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Science is at its best when shared widely both within the scientific community and to the broader public. To maintain trust in science while simultaneously maximizing openness, it is important that every paper is carefully vetted. For this we rely on many volunteers, especially our peer reviewers. Their work is key for maintaining the integrity of scientific journals and to ensure the highest standards of scientific rigor. Reviewers verify that the assumptions, the methods, and the inferences made in our papers are sound. Many spend hours going over derivations, analyses, and evaluating the novelty and significance of the submitted manuscript. In 2019, JGR Planets benefited from 901 reviews provided by 584 unique reviewers. On behalf of the entire editorial board of JGR Planets and the entire planetary science community, we want to express here our most heartfelt gratitude to all of you. Thank you! Plain Language Summary: Peer review is key to vetting every paper submitted to JGR Planets in order to ensure the highest standards of scientific rigor in the journal. Reviewers verify that the assumptions, the methods, and the inferences made in our papers are sound. We depend on, and are grateful to, all the volunteers who spent going over and evaluating the submitted manuscript. In 2019, JGR Planets benefited from 901 reviews provided by 584 unique reviewers. On behalf of the entire editorial board of JGR Planets and the entire planetary science community, we want to express here our most heartfelt gratitude to all of you. Thank you! Key Points: Peer review is a crucial component of the high‐quality planetary science published in JGR: PlanetsIn 2019, 584 individuals provided 901 reviews of papers submittedPeer reviewers: thank you for your service to the planetary community [ABSTRACT FROM AUTHOR]

In 2016, JGR: Planets became the first AGU journal to begin accepting plain language summaries as a component of research and review articles. Authors began widely participating in this new approach to sharing planetary science in clear language with growth from 18% to 52% of published articles having a summary from the first to last quarters of 2017. In 2018, JGR: Planets joined several other AGU journals in making a plain language summary an expectation of all papers. Plain language summaries are an opportunity to make the rationale for, relevance of, and results from our work clear to a broad audience including students, science journalists, funders, and colleagues outside our specialty, among others. As adoption of and familiarity with, plain language summaries expands the impact of our work within our community and our society will grow too. Plain Language Summary: Scientists write papers about their work to share the results of their scientific studies. Research in the planetary science is increasingly focused on complex problems that require approaches from multiple scientific specialties. However, the language generally used in these articles that is intended to precisely explain the work can also, unfortunately, be a barrier to understanding for many readers. Three years ago, JGR: Planets introduced plain language summaries, such as this one, in order to facilitate broader understanding of why published research studies are valuable and of the fundamental outcomes of their work. Students, journalists, colleagues in other specialties, and really anyone, benefit from clear descriptions of the main ideas that drive and come out of research. While adoption of this new element of papers was swift and largely positive, authors have also expressed a desire for guidelines for writing effective summaries. I share such guidance and point to a few relevant examples of clear and effective plain language summaries in JGR: Planets. Ultimately, plain language descriptions of the value and outcomes of our work broaden its impact and benefit our community. Key Points: Plain language summaries are a valuable way to make the value and outcomes of scientific papers clear to a broader audienceAdoption of plain language summaries was swift and positive in the first 2 years of availability. Since July 2018, they are now expectedI provide guidance for effective plain language summaries with examples from recent JGR: Planets papers [ABSTRACT FROM AUTHOR]

This paper presents a theoretical analysis of the data on wave velocity measurements at small stresses presented in a companion paper describing the experimental results on a Martian regolith loose sandy simulant (Fontainebleau sand) of the soil at the InSight landing site on Mars (Elysium Planitia). Experimental data of wave velocities and Poisson's ratio are interpreted in the light of a granular contact mechanics theory and completed accounting for rugosity effects that are suspected to have stronger effects in sands under low stresses. The asperities of a grain of Fontainebleau sand were investigated through Atomic Force Microscopy, but larger asperities had to be adopted so as to better fit the model prediction with experimental data. A good agreement between the experimental data and the model predictions is obtained for stress above 10 kPa. Below 5 kPa, an area in which asperities are suspected to have a stronger influence, the model is not fully satisfactory, showing that further experimental and theoretical investigation is necessary in a stress zone particularly relevant to surface soils in planets, with probably enhanced effects of asperities on the intergrain contact mechanics. Plain Language Summary: To better understand seismic wave propagation at the surface of the InSight landing site on Mars, this paper presents a theoretical interpretation of the seismic wave velocities at low stress measured in a sand sample used as a Martian regolith simulant and presented in a companion paper. The theory is based on both a theoretical elastic model of a pack of smooth spheres of same diameter and approach accounting for the effects of local rugosity at the contact between sand grains (rugosity is supposed to have a stronger influence at the low stresses resulting from the smaller gravity on Mars). The sphere pack model shows that some slippage between grains is necessary to properly account for the radial deformation of a cylindrical sample subjected to an increase in axial stress (as described by an elastic parameter called the Poisson ratio). Accounting for rugosity effects allows a better prediction of the changes in wave velocity with respect to stress. The decrease in velocity under decreased stress is confirmed, but the rugosity parameter measured using an Atomic Force Microscope was under‐estimated, probably due to the over‐simplistic hypothesis of considering spheres. A proper prediction was obtained by fitting this parameter at a twice larger value. Key Points: A novel theory based on contact mechanics was proposed to calculate Vp and Vs of uniform granular materialsThe theory considers the roughness of the particles, the stress, the porosity of the granular assembly, and the properties of the grainsTheoretical results agree with experimental measurements on loose samples of Fontainebleau sand used as a Mars regolith simulant [ABSTRACT FROM AUTHOR]

This is the second part of Stolzenbach et al. (2023, https://doi.org/10.1029/2022JE007276), named hereafter Paper I, extends the period to the end of MY 34 and the first half of MY 35. This encompasses the end phase of the MY 34 Global Dust Storm (GDS), the MY 34 C‐Storm, the Aphelion Cloud Belt (ACB) season of MY 35, and an unusual early dust event of MY 35 from LS 30° to LS 55°. The end of MY 34 overall aerosol size distribution shows the same parameters for dust and water ice to what was seen during the MY 34 GDS. Interestingly, the layered water ice vertical structure of MY 34 GDS disappears. The MY 34 C‐Storm maintains condition like the MY 34 GDS. A high latitude layer of bigger water ice particles, close to 1 μm, is seen from 50 to 60 km. This layered structure is linked to an enhanced meridional transport characteristic of high intensity dust event which put the MY 34 C‐Storm as particularly intense compared to non‐GDS years C‐Storms as previously suggested by Holmes et al. (2021, https://doi.org/10.1016/j.epsl.2021.117109). Surprisingly, MY 35 began with an unusually large dust event (Kass et al., 2020, https://ui.adsabs.harvard.edu/abs/2020AGUFMP039...01K) found in the Northern hemisphere during LS 35° to LS 50°. During this dust event, the altitude of aerosol first detection is roughly equal to 20 km. This is close to the values encountered during the MY 34 GDS, its decay phase and the C‐Storm of the same year. Nonetheless, no vertical layered structure was observed. Plain Language Summary: Mars has a peculiar tendency, in one in every three Martian year, the entire planet is covered by dust suspended in the atmosphere. These events are referred as Global Dust Storms (GDS). The dust is lifted by fierce winds from the ground up to 80 km high. The mixture of water ice crystals and dust particle in the air of Mars change how the incoming Sun light warms the atmosphere. It is then of a crucial importance to properly characterize the nature and size of the aerosols, especially during a GDS, to better understand the dynamics of the Martian atmosphere. The spacecraft (ESA/Roscosmos) ExoMars Trace Gas Orbiter has been studying the Martian atmosphere since April 2018 and observed a GDS but also other and less intense dust storms. These data help us distinguish the aerosols nature (dust and water ice) and sizes during these distinct types of dust events. Our study confirms that, globally, the particles of dust and water ice are quite small, close to 1 μm or even less. Their distribution varies a lot, meaning that one may find a lot of different particle sizes or lot of similar sized particles. Key Points: MY 34 C‐Storm aerosol effective size and vertical structure is similar to MY 34 Global Dust Storm (GDS)MY 35 early dust event aerosol shows similar effective size and vertical structure as MY 34 GDSMY 34 GDS leads to more intense C‐Storm and an unusual early storm at the start of MY 35 [ABSTRACT FROM AUTHOR]

For many binary mixtures, the three‐phase solid‐liquid‐vapor equilibrium curve has intermediate pressures that are higher than the pressure at the two pure triple points. This curve shape results in a negative slope in the high‐temperature region near the triple point of the less volatile component. When freezing mixtures in the negative slope regime, fluid trapped below confined ice has latent heat released with more vapor upon cooling, and thus increases in pressure. If the rising pressure of the confined fluid overcomes the strength of the confining solid, which may be its own ice, it can produce an abrupt outburst of material and an increase in the system's overall pressure. Here, we report experimental results of freezing‐induced outbursts occurring in the N2/CH4, CO/CH4, and N2/C2H6 systems, and provide insight into the phenomenon through a thermodynamics perspective. We also propose other binary systems that may experience outbursts and explore the geological implications for icy worlds such as Titan, Triton, Pluto and Eris as well as rocky bodies, specifically Earth and Mars. Plain Language Summary: The solar system is host to a number of active worlds whose subsurface materials can well up and breach the surface through explosive and effusive means. Typically, these processes are initiated when the substance is heated and consequently pressurized, leading to the affected materials finding an upward means of escape. Interestingly, we have seen in the laboratory that mixtures can undergo overpressurization as a result of freezing. In this paper, we report our findings in experiments with mixtures of nitrogen/methane, carbon monoxide/methane, and nitrogen/ethane and refer to the process of overpressurized material breaching the surface during freezing as "outbursts." We represent these events through the use of phase diagrams, which are useful tools for understanding surface processes as they indicate where phase changes happen and what phases (e.g., vapor, liquid, solid, or a combination of these) will be most stable for any temperature, pressure, and composition conditions. It is likely that this outburst process occurs in many mixtures beyond those explored in the experiments in this paper. We discuss more mixtures that could have outbursts, and give examples of how this process is relevant on icy worlds like Titan, Triton, Pluto, and Eris, and rocky bodies, specifically Earth and Mars. Key Points: Three binary systems exhibited outbursts of volatile materials due to freezing‐induced overpressurizationSolid‐liquid‐vapor curves can be used to predict when outbursts occurOutbursts have implications for explosive/effusive processes throughout the solar system and likely occur in many other volatile mixtures [ABSTRACT FROM AUTHOR]

Peer review is the key to ensuring that the articles published in scientific journals such as JGR: Planets are based on sound scientific principles, follow state‐of‐the‐art techniques, and present exciting discoveries or novel understanding of the fundamental processes that affect solar system objects. JGR: Planets covers an extraordinarily broad range of topics addressing every aspect of geoscience with the only requirement that the work addresses planetary processes. The breadth of topics published is reflected by our editorial team composed in 2022 of associate editors Jun Cui, Joel Davis, Leigh Fletcher, Matthias Grott, Ananya Mallik, German Martínez, Molly McCanta, Katarina Miljkovic, Ryan Park, Andrew Poppe, Beatrix Sánchez‐Cano, Mariek Schmidt, Yasuhito Sekine, Kelsi Singer, Sonia Tikoo, David Trang, and Zhiyong Xiao in addition to the editors who authored this note. We rely on the expertise of the community to vet the articles submitted to the journal. In 2022, JGR: Planets benefited from 1,299 reviews provided by 775 unique volunteer reviewers. We also received help from 29 guest editors working on seven active special collections. We know that all our volunteers juggle many duties, both professional and personal. We are forever grateful that they chose to dedicate their time and energy to evaluating submitted manuscripts and advising us on the suitability of each manuscript for our journal, often suggesting ways to improve the paper. Scientific publishing is truly a selfless community effort. The entire editorial board of JGR: Planets wishes to express their most heartfelt gratitude to the many scientists who chose to support their journal. Thank you! Plain Language Summary: The expertise and dedication of peer reviewers are the keys to ensuring that the articles published in JGR: Planets are thoroughly vetted and present exciting discoveries or novel understanding of the fundamental processes that affect solar system objects. In 2022, JGR: Planets benefited from 1,299 reviews provided by 775 unique reviewers and from the help of 29 guest editors working on seven active special collections. We are forever grateful that so many scientists chose to dedicate their time and energy to evaluating and suggesting ways to improve each submitted manuscript. Scientific publishing is truly a community effort. The entire editorial board of JGR: Planets wishes to express here our most heartfelt gratitude to the many scientists who chose to support their journal. Thank you! Key Points: Peer review ensures that JGR: Planets publishes planetary science papers of the highest qualityIn 2022, 775 individuals provided 1,299 reviews of the submitted papersPeer reviewers: thank you for your service to the planetary community [ABSTRACT FROM AUTHOR]

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°–360°) 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. Plain Language Summary: The ExoMars Trace Gas Orbiter (TGO) has observed three Martian dusty seasons (summer in the southern hemisphere). In mid‐2018, we made the first detection of hydrogen chloride (HCl) in the Martian atmosphere using the Atmospheric Chemistry Suite (ACS). Finding this gas, among others, was a priority of ExoMars because its primary source may be volcanic. Since then, we have observed two more dusty periods with the reappearance of HCl each time. In part 1, we presented the climatology of HCl 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, part 2, we quantify the correlations between HCl, water vapor, ozone, dust, temperature, and water ice. We show that HCl is closely correlated to water vapor, which is, in turn, correlated to temperature. There is only a very low correlation between HCl and dust or water ice, which both impact water vapor. Possible mechanisms for seasonal HCl production and destruction are discussed, concluding that the freeze‐thaw cycle of atmospheric water is most likely. Key Points: Comparison of trace gases and aerosol opacity between ACS on ExoMars TGO and MCS on MROLow correlations between HCl and the opacities of dust or water ice, but very strong correlation with water vapor and the formation of icesHCl source and sink possibilities are discussed, concluding that frost activity may be the most likely possibility for both [ABSTRACT FROM AUTHOR]

