless of subsurface shape). Selected global temperature maps are also presented. They show the influence of polar processes, the latitudinal distribution of exposed ices, and physical properties of the Martian regolith on surface temperatures. Temperature controls at regional and global scales include the variation of insolation with season and latitude, atmospheric composition, global circulation, the presence of snow precipitations, and various regolith properties (i.e., albedo and thermal inertia).

Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO) started science measurements on 21 April, 2018. Here, we present results on the retrievals of water vapor vertical distributions in the Martian atmosphere from three years of TGO/NOMAD science operations. NOMAD is a spectrometer operating in the spectral ranges between 0.2 and 4.3 μm onboard ExoMars TGO. NOMAD has 3 spectral channels: a solar occultation channel (SO – Solar Occultation; 2.3–4.3 μm), a second infrared channel capable of nadir, solar occultation, and limb sounding (LNO – Limb Nadir and solar Occultation; 2.3–3.8 μm), and an ultraviolet/visible channel (UVIS – Ultraviolet and Visible Spectrometer, 200–650 nm). The infrared channels (SO and LNO) have high spectral resolution (λ/dλ~10,000–20,000) provided by an echelle grating used in combination with an Acousto Optic Tunable Filter (AOTF) which selects diffraction orders. The sampling rate for the solar occultation measurement is 1 second, which provides a good vertical sampling step (~1 km) with higher resolution (~2 km) from the surface to 200 km. Thanks to the instantaneous change of the observing diffraction orders achieved by the AOTF, the SO channel is able to measure five or six different diffraction orders per second in solar occultation mode. In this study, we analyze the solar occultation measurements at diffraction order 134 (3011-3035 cm-1), order 136 (3056-3080 cm-1), order 168 (3775-3805 cm-1), and order 169 (3798-3828 cm-1) acquired by the SO channel in order to investigate water vapor vertical distributions. Knowledge of the water vapor vertical profile is important to understand the water cycle and its escape process. Solar occultation measurements by two new spectrometers onboard TGO - NOMAD and Atmospheric Chemistry Suite (ACS) - allows us to daily monitor the water vapor vertical distributions through the whole Martian Year and obtain a good latitudinal coverage for every ~20° of Ls. In 2018, for the first time after 2007, a global dust storm occurred on Mars. It lasted for more than two months (from June to August). Moreover, following the global dust storm, a regional dust storm occurred in January 2019. The NOMAD and ACS observations therefore fully cover the majority of the global and regional dust storms and offer a unique opportunity to study the trace gases distributions during the dust storms. We analyzed those datasets and found a significant increase of water vapor abundances in the middle atmosphere (40-100 km) during the global dust storm from June to mid-September 2018 and the regional dust storm in January 2019. In particular, water vapor reaches very high altitude, at least 100 km, during the global dust storm (Aoki et al., 2019, Journal of Geophysical Research, Volume124, Issue12, Pages 3482-3497, doi:10.1029/2019JE006109). A GCM simulation explained that dust storm related increases of atmospheric temperatures suppress the hygropause, hence reducing ice cloud formation and so allowing water vapor to extend into the middle atmosphere (Neary et al., 2020, Geophysical Research Letters, accepted, Volume47, Issue7, e2019GL084354, doi: 10.1029/2019GL084354). This study presents the results with the extended dataset, which covers a full Mars year. The extended dataset newly includes aphelion season that involves interesting phenomena such as sublimation of water vapor from the northern polar cap and formation of the equatorial cloud belt, which are known as key periods to understand the large north-south hemispheric asymmetries of Mars water vapor. Yet, only a few papers report the water vapor vertical distributions in the aphelion season. The extended dataset also includes the southern summer season (dusty season) in MY 35, which will allow us to compare the water vapor distributions in the global dust storm year with those in the non-global dust storm year. In the presentation, we will discuss the water vapor vertical profiles as well as the aerosols vertical distributions retrieved from the three-year measurements of the TGO/NOMAD.

