Exchangeable ice deposits are present today on the surface of Mars in polar caps and in the shallow subsurface in mid-latitudes. Geologic observations indicate these deposits waxed and waned in the past, at times, along with the emplacement and loss of equatorial glaciers. Here, we couple a climate model with an ice stability criterion, to self-consistently determine the distribution of the mid-latitude ground-ice deposits in diffusive equilibrium with the atmosphere, at present and under past orbital configurations. This new coupling and iteration between the short-term and long-term models improves upon past calculations that do not allow the ice table to evolve over timescales much longer than the annual climate dynamics. The model predictions for the extent of the equilibrium ice-table in the past match the latitudinal distribution of terrain softening geologic features previously mapped. At past times, thermally stable shallow ground ice is expected even in equatorial regions, in parts of Tharsis and Arabia Terra.

The homopause marks the transition in a planetary atmosphere from turbulent mixing, which maintains a well-mixed composition, to molecular diffusion, which causes the diffusive separation of chemical species, impacting how these are distributed in the upper atmosphere and potentially escape to space. Here, we analyse simulations from the Mars Planetary Climate Model (Mars PCM) to investigate the variability of the Martian homopause in diurnal, seasonal and interannual timescales. The simulations reveal strong seasonal and latitudinal trends, with homopause altitudes peaking in the summer polar regions (∼120–130 km) and showing minimum values during the winter southern polar region (∼60–90 km). The simulations predict diurnal variations in the homopause altitude typically within 5–15 km and suggest that dust events can raise the value of the homopause altitude by 10–20 km, depending on the intensity of the event. When comparing the Mars PCM results with empirical estimates of the homopause altitude and density derived from Martian atmospheric data, we find that the model captures the overall magnitude and seasonal/latitudinal variability of the homopause, but appear to underestimate the strength of the diurnal cycle. Finally, we use the Mars PCM data to constrain the variability of the eddy diffusion coefficient, which can vary by one or two orders of magnitude across latitude and season. The derived parameterisation and variability of the eddy diffusion coefficient is suitable for use in one-dimensional models devoted to understanding seasonal difference in atmospheric photochemistry and escape.

The formation of gullies on Mars has often been attributed to the melting of (sub)surface water ice. However, melting-based hypotheses generally overlook key processes: (a) sublimation cooling by latent heat absorption, (b) the non-stability of ice where melting conditions can be reached, and (c) the particular microclimates of gullied slopes. Using state-of-the-art climate simulations, we reassess ice melting scenarios over the past 4 million years (obliquity 35 degrees), beyond the estimated period of gully formation. We find that the melting of opaque water snow or ice at the surface of Mars is unlikely anywhere due to sublimation cooling, while (quasi-) stable subsurface ice is typically too deep to reach melting temperatures. We propose an alternative mechanism in which seasonal frost sublimation destabilizes the regolith and brings the underlying water ice close to the surface, allowing rapid heating. Even under these optimal conditions, melting requires unrealistic assumptions. Ice containing a small amount of dust could melt via a solid-state greenhouse effect, but both its possibility and frequency in Mars' recent past remain uncertain.

There exists strong geomorphological, sedimentary and mineralogical evidence that Mars had an active surface hydrological cycle during the Noachian period, about 3.8 Gyr ago (Ga). However, how surface temperatures compatible with perennial liquid water could be sustained in spite of a Sun that only had 75% of its present-day brightness has remained elusive, leading to the faint young Sun paradox for Mars. Recently, the greenhouse effect of hydrogen peroxide (H2O2) has been proposed as a solution by Ito et al. (2020). Radiative transfer models have shown that a few ppmv of H2O2 in a 1 or 2 bar CO2 atmosphere could solve the faint young Sun paradox on early Mars. In a warm and wet CO2 atmosphere, H2O2 is produced by photochemistry and contributes to the stability of the CO2 atmosphere along with the HOx (H, OH and HO2) catalytic cycles. Nevertheless, a thorough assessment of the viability of such a high H2O2 abundance is still lacking. Using 1D and 3D climate models coupled with a C-H-O photochemistry solver, we show that in the most favorable case for H2O2 to build up, its steady-state abundance is several orders of magnitude short from its required abundance of similar to 1 ppmv to have a significant radiative effect. Furthermore, we also show that a transient warming episode associated with massive H2O2 release cannot exceed 10 Martian years. We therefore rule out H2O2 as a warming agent for early Mars.