Wind speeds measured by the Mars 2020 Perseverance rover in Jezero crater were fitted as a Weibull distribution. InSight wind data acquired in Elysium Planitia were also used to contextualize observations. Jezero winds were found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observed. However, a great influence of turbulence and wave activity was observed in the wind speed variations, thus driving the probability of reaching the highest wind speeds at Jezero, instead of sustained winds driven by local, regional, or large‐scale circulation. The power spectral density of wind speed fluctuations follows a power‐law, whose slope deviates depending on the time of day from that predicted considering homogeneous and isotropic turbulence. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. The signature of convection cells was also found during dust storm conditions, when prevailing winds were consistent with a tidal drive. Nighttime fluctuations were also intense, suggesting strong mechanical turbulence. Convective vortices were usually involved in rapid wind fluctuations and extreme winds, with variations peaking at 9.2 times the background winds. Transient high wind events by vortex‐passages, turbulence, and wave activity could be driving aeolian activity at Jezero. We report the detection of a strong dust cloud of 0.75–1.5 km in length passing over the rover. The observed aeolian activity had major implications for instrumentation, with the wind sensor suffering damage throughout the mission, probably due to flying debris advected by winds. Plain Language Summary: Jezero winds as measured in the crater floor by Perseverance were found to be much calmer on average than in previous landing sites. Turbulence and wave activity provoked rapid fluctuations that changed wind speed from calm conditions to more than 10–15 ms−1 in the timescale of seconds to minutes. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. These convection cells are produced under strong thermal gradients typically present during daytime. Pressure drops, associated with convective vortices, were usually involved in rapid wind fluctuations and, in some cases, in extreme winds as measured by Perseverance. An intense aeolian activity was observed at Jezero crater produced by transient high wind events. This aeolian activity had major implications for instrumentation, with the Perseverance wind sensor suffering damage probably due to flying debris advected by winds. Also, we report the detection of a strong dust cloud of 0.75–1.5 km passing over the rover. This paper has a companion paper (part 1) in the same issue, which is focused on wind patterns and analyzed the mechanisms driving atmospheric circulation at Jezero. Key Points: Jezero winds are found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observedTurbulence, wave activity, and convective vortices drive the peak wind speeds observed at JezeroWe report the detection of a dust cloud of 0.75–1.5 km in length passing over the rover [ABSTRACT FROM AUTHOR]

This paper presents estimates of the water and chlorine contents in the subsurface of Gale crater based on the measurements by the Dynamic Albedo of Neutrons (DAN) instrument onboard the NASA Curiosity rover. It is Part 1 of a two‐paper series. Data derived both from DAN active and passive measurements are presented in discrete surface areas (pixels) assuming a homogeneous distribution of water within the DAN sensing depth (60 cm) along the traverse of the rover. It is shown that the content of hydrogen, reported as Water Equivalent Hydrogen, varies between almost zero and a maximum of (6.1 ± 0.7) wt.%. The content of absorption equivalent chlorine varies between almost zero and (2.6 ± 0.2) wt.%. Such variations are thought to be related to the different geological processes and environmental conditions present in the strata along the traverse during the evolutionary history of Gale crater. The second paper (Part 2) studies particular properties of water and abundances of neutron absorbing elements at distinct geological regions, that the rover crossed on its way. Plain Language Summary: The Dynamic Albedo of Neutrons (DAN) is a neutron spectrometer that searches for signs of water by measuring the content of neutron absorbing elements in the ground below the NASA's Curiosity rover. Data derived from DAN are presented in discrete pixels along the traverse of the rover. Each pixel contains estimations of water equivalent hydrogen (WEH) and absorption equivalent chlorine (AEC) content from neutron sensing. DAN observations were combined into the two types of 3 × 3 m pixels: Pixel with Active Data includes WEH and AEC content and Pixel of Passive Data includes WEH content. Each pixel has been assigned to the associated geological member of the Mars Science Laboratory stratigraphic column. Full DAN data volume for 27 km of the traverse were formatted as a table with more than 10,000 pixels. The large fraction of stratigraphic members (Bradbury, Pahrump Hills and others) have the mean WEH values between 2 and 3 wt.%, while other members of the second part of the traverse (Jura, Knockfarrill Hill, Glasgow, Pontours) have the mean values of WEH above 3 wt.%. Mean AEC value has no large variations for all tested stratigraphic members; it is equal to around 1 wt.% for the majority of the pixels. Key Points: Dynamic Albedo of Neutrons (DAN) neutron sensing data were analyzed for 27 km of the Curiosity rover traverseDAN data were derived as single pixels and associated with the Gale crater stratigraphic column divisionsDAN measurements indicate area with maximum water content of (6.1 ± 0.7) wt.% [ABSTRACT FROM AUTHOR]

In a previous work (Hernández‐Bernal et al., 2021, https://doi.org/10.1029/2020je006517) we performed an observational analysis of the Arsia Mons Elongated Cloud (AMEC), which stands out due to its impressive size and shape, quick dynamics, and the fact that it happens during the Martian dusty season. Observations show that its morphology can be split in a head, on the western slope of the volcano of around 120 km in diameter; and a tail, that expands to the west reaching more than 1,000 km in length, making the AMEC the longest orographic cloud observed so far in the solar system. In this work we run the Laboratoire de Météorologie Dynamique Mesoscale Model to gain insight into the physics of the AMEC. We note that it is coincident in terms of local time and seasonality with the fastest winds on the summit of Arsia Mons. A downslope windstorm on the western slope is followed by a hydraulic‐like jump triggering a strong vertical updraft that propagates upwards in the atmosphere, causing a drop in temperatures of down to 30 K at 40–50 km in altitude, spatially and temporarily coincident with the observed head of the AMEC. However the model does not reproduce the microphysics of this cloud: the optical depth is too low and the expansion of the tail does not happen in the model. The observed diurnal cycle is correctly captured by the model for the head of the cloud. This work raises new questions that will guide future observations of the AMEC. Plain Language Summary: This is the second paper of our research on the Arsia Mons Elongated Cloud (AMEC), which is a visually impressive cloud on Mars. It appears on the western flank of the Arsia Mons volcano during a specific season right at sunrise. For 3 hr it grows, developing a thin elongated tail that has been observed to be as long as 1,800 km. In our previous work we described available observations. In this work we run a high resolution atmospheric model that captures the effect of the Arsia Mons volcano on the atmosphere. This model shows that due to the presence of the volcano and its effect on the wind, air is forced upwards next to the volcano, leading to a drop in temperatures of 30°C, which causes the formation of the cloud under extreme conditions of humidity. This is a success of the model that provides a new understanding of this outstanding cloud, however, the accurate physics behind the extreme expansion of the AMEC are not fully understood yet. This work solves some questions and raises many new ones, which will be an aid in the planning of new observations. Key Points: We performed mesoscale model dynamic simulations of the Arsia Mons Elongated Cloud observed in the Martian southern solsticeTopography‐induced circulation causes temperatures to drop by about 30 K at the observed origin location and local time of the cloudThe cloud tail is much more elongated in the observations than in the model, which challenges our understanding of winds and microphysics [ABSTRACT FROM AUTHOR]

In this paper, the non‐orographic gravity waves (GW) parameterization of the Mars Planetary Climate Model (PCM) previously implemented by Gilli et al. (2020, https://doi.org/10.1029/2018JE005873) is revisited and extended to the exobase (∼250 km). The simulations performed with the new scheme correct some known biases in the modeled thermal tide amplitudes and polar warming, improving the agreement with Mars Climate Sounder (MCS) observed thermal structures and tides below ∼100 km. Additionally, we find that the simulated densities above 150 km are compatible with NGIMS (Neutral Gas and Ion Mass Spectrometer) measured abundances. Large drag depositions ranging up to >∼950 m s−1 sol−1 are induced at altitude of 90–170 km due to the wave saturation (breaking) and depletion, leading to winds damped to magnitudes of ∼150–225 and ∼80 m s−1 in the zonal and meridional directions, respectively. Resulting temperature variations are ∼±10–30 K or 5%–10% at most latitudes except in the polar regions (where they can reach ∼±30–60 K). The results indicate that non‐orographic GW play a significant role in the dynamics of the middle‐upper atmosphere of Mars via the induced transfer of momentum and energy from the lower atmosphere. Plain Language Summary: Atmospheric gravity (buoyancy) waves are oscillations that result from flows over topography (orographic gravity waves (GW)) or perturbations caused by convective forcing, front systems, and jet streams (non‐orographic GW). In this paper, we model non‐orographic GW as a Gaussian wave packet, which is the sum of several monochromatic waves of random wavenumbers and frequencies. The wave packets are launched vertically from low altitudes (∼6 km), where the mean Mars Planetary Boundary Layer is located. We reproduce several features of the thermal structure and tides observed by the MCS by implementing our scheme to the Mars PCM. The scheme has a strong effect on the model's wind fields above an altitude of 35–40 km, transporting momentum from the source layers to higher layers, and even up to the thermosphere (altitude >∼100 km). The temperature and density variations generated by the scheme are comparable to the observations made by the Neutral Gas and Ion Mass Spectrometer. Our results demonstrate that non‐orographic GW play a crucial role in the momentum‐energy transport that affects the dynamics of the middle and upper atmosphere of Mars. Key Points: A stochastic parameterization of non‐orographic gravity waves is implemented up to the exobase (∼0–250 km)Simulation results are in very satisfying agreement with Mars Climate Sounder observed thermal structures and tidesThe simulated upper atmospheric densities are compatible with Neutral Gas and Ion Mass Spectrometer abundances [ABSTRACT FROM AUTHOR]

Aeolian landforms are widespread in our solar system. Understanding the exact nature and processes of formation of these features are challenging tasks necessitating a strong collaboration between scientists with different skills and scientific backgrounds. This paper describes the special issue for the 5th International Planetary Dunes Workshop, which includes 15 research papers and three commentaries. Among the 18 papers included in this collection, 16 cover Martian aeolian science and two Titan aeolian science. The papers presented focus on bedform morphology and dynamics via remote sensing data, modeling, analogs studies and laboratory experiments. Here we put the main results of the papers in their appropriate scientific context and discuss potential future lines of research. Plain Language Summary: Wind‐sculpted (aeolian) landforms are found on many bodies of our solar system. Their study is fundamental to understanding the geology and climate of these bodies and to safely plan extraterrestrial missions. Here we introduce a collection of papers describing aeolian features on Mars and Titan. Collectively, the papers presented in this special issue show the importance of an interdisciplinary approach in comprehending what we are seeing on other planets. The study of wind landforms in extraterrestrial planets is sparking a new interest on terrestrial aeolian geomorphology highlighting the significance of planetary studies in advancing the understanding of our Earth. Key Points: Results from the 5th International Planetary Dunes Workshop are presentedStudy of aeolian landforms can shed light on climatic changes and are key for landing site characterizationCombined use of imaging and high‐resolution flow modeling can provide new insights into the mechanism and processes that form bedforms [ABSTRACT FROM AUTHOR]

This paper is Part II of a double‐paper series that presents the abundance of water and chlorine along with other neutron‐absorbing elements in the shallow subsurface of Gale crater based on measurements by the Dynamic Albedo of Neutron (DAN) instrument onboard NASA's Curiosity rover. Initial results were represented as pixels on map data products from both DAN active and passive measurements made along the 27‐km traverse of the rover, corresponding to the mission period from landing on the martian surface in August 2012 through December 2021. In Part II, the contents of water and chlorine along with other neutron absorbers are studied separately for distinct geological regions along the traverse. Mean values and sample variances of these values are presented for each region. Water‐equivalent hydrogen (WEH) measurements show variability within the Jura member of the Murray formation and increase within the Carolyn Shoemaker formation. A large fraction of stratigraphic units (e.g., Bradbury, Sheepbed, Pahrump Hills and others) have mean WEH values between 2 and 3 wt.%, while units in the second part of the traverse (Jura, Knockfarrill Hill, Glasgow, Pontours) have mean values of WEH above 3 wt.%. The mean absorption‐equivalent chlorine value has no large variations for all tested geologic units; it is equal to around 1% for all of them. Plain Language Summary: The Dynamic Albedo of Neutrons instrument installed on NASA's Curiosity rover provides measurements of hydrogen (as an indicator of water) and other neutron‐absorbing elements (primarily chlorine) in the shallow subsurface of Gale crater on Mars. The initial data set for this study consists of pixel data products obtained from the instrument measurements taken over the rover's traverse of 27 km from August 2012 to December 2021. The data has been analyzed for distinct geological members defined by the Mars Science Laboratory team, allowing independent examination of various geological locations. The Jura, Knockfarrill Hill, and Pontours members exhibit mean water estimation values above 3 wt.%, while the majority of stratigraphic units, such as Bradbury and Sheepbed, have mean water values ranging between 2 and 3 wt.%. Measurements of water reveal differentiation in the Jura member of the Murray formation. The highest water values, reaching up to 4 wt.%, are observed in locations of the Carolyn Shoemaker formation. For other neutron‐absorbing elements, the mean value is roughly 1 wt.% for all geological units tested and does not vary significantly. Key Points: The Dynamic Albedo of Neutron (DAN) data was analyzed for the period of time from August 2012 to December 2021, for almost 27 km of the MSL traverse15 distinct MSL geological regions were examined using the DAN dataThe DAN data recognized the presence of two subregions within the Jura member of the Murray formation based on Water‐equivalent hydrogen concentration [ABSTRACT FROM AUTHOR]