This study involving both observations and simulations furthers our understanding of water transport in the Martian northern polar region, a critical component of the global water cycle, and explores strengths and weaknesses in simulations of the polar atmosphere. Observations of the northern polar winter by the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter show extensive water ice clouds over the polar ice cap throughout the 300-3 Pa (similar to 10-50 km) vertical column within the vortex during the entire winter season. The observations also indicate that the vortex evolves throughout its depth on a broad range of timescales, from sub-diurnal to seasonal. Time sequences of these data together with results from a Mars global circulation model and Ensemble Mars Atmosphere Reanalysis System reanalysis (EMARS) are used to study the evolution of the winter polar atmosphere and to examine dynamic mechanisms for transporting water across the vortex boundary. Model simulations and reanalysis show a similar temperature structure to observations, although they struggle to reproduce some of the detailed features such as the extent of polar warming above the vortex and the magnitude of the temperature minima inside the vortex. The free run simulation also fails to capture the vertically distributed water ice cloud due to a general absence of transport across the vortex boundary. EMARS results, with assimilated MCS temperatures, show a greater amount of water entering the vortex at pressures below 200 Pa, leading to a more vertically extended cloud within the vortex and improving agreement with observations. This study explores the winter atmosphere of the northern polar region of Mars and examines how winds move water into this region, which is important for understanding the cycling of water between the atmosphere and the Martian ice caps. The study uses both spacecraft observations of the atmosphere (from which we can infer temperature, dust, and water ice clouds), along with computer simulations that are guided by observations (from which we get greater space and time coverage, and gain additional insights on winds and water vapor). The Martian polar vortex, which serves as a boundary to the polar regions, varies considerably on sub-daily, daily, and seasonal time scales. Computer simulations that are guided by observations show a greater transport of water by the wind at higher altitudes, which is consistent with the greater vertical extent of water ice clouds in winter polar regions, as seen in observations. Similarities and differences between observations and model simulations are also discussed, which motivate future modeling to better represent water transport into the vortex aloft and therefore a better depiction of the water cycle. Improved Mars Climate Sounder (MCS) observations reveal a complex northern winter polar vortex evolving on multiple timescales Water enters the northern winter polar vortex in the middle atmosphere (10-50 km) on Mars, forming a vertically extended ice cloud Agreement of model and observed ice cloud distribution in the north polar region is improved by assimilation of temperature observations

The detection of hydrogen chloride (HCl) in the atmosphere of Mars was among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Its discovery using the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) showed a distinct seasonality and possible link to dust activity. This paper is part 2 of a study investigating the link between HCl and aerosols by comparing gas measurements made with TGO to dust and water ice opacities measured with the Mars Climate Sounder (MCS). In part 1, we showed, and compared, the seasonal evolution of vertical profiles of HCl, water vapor, temperature, dust opacity, and water ice opacity over the dusty periods around perihelion (solar longitudes 180 degrees-360 degrees) across Mars years 34-36. In part 2, we investigated the quantitative correlations in the vertical distribution between each quantity, as well as ozone. We show that there is a strong positive correlation between HCl and water vapor, which is expected due to fast photochemical production rates for HCl when reacting with water vapor photolysis products. We also show a strong positive correlation between water vapor and temperature, but are unable to show any correlation between temperature and HCl. There are weak correlations between the opacities of dust and water ice, and dust and water vapor, but only very low correlations between dust and HCl. We close with a discussion of possible sources and sinks and that interactions between HCl and water ice are the most likely for both, given the inter-comparison.

AbstractRecent findings of NO near Gale Crater on Mars have been explained by two pathways: formation of nitric acid (HNO3) in a warm climate or formation of peroxynitric acid (HO2NO2) in a cool climate. Here, we put forth two hitherto unexplored pathways: (a) deposition of nitric/peroxynitric acid onto ice particles in a cold atmosphere, which settle quickly onto Mars' surface and (b) solar energetic particle‐induced production of nitric/peroxynitric acid. The deposition rates are enhanced and NO production is more efficient under the higher atmospheric pressures typical of Mars' ancient atmosphere. Depending on the unknown rate at which nitric/peroxynitric acid is lost from the surface, the new pathways could result in larger NO‐levels than those detected by the Mars Science Laboratory. We predict a 2:1 ratio of nitrite:nitrate would have deposited in cool surface climates with an icy atmosphere, whereas orders of magnitude more nitrate than nitrite is expected from warm surface climates.

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 mu m 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 mu m 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.

As tracers of the major volatile cycles of MarsCO2, H2O, 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.

Detecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid-infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl abundance from below detection limits to 1-3 ppbv in both hemispheres coincident with the start of dust activity, followed by very sudden and rapid loss at the southern autumnal equinox. In this study, we have investigated the relationship between HCl and atmospheric dust by making comparisons in the vertical distribution of gases measured with ACS and aerosols measured co-located with the Mars Climate Sounder (MCS). This study includes HCl, water vapor, and ozone measured using ACS MIR, water vapor and temperature measured with the near infrared channel of ACS, and temperature, dust opacity, and water ice opacity measured with MCS. In part 1, we show that dust loading has a strong impact in temperature, which controls the abundance of water ice and water vapor, and that HCl is very closely linked to water activity. In part 2, we investigate the quantitative correlations between each quantity and discuss the possible source and sinks of HCl, their likelihood given the correlations, and any issues arising from them. Plain Language Summary After four full Martian years in orbit since 2018, the ExoMars Trace Gas Orbiter (TGO) has observed three Martian dusty seasons, which occur when it is spring and summer in the southern hemisphere. The first, starting in summer 2018, featured a global dust storm (GDS) after which we made the first detection of hydrogen chloride (HCl) in the Martian atmosphere using the Atmospheric Chemistry Suite (ACS) instrument. Finding this gas was a priority of ExoMars because its presence may indicate that the planet is volcanically active. Since then, we have observed two more dusty periods without a GDS and observed the reappearance of HCl each time. Here, we present the climatology of HCl in both hemispheres over these three dusty periods (in Mars years 34, 35, and 36) and investigate their relationships with temperature and water vapor measured by ACS, and with airborne dust and water ice measured with the Mars Climate Sounder (MCS) on the Mars Reconnaissance Orbiter (MRO). In this paper, we examine how the vertical structure of each quantity changes over time. We show that there is a close relationship between HCl and H2O, and that both are controlled by temperature, driven by dust loading.