We investigate thermosphere responses to non-orographic gravity waves (GWs) using wind measurements from the Neutral Gas and Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile EvolutioN mission, alongside simulations from the Mars Planetary Climate Model. We focus on zonal jets in high-latitude regions of the upper atmosphere. Jet acceleration and deceleration (280 m ) arise from momentum divergence (1,300 m ) driven by wave saturation and wind filtering. Simulations and observations indicate that GWs modulate these jets in the hemisphere associated with the descending branches of the Hadley Cell, due to the absence of wave critical layers in the middle atmosphere. Interactions between GWs and the mean flow can shape the circulation and dynamics of the upper atmosphere of Mars.

Surface runoff shapes planetary landscapes, but global hydrological models often lack the resolution and flexibility to simulate dynamic surface water bodies beyond Earth. Recent studies of Mars have revealed abundant geological and mineralogical evidence for past surface water, including valley networks, crater lakes, deltas and possible ocean margins dating from late Noachian to early Hesperian times. These features suggest that early Mars experienced periods allowing liquid water stability, runoff and sediment transport. To investigate where surface water could accumulate and how it may have been redistributed, we developed a global high-resolution (km-scale) surface hydrological model. The model uses a pre-computed hydrological database that maps topographic depressions, their spillover points, hierarchical connections between basins, and lake volume-area-elevation relationships. This database approach greatly accelerates simulations by avoiding repeated geomorphic processing. The model dynamically forms, grows, merges and dries lakes and putative seas without prescribing fixed coastlines, by transferring water volumes between depressions according to their storage capacities and overflow rules. We explore model behavior over the present-day Mars' topography measured by MOLA (Mars Orbiter Laser Altimeter) topography for a range of evaporation rates (from 0.1 m/yr to 10 m/yr) and total water inventories expressed as Global Equivalent Layer (from 1 mGEL to 1000 mGEL). 48 Simulations are iterated to reach the steady state. The model outputs the extent and depth of surface water bodies and identifies main drainage pathways using overflow fluxes as runoff indicators. Results show a transition toward a contiguous northern ocean between low (1-10 m) GEL values and increasing concentration of water in northern lowlands and major impact basins at higher GEL.

The stability of Mars's CO2-dominated atmosphere remains incompletely understood because traditional onedimensional photochemical models underestimate atmospheric CO mixing ratios by up to a factor of seven. Here we present the first long-term three-dimensional simulations for long-lived chemical species in the Martian atmosphere using a general circulation model coupled with photochemistry, the Mars Planetary Climate Model, allowing the equilibrium concentration of CO to be determined. When heterogeneous reactions on water-ice clouds and the condensation and surface deposition of H2O2 are included, the model predicts an equilibrium, annually and globally averaged CO volume mixing ratio of 750 ppmv. After correcting for a small underestimation of atmospheric water content relative to observations, the simulated CO mixing ratio becomes 700 ppmv, representing a substantial improvement over previous one-dimensional results and consistent with Earth-based measurements, although still slightly lower than retrievals from multiple satellites orbiting Mars (800-960 ppmv). Sensitivity experiments show that adjusting key chemical reaction rates within up to twice their laboratory-measured uncertainties, or including heterogeneous chemistry on dust particle surfaces, can reproduce the CO mixing ratios measured by the Mars orbiters and in some cases even lead to an overestimation. However, these CO-realistic simulations produce an equilibrium H2 mixing ratio that is substantially higher than observed, converting the long-standing CO-deficit problem in models into a new H2-surplus problem.