Gravity waves (GWs) are ubiquitous and important dynamical processes in planetary atmospheres. But their properties and impact on the lower atmosphere remain unclear for most of planets due to the lack of data. The recent in situ observation from Huygens reveals GW activity in Titan's lower stratosphere. This paper investigates the upward propagation of GWs from Titan's lower atmosphere and their thermal effects using a full‐wave model. We reproduce the observed temperature perturbations with a superposition of three GW solutions with λx = 50 km and λz = 5.3, 8.9, and 28 km, with the longer wavelength one overlooked in previous studies. The simulation suggests that the propagation of GWs in Titan's lower atmosphere is almost nondissipative and thus has no wave‐induced thermal effect on the stratosphere, which is very different from the planetary thermosphere. The temperature minimum at the tropopause leads to a rapid local GW growth, which may be the primary reason for the significant GW signals above 60 km. We also find that the zonal wind may filter out the majority of GWs except for those traveling perpendicular to the wind direction, which can propagate upward to above 100 km, as observed by the Huygens probe. Plain Language Summary: Gravity waves are wave‐like air motions in the atmosphere of various planets. However, due to the lack of observations, their detailed nature and impact on the lower atmosphere are unclear for most planets, including Titan, a moon of Saturn. Fortunately, recent observations from the Huygens spacecraft obtained the temperature profile of Titan's lower atmosphere, enabling the study of gravity wave (GW) activity there. This paper aims to use a model to simulate and study the observed GWs on Titan. We reproduce the observed temperature wave structure with three kinds of GWs of different spatial scales. We found that when GWs travel in the lower atmosphere of Titan, they are not weakened by the air as that occurs in the upper atmosphere. As a result, there is also no heating effect from the waves in the lower atmosphere. Instead, a layer of cool air at around 60 km altitude may lead to a rapid growth of GWs, which may be the main reason for much larger GW signals above 60 km than below. We also found that atmospheric wind can stop most of the GWs from traveling upward into the higher atmosphere of Titan. Key Points: The temperature fluctuations observed by the Huygens probe between 10 and 140 km are well reproduced by our full‐wave modelThe propagation of gravity waves (GWs) in Titan's lower stratosphere is almost nondissipative and has limited wave‐induced thermal effectThe rapid GW growth at the tropopause is caused by the thermal structure rather than the wind effect [ABSTRACT FROM AUTHOR]

On 1 October 2021, Bepi‐Colombo performed its first flyby of Mercury. During the maneuver, the short wavelength channel (55–155 nm) of "Probing the Hermean Exosphere by UV Spectroscopy" (PHEBUS) was activated for a total duration of 1 hr. The helium resonance line at 58.4 nm was clearly observed during the whole sequence. At large distance from the planet, the emission was due to helium atoms in the interplanetary medium (interplanetary UV glow). Just after crossing the terminator of the planet and entering the dawn side of the exosphere, PHEBUS observed a clear additional emission due to scattering of solar photons by helium atoms in the exosphere of Mercury. The first detection of the 58.4 nm line in the exosphere of Mercury was reported by Broadfoot et al. (1976, https://doi.org/10.1029/gl003i010p00577) following the Mariner 10 flybys in 1974. The PHEBUS observation of exospheric helium emissions is the first for this element since the UVS measurements. In this paper, we present the results of our analysis of the PHEBUS data at 58.4 nm. Calibration of both instruments are compared with observations of the interplanetary glow, showing that the measurements of both instruments are accurate. However, we find that the exospheric density of helium atoms deduced from the PHEBUS data is 4.5–7.5 times lower than the previous estimate from UVS on Mariner 10. Possible explanations are considered. We show that some of the helium atoms present in the exosphere of Mercury could originate from the local interstellar cloud. Plain Language Summary: On 1 October 2021, The Bepi‐Colombo mission performed its first flyby of Mercury. During this maneuver, the "Probing the Hermean Exosphere by UV Spectroscopy" (PHEBUS) ultraviolet spectrometer was activated for 1 hour, starting 30 min before the time of the closest approach to the surface of Mercury. This instrument is able to measure the emission of helium atoms either in the interplanetary medium or in the tenuous atmosphere of Mercury. The presence of helium atoms around Mercury was first discovered by the ultraviolet spectrograph (UVS) of the Mariner 10 mission in 1974. The observations of PHEBUS confirm the detection of helium atoms in the atmosphere of Mercury. However, the amount of helium atoms detected by PHEBUS is 4.5–7.5 times lower than the value reported by UVS after the Mariner 10 flybys. We discuss possible explanations for this difference. Key Points: This paper presents helium measurements obtained during the first flyby of Mercury by the Bepi‐Colombo missionCalibrations of UVS/Mariner 10 and of PHEBUS are cross‐checked with observations of the interplanetary glow at 58.4 nmThe helium content in the exosphere of Mercury derived by PHEBUS is 4.5–7.5 times lower than the previous estimate from the Mariner 10 mission [ABSTRACT FROM AUTHOR]

During the first year of NASA's Mars 2020 mission, Perseverance rover has investigated the dark crater floor unit of Jezero crater and four samples of this unit have been collected. The focus of this paper is to assess the potential of these samples to calibrate the crater‐based Martian chronology. We first review the previous estimation of crater‐based model age of this unit. Then, we investigate the impact crater density distribution across the floor unit. It reveals that the crater density is heterogeneous from areas which have been exposed to the bombardment during the last 3 Ga to areas very recently exposed to bombardment. It suggests a complex history of exposure to impact cratering. We also display evidence of several remnants of deposits on the top of the dark floor unit across Jezero below which the dark floor unit may have been buried. We propose the following scenario of burying/exhumation: the dark floor unit would have been initially buried below a unit that was a few tens of meters thick. This unit then gradually eroded away due to Aeolian processes from the northeast to the west, resulting in uneven exposure to impact bombardment over 3 Ga. A cratering model reproducing this scenario confirms the feasibility of this hypothesis. Due to the complexity of its exposure history, the Jezero dark crater floor unit will require additional detailed analysis to understand how the Mars 2020 mission samples of the crater floor can be used to inform the Martian cratering chronology. Plain Language Summary: Perseverance rover landed within Jezero crater (Mars) in 2021 and is collecting rocks that will be returned to Earth. In terrestrial state‐of‐the art labs, these rocks will be dated. It will allow to test the method planetary scientists are using to assess the age of Martian surfaces: their impact crater statistics. As impact craters are forming regularly, their statistics are used as a timeline in planetary sciences. The present paper studies the statistic of impacts craters within Jezero crater to reveal that the floor of Jezero has been protected from bombardment during years. These results imply that the collected rocks of the Jezero crater floor will not be ideal to test the method of using impact crater as chronometer of Martian surfaces. Key Points: The dark floor unit of the Jezero crater displays an unusual inhomogeneous crater density, suggesting a complex exhumation historyThe crater density of the dark crater floor unit corresponds to the exposure time post exhumationThe crater density of this unit will not allow the calibration of the Martian crater chronology from return samples [ABSTRACT FROM AUTHOR]

This paper reports the study of potential water retention mechanisms based on the migration process of water molecules in the lunar regolith. First, we propose innovative definitions for the adsorption barrier coefficient, the collision barrier coefficient, and the total barrier coefficient for water molecule migration in the lunar regolith. Second, we derive an equation that relates the different barrier coefficients. Finally, we develop an equation for the diffusion coefficient of water in the regolith. The results are as follows. The collision barrier coefficient is a function of the soil layer porosity and pore number density and increases with increasing pore number density and decreasing porosity. The adsorption barrier coefficient depends on the soil temperature, adsorption heat, porosity, and pore number density and decreases sharply with increasing temperature. The adsorptive capacity of lunar soil to adsorb water molecules increases with increasing the adsorption barrier coefficient. For soil layers with a lower temperature, adsorption plays a more important role than collision in hindering the migration of H2O ${\mathrm{H}}_{2}\mathrm{O}$ molecules in the regolith. The total barrier coefficient increases with decreasing temperature, increasing adsorption heat, decreasing porosity, and increasing pore number density. Our results suggest that the water loss rate is inversely proportional to the total barrier coefficient. The water loss rate decreases with decreasing soil temperature, increasing adsorption heat, decreasing porosity, and increasing pore number density. Namely, water is more likely to be retained in areas where the regolith has a low temperature, large adsorption heat, and both low porosity and large pore number density. Plain Language Summary: Past experience has suggested that water is not present on the Moon. However, increasing evidence has shown that water is present in the regolith of permanent low‐temperature regions on the Moon. Our research aims to understand why water remains in the regolith. This paper develops a model to calculate how the lunar regolith hinders water migration. We derive an equation for the barrier coefficient of the water migration process in lunar regolith under various conditions. The adsorption effect can potentially impede the loss of H2O ${\mathrm{H}}_{2}\mathrm{O}$ molecules in the lunar regolith. The potential water retention area may extend to some regions covered by the regolith with large adsorption heat compared with the results of previous research. The area covered by the regolith with both great adsorption heat and low temperatures is most likely to retain water ice. Key Points: The collision effect and the adsorption effect influence the migration of water molecules in the lunar regolithEquations for barrier coefficients and the effects of different soil properties on water diffusion are establishedThe adsorption barrier coefficient may explain the presence of buried water ice if it is retained in the regolith [ABSTRACT FROM AUTHOR]

The Perseverance rover, Mars 2020 mission, landed on the surface of the Jezero crater, on 18 February 2021. This Martian crater is suspected to have hosted a paleolake as evidenced by the numerous detections of aqueously altered phases and thus is a promising candidate for the search for past Martian life. The SuperCam instrument, a collaboration by a consortium of American and European laboratories, plays a leading role in this investigation, thanks to its highly versatile payload providing rapid, synergistic, fine‐scale mineralogy, chemistry, and color imaging. After its landing, the first measurements of Martian targets with the infrared spectrometer of SuperCam (IRS) showed new instrumental behaviors that had to be characterized and calibrated to derive unbiased science data. The IRS radiometric response has thus been calibrated using periodic observations of the Aluwhite SuperCam Calibration Target (SCCT). Parasitic effects were understood and mitigated, and the instrumental dark and noise are characterized and modeled. The reflectance calibrated data products, provided periodically on the NASA Planetary Data System, are corrected for the main instrumental features. This radiometric calibration allowed us to study the 2.5 μm absorption band, which has been discovered in the Séítah unit and is associated with phyllosilicates‐carbonates mixtures. Plain Language Summary: This paper is an instrumental investigation of the infrared spectrometer (IRS) portion of the SuperCam instrument on the Mars Science Laboratory Perseverance rover. Work performed prior to and during flight operations enabled the derivation of a proper instrumental response suitable for calibration of infrared point spectra of rocks and soils observed along the rover traverse. The paper describes development of a full data reduction pipeline in which the radiometric response, sensitivity of the IRS electronic board to temperature, and electromagnetic interference artifacts were removed. A companion paper (Mandon et al., 2022, https://doi.org/10.1029/2022JE007450) investigates the IRS data set through Sol 379 in more detail. Here, we specifically explore the 2.5 μm band attributed to carbonates in Séítah unit's phyllosilicate‐carbonates mixtures. We found that such mixtures likely have a low carbonate content, which may indicate low amounts of chemical alteration or an alteration by a carbon‐poor fluid. Key Points: The infrared spectrometer of SuperCam on Perseverance has been successfully flight calibrated using the onboard calibration targetsCalibration permitted to study the most challenging long wavelengths and thus to discover an absorption band at 2.5 μmStudy of the 2.3 and 2.5 μm absorption bands showed the Séitah unit has variable clay and carbonate mixtures with low carbonate content [ABSTRACT FROM AUTHOR]

Hyperactive comets have attracted attention due to their high water production rate with an unclear mechanism, though some hypotheses have been proposed to explain it. Based on the thermal theories of the comet nuclei, this paper studied a comet surface thermal model considering the sublimation of H2O. In this paper, a method for solving the sublimation rate of water ice by infrared spectroscopy is proposed. The method adopts the assumption of subpixel temperature nonuniformity on the comet nucleus surface and uses the numerical solution of the Fredholm equation. We use the HRI‐IR spectrum (1.05–4.8 μm) data by EPOXI to analyze the pixel‐by‐pixel water sublimation rate of hyperactive comet 103P/Hartley 2. The results show that sublimation exists in most areas of the surface with or without surface roughness, and most of the water production rate (70%–90%) may come from the comet nucleus when Gauss random surface roughness is adopted. According to the sublimation law, it is estimated that the sublimation temperature of water ice on 103P is above 180 K. If the dust‐to‐ice volume ratio is 3:1, the sublimation temperature is about 200–210 K, which indicates that the water ice may sublimate underneath. This may explain why exposed water ice on the surface can hardly be observed while the active fraction of this comet is up to 100%. Plain Language Summary: Extremely active comets, known as hyperactive comets, release water at very high rates, although observations indicate that they have little exposed water ice on their surfaces. We found a significant energy deficit in the comet surface spectroscopic data, suggesting that the sublimation of water ice may be prevalent in the layers just below the nucleus surface. The sublimation of these water ices may have provided an appreciable water production rate by hyperactive comets. Key Points: The sublimation of the water ice on the comet nucleus absorbs energy, which significantly reduces the infrared radiation on the surfaceThe high water production rate of hyperactive comets may be due to the sublimation of water ice in the surface layer of the comet nucleusThe solution of the Fredholm equation is proved effective to solve the surface temperature distribution based on the observed spectrum [ABSTRACT FROM AUTHOR]

