Kevin W. Lewis

The basal strata of the orbitally defined Layered Sulfate unit (LSu) in Gale crater, Mars, marks the end of a major mineralogical transition from clay‐to sulfate‐bearing stratigraphy. This has previously been attributed to a period of significant aridification of Gale's climate. Here, we present ground‐based observations by the Curiosity rover of the sedimentary facies and architecture of the basal strata of the LSu, and reconstruct the past depositional environments. The succession predominantly comprises four members of the Mirador formation: the Contigo, Catrimani, Amapari, and Chenapau members. The Contigo member records the migration of simple aeolian dunes. The Catrimani and Chenapau members record a facies characteristic of wind‐ripple migration in aeolian sand sheets. Periodic damp sand sheet deposition is inferred through candidate adhesion structures and evaporite crystals. Evidence for periodic surface water occurs as lacustrine deposits (the Amapari member) and an embedded fluvial lens. Near‐surface water likely restricted the availability of loose sediment, suppressing dune formation and promoting the accumulation of a thick sandsheet succession that, at ∼60 m, exceeds known terrestrial equivalents. Deposition under these conditions would have required a continuously rising water table, but the rate of sediment accumulation was likely high enough for a predominantly aeolian environment. Contrary to earlier aridification models for the LSu, this basal succession likely records an overall aeolian environment controlled by a rising water table in Gale crater during the Hesperian.

NASA's Curiosity rover is exploring a 5 km tall sedimentary mound that is hypothesized to record the transition from a warm and wet (phyllosilicate‐rich) to a cold and drier (sulfate‐rich) Mars. Evidence of magnesium sulfate‐bearing rock has shown that Curiosity has crossed through this phyllosilicate‐sulfate transition. Recently, Curiosity arrived at the Amapari Marker Band, a darker, indurated unit that can be traced laterally for tens of kilometers in orbiter images. Here, Curiosity found evidence for a very broad lake, and bedforms interpreted as wave‐ripple laminated sedimentary rock that likely was deposited in shallow water in the explored location, before becoming a deeper lake. These rocks are enriched in Fe, Mn, and Zn which has major implications for groundwater paleohydrology in Gale crater. Three formation hypotheses are considered: concretion formation during early diagenetic alteration of shallow lake sediments, laterization or leaching of the sediments, and addition of Fe, Mn, and Zn by a mildly acidic and reducing groundwater interacting with a redox and/or pH front in a stratified lake. The preferred interpretation of the metal enrichments within the Amapari Marker band sedimentary rocks is that they formed in a shallow water environment at a redox and/or pH front within the ripple unit, which drove precipitation and concentration of metals. If the enrichments are due to groundwater alteration, these processes could link subsurface and surface environments. Water and the presence of high amounts of redox sensitive elements and other metals are favorable indicators for habitability.

Key Points: The editors thank the 2025 peer reviewers

We needed to evaluate available orbital data of NASA’s Volatiles Investigating Polar Exploration Rover (VIPER) mission area in order to derive a variety of maps to help the science team identify scientifically interesting places for the rover to visit and to provide scientific context for our mission. Some of these maps also fulfilled engineering and mission design needs to enable safe and efficient landing and roving. We incorporated data from the Lunar Reconnaissance Orbiter Camera, the Lunar Orbital Laser Altimeter, the Mini-RF instrument, the Chandrayaan-2 Orbital High Resolution Camera, the Korean Pathfinder Lunar Orbiter’s Shadowcam, the Kaguya Spectral Profiler and Multiband Imager, and the Chandrayaan-1 Moon Mineralogy Mapper. We used a variety of techniques to build these maps, including stereogrammetry, shape-from-shading, ice stability depth and surface temperature calculations, and the horizon method for solar illumination and direct-to-Earth communications maps. Altogether, these maps allowed us to survey for boulders, evaluate features in permanently shadowed regions that VIPER might explore, provide mineralogic context for what VIPER’s instruments may learn, estimate the ages and radar properties of craters in the VIPER mission area, and evaluate the potential for gravity traverses with the rover. These data and techniques provided a rich set of information from which both the VIPER science team and engineering teams were able to draw in order to plan a safe landing and to plan a VIPER surface mission that will be both scientifically valuable and robust from an operational perspective.