Thermal tides significantly influence Martian atmospheric dynamics and radiative transfer. However, the excitation mechanisms of non-migrating tides, exhibiting diverse spatial structures, remain poorly understood due to the limited number of observations. We investigated these tides using temperature observations from the Emirates Mars InfraRed Spectrometer (EMIRS) onboard the Emirates Mars Mission (EMM). The comprehensive local time coverage of the EMIRS data allows alias-free tidal mode decomposition, revealing detailed seasonal variability, spatial structure, and vertical propagation characteristics. We detect a terdiurnal westward-propagating wavenumber-1 (TW1) mode with amplitudes exceeding 1 K during a regional dust storm (C-storm) and characterize its alias-free structure in the lower atmosphere. The results demonstrate a robust link between the seasonal variations in tides and dust events. Observed vertical phase shears and partial wave reflections imply significant nonlinear interactions and feedback processes within the dusty Martian troposphere. These findings underscore the critical influence of aerosols on tidal dynamics, enhancing our understanding of atmospheric processes on Mars.

The knowledge of the Venus near-surface atmosphere is sparse. Few spacecrafts landed on the surface and measured winds with amplitudes below 1 m/s. The diurnal cycle of the wind amplitude and orientation is not known. Recent numerical simulations showed that slope winds along topographic structures could strongly impact the direction of winds. This study presents the first mesoscale modeling of such winds on Venus. A change of direction is occurring during the day in the main slopes, with upslope winds at noon due to solar heating and downslope winds at night. This is due to efficient IR cooling of the surface during the night, being colder than its surroundings slope atmospheric environment and leading to displacement of air. The temperature is impacted by the adiabatic cooling/warming induced by those winds. A strong heating effect is occurring for the downslope winds, leading to an anti-correlation between the surface temperature diurnal amplitude and the topography. This diurnal amplitude reaches 4 K in the plains and below 1 K in the mountains. The saltation of sediment by those winds was also quantified, with a higher probability at night along the slopes on the western flanks.

The Martian atmosphere experiences large diurnal variations due to its small thickness and low heat capacity. Driven by diurnal solar insolation and influenced by topography and radiative drivers (clouds and dust), diurnal temperature changes propagate from lower atmosphere into higher altitudes as forms of atmospheric tides. However, our understanding of diurnal variations in the Martian atmosphere is poor due to the lack of observations, especially those covering the entire planet and all local times, until recent. In its novelly designed high-altitude orbit, instruments onboard the Hope probe of the Emirates Mars Mission (EMM) could obtain a full geographic and local time coverage of Mars every 10 Martian days (Almatroushi et al., 2021). The Emirates Mars InfraRed Spectrometer (EMIRS, Edwards et al., 2021) observes surface temperature, temperature profile, dust content, water clouds, and water vapor in the lower atmosphere. Diurnal variations of such properties are derived on a planetary scale for the first time without significant gaps in local time or interference from seasonal changes. Such a rapid full planetary-scale coverage is ideal for investigating the fast-changing dust storms on Mars. In this talk, we present results of diurnal temperature variations and thermal tides before, during, and after several regional dust storms in Martian Year (MY) 36 and 37, and their coupling with dust and clouds. The results are also compared with numerical simulations by the Mars Planetary Climate Model (PCM), providing valuable information on physical processes controlling the diurnal climate of Mars. Almatroushi, H., AlMazmi, H., AlMheiri, N., AlShamsi, M., AlTunaiji, E., Badri, K., et al. (2021). Emirates Mars Mission Characterization of Mars Atmosphere Dynamics and Processes. Space Science Reviews, 217(8), 89. https://doi.org/10.1007/s11214-021-00851-6 Edwards, C. S., Christensen, P. R., Mehall, G. L., Anwar, S., Tunaiji, E. A., Badri, K., et al. (2021). The Emirates Mars Mission (EMM) Emirates Mars InfraRed Spectrometer (EMIRS) Instrument. Space Science Reviews, 217(7), 77. https://doi.org/10.1007/s11214-021-00848-