Atmospheric eddies serve as a crucial component of global circulation on Mars. It is of great scientific and engineering significance to study the spatio‐temporal characteristics of atmospheric eddies and eddy sequences. In this paper, based on the Open access to Mars Assimilated Remote Soundings (OpenMARS) dataset, we used an eddy detection and sequence tracking method to extract a total of 4,089 eddies in LS = 109° MY28 to LS = 109° MY29, which comprised 3,385 cyclones and 704 anticyclones. Notably, the northern hemisphere exhibited a markedly higher eddy number compared with the southern hemisphere, and more than 82% of eddies had an equivalent radius greater than 400 km. These eddies mainly gathered in the interior and vicinity of basins, mountains, and broad plains in the northern hemisphere. Martian atmospheric eddies can be classified into two categories: topographic eddies, which are related to the various landforms on Mars and non‐topographic eddies, which do not show obvious terrain dependence. Additionally, our analysis yielded 83 extracted eddy sequences, with nearly 90% of them exhibiting short life cycles of less than 40 Martian days (sols). Eddy sequences mainly arise in the spring and summer seasons of the northern hemisphere. Considering the instrument criterion of minimal atmospheric eddy activity for the Tianwen‐3 mission, Chryse Planitia was identified as the optimal landing area. Plain Language Summary: Atmospheric eddies, or "a movement of air, dust, or water in a circle" are a crucial component of Mars' global circulation. Studying eddy sequences is essential to understand the atmospheric circulation on Mars. During LS = 109° MY28 to LS = 109° MY29, a total of 4,089 eddies were identified using the OpenMARS dataset, with a higher occurrence of cyclones in both hemispheres during spring and summer compared to autumn and winter. Based on the distribution and density of the eddy centers, we classified the eddies into two categories: topographic eddies and non‐topographic eddies. Topographic eddies are influenced and restricted by the topography of the Martian surface, mainly concentrated in areas with positive and negative topographical features. Non‐topographic eddies are predominantly dispersed over high‐latitude regions. The moving distances of the topographic eddies are usually short, and they are more stable than the non‐topographic eddies. In addition, we extracted 83 eddy sequences, of which nearly 90% had short lifecycles less than 40 Martian days (sols). China's Tianwen‐3 mission is scheduled to return samples in 2030. We think Chryse Planitia is the optimal landing area for the Tianwen‐3 mission according to the spatio‐temporal distribution characteristics of Martian eddies. Key Points: A total of 4,089 eddies were extracted in MY28‐29 using the eddy detection and sequence tracking methodEddies can be classified into topographic eddies (related to landforms) and non‐topographic eddies (no obvious terrain dependence)Considering the features of Martian atmospheric eddies, Chryse Planitia was identified as the optimal landing site for Tianwen‐3 mission [ABSTRACT FROM AUTHOR]

Impact cratering is one of the most important processes shaping planetary surfaces, offering valuable clues about the target body's geologic history and composition. However, crater mapping has historically been done manually, a process that has proven to be both arduous and time consuming. This paper outlines a machine learning crater mapping approach for bodies with limited elevation data available (Digital Elevation Models). We applied a Convolutional Neural Network for the detection and morphometry of impact craters on Saturn's moon Enceladus using light‐shadow labels trained on data from the Cassini Imaging Science Subsystem. Our algorithm identified a total of 5,240 features which were used to quantify crater distribution; this included the highest number of small craters (<1–2 km in diameter) recorded on Enceladus by any previous published study. The pool of features was later down‐selected to craters between 0 and 30°N (latitude) imaged at high incidence (>60°) and phase angles (>26°). The down selection was necessary to accurately perform diameter measurements and derive depths from shadow estimation techniques to calculate depth–diameter ratios (d/D); a well‐studied relationship used to constrain planetary surface properties. Results show that the d/D ratio of craters in the equatorial region of Enceladus range from ∼0.06 to 0.37, with a median of 0.19. Our results will inform efforts to constrain the surface properties of this region of Enceladus, potentially also supporting future mission concept design for the Saturnian moon. Future work will explore the simple‐to‐complex crater transition and differences between this area's d/D and Enceladus' northern and southern latitudes. Plain Language Summary: Saturn's moon Enceladus epitomizes the statement "tiny but mighty." A mere ∼500 km in diameter, this body boasts a remarkable array of diverse terrains, a subsurface ocean, and is a strong candidate in the search for extraterrestrial life. As such, it is no surprise that it was named a top research priority in the 2023–2032 Planetary Decadal Survey. The study of impact craters is regarded as one of the most important tools in the study of planetary surfaces, providing insights into the history of celestial bodies. However, traditional manual approaches have proven to be arduous and time intensive. Motivated by the abundance of Cassini mission imagery, the lack of updated studies of Enceladus' cratered terrains and seeking a method to avoid intensive hand‐mapping, we developed a machine learning approach for crater identification and morphometry determination on Enceladus. This approach recorded more smaller craters (<1–2 km in diameter) on Enceladus than any previous published study and supports a history of intense geologic activity and heat flow in the leading and trailing hemispheres. Our method also allowed us to calculate the depth‐diameter relationship for craters in the equatorial region, a relationship that will inform the community's knowledge of the geological characteristics of the region. Key Points: Crater counting and identification provides information about a planet's history and properties, but is time‐consuming when done by handWe used a machine learning approach to map over 5,000 craters on the surface of Enceladus, using highlight‐shadow labelsWe present the depth–diameter ratio for craters on Enceladus' equator to aid future efforts to constrain surface properties of the region [ABSTRACT FROM AUTHOR]

This is the first part of a two‐part paper. NASA's Mars 2020 Perseverance rover measured winds on the Jezero crater floor close to the delta of an ancient river. A mostly repeatable diurnal cycle was observed and presented two regimes: (a) a convective regime, from dawn to sunset, with average easterly to southeasterly winds, during which maximum wind speeds were measured, and (b) a nighttime regime with westerly‐northwesterly winds followed by a relatively calm period with highly variable wind directions as a function of sol and time of night. The timing and magnitude of the observed regimes are consistent with primary control by regional and local slope flows. Data suggest that the surface circulation at Jezero region in northern spring and summer is highly unaffected by large‐scale circulation except during particular periods in the diurnal cycle or generally during dust storms, which is supported by MarsWRF model simulations. Consequently, the observed seasonal variability was weak. However, sol‐to‐sol and seasonal variability were measured, most of it during certain nighttime periods. Traveling waves consistent with baroclinic instability were clearly observed in surface winds at Ls ∼ 75°. The early MY36/2022A regional dust storm at Ls ∼ 153° disturbed the wind patterns with changes suggesting enhanced tidal flows. After sunset, the dust storm also produced detectable gravity wave activity, increasing the mixing in the nighttime planetary boundary layer during storm conditions. Inferred wind directions from dust devil movies strongly suggest that prevailing winds continued to be slope‐driven during the late summer, fall and early winter seasons. Plain Language Summary: This is the first part of a two‐part paper. NASA's Mars 2020 Perseverance rover landed close to the western rim of Jezero crater (18.44°N, 77.45°E) on 18 February 2021. The wind data acquired by the rover measured a mostly repeatable diurnal cycle with two regimes: (a) a convective period, from dawn to sunset with average easterly to southeasterly winds, in which maximum wind speeds were measured, and (b) a nighttime regime with westerly‐northwesterly winds followed by a relatively calm period with highly variable wind directions as a function of sol and time of night. The timing and magnitude of the observed regimes are consistent with primary control by regional and local slope flows, as have been observed to varying degrees at other landing sites on Mars. Data suggest that the surface circulation at Jezero is highly unaffected by large‐scale circulation, except during particular periods. An early regional dust storm prior to fall equinox also disturbed wind patterns with changes suggesting the strengthening of flows linked to large‐scale atmospheric oscillations called tides. The latter are primarily driven by the daily pattern of solar heating around Mars and its interaction with topography. Key Points: A mostly repeatable diurnal cycle of winds is consistent with primary control by regional and local slope flowsGreat sub‐diurnal and sol‐to‐sol variability in winds was observed during the calm period at nightTraveling and gravity waves were observed, along with the probable effect of enhanced tidal flows [ABSTRACT FROM AUTHOR]

This paper is the first of two companion papers presenting a theoretical and experimental study of the evolution of crystallizing magma oceans in planetesimals. We aim to understand the behavior of crystals formed in a convective magma ocean, and the implications of crystal segregation for the thermal and structural evolution of the convective system. In particular, we wish to constrain the possibility to form and preserve cumulates and/or flotation crusts by sedimentation or flotation of crystals respectively. We use lab‐scale analog experiments to study the stability and the erosion of a floating lid composed of plastics beads lying over a convective viscous fluid volumetrically heated by microwave absorption. We propose a law for erosion and re‐entrainment that depends only on two dimensionless numbers that govern these phenomena: (a) the Rayleigh‐Roberts number, characterizing the vigor of convection and (b) the Shields number, that encompasses the physics of the flow‐particle interaction. We further consider the formation of a cumulate at the base of the convective layer by sedimentation of beads that are denser than the fluid. We find that particle deposition occurs at a velocity that scales with the Stokes velocity, a result consistent with previous experimental studies. We build up a model that describes the transient evolution of the convective system's thermal state and the fraction of particles that segregated from the flow or that remain in suspension. Plain Language Summary: At early times in planetary formation, the heat supplied by 26Al was large enough to generate massive melting episodes in planetesimals and to produce magma oceans. Following melting and iron core differentiation, the proto‐mantle of these 10–100's km radius rocky bodies was a magma ocean that behaved like a convective fluid bearing solid crystals. The goal of this study is to understand the crystal‐melt segregation in such a magma ocean. In order to characterize the behavior of crystals in a convective flow we build a theoretical model based on lab‐scale experiments. We use a viscous fluid heated by microwave absorption to simulate the convection regime expected in magma oceans. We use plastic beads as analog of crystals in the experiments and we study the formation of either a flotation lid or a sedimented basal cumulate. We first demonstrate that both types of deposit form only if the buoyancy of the particles balances the shear imposed by convection, we then propose a complete model to account for the transient evolution. Key Points: We study experimentally the transient behavior of a convective fluid bearing particles that can float or sediment to form depositsThe erosion process is described by a modified Shields number that compares the convective shear to the buoyancy of the beadsThe deposition rate scales with the Stokes velocity of beads in the absence of convection [ABSTRACT FROM AUTHOR]

Models of asteroid collisional evolution suggest that many asteroids are gravitationally‐bound rubble piles. Although rubble piles may be expected to retain large void fractions, compaction may reduce the porosity. We apply models for cold compaction of rubble‐pile bodies developed in a companion paper toward observations of asteroid densities. The model for chondritic boulders is applied to S‐type (stony) and C‐type (carbonaceous) asteroids. The relation between density and size of S‐type asteroids is largely explained by cold compaction of rubble piles through fracturing of boulders, under the assumption that boulder size distributions are narrow before fracturing and fractal‐like afterward. The density variation of C‐type asteroids can only partly be explained by this mechanism, and the removal of micro‐voids inside the boulders would be required to match observations. The model for metal boulders is applied to M‐type asteroids, and the results suggest that, because of cold welding between metal boulders and the high yield strength of metal for either ductile or brittle‐like deformation, metallic rubble piles can preserve large (≳50%) porosities if the boulders are ∼1 m in size. This implies that M‐type asteroids such as Psyche and Kleopatra may be purely metallic, even though their densities are less than half that of iron. We also consider the hypothesis that Psyche is a primitive body of a CB chondrite‐like material. Assuming that the strength of CB chondrite is controlled by a silicate matrix, we predict that the density of a Psyche‐sized rubble pile of CB chondrite is higher than that of Psyche. Plain Language Summary: Many asteroids are gravitationally‐bound rubble piles that are expected to contain large void fractions. Mass and volume estimates of asteroid bodies imply that (a) for asteroids of similar materials, larger bodies tend to have smaller void fractions, and (b) bodies that are suspected to be metallic appear to retain larger void fractions than stony bodies of similar sizes. The difference between stony and metallic asteroids is possibly caused by the variation in the extent of compaction under self‐gravitation. We use theoretical models for cold compaction of rubble‐pile bodies, developed in a companion paper, to evaluate this hypothesis. We suggest that (a) the density versus size relation of stony asteroids can be partly explained by a cold compaction mechanism, and (b) metallic rubble piles can retain large (over 50%) porosities. Therefore asteroids that have been classified as metallic based on spectral properties but have lower than expected density, such as (16) Psyche and (216) Kleopatra, could be entirely metallic. Key Points: We model cold compaction in stony and metallic rubble piles, under the assumption of initially narrow boulder size distributionsStony asteroid density increases with size, which can be largely explained by cold compaction under self‐gravitationMetallic rubble piles can preserve large enough porosities to explain the densities of M‐type asteroids like Psyche and Kleopatra [ABSTRACT FROM AUTHOR]