Introduction: Like Earth, Mars possesses dynamical atmospheric features known as polar vortices. These are regions of cold, isolated polar air surrounded by powerful westerly wind jets which can create barriers to transport of atmospheric dust, water, and chemical species. They have a complex and asymmetrical (north/south) relationship with atmospheric dust loading [1]. Regional and global dust events have been shown to cause rapid vortex displacement [2,3] in the northern vortex, while the southern vortex appears more robust. Unlike Earth, Mars also experiences planet-encircling Global Dust Storms: spectacular, planet-spanning events which dramatically increase atmospheric dust loading. The most recent such event in 2018 (beginning at northern autumn equinox) [4] was observed by multiple spacecraft, including the ExoMars Trace Gas Orbiter (TGO) and the Mars Reconnaissance Orbiter (MRO), enabling the opportunity to study its effects on the polar vortices in detail. We do this by assimilating [5] spacecraft data from TGO’s Atmospheric Chemistry Suite (ACS) [6,7] and MRO’s Mars Climate Sounder (MCS) [8,9] into the LMD-UK Mars Global Climate Model [10], a 4D numerical model of the martian atmosphere. Results: We present our recently published results [11], where we find that the 2018 GDS had asymmetrical impacts in each hemisphere: the northern polar vortex remained relatively robust, while the southern polar vortex was significantly disrupted. This asymmetry was due to both the storm’s latitudinal extent, which was greater in the south than in the north, and its timing, occurring as the southern vortex was already decaying after equinox. Both polar vortices and especially the northern showed reductions in their ellipticity, and this correlated with a reduction in high-latitude stationary wave activity in both hemispheres. We show that the characteristic elliptical shape of Mars’ polar vortices is the pattern of the stationary waves; this was suppressed during the storm by the shifting of the polar jet away from regions of high mechanical forcing in the north, and by the reduced polar jet due to the decreased meridional temperature gradient in the south. These asymmetric effects suggest enhanced transport into the southern, but not northern, polar region during GDS around northern autumn equinox, as well as more longitudinally symmetric transport around both poles. References: [1] Waugh, D. W. et al (2016) J. Geophys. Res. Planets, 121, 1770-1785. [2] Guzewich, S. D. et al (2016) Icarus, 278, 100-118. [3] Mitchell, D. M. et al (2015) Q.J.R. Meteorol. Soc., 141, 550-562. [4] Kass, D. M et al (2019) GRL, 47(23). [5] Lewis, S. R. et al (2007) Icarus, 192(2). [6] Korablev, O. et al (2018) Space Sci. Rev., 214(7). [7] Fedorova, A. A. et al (2020) Science, 367(6475). [8] McCleese, D. J. et al (2007) JGR (Planets), 112(E5). [9] Kleinböhl, A. et al (2009) JGR (Planets), 114(E10). [10] Forget, F. et al (1999) JGR (Planets), 104(E10). [11] Streeter, P. M. et al (2021) JGR (Planets), e2020JE006774.

Recent findings by the Mars Science Laboratory (MSL) have confirmed the presence of nitrates near Gale Crater on Mars. In this work, we consider the formation and deposition of HNOx species in cold early Mars climates. We find that solar energetic particles could facilitate nitrogen fixation by photochemically generating pernitric and nitric acid, which then deposit onto icy particles that settle onto Mars’ surface. This study demonstrates that such deposition would be more efficient under higher atmospheric pressures, consistent with Mars’ ancient atmosphere, and could account for the nitrate levels detected by the MSL. We find a more rapid deposition rate for pernitric acid over nitric acid (in agreement with Smith et al., 2014), and a significant enhancement of deposition rates through consideration of deposition onto icy particles. This distinction could be crucial for interpreting the MSL data.