Due to gravitational perturbations from nearby planets, Mars has undergone large obliquity variations through its history. Modeling suggested that in the past 10 million years, the obliquity of Mars has varied by up to 20°, from 15° to 35°. During time periods of high obliquity, the polar regions of Mars received more solar insolation and became warmer, leading to more rapid sublimation of water ice and higher atmospheric water content. During periods of low obliquity, on the contrary, water vapor condensed in polar regions and the atmosphere became dry. This variation has a significant impact on the photochemistry of the Martian atmosphere, as HOx radicals, which are photolytic products of water vapor, are key catalysts to the photochemistry of the Martian atmosphere. It is then of interest to explore the photochemistry of Mars at different obliquities and its effects on the climate and surface of Mars, as part of the objectives of the “Mars Through Time” European Research Council project. In preparation for future Mars sample return missions, it is important to evaluate the preservability of potential organic matter buried in the shallow subsurface with different oxidizing capacities of the atmosphere at different obliquities. In view of the three-dimensional nature of the sublimation, transport, and condensation of water, we employ a fully coupled photochemical-radiative-dynamical model—the Mars Planetary Climate Model, developed at LMD in collaboration with other institutions—to simulate the photochemistry of the recent Martian atmosphere at obliquities between 15° and 35°. We find that at high obliquities, water content of the Martian atmosphere could exceed the present-day value by more than one order of magnitude, and the OH concentration could be higher by up to two orders of magnitude. These drastic changes result in a significantly lower CO concentration. Opposite effects are observed from low-obliquity simulations. The nonlinearity in the photochemical system, however, has led to more complex behaviors of the HO2 and H2O2 concentrations. We will explain the mechanisms behind these effects and discuss their implications in the paleoclimate of Mars and the preservation of potential biogenic organic matter in the shallow subsurface. We will also address the long-standing “CO-deficit” problem in Mars photochemical modeling, and show how the state-of-the-art 3D photochemical modeling helps to mitigate the problem.

The Martian South Polar Layered Deposits (SPLD) are composed mostly of ice and dust with a thin perennial CO2 cover and some internal CO2 ice layers. In the North, the seasonal CO2 cap is lost during summer, allowing H2O ice to sublimate into the atmosphere. In the South, the perennial CO2 cover prevents H2O ice sublimation. This work uses the Mars Planetary Climate Model to investigate how the H2O and CO2 cycles are affected if the thin perennial CO2 SPLD cover is lost. We find that during southern summer, the atmospheric water content will more than double in the south polar region. However, on a global scale, the NPLD is still the dominant source of humidity because of its larger surface area. When exposing some of the South Polar Cap buried water ice, the south polar cap becomes the dominant source of atmospheric humidity due to Mars's spin-orbital alignment.

The transfer of momentum due to non‐orographic Gravity Waves (GWs) significantly regulates the Martian middle‐upper atmospheric dynamics. Thus, these waves influence the transport of tracers and escape in the thermosphere. However, models assume that the non‐orographic GWs are emitted from a constant source level that approximates the averaged Planetary Boundary Layer (PBL). We move on to impose that the emission of the waves follows the top of a real‐time evaluated PBL to account for the diurnal cycle of the waves' source altitudes and implement this improvement in the Mars Planetary Climate Model (Mars PCM). In the absence of the PBL during the night, the non‐orographic GWs are assumed to be launched at altitudes near the surface following Hinson and Wilson (2023, https://doi.org/10.1016/j.icarus.2022.115420 )'s results. Sensitivity tests with the Mars PCM show that non‐orographic GWs are built up efficiently during the (polar) night. With the new scheme, the angular momentum in the upper atmosphere is enhanced. Additionally, simulations recover the “cold pockets” in temperature observed by the Mars Climate Sounder at 80–100 km and capture “deep drops” of the atmospheric species recorded by the Neutral Gas and Ion Mass Spectrometer in the polar night.