Many asteroids are likely to be have been shattered by collisions into fragments and reaccumulated as gravitationally‐bound rubble piles. These bodies may contain large porosities, although this picture may be complicated by compaction inside the asteroid body. Estimates of asteroid mass and volume imply a negative correlation between size and porosity for stony asteroids. Asteroids that are suspected to be metallic appear to contain larger porosities than stony asteroids of similar sizes. To understand these observations, we develop models for cold compaction of fragments of different materials. The initial boulder size distributions are assumed to be narrow. We focus on macro‐porosity between the boulders and do not consider micro‐porosity inside the boulders. In our model of silicate/chondritic boulders, compaction is assumed to occur through cataclastic fracturing, which creates small pieces that fill the pores between residual large boulders, leading to fractal‐like distributions. This fracturing occurs when compression leads to stresses exceeding the tensile strength of the boulders. Combining this model with data on meteorite strength, we suggest that the compaction of chondritic boulders can be significant at pressures of several megapascals. In our model of metal boulders, we consider cold welding and boulder deformation (through ductile yielding or brittle‐like fracturing, depending on the stress and intrinsic crack size). Given the properties of iron meteorites, we infer that compaction in metallic rubble piles, caused by ductile or brittle deformation, is small, and that cold welding may lead to large (≳50%) porosities if the boulders are of ∼1 m sizes. Plain Language Summary: The asteroid belt is a remnant of early planetary formation, and it has been continuously modified in the following ∼4.5 billion years through collisional evolution. Many asteroids are likely to have experienced disruptive collisions and exist as gravitationally‐bound rubble piles. Voids are expected to exist between the rubble fragments, but they may be removed by the pressure inside the asteroid body. In this paper, we develop models for compaction of rubble piles of different compositions. At pressures inside asteroid bodies with diameters of a few hundred kilometers, assemblages of stony boulders can be substantially compacted through boulder fracturing, which creates smaller pieces that fill the pores between large ones. Assemblages of metal boulders likely preserve larger fractions of voids because of their greater strength and inter‐boulder adhesion through cold welding. These models are used to discuss asteroid observations in a companion paper. Key Points: Models are developed for cold compaction of rubble piles of different materialsSilicate and chondritic assemblages may be substantially compacted through boulder fracturing at pressures of several megapascalsMetal assemblages can preserve large porosities at least up to ∼10 MPa pressures [ABSTRACT FROM AUTHOR]

HDO and the D/H ratio are essential to understand Mars past and present climate, in particular with regard to the evolution through ages of the Martian water cycle. We present here new modeling developments of the HDO cycle with the Laboratoire de Météorologie Dynamique Mars Global Climate Model (GCM). The present study aims at exploring the behavior of the D/H ratio cycle and its sensitivity to the modeling of water ice clouds and the formulation of the fractionation by condensation. Our GCM simulations are compared with observations provided by the Atmospheric Chemistry Suite (ACS) on board the ESA/Roscosmos Trace Gas Orbiter (TGO), and reveal that the model quite well reproduces the temperature and water vapor fields, which offers a good basis for representing the D/H ratio cycle. The comparison also emphasizes the importance of modeling the state of supersaturation, resulting from the microphysical processes of water ice clouds, to correctly account for the water vapor and the D/H ratio of the middle‐to‐upper atmosphere. This work comes jointly with a detailed comparison of the measured D/H profiles by TGO/ACS and the model outputs, conducted in the companion paper of Rossi et al. (2022, https://doi.org/10.1029/2022JE007201) (this issue). Plain Language Summary: The D/H ratio observed in a planetary atmosphere is traditionally used as a proxy to estimate the initial water reservoir of the planet. We present here an improved global circulation model including HDO, the main isotope of water on Mars. The updated model takes into account the details of the formation of clouds and their radiative effect. It also includes the effect of photochemistry on HDO and deuterated species. We find that supersaturation is key to the representation of the D/H cycle by making the hygropause more porous, allowing more HDO in the upper atmosphere. It also reduces the efficiency of the isotopic fractionation occurring at condensation. We present here comparisons between observations by the Atmospheric Chemistry Suite spectrometer onboard the orbiter TGO. While the model is globally in agreement with the observations, the inability of the model to reproduce the observed vertical distribution of dust, especially during the Global Dust Storm, causes discrepancies in the representation of water vapor. The model reveals the importance of representing the state of supersaturation to correctly account for the water vapor amount reaching the top of the atmosphere and estimate the D/H ratio at escape. Key Points: The HDO cycle has been implemented in the last version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model including microphysics and radiative effect of water ice cloudsKinetics effect is now included in HDO fractionation during condensation and proves to be significantSupersaturation, the presence of which is regulated by cloud processes, alters significantly the relative abundances of HDO and H2O [ABSTRACT FROM AUTHOR]

Nanoparticles within lunar soil grains are a primary product of space weathering. The microstructural and chemical characteristics of the nanoparticles are diverse and their formation mechanisms are still under debate. In this paper, for the first time, tadpole‐shaped nanoparticles (with Fe‐Ni(‐S) head and Fe‐Ti‐O tail) were found in the impact melt glass spherule of an agglutinate in the returned Chang'e‐5 lunar soil, and their possible formation mechanisms were discussed. In terms of the Fe‐Ni(‐S) "head" formation mechanisms, they probably produced by shock‐induced dissemination. Another possibility is that the Fe‐Ni(‐S) heads were derived from the impact glass due to liquid immiscibility. The S degassing of FeS was contributed to nanophase Fe‐Ni metal. For the Fe‐Ti‐O tails, they are devitrified ilmenites, nucleated as a result of the passage of the Fe nanoparticles through the melt. These nanoparticles formed though impact‐induced nonequilibrium growth and recorded the movement and migration of the Fe‐Ni‐S nanoparticles within the melt. The tadpole‐shape nanoparticles provide a new example of viscous fingering in impact melts and the associated ilmenite dendrites point to the formation of high‐temperature chemical gardens in lunar impact melt. Plain Language Summary: The Moon's surface is a hostile environment. Micrometeorite impacts and solar wind irradiation have modified lunar surface for billions of years, producing large amount of nanoscale metal particles within lunar soil grains. These nanoparticles show a variety of microstructure and chemical characteristics, and the formation mechanisms are still controversial. From the returned Chang'e‐5 lunar soil, we first discovered tadpole‐shaped Fe‐Ni‐S‐Ti nanoparticles in the impact melt glass of an agglutinate particle. The composition of the nanoparticles "metal heads" indicates they could derive from impact‐induced dissemination in melts or unmixing of two immiscible melts between Fe‐S melt and silicate melt. S outgassing from FeS led to the creation of nanoscale Fe‐Ni metallic particles. The Fe‐Ti‐O tails of the nanoparticles indicate that these particles are most likely exsolution from melt during the movement and migration of the Fe‐Ni‐S nanoparticles. Our observations provide a viscous fingering in impact melts of nano‐phase metal particles in lunar soil. The presence of ilmenite dendrites suggests the occurrence of high‐temperature chemical gardens (self‐organizing nonequilibrium process) during the impact process. Key Points: Tadpole‐shaped Fe‐Ni‐S‐Ti nanoparticles have been found in the impact melt glass spherule of an agglutinate in Chang'e‐5 lunar soilThe nanoparticles made up of Fe‐Ni‐S heads and with tails made up of Fe‐Ti‐O (ilmenite)The nanoparticles provide a new example of viscous fingering and ilmenite dendrites point to high‐T chemical gardens in lunar impact melt [ABSTRACT FROM AUTHOR]

Irradiation by energetic ions, electrons, and UV photons induces sputtering and chemical processes (radiolysis) in the surfaces of icy moons, comets, and icy grains. Laboratory experiments, both of ideal surfaces and of more complex and realistic analog samples, are crucial to understand the interaction of surfaces of icy moons and comets with their space environment. This study shows the first results of mass spectrometry measurements from porous water ice regolith samples irradiated with electrons as a representative analogy to water‐ice rich surfaces in the solar system. Previous studies have shown that most electron‐induced H2 ${\mathrm{H}}_{2}$O radiolysis products leave the ice as H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$ and that O2 ${\mathrm{O}}_{2}$ can be trapped under certain conditions in the irradiated ice. Our new laboratory experiments confirm these findings. Moreover, they quantify residence times and saturation levels of O2 ${\mathrm{O}}_{2}$ in originally pure water ice. H2 ${\mathrm{H}}_{2}$O may also be released from the water ice by irradiation, but the quantification of the released H2 ${\mathrm{H}}_{2}$O is more difficult and the total amount is sensitive to the electron flux and energy. Plain Language Summary: The surface of any airless body exposed to space is irradiated by charged and neutral particles. This irradiation can alter the surface and trigger chemical reactions (so‐called radiolysis) and is an important factor in the context of icy satellites and their environment in the solar system. This paper presents one approach to better understand these irradiation processes by irradiating porous water ice samples with electrons at laboratory conditions representative of the icy moons of Jupiter. We find that most of the radiolysis products of water leave the ice as H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$, while some of the produced O2 ${\mathrm{O}}_{2}$ remains trapped in the irradiated ice. This could help explain remote measurements of the surfaces of Europa and Ganymede. Water may also be released from the irradiated ice. Key Points: Laboratory irradiation experiments are crucial to understand the surfaces of icy bodies in spaceFirst mass spectrometer results from water ice regolith samples irradiated with electrons are shownThe dominant radiolysis products released from the ice samples are H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$, with a fraction of the O2 ${\mathrm{O}}_{2}$ being retained in the ice [ABSTRACT FROM AUTHOR]

The formation of complex craters requires some form of transient weakening of target rocks. Acoustic fluidization is one proposed mechanism applied in many numerical simulations of large crater formation. In a companion paper, we describe implementing the Melosh model of acoustic fluidization in the iSALE shock physics code. Here, we explore the effect of Melosh model parameters on crater collapse and determine the range of parameters that reproduce observed crater depth‐to‐diameter trends on the Earth and Moon. Target viscosity in the Melosh model is proportional to the vibrational wavelength, λ $\lambda $, and the longevity of acoustic vibrations is ∝λQ $\propto \lambda Q$ (Q $Q$—quality factor). Our simulations show that λ $\lambda $ affects the size of the fluidized region, its fluidity, and the magnitude of the vibrations, producing a variety of crater collapse styles. The size of the fluidized region is strongly affected by the Q $Q$. The regeneration factor, e $e$, controls the amount of (re)generated acoustic energy and its localization. We find that a decrease in e $e$ leads to less crater collapse and that there are trade‐offs between e $e$ and Q $Q$. This trade‐off contributes to the more realistic Q $Q$ values than those used in the Block model. The diffusion of vibrations in regions with high stress and strain is controlled by the scattering term, ξ $\xi $. Compared to the Block model, the Melosh model results in a shallower zone of weakening in complex craters and enhanced strain localization around the crater rim. The parameter set that produces best depth‐diameter trends is λ $\lambda $ = 0.2× ${\times} $impactor radius, Q $Q$ = 10–50, e $e$ = 0.025–0.1, and ξ $\xi $ = 10–105 ${10}^{5}$m2s−1 ${\mathrm{m}}^{2}{\mathrm{s}}^{-1}$. Plain Language Summary: Impact craters are the most common geological features on planetary surfaces. As craters get larger in diameter, they express more complex morphology. Complex crater morphology results from target rocks behaving fluidly upon hypervelocity impact. The rocks behave as fluids because of the temporary reduction in strength, which results from vibrations that remain in the target after the passage of the shock wave. These vibrations cause pressure fluctuations and reduce the rock's strength. This concept is called acoustic fluidization. Here, we explore how vibrations' wavelength, dissipation, scattering, and (re)generation affect crater collapse. Our models show a larger subsurface deformation variety, which the previous simplified model lacked. We determine bounds for each explored parameter, show trade‐offs between them, and their success in reproducing depth‐to‐diameter trends on Earth and the Moon. Key Points: We explore the parameters of the Melosh model of acoustic fluidization (full model) and their effect on crater collapseWe find that the final crater size is a product of the trade‐off between the quality (Q) $(Q)$ and regeneration (e) $(e)$ factors and that vibrational wavelength, λ $\lambda $,affects the style of crater collapseConsidering the full model of acoustic fluidization produces a variety of subsurface deformation in the complex craters lacking in the hitherto applied models [ABSTRACT FROM AUTHOR]

We investigated the age and origin of 41 light plain deposits around the lunar south pole, using LRO data sets and successfully dated 22 of them using crater size‐frequency distribution measurements. We find that deposits related to Schrödinger basin are ∼3.8 Ga old, including deposits on the Amundsen crater floor, which we interpret as Schrödinger basin ejecta. Six study areas date to ∼3.7 Ga, which reflect an additional large‐scale or basin impact event post‐Schrödinger ‐ possibly Orientale. An additional two areas with younger isolated ages likely represent local impact sources. Finally, smooth, light plains‐like, ejecta deposits around Shackleton crater were found to be Eratosthenian in age. Our findings show that light plain deposits originate from a combination of both basin and local ejecta materials. Thus, samples collected in the region will exhibit a diverse range of basin materials and ages representative of the ejecta from multiple impact events. Plain Language Summary: The southern polar region of the Moon is the focus of future human exploration missions due to the presence of essential resources like water and the possible capability to sustain a prolonged human presence on the Moon. Thus, it is vital to understand the geologic context of the region for a correct interpretation of returned samples. In this paper, we used the Lunar Reconnaissance Orbiter data sets to investigate the age and origin of 41 smooth deposits around the lunar south pole. Of these, 22 could be accurately dated using the size‐frequency distribution of craters, and we found groups of smooth deposits of similar ages that can be related to a one or more source impacts. Some of them are related to the Schrödinger basin impact, while others are likely related to the Orientale basin on the farside of the Moon. We also found two deposits of younger age, likely related to fresh nearby craters, and other ejecta deposits that represent the formation age of the Shackleton crater. Therefore, these deposits appear to have formed from both basin/crater ejecta and local materials. Samples collected in this region will exhibit a diverse range of materials and sources from multiple events. Key Points: Forty‐one light plain deposits in the lunar southern polar region are investigated for origin and absolute model age determinationSchrödinger basin materials date to ∼3.8 Ga, Orientale materials to ∼3.7 Ga and Shackleton impact to ∼2.4 GaMultiple impact events and local sources contributed to light plain deposits across the lunar south pole [ABSTRACT FROM AUTHOR]