Martian dust is mobilized throughout the year by local, regional, and global dust storms, influencing atmospheric opacity and inhibiting observations of the surface. Using Mars Hand Lens Imager (MAHLI) images and methods of Schmidt et al. (2018), https://doi.org/10.1029/2018je005553 , areal dust coverages on 368 near‐horizontal, undisturbed rock surfaces were estimated along six Mars Years of the Mars Science Laboratory (MSL) Curiosity rover's geologic traverse from mission sols 46–3,409, corresponding to Mars Year 31 (MY#; Clancy et al., 2000, https://doi.org/10.1029/1999JE001089 ), areocentric solar longitude (Ls) 175.9° through MY36, Ls 177.6°. Targets were evaluated for potential geospatial and seasonal (Ls) correlations. Dust coverages increased at each spring equinox (Ls = 0°) with the highest coverage (76.6%) recorded at the top of Vera Rubin Ridge (VRR) prior to the Mars Year 34 (MY34) global dust storm. Dust coverages annually decreased as prevailing wind strengths in the Gale crater increased during southern summer. Following the ascent of VRR, New Year maximums have decreased by approximately 15% annually (MY35 60.7%, and MY36 52.8%), suggesting that dust is less abundant at higher elevations on Mount Sharp, and/or that dust suspension or removal is enhanced at higher elevations by stronger winds at higher elevations. Two regions with relatively low dust coverages (<20%) were found in proximity to active aeolian sand dunes and are interpreted to result from saltating sand particles striking and lofting dust particles. This research represents the single longest recording of surface dust deposits to date for landed missions.

The Amapari Marker Band (AMB) is a layer within the Mount Sharp stratigraphy that has been mapped around the Gale crater in orbital images and was recently investigated up close by the Curiosity rover. Symmetric wave ripple marks within the AMB indicate a lacustrine depositional environment in the area investigated along the Curiosity traverse. The wavelength and morphology of the ripples constrain the water depth to a few meters or less. The lateral continuity of the ripple unit defines a minimum extent of the lake during ripple formation. The stratigraphy of the AMB is consistent with an environment of increasing water depth during sedimentation and the lateral correlation of the AMB stratigraphy suggests a transgressive depositional system building upon an eroded surface. The location of the AMB within the surrounding aeolian stratigraphy, coupled with the progression of depositional environments through the Mirador formation, records a pattern of a rising water table relative to sedimentation rates. The potential regional extent of the lacustrine environment, based on orbital mapping of the AMB's variable elevation, spans at minimum 2.0 km of the lateral AMB deposit in the area around Marker Band Valley and may have extended up to 14 km to the west across the northern Gale crater.

Mars' sedimentary rocks record Gyrs of environmental change. New data enable the first global analysis of paleo-environment relevant physical properties of these rocks, including layer thickness and accumulation rate. We find that layer thicknesses of post-3.5 Ga sedimentary rocks across the Martian surface show coherent variations at ~1000 km-scale that are inconsistent with simple volcanic and climatic hypotheses for formation, which are consistent with global compositional homogeneity at orbital scales. These data, in combination with new analyses of outcrop age and total rock volume demonstrate a global decrease in layer thickness that predates the eventual drop off in preserved sedimentary rock volume per Myr. The new constraints confirm a diachronous transition in Mars' global sedimentary rock record while also highlighting a regional dichotomy in young sedimentary rock deposits that has not been quantified before.

Various active and passive orbital measurements have provided evidence for surficial water ice within some lunar permanently shadowed regions (PSRs), especially from near-infrared observations by the M³ instrument. However, radar identification of lunar ice has so far remained ambiguous. Here, we examine the radar-inferred dielectric properties of lunar PSRs and illuminated craters to investigate the potential for ice. We show that the dielectric permittivity of proposed surficial ice-bearing PSRs is lower and has a different distribution than illuminated crater floors of the same diameter range. This difference is confirmed via polarimetric analysis. However, we find that regions with fewer or greater numbers of M³ detections do not have meaningfully different dielectric properties. The lack of correlation with M³ detections suggests the differences in radar properties are likely due to a smoother surface at the wavelength scale, perhaps as a consequence of the presence of deeper ice, as suggested by prior studies.

On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts in 2024. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 5,225 submissions in 2024, and 5,597 reviewers contributed to their evaluation by providing 9,697 reviews in total. We deeply appreciate their contributions. We would also like to acknowledge the passing of our beloved colleague, Harihar Rajaram. An AGU Fellow and longtime affiliate of AGU's Hydrology Section, Hari was the Editor‐in‐Chief of Geophysical Research Letters since 2019, a former editor on Water Resources Research, and served on the AGU Publications Committee.