This paper presents a formalism of mixing induced by non-orographic gravity waves (GWs) to integrate with the stochastic GWs scheme in the Mars Planetary Climate Model. We derive the formalism of GWs and their mixing under the same assumptions, integrating the two schemes within a unified framework. Specifically, a surface-to-exosphere parameterization of GW-induced turbulence has been derived in terms of the eddy diffusion coefficient. Simulations show that the coefficient is on the order of to and a turbopause is at altitudes of 70-140 km, varying with seasons. The triggered mixing has minor effects on model temperatures, yet it substantially impacts upper atmospheric abundances. Simulations are consistent with observations from the Mars Climate Sounder and the Neutral Gas and Ion Mass Spectrometer. Mixing enhances the tracer transports in the middle and upper atmosphere, governing the dynamics of these regions. The scheme reveals how non-orographic GW-induced turbulence can regulate upper atmospheric processes, such as tracer escape.

Surface water ice is unstable on present-day Mars outside of the polar regions. However, prominent geological features show that during its recent past the surface of Mars was covered, on multiple occasions, by a « latitude-dependent mantle » (LDM) of water ice, from the polar regions to the tropics [1].Different studies conducted with Global Climate Models, in particular the Mars PCM (previously Mars LMD-GCM) led to the formulation of a climate scenario for the emplacement of ice ages : during high obliquity phases (>45°, as opposed to present-day ~25°), strong destabilization of the Northern Cap allowed for the aerial deposition of ice on the flank of tropical volcanoes, forming glaciers. When returning at lower obliquity, these glaciers were in turn destabilized but ice accumulated in the mid and high latitudes, and thus formed the observed surface ice deposits (LDM) [2]. However, the 45° obliquity excursions occurred before the last 5 million years, while the last ice age occurrence is dated of 400 000 years at most.Previous numerical experiments did not account for the radiative effect of water-ice clouds. Previous studies show that, even though somewhat negligible in the present-day Martian climate, this effect is overriding at higher obliquity with the intensification of the water cycle [3]. We have conducted new experiments at 35° obliquity with the Mars PCM using an improved physical package for the radiatively active clouds (RACs) and surface ice. Here, we present the resulting climate regime in our simulations. At 35° obliquity, the atmosphere is almost two orders of magnitude wetter than present-day, due to the greenhouse effect of RACs over the polar regions. In the high to mid latitudes, the seasonal winter ice accumulation is increased dramatically, while the summer sublimation is dampened by the latent heat cooling. Surface water ice thus accumulates at rates corresponding to tens of meters at each high obliquity excursion, reconciling the climatic scenario with the inferred age of emplacements of the LDM.References:[1] Head et al. (2003), Recent ice ages on Mars, Nature, 426, 797.[2] Madeleine et al. (2009), Amazonian northern mid-latitude glaciation on Mars: A proposed climate scenario, Icarus, 203, 390[3] Madeleine et al. (2014), Recent Ice Ages on Mars: The role of radiatively active clouds and cloud microphysics, Geophysical Research Letters, 41, 4873Acknowledgements:This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 835275).

It is still unknown how much water has escaped from Mars during its history. Hydrogen escape from Mars's atmosphere probably played a major role in drying the planet, but present-day Hloss rates (about 3x10^26 atoms per second on average) cannot explain the geological evidence for the large volumes of liquid water on ancient Mars. Here we used the three-dimensional Mars-Planetary Climate Model to show that H loss rates could have increased by more than one order of magnitude (6x10^27 atoms per second) during higher spin axis obliquity periods, notably in the last few million years when Mars's obliquity was about 35 deg on average. The resulting accumulated H escape over Mars's history translates into an approx. 80 m global equivalent layer, which is close to the lower limit of geological estimates, assessing the major role of atmospheric escape in drying Mars.