Our understanding of the paleoclimate of Mars and the dominant forcing functions that drive large‐scale changes largely remains a mystery. However, the Martian Polar Layered Deposits (PLD) offer a promising avenue for unraveling the planet's recent paleoclimate history. Despite recent progress to detect a climate signal in the PLD, definitive evidence of a correlation between the stratigraphic record and Mars' recent orbital history remains elusive. We utilized new and updated techniques, including high‐resolution stratigraphic reconstruction from High Resolution Imagine Science Experiment stereo imagery and digital terrain models, combined with a technique of variable dip correction to account for three dimensional bedding orientations. Signal processing methods, such as wavelet and Fourier analysis, were applied to perform detailed time‐series analysis. These analyses revealed a quasi‐periodic signal indicative of fine bedding at a scale of approximately 2 m. Based on previously proposed deposition rates, these fine layers appear to correspond to timescales centering around 4,000 years. This suggests that the meter‐scale layers may not be the result of orbital forcing and hint at an unknown sub‐Milankovitch climate forcing mechanism. We discuss potential exogenic causal mechanisms, such as cyclic variations in solar activity, and endogenic factors, including large‐scale changes in dust distribution. Understanding the formation processes of these fine layers may significantly enhance our knowledge of Martian climate history and its driving forces. Plain Language Summary: In this paper, we investigate the presence, distribution, and significance of small‐scale layers in the northern polar layered deposits (PLD) of Mars. Solar radiation is a key variable in determining the long‐term evolution and stability of the polar caps, and it is closely linked to changes in the planet's orbit, such as precession, eccentricity, and obliquity. We argue that the thickness and periodicity of these fine layers reflect a causal mechanism that cannot be attributed solely to climate changes resulting from the long‐term evolution of Mars' orbit. Based on proposed accumulation rates of the polar ice, we suggest that the quasi‐periodic bedding may represent a climate signal occurring on timescales several orders of magnitude shorter than those of the orbit changes in Mars, centering around 4,000 years. These observations help to characterize and better understand the PLD and their formation. Key Points: Meter‐scale bedding is widespread throughout Mars northern polar layered depositsUsing image and topographic data, we show that the fine bedding is quasi‐periodic within a confined range of wavelengths from 1.5 to 3 mUnder previous deposition rate estimates, meter‐scale bedding corresponds to ∼ ${\sim} $4,000‐year, sub‐Milankovitch timescales [ABSTRACT FROM AUTHOR]

The paper addresses the possibility of a young Mars having had a massive moon, which synchronized the rotation of Mars, and gave Mars an initial asymmetric triaxiality to be later boosted by geological processes. It turns out that a moon of less than a third of the lunar mass was capable of producing a sufficient initial triaxiality. The asymmetry of the initial tidal shape of the equator depends on timing: the initial asymmetry is much stronger if the synchronous moon shows up already at the magma‐ocean stage. From the moment of synchronization of Mars' rotation with the moon's orbital motion, and until the moon was eliminated (as one possibility, by an impact in the beginning of the Late Heavy Bombardment), the moon was sustaining an early value of Mars' rotation rate. Key Points: A synchronous moon of 1/3 of the lunar mass could create Mars' present triaxiality and asymmetry, when the lithosphere was absent or weakVolcanism near the frozen tidal bulges was, hypothetically, more active than in other provinces. Hence Tharsis and Syrtis MajorThe putative moon perished in the Late Heavy Bombardment, Phobos and Deimos being remnants of its large fragment destroyed later [ABSTRACT FROM AUTHOR]

The thermal and dynamical structure of Titan's middle atmosphere (the stratosphere and mesosphere) has been observed to evolve over seasonal timescales. Measurements from the Composite Infrared Spectrometer on the Cassini spacecraft indicated the presence of a westerly jet with the strongest winds exceeding 200 m s−1 near the autumn and spring poles. The strength of the winds varied substantially with latitude and altitude, and weakened throughout most of winter. The strong winds also served to trap short‐lived trace molecules near the winter pole, leading to a chemically enriched environment. Here, to better understand the evolution of the middle atmosphere jet, we quantify the heat and zonal momentum budgets in Titan's stratosphere and mesosphere over the course of a Titan year using a three‐dimensional general circulation model. We confirm that the dominant heating balance is between the net radiative and adiabatic heating rates, and also show that the convergence of sensible heat by the atmospheric flow is important at low stratospheric altitudes above the winter pole. We show that the polar jet is maintained by the convergence of zonal momentum by the mean meridional flow, while the low‐latitude winds are maintained by an up‐gradient transport of momentum by eddies that occur on time scales of less than one Titan day. The heat and momentum budgets we determine here will be useful in constraining the factors controlling the evolution of Titan's middle atmosphere over the coming decades. Plain Language Summary: Titan is the largest moon of Saturn, and possesses a thick, nitrogen dominated atmosphere with surface pressure that is greater than Earth's. The middle atmosphere of Titan includes the stratosphere and mesosphere and extends from about 60 km through about 600 km above its surface. Winds in Titan's middle atmosphere can be up to 200 m s−1 (∼450 miles per hour) and play an important role in moving trace molecules throughout the atmosphere. In this paper, we perform the first numerical study to determine what causes the temperature and winds in Titan's middle atmosphere to change over the course of Titan's 29.5 Earth‐year trip around the sun. We find that the temperatures in Titan's middle atmosphere are driven primarily by solar heating and infrared cooling, and by compressional heating of descending air; however, the transport of heat by the winds is also important. The winds are primarily accelerated by the large‐scale transport of momentum toward the high winter latitudes, but are balanced by the transport of momentum toward the equator by atmospheric waves. Our results will be important to predicting the changes that will be seen in Titan's atmosphere as it progresses through northern autumn in 2025. Key Points: Heating in the middle atmosphere is balanced between radiative and adiabatic effects, but convergence of heat is important near the polesThe acceleration of the polar jet is balanced between the transport of momentum by the mean flow and deceleration by transient eddiesThe transition of the polar jet from one hemisphere to the other is a complex process that involves changing structure over all latitudes [ABSTRACT FROM AUTHOR]

Wave velocity measurements were performed on Fontainebleau sand samples used as a Martian regolith simulant to investigate the elastic properties of the surface material at the InSight landing site on Mars (Elysium Planitia). Loose samples (density 1.4 Mg/m3, density index 6%) were prepared by using the pluviation method to mimic the low regolith density at the InSight landing site. A novel device derived from triaxial testing was designed to measure wave velocities at low stresses along a horizontal cylindrical specimen. Four tests were made, in which the confining stress was applied by applying a vacuum between 1 and 80 kPa. Wave velocities were measured by using bender elements under stresses as low as 1.75 kPa, a very low value compared to the standard stress ranges generally considered in terrestrial geotechnics (>10 kPa). The changes in compression and shear wave velocities obey a standard power law, with two slightly different exponents for Vp and Vs, indicating a not perfectly elastic behavior. Data showed greater variability below 5 kPa, indicating some limitations of the bender element technique in this range. A slight decrease in Poisson ratio was detected below 5 kPa, which certainly deserves more investigation. This investigation is useful to better analyze the data of the InSight mission, both in terms of wave propagation at the surface and to interpret some in situ elastic tests carried out with the scoop. These data are interpreted in the light of a granular contact mechanics theory in a companion paper. Plain Language Summary: The InSight mission lander is a highly sensitive seismometer placed on Mars to detect seismic waves from Marsquakes and meteorite impacts. To help analyzing the seismic waves, a special laboratory device was developed to measure wave velocities on a loose sandy simulant mimicking the surface regolith, with wave emitting/receiving transducers placed on both sides of a cylindrical sand sample subjected to external stress. Due to reduced gravity, the stresses that control wave velocity (larger stress, faster waves) are very low at the Mars surface. To impose low stresses (down to 1.75 kPa), we used a horizontal sample with a careful stress calculation along the sample axis, along which waves propagate. We determined the changes in wave velocity with stress (governed by a power law) and observed greater variability under 5 kPa, showing the limit of our transducers at low stresses. Based on elasticity theory, derived from wave velocity parameters governing sample deformations under strain changes. We suspect a slight decrease in the Poisson ratio (that characterizes radial elastic strain of a cylindrical sample under axial loading) below 5 kPa. Data is useful to better constrain the response of Mars sandy surface materials at the InSight site and other areas for future Mars missions. Key Points: Bender Element measurements of Vp and Vs carried out on a novel device on loose Fontainebleau sand samples used as a Mars regolith simulantInvestigation carried out within a so far unexplored low stress range, below 10 kPa and as low as 1.75 kPaPoisson ratio observed to remain mostly constant at low stress, with a possible decrease in the zone below 5 kPa [ABSTRACT FROM AUTHOR]

The ExoMars Trace Gas Orbiter instruments, Nadir and Occultation for Mars Discovery and Atmospheric Chemistry Suite have provided vertical profiles of H2O and HDO mixing ratios in the presence and absence of Global Dust Storm 2018. These observations are carried out in the lower atmosphere at SZA ∼ 71° of northern mid latitude (∼55°N, 130°E). Using NOMAD observations, we developed chemical models 1 and 2 for dust and non‐dust storm conditions, respectively. The production/loss rates of H2O, HDO and D2O and the densities of water, nitrogenated and deuterated cluster ions (H2O+, D2O+, HDO+, H+(H2O)n, D+(D2O)n, H+(HDO)n, CO4−, CO3−, CO3−(H2O)n, CO3−(D2O)n, CO3−(HDO)n, NO2−(H2O), NO2−(HDO), and NO2−(D2O) (for n = 1 to 5)) are estimated from these models between 10 and 70 km. We have used two ionization sources: (a) Galactic Cosmic Rays (GCR) and (b) hard X‐ray in both models. The D peak density produced by GCR is lower by a factor of ∼5 than that produced by the hard X‐rays. In model 1, the electron densities were increased by a factor of ∼2–3 than that produced by model 2. The estimated production/loss rates of H2O, HDO, and D2O are also larger by an order of magnitude in model 1 than that estimated by model 2. Plain Language Summary: The chemistry of the lower ionosphere of Mars is very complex because of the rapidly increasing number of ion species at larger masses. It couples ion‐neutral, electron‐neutral, dissociation of positive/negative ions, electron detachment, ion‐ion and ion‐electron recombination processes through various chemical reactions. In this paper we have performed ionospheric modeling and used Nadir and Occultation for Mars Discovery observations as inputs for the model. The chemistry of water, nitrogenated and deuterated cluster ions were studied. The production/loss of H2O, HDO, and D2O are also estimated during Global Dust Storm 2018. One‐dimensional chemical model 1 and 2 have been developed to estimate the densities of these cluster ions in the presence and absence of dust storm respectively. In this modeling, two ionization sources, (a) Galactic Cosmic Rays and (b) hard X‐ray, are used. In model 1 estimated production/loss rates of H2O, HDO, and D2O are increased by an order of magnitude than that estimated by model 2. The estimated densities are also larger in model 1 by a factor of 2–3 than that produced by model 2. There is no measurement of ion/electron density in the lower ionosphere of Mars. In the absence of measurements, predicted results improve our understanding of the ionospheric chemistry of Mars and its environment. Key Points: The densities of water, nitrogenated and deuterated cluster ions in the presence and absence of Global Dust Storm 2018 are estimatedThe Trace Gas Orbiter data (H2O and HDO) are used for the first time in the modeling of the lower ionosphere of MarsThe production and loss rates of H2O, HDO, and D2O are increased by one order of magnitude during the dust storm [ABSTRACT FROM AUTHOR]

The Mg‐spinel‐bearing rocks are an essential component of the lunar crust. They are widely distributed on the Moon's near and far sides. The study of petrogenesis provides a rare opportunity to understand the early lunar crust‐mantle interaction. Northwest Africa 12279 is a unique Mg‐spinel‐bearing troctolitic anorthosite lunar meteorite that is unbrecciated and pristine. The modal content of the spinel‐bearing troctolitic component is ∼95%. Five hundred sixty spinels larger than 100 μm are investigated in this paper, and 38 of them are found to contain magma melt inclusions. These magma melt inclusions represent the mixed composition between mantle melt and crust material during the crystallization of a spinel. They generally show an immiscible liquid phase separation, which can be divided into a magnesium‐rich and silica‐rich phase. The former is similar in chemical composition to the lunar mantle's peridotite or plutonic ultramafic Mg‐suite. The mantle melt reacts successively with Ferroan Anorthosites in the crust to crystallize Mg‐spinel, plagioclase, olivine, pyroxene, and silica phases. The large grains of euhedral Mg‐spinels with homogeneous chemical composition indicate they are the products of equilibrium crystallization, while plagioclase, olivine, pyroxene, and silica phase are the products of differentiated crystallization of mixed magma. This reaction may represent a typical process of lunar Mg‐suite. This work is the first to find petrological evidence that the melt from the lunar mantle intrudes into the crust and then undergoes a liquid phase immiscible separation to form the Mg‐spinel‐bearing troctolitic anorthosite. Plain Language Summary: Northwest Africa 12279 (weight 1,830 g) is the only known pristine and unbrecciated lunar spinel‐bearing troctolitic anorthosite, including lunar meteorites and returned samples. These pristine samples are indispensable for understanding Moon's history and chemical evolution. The spinel‐bearing rocks are an essential component of the lunar crust. They are widely distributed on the Moon's near and far sides. The study of petrogenesis provides a rare opportunity to understand the early lunar crust‐mantle interaction. Five hundred sixty spinels larger than 100 μm are investigated in three polished thin sections, and 38 contained magma melt inclusions. These magmatic inclusions represent the mixed composition between early mantle melt and crust material during the crystallization of a spinel. This work is the first to find evidence that the melt from the lunar mantle intrudes into the crust. The magnesium‐rich melt from the mantle assimilates with ferroan anorthosite (an igneous rock is composed of >90 vol.% plagioclase rich in calcium with minor iron‐rich pyroxene and olivine) from the early crust to form an Mg‐spinel‐bearing hypabyssal igneous rock. Key Points: Northwest Africa (NWA) 12279 is a unique lunar meteorite that is both unbrecciated and pristineThe magmatic melt inclusion in Mg‐spinel represents the mixed composition between lunar mantle melt and anorthositic crustThe formation of Mg‐spinel in NWA 12279 results from the interaction between the mantle material and the anorthositic crust of the Moon [ABSTRACT FROM AUTHOR]