One of the main objectives of the ExoMars Trace Gas Orbiter (TGO) mission is to hunt for any gases that may be diagnostic of active geological of biogenic processes. Of key interest was methane (CH4) due to its link to biological production mechanisms on Earth. While this has so-far not been observed (Montmessin et al., 2021), the discovery of hydrogen chloride (HCl) was announced after the first full Martian year of observations (Korablev et al., 2021). Like CH4, HCl will photolyze readily in the Martian atmosphere and have a short lifetime, requiring an active source. One of the dominant sources on Earth is active volcanism, making its characterization a high priority for TGO.HCl was discovered using data from the mid-infrared channel of the TGO’s Atmospheric Chemistry Suite (ACS MIR). This is a cross-dispersion spectrometer operating in solar occultation geometry. The instrument consists of a telescope and foreoptics, a primary echelle grating to access the mid-infrared spectral region, and a secondary diffraction grating to separate overlapping diffraction orders. The solar occultation method is self-calibrating, provides a very long optical path length, and very high signal-to-noise ratios.It was quickly revealed that HCl was linked to water vapour and had its own seasonal cycle, possibly associated with dust activity (Korablev et al., 2021; Olsen et al., 2021). In this presentation, we will present the results of our work to further characterize HCl and explore its possible origins and seasonality. We present a direct comparison over altitude between the volume mixing ratios (VMR) of HCl with: the water vapour VMR, temperature, water ice extinction, and dust extinction. Water vapour is measured simultaneously with ACS MIR, temperature is measured simultaneously with the near-infrared channel of ACS (Fedorova et al., 2020; 2023), and aerosol extinctions are taken form co-located measurements made with the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (Kleinböhl et al., 2009; 2017).Our results reveal that regardless of the photochemical origins of HCl, seasonal dust activity very strongly controls its behaviour. At the start of southern spring, dust is lifted into the atmosphere and warms the vertical extent over which dust is present. Temperature strongly controls water vapour, and HCl is tightly correlated with water vapour over altitude. We do not find direct evidence that the abundance of dust aerosols impacts the HCl VMR, but observed a pronounced difference between the altitude range where HCl (and water vapour) is present and where water ice forms (controlled by temperature).We have explored, and will discuss, the likelihood of serval hypothesized HCl formation and destruction mechanisms. These include heterogeneous reaction on chloride-bearing dust aerosols, emissions from the surface, year-round atmospheric residence (low altitudes? alternative form of chloride?), the formation of perchlorate and surface deposition, and the adhesion of HCl on aerosol surfaces and eventual deposition.ReferencesMontmessin, F. et al. Astron. Astrophys. 650, A140 (2021). DOI:10.1051/0004-6361/202140389.Korablev, O., Olsen, K. S. et al. Sci. Adv. 7, eabe4386 (2021). DOI:10.1126/sciadv.abe4386.Olsen, K. S., et al. Astron. Astrophys. 647, A161 (2021). DOI:10.1051/0004-6361/202140329.Fedorova, A. A., et al. Science 367, 297-300 (2020). DOI:10.1126/science.aay9522.Fedorova, A. A., et al. J. Geophys. Res. 128, e2022JE007348 (2023). DOI:10.1029/2022JE007348.Kleinböhl, A., et al. J. Geophys. Res., 114, E10006 (2009). DOI:10.1029/2009JE003358.Kleinböhl, A., Friedson, A. J., & Schofield, J. T. J. Quant. Spectrosc. Radiat. Transfer. 187, 511-522 (2017). DOI:10.1016/j.jqsrt.2016.07.009.

The kinetic temperature of the Martian seasonal caps is controlled by the partial pressure of atmospheric CO2 at the surface. When carbon dioxide condenses, typically near the poles, light non‐condensable species (Ar, N2, CO, etc.) accumulate in the atmosphere, resulting in a decrease of the CO2 partial pressure and depressing the local frost point temperature. The buoyant air should mix laterally and vertically within the polar vortices. Observations show that the Martian seasonal caps' kinetic temperatures are ∼0–4 K below the expected CO2 frost point, depending on latitude and season, indicating atmospheric CO2 gas depletion at the surface/atmosphere interface. In the North and South, we find relatively similar non‐condensable peak enhancement factors (e.g., EFNC ∼ 6–8, up to ∼8.7 in the North) at most latitudes, confirming the efficient meridional mixing within the polar vortices, despite steep surface condensation gradients. In the South, this surface enhancement is similar to column‐integrated values derived from Gamma Ray Spectrometer data, indicating efficient vertical mixing. But in the North, the surface depletion is much larger than in the entire column, suggesting poor vertical mixing. Reduced infrared emission of the seasonal caps stemming from CO2 depletion is not a major energy balance factor. This work illustrates how the atmosphere's composition at the surface can be significantly different from column‐integrated values.