The Martian atmosphere experiences large diurnal variations due to the similar to 24.6 hr planetary rotation and its low heat capacity. Understanding such variations on a planetary scale is limited due to the lack of observations, which are greatly addressed with the recent advent of the Emirates Mars Mission (EMM). As a result of its unique high-altitude orbit, instruments onboard are capable of obtaining a full geographic and local time coverage of the Martian atmosphere every 9-10 Martian days, approximately similar to 5 degrees in solar longitude (L-S). This enables investigations of the diurnal variation of the current climate on Mars on a planetary scale without significant local time (LT) gaps or confusions from correlated seasonal variations. Here, we present the results of diurnal temperature variations and thermal tides in the Martian atmosphere using temperature profiles retrieved from the Emirates Mars InfraRed Spectrometer (EMIRS) observations. The data during the primary mission is included, covering an entire Martian Year (MY) starting from MY 36 L-S = 49 degrees. The diurnal temperature patterns suggest a dominant diurnal tide in most seasons, while the semi-diurnal tide presents a similar amplitude near the perihelion. The seasonal variation of the diurnal tide latitudinal distribution is well explained by the total vorticity due to zonal wind, while that of the semi-diurnal tide following both dust and water ice clouds, and the ter-diurnal tide following only dust. Comparison with the updated Mars Planetary Climate Model (PCM, version 6) suggests improvements in simulating the dust and water cycles, as well as their radiative processes. Plain Language Summary As a result of Mars' planetary rotation of similar to 24.6 hr and the low mass of its atmosphere, the atmospheric temperature changes dramatically every day following the cycle of the incoming sunlight. Such rapid and large changes, together with dust and clouds, can excite a number of waves propagating in the Martian atmosphere. These strong waves are one of the key factors controlling the weather and climate on Mars. Although these temperature variations and waves are important, our understanding is currently limited due to the lack of observations. A recent spacecraft mission located in an orbit distant from Mars has largely solved this problem by providing observations with good coverage in both location and time, enabling the investigations on the temperature variations and waves on a global and hourly scale. Here, we present the results of such investigations during the first Martian year of this mission, as well as comparison with the outputs from a Mars climate model, which represents our current knowledge. The model and the observations show generally good agreements, with differences suggesting directions in future model improvements, among which are the location and time of the dust and clouds in the Martian atmosphere.

In past decades, the study of Mars revealed geological and geochemical indicators regarding the past presence of valley networks, lakes and even possibly oceans, particularly at the end of the Noachian and the beginning of the Hesperian epochs. These indicators suggest a potentially long stable period during which early Mars could have had a climate warm and wet enough to allow the presence of surface liquid water and precipitation.To understand the evolution of the planet's hydrological cycle, we developed a new high-resolution surface hydrological model to constrain hydrological processes. The resolution of our model is obtained from the data of MOLA (Mars Orbiter Laser Altimeter) topographic map (1/128°). Our approach is based on the creation of a global hydrological database to define the location of topographic depressions and their spillover flow point, the hydrological connection between watersheds, and a relationship between elevation, volume, and area of generated lakes. This database significantly accelerates the computation speed of the hydrological model. The hydrological model can simulate the drying up or formation of lakes and oceans based on various parameters. In this conceptual study, we primarily explore the variability in the location of precipitation and the amount of available water on the surface of Mars, in Global Equivalent water Layer (GEL) units.The hydrological model performs detailed simulations by transferring water between watersheds according to their water storage capacity. Simulations are run in steady state, to ensure that inflow (precipitation) and outflow (evaporation and overflow) are equal. The model provides the location and extent of lakes and oceans depending on the amount and location of precipitation. Lake overflow rates are used as markers to identify runoff. To align with one of our plausible conceptual models, we use the relative climate aridity indicator (X-ratio) to position our study in relation to previous studies and highlight the contribution of this new model. Simulation results are compared with geological and geomorphological observations such as opened and closed lakes, deltas, and valley networks.The next steps in our work aim to enhance the robustness of our hydrological model by integrating it into a global climate model (GCM) and a planetary evolution model (PEM). This connection will provide a better understanding of the interactions between Mars' hydrological regime and its global climate. We also plan to add subsurface/groundwater flows to our model, providing a perspective on the distribution and dynamics of water under the Martian surface.