We explore the role of various equations of state (EoS) in controlling the composition of the Moon formed by a giant impact (GI) using a density‐independent SPH code. A limitation in our previous model Hosono et al. (2019), https://doi.org/10.1038/s41561-019-0354-2 is improved by replacing the EoS of the solid from Tillotson EoS to M‐ANEOS, and we also explored two recently proposed EoSs by Stewart et al. (2020), https://doi.org/10.1063/12.0000946 and Wissing and Hobbs (2020a), https://doi.org/10.1051/0004-6361/201935814; Wissing and Hobbs (2020b), https://doi.org/10.1051/0004-6361/201936227. The goal is to investigate to what extent we can explain the observed composition of the Moon including the similarity in the isotopic composition and the dissimilarity in the FeO/(FeO + MgO) ratio as compared to that of Earth by the different types of EoS assuming the conventional collision conditions. We found that changing the EoS for solids from Tillotson to M‐ANEOS EoS resolves the issues of latent heat, but its effect on the composition of the disk is small compared to the influence of the hard‐sphere EoS of magma ocean in controlling the composition of the disk. Similarly, two recently proposed EoSs have small effects on the composition of the disk in comparison to the model where the hard‐sphere EoS is used for preexisting magma ocean. We attribute this difference to a fundamental difference in thermodynamic behavior of silicate melts captured by the hard‐sphere EoS and by newly proposed EoSs; in the hard‐sphere model of silicate melts, configurational entropy dominates in free energy, whereas in the newly proposed model, entropy is dominated by vibrational entropy similar to entropy of solids. Plain Language Summary: Explaining the observed close similarities in the isotopic compositions of the Moon and Earth is a big challenge in planetary science. Many models invoke unconventional collision conditions but their plausibility is questionable. In our recent paper, we explored an alternative model where we assume a conventional collision condition but use recently developed new types of equation of state for melts, assuming that the magma ocean (MO) covered the growing Earth on which a giant impact occurred. In the present paper, we extend our previous work in modifying the equation of state of solid and also tested two new types of equations of state. We conclude that the modification of the equation of state for solid does not make a large difference in the composition of the Moon and that the recently proposed new equations of state fail to explain the composition of the Moon. We conclude the magma ocean on the growing Earth played a major role to control the composition of the Moon similar to that of Earth, and for the magma ocean, an equation of state that captures the unique thermodynamic properties of silicate melts needs to be used. Key Points: We surveyed the effect of equation of state for Earth's mantle on mantle ejection by a canonical Moon‐forming impact modelThe use of modified hard‐sphere equations of state (EoS) for the magma ocean provides a reasonable explanation for the composition of the MoonThe EoS that treats the solid and liquid in a similar way cannot explain the composition of the Moon [ABSTRACT FROM AUTHOR]

Massive, kilometer thick deposits of carbon dioxide (CO2) ice have been detected at the south polar cap of Mars by radar investigations. These deposits are divided into several units that are separated by thin water ice bounding layers. Recent studies investigated the accumulation history of CO2 ice and found that the deposits most likely formed during several episodes in the past, when Martian obliquity was much lower than now. Those studies, while able to predict total volumes of CO2 ice consistent with those observed, did not attempt to explain the anomalous three‐dimensional distribution (thickness or extent) of CO2 ice or the ice's offset from the topographic high of the polar cap. In this paper we use a combination of feature analysis and numerical modeling to demonstrate that the CO2 deposits flow as glaciers and that glacial flow distributes the ice into its current position. Further, this distribution allows the ice to survive during high obliquity excursions. Plain Language Summary: Carbon dioxide (CO2) ice is found in a stack of deposits at Mars' south pole. These deposits are situated in basins, where they reach more than 1 km thick. Previous work suggested that the CO2 ice should be deposited when the axial tilt of the planet was lower, making the poles colder than they are now; however, the thickness and distribution of this ice should be much thinner than observed if only atmospheric effects are working on the ice. Therefore, the CO2 ice deposit distribution cannot be explained by atmospheric deposition alone. In this paper, we use glacial modeling and feature analysis to demonstrate that glacial flow better explains the distribution of ice in its present state. In addition, we show that the slopes on the south polar cap act to focus glacial flow into the basins, where it can survive warm periods by sublimating only the uppermost sections when the tilt of the planet is larger than present day. Key Points: Carbon dioxide ice has been mapped and modeled at the south pole of MarsDeposition models cannot explain the volumetric distribution of CO2 ice, thickness, or distributionWe present geomorphic and modeling evidence that the CO2 deposits flow as glaciers into basins to reach their observed volumetric distribution [ABSTRACT FROM AUTHOR]

China's first Mars rover, Zhurong, has carried out an exploration mission in southern Utopian Planitia. As rocks and their three‐dimensional (3D) morphology are essential for studying the geological characteristics of the Martian surface, this paper presents an approach for automatically extracting 3D rocks from stereo rover images. With our approach, 6,185 cm‐scale rocks are extracted using the Zhurong NaTeCam images, with the F1‐score achieving 91.86%. Approximate 3D morphological features are then calculated for each rock, and the statistical result shows that small rocks in centimeters make up the majority. Comparative morphological analyses of rocks in different areas show that landscape type affects the number of rocks but has no clear relationship to the proportions of rocks of different sizes. The outcrops of sedimentary rocks, the predominant type of rocks in the Zhurong landing area, and the float rocks are distributed differently regarding all morphological features, making the size‐frequency distribution of rocks around craters significantly different from other areas. Further comparative analysis with the Bonneville crater ejecta rocks extracted from the Spirit rover data is also conducted. Some similarities between the Zhurong outcrop rocks and the Bonneville crater ejecta rocks are found in 2D compactness. Meanwhile, the Bonneville crater ejecta rocks have a more scattered distribution in sphericity than both the float and outcrop rocks in the Zhurong landing area, indicating that the shapes of the Bonneville crater ejecta rocks may be more diverse. In addition, the sphericity is found to converge with height for rocks in both landing areas. Plain Language Summary: Rocks on the Martian surface and their three‐dimensional (3D) morphology record the geological evolution of Mars and its interaction history with outer space. Currently, China's Zhurong Mars rover has traveled nearly 2 km on Mars and collected a wealth of images, allowing the fine‐scale measurement and 3D morphological analysis of Martian rocks with statistical significance. We therefore propose an automatic approach for extracting individual 3D rocks from the stereo images and apply it to the Zhurong NaTeCam images to facilitate fine‐scale morphological analyses of the Martian rocks in the Zhurong landing area. Our morphological analyses lead to several remarkable findings about the interior relations between Martian rocks and geomorphology, giving new clues about the topographical and geomorphic evolution of Mars. Key Points: We extracted 6,185 three‐dimensional rocks at centimeter‐scale in the Zhurong landing area and computed their morphological featuresWe analyzed the morphology of the rocks and revealed the relationships between the rocks and landscape type of the Martian surfaceCompared to the rocks from the Spirit data, we found morphological similarity and difference for the outcrop and float rocks, respectively [ABSTRACT FROM AUTHOR]

Parameterised by the Love number k2 and the tidal quality factor Q, and inferred from lunar laser ranging (LLR), tidal dissipation in the Moon follows an unexpected frequency dependence often interpreted as evidence for a highly dissipative, melt‐bearing layer encompassing the core‐mantle boundary. Within this, more or less standard interpretation, the basal layer's viscosity is required to be of order 1015–1016 Pa s and its outer radius is predicted to extend to the zone of deep moonquakes. While the reconciliation of those predictions with the mechanical properties of rocks might be challenging, alternative lunar interior models without the basal layer are said to be unable to fit the frequency dependence of tidal Q. The purpose of our paper is to illustrate under what conditions the frequency‐dependence of lunar tidal Q can be interpreted without the need for deep‐seated partial melt. Devising a simplified lunar model, in which the mantle is described by the Sundberg‐Cooper rheology, we predict the relaxation strength and characteristic timescale of elastically accommodated grain boundary sliding in the mantle that would give rise to the desired frequency dependence. Along with developing this alternative model, we test the traditional model with a basal partial melt; and we show that the two models cannot be distinguished from each other by the available selenodetic measurements. Additional insight into the nature of lunar tidal dissipation can be gained either by measurements of higher‐degree Love numbers and quality factors or by farside lunar seismology. Plain Language Summary: As the Moon raises ocean tides on the Earth, the Earth itself gives rise to periodic deformation of the Moon. Precise measurements of lunar shape and motion can reveal those deformations and even relate them to our natural satellite's interior structure. In this work, we discuss two interpretations of those measurements. According to the first one, the lunar interior is hot and a small part of it might have melted, forming a thick layer of weak material buried more than 1,000 km deep under the lunar surface. According to the second one, there is no such layer, and the measured deformation can be explained by the behavior of solid rocks at relatively low temperatures. We show that the two possibilities cannot be distinguished from each other by the existing data. Key Points: A lunar mantle governed by the Andrade model fits selenodetic constraints only with a very weak frequency dependence of tidal dissipationWe seek the parameters of two more complex models that may explain the anomalous frequency dependence of tidal Q measured by lunar laser rangingBoth a dissipative basal layer and elastically accommodated grain‐boundary sliding in the deep mantle can result in the same tidal response [ABSTRACT FROM AUTHOR]

Almost three decades ago, the problem of long‐term polar wander on a dynamic planet was formulated and simplified within the framework of normal mode theory. The underlying simplifications have been debated ever since, recently in a series of papers by Hu et al. (2017a, https://doi.org/10.1002/2016je005114, 2017b, https://doi.org/10.1002/2017je005365, 2019, https://doi.org/10.1016/j.icarus.2018.08.013), who clarify the role of neglecting short‐term relaxation modes of the body. However, the authors still do not solve the governing equations in full, because they make approximations to the Liouville equation (LE). In this paper, I use a time domain approach and, for previously studied test loads, both the planet's relaxation and the LE are solved in full. In order to analyze the existing LE approximations, I compute the energy balance of true polar wander (TPW). For fast rotating bodies such as Earth, the rotation axis becomes aligned with the main inertia axis (ω‖MIA) once free oscillations are damped. The ω‖MIA assumption is re‐derived theoretically–contrary to previous beliefs, I demonstrate that it is not necessarily linked to the quasi‐fluid limit of the viscoelastic response to loading and rotation, but that it is an expression of neglecting the Coriolis and Euler forces from the equation of motion. It is thus important to distinguish between simplifying the LE and simplifying the planet's response to forcing. For slow rotators such as Venus, the full LE together with energy analysis indicate that previous estimates of TPW rate need to be revisited. The numerical code LIOUSHELL is released on GitHub. Plain Language Summary: The rotational poles of Earth (the North and South poles) are slowly moving with respect to Earth's surface. This is because large mass anomalies such as cold subducting slabs or big mountains tend to go away from the rotation axis and, conversely, large negative anomalies such as surface depressions tend to go toward the rotation axis. This phenomenon is referred to as true polar wander (TPW), and it takes place also on other planets and moons. As a result of TPW, various prominent craters or mountains have different latitudes now than they had at the time of their formation. In this paper I present a non‐approximative method to compute the TPW and, for fast rotating bodies such as Earth, I show that a very simple equation can be used for accurate predictions. For slowly rotating planets such as Venus, the present method indicates that previously derived estimates of the TPW rate need to be revisited. The code used for the calculations is standalone, has a simple interface, and is made freely available. Key Points: Quasi‐fluid approximation of the Liouville equation accurately predicts the reorientation of fast rotating planetsTrue polar wander rates of slowly rotating planets such as Venus need to be revisitedA custom written code LIOUSHELL, designed for computing true polar wander of planets with arbitrary rotation, is made freely available [ABSTRACT FROM AUTHOR]

Magma ocean crystallization models that track fO2 evolution can reproduce the D/H ratios of both the Earth and Mars without the need for exogenous processes. Fractional crystallization leads to compositional evolution of the bulk oxide components. Recent work suggests that metal‐saturated magma oceans may contain near‐present‐day Fe3+ concentrations. We model the fractional crystallization of Earth and Mars, including Fe2+ and Fe3+ as separate components. We calculate Fe3+ partition coefficients for lower mantle minerals and compare the results of fractional crystallization for both Earth and Mars. We calculate oxygen fugacity (fO2) at the surface as the systems evolve and compare them to constraints on the fO2 of the last magma ocean atmosphere from D/H ratios, both with and without metal saturation. For Earth, we find that Fe3+ likely behaves incompatibly in the lower mantle in order to match the D/H constraint for whole mantle models, but shallow magma ocean models also provide reasonable matches. Disproportionation in whole mantle magma oceans likely overpredicts the amount of Fe3+ and metal that form or require subsequent reduction to return to present‐day values. For Mars, we cannot match the D/H constraints on last fO2 unless the magma ocean begins with <50% of the predicted Fe3+, but better match the present day mantle redox. We show that Fe3+ partitioning has a measurable effect on magma ocean redox, and that it evolves throughout the magma ocean's lifetime. We highlight the need for additional experimental constraints on ferric iron mineral/melt partitioning and more thermodynamic data for the Fe‐disproportionation reaction. Plain Language Summary: During the formation of planets such as Earth and Mars, many undergo a stage in which their rocky mantles are almost fully molten, called a magma ocean. The crystallization of different minerals as the magma ocean cools leads to changes in the composition of the melt. The melt maintains contact with the atmosphere throughout this stage, so changes in the melt's composition, especially the availability of oxygen, can lead to changes in the atmosphere's composition. In this paper, we use a model that looks at how the melt's composition changes, with a special focus on how different types (or species) of iron (Fe2+, Fe3+) behave as minerals form. We tested different depths of the magma oceans and initial compositions. For Earth, shallow magma ocean models produce results that agree with information that we have about the composition of the last atmosphere in contact with the magma ocean better than whole mantle models. For Mars, all of the models produce similar results but predict too much Fe3+ formation to be successful in recreating the last atmosphere, but match present‐day Mars relatively well. Key Points: We model mineral/melt partitioning during magma ocean crystallization using new Fe3+ partition coefficients for lower mantle mineralsWe calculate oxygen fugacity (fO2) of outgassing at the surface for Earth and Mars magma oceans and match to constraints from planetary D/HFor Earth, fractionated shallow magma ocean models (<1,000 km) match constraints, but Mars must start Fe3+‐depleted to match constraints [ABSTRACT FROM AUTHOR]

On Mars, the lack of either plate tectonics or a prominent erosional hydrological cycle since the Noachian means geological changes caused by asteroid and comet impact events have been preserved. On Earth, surviving impact‐induced fractures are localized to the relatively few preserved craters on the planet. We estimate that the shell of impact‐fractured rock on Mars (the "impact‐sphere") could provide between 9,200 times the surface area of a Mars radius sphere and up to 100 times this value, depending on the assumptions made, as potential microbially accessible space. Although >93% of craters we consider are smaller than 10 km in diameter, they contribute only about 5% of the total fracture surface area generated by all craters, making complex craters the dominant process for potential habitat formation. Microbiological data from terrestrial impact craters suggest that these fractures could have significantly enhanced local habitability by providing pathways for fluid flow, and thus nutrients and energy. However, unlike on Earth, the geological history of Mars means that pervasive impact fractures may also have provided pathways for Hesperian and Amazonian brines to infiltrate the subsurface and locally reduce habitability. Combining the fracture data with previous microbiological observations provides testable hypotheses for Martian drilling missions. Plain Language Summary: The Martian surface and subsurface is pervasively fractured by asteroid and comet impacts, largely because, compared to Earth, they have not been subducted by plate tectonics or eroded by a vigorous water cycle. In this paper, we estimate the surface area of fractures generated by these impacts. We find that these impact fractures could be 9,200 times the equivalent surface area of a flat Mars‐sized sphere and up to a hundred times this, depending on the assumptions made. On the one hand, these fractures could have provided surfaces for life and improved the flow of nutrients, on the other hand they could have channeled deleterious salty waters into the subsurface, suggesting that habitability of the subsurface is profoundly influenced by impact, but its positive or negative effects depend on the history of water flow and brine formation. This study leads to several testable hypotheses about the subsurface habitability of Mars. Key Points: Impact have fractured the deep subsurface of Mars pervasivelyWe estimate this fracture area to be between 9,200 times the surface area of a Mars radius sphere and up to 100 times this valueThis fracturing has likely profoundly influenced habitability [ABSTRACT FROM AUTHOR]

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. Key Points: Comparison of trace gas abundance and aerosol extinction between Atmospheric Chemistry Suite on ExoMars TGO and Mars Climate Sounder on Mars Reconnaissance OrbiterReveal the seasonal changes in hydrogen chloride on MarsShow that dust affects atmospheric temperature, which controls water vapor abundance and ice extinction, and thus impacts HCl abundance [ABSTRACT FROM AUTHOR]

Magnetic reconnection, an essential mechanism in plasma physics that changes magnetic topology and energizes charged particles, plays a vital role in the dynamic processes of the Jovian magnetosphere. The traditional Vasyliūnas cycle only considers the effect of magnetic reconnection at the nightside magnetodisk. Recently, magnetic reconnection has been identified at the dayside magnetodisk in Saturn's magnetosphere and can impact dayside auroral processes. In this study, we provide the first evidence that the dayside magnetodisk reconnection can also occur at Jupiter. Using data from the Galileo and Voyager 2 spacecraft, we have identified 18 dayside reconnection events with radial distances in the range of 30–60 Jupiter radii (RJ). We analyzed the particle (electron and ion) flux, energy spectra, and characteristic energy of these dayside events and compared them to the nightside events. The statistical results show that the energy spectra and characteristic energy of electrons/ions in dayside and nightside magnetic reconnection events are comparable. On average, the characteristic energy of ions on the dayside is higher than that on the nightside. Based on the limited data set, we speculate that the occurrence rate of dayside magnetodisk reconnection should be significant. The dayside Jovian magnetodisk reconnection seems to have a comparable effect on providing energetic particles as that at nightside and to be one of the key processes driving dynamics within the Jovian magnetosphere. Plain Language Summary: Jupiter has the strongest magnetic field among the planets in our solar system. Particles in the space around Jupiter are affected by its magnetic field. Changes in the strength and structure of the local magnetic field, known as magnetic reconnection, cause those particles to undergo rapid acceleration leading to their energization. In the past, magnetic reconnection was thought to be difficult to occur on Jupiter's dayside magnetodisk. In contrast, the recent observation of reconnection at Saturn's dayside magnetodisk strongly implied the possibility of a similar process at Jupiter. In this paper, taking advantage of the archived data from the Galileo and Voyager 2 spacecraft, we provide the first evidence that the dayside magnetodisk reconnection can occur at Jupiter and have an important effect on the energization of particles. Key Points: Dayside magnetodisk reconnection events at 30–60 RJ are identified in the Jovian magnetosphere for the first time18 dayside events are analyzed to compare with nightside events regarding particle flux, energy spectra, and characteristic energyMagnetodisk reconnection at the dayside is as effective as that at the nightside in providing energetic particles to the magnetosphere [ABSTRACT FROM AUTHOR]

This paper introduces the new Venus global ionosphere‐thermosphere model (V‐GITM) which incorporates the terrestrial GITM framework with Venus‐specific parameters, ion‐neutral chemistry, and radiative processes in order to simulate some of the observable features regarding the temperatures, composition, and dynamical structure of the Venus atmosphere from 70 to 170 km. Atmospheric processes are included based upon formulations used in previous Venus GCMs, several augmentations exist, such as improved horizontal and vertical momentum equations and tracking exothermic chemistry. Explicitly solving the momentum equations allows for the exploration of its dynamical effects on the day‐night structure. In addition, V‐GITM's use of exothermic chemistry instead of a strong heating efficiency accounts for the heating due to the solar EUV while producing comparable temperatures to empirical models. V‐GITM neutral temperatures and neutral‐ion densities are compared to upper atmosphere measurements obtained from Pioneer Venus and Venus Express. V‐GITM demonstrates asymmetric horizontal wind velocities through the cloud tops to the middle thermosphere and explains the mechanisms for sustaining the wind structure. In addition, V‐GITM produces reasonable dayside ion densities and shows that the neutral winds can carry the ions to the nightside via an experiment advecting O2+ ${\mathrm{O}}_{2}^{+}$. Plain Language Summary: We present a new, state‐of‐the‐art Venus global circulation model, the Venus global ionosphere thermosphere model. The new model will be useful for answering unknown questions about Venus' atmosphere, the physics of CO2 rich planets, and Venus missions utilizing the aerobraking maneuver. We performed many model simulations and compared the results to some of the existing Venus data sets to assess the accuracy of the model. The results showed that the dayside extreme ultraviolet heating can be captured using the dominant ion‐neutral chemistry rather than relying on simplified legacy methods. Furthermore, the cloud layer below the thermosphere has a unique wind pattern and its impact on the thermospheric temperatures and winds were explored. This work also revealed that the thin nightside ionosphere forms as a result of the transport of a single ion species. Key Points: A new, non‐hydrostatic Venus ionosphere‐thermosphere model is introduced with new physics not previously included in Venus GCMsSimulations during solar minimum conditions are used for data‐model comparisons of the temperatures, plasma, and neutral densitiesThe influence of the retrograde superrotating zonal flow is explored in relation to how it affects the neutral temperature and velocities [ABSTRACT FROM AUTHOR]

The Mars 2020 Perseverance rover landed in Jezero crater on 18 February 2021. After a 100‐sol period of commissioning and the Ingenuity Helicopter technology demonstration, Perseverance began its first science campaign to explore the enigmatic Jezero crater floor, whose igneous or sedimentary origins have been much debated in the scientific community. This paper describes the campaign plan developed to explore the crater floor's Máaz and Séítah formations and summarizes the results of the campaign between sols 100–379. By the end of the campaign, Perseverance had traversed more than 5 km, created seven abrasion patches, and sealed nine samples and a witness tube. Analysis of remote and proximity science observations show that the Máaz and Séítah formations are igneous in origin and composed of five and two geologic members, respectively. The Séítah formation represents the olivine‐rich cumulate formed from differentiation of a slowly cooling melt or magma body, and the Máaz formation likely represents a separate series of lava flows emplaced after Séítah. The Máaz and Séítah rocks also preserve evidence of multiple episodes of aqueous alteration in secondary minerals like carbonate, Fe/Mg phyllosilicates, sulfates, and perchlorate, and surficial coatings. Post‐emplacement processes tilted the rocks near the Máaz‐Séítah contact and substantial erosion modified the crater floor rocks to their present‐day expressions. Results from this crater floor campaign, including those obtained upon return of the collected samples, will help to build the geologic history of events that occurred in Jezero crater and provide time constraints on the formation of the Jezero delta. Plain Language Summary: The Mars 2020 Perseverance rover, along with the Ingenuity Helicopter technology demonstration, landed in Jezero crater, Mars on 18 February 2021. Here, we detail results from the first science campaign of the mission, the purpose of which was to explore the enigmatic Jezero crater floor. By the end of the campaign, Perseverance traversed more than 5 km, created seven abrasion patches, and sealed a total of nine samples and a witness tube for return to Earth. Analysis of the rocks in the crater floor revealed two distinct geologic formations, named the Máaz and Séítah formations. Both formations were determined to be igneous in origin, the former likely from a series of lava flows and the latter formed from a slowly cooling melt or magma body. The composition of the Máaz and Séítah formation rocks also indicate that they experienced significant alteration from water in the past, consistent with Jezero crater having once been filled with water. Results from Perseverance's Jezero crater floor campaign, including those obtained upon return of the collected samples to Earth, will help build the geologic history of Jezero crater and reveal its past habitability. Key Points: The Máaz and Séítah formations are igneous, with Séítah representing olivine‐rich cumulates and Máaz representing separate lava flowsCrater floor rocks have been variably altered to produce carbonates and other alteration minerals during multiple aqueous episodesPost‐emplacement processes tilted these rocks near the Máaz‐Séítah contact, and erosion produced varied textures and olivine‐rich regolith [ABSTRACT FROM AUTHOR]

Planetary waves play an essential role in the momentum budget of Venus's atmosphere superrotation. Using the Longwave Infrared Camera (LIR) observations of the Akatsuki and their cloud‐tracking wind fields, we study the characteristics of planetary waves and their long‐term variations. The dominant planetary wave is a wavenumber 1 wave with a period of 5–6 days (Quasi‐Five‐Day wave or QFDW) and is symmetrically distributed in the midlatitudes of both hemispheres. The wave phase speed is close to the background zonal wind, indicating the possible existence of a critical level near the LIR emission altitude of 65 km. The QFDW tends to appear at ∼8:00 a.m. local time, suggesting a modulation by the thermal tide. The QFDW amplitude is correlated with the vertical shear of the background zonal wind modulated by the thermal tide. This indicates some nonlinear interaction between the thermal tide and the QFDW. Plain Language Summary: Venus has a mysterious phenomenon in its atmosphere called "superrotation," which manifests as an atmospheric zonal wind speed of ∼100 m/s at the cloud top (∼70 km). Some studies suggest that planetary waves may be a contributing mechanism to their momentum budgets, but the characteristics of these waves remain unknown, or there is no clear observational evidence. In this paper, we used the Akatsuki mission data to study the long‐term variations of planetary waves in the upper layer of Venus' atmospheric clouds (∼65 km). This planetary wave is characterized by 5–6 day periods with a wavenumber of 1, symmetrically distributed across the midlatitudes of both hemispheres. The amplitude of a 5–6 days planetary wave typically peaks around 8 a.m. The 5–6 days wave amplitude has the local time dependence, likely suggesting some nonlinear interaction between the thermal tide and the planetary wave. Key Points: Akatsuki/LIR measurements indicate the possible presence of critical levels at ∼65 km altitude for the quasi‐five‐day wave (QFDW)The QFDW tends to appear at ∼8:00 a.m. local time, suggesting a modulation by the thermal tidesThe QFDW amplitude is correlated with the vertical shear of the background zonal wind modulated by the thermal tide [ABSTRACT FROM AUTHOR]