Bright-toned aeolian bedforms are abundant in Oxia Planum, the future landing site of ESA’s ExoMars rover mission [1-3]. Their NE-SW orientation differs from other aeolian landforms in the area, such as the E-W oriented ridges carved in the bedrock (periodic bedrock ridges – PBRs [4, 5]), suggesting major changes in wind and climatic conditions [2, 3]. At Oxia Planum, bedforms formative winds have been interpreted as blowing from the NW to the SE based on the difference between dark stoss slopes and bright lee slope albedo, with darker surfaces interpreted as coarse grained materials and brighter surfaces interpreted as fine grained material, a relationship recognized in terrestrial megaripples observed on the Argentinian Puna Plateau [2]. In another interpretation [3], bedform formative winds were interpreted as coming from the SE, as evidenced by the presence of regularly spaced low albedo bands found on bedforms SE slopes and interpreted as exposed cross-beds at their windward sides. The same interpretation was given by other authors for similar bandings found on bright bedform slopes in other areas of Mars [6]. Here we propose an alternative explanation for these bands, which we interpret as potential “sorting streaks”, analogous to what is observed over dunes in Great Sand Dunes National Park (CO, USA). The morphology of some crescent-shaped examples visible in the study area, with their tips pointing to the SE, confirms a formative wind from the NE. This scenario implies a complex wind regime where bright bedforms were first formed by winds blowing from the NE [2], and subsequently shaped by winds coming from the ESE (assuming that an oblique/parallel wind direction is necessary to deposit darker material in bands over the SE slopes). The presence of dark wind streaks pointing WSW supports this scenario. We also report the presence of similar regular bands on bright bedform slopes at the Zhurong rover landing site in Utopia Planitia. Due to the widespread nature of these banded landforms [6-8], this new interpretation might help to interpret paleo-wind conditions on Mars. References [1] Balme et al. 2017, Geomorphology, 101(4), 703–720. [2] Favaro et al. 2021, JGR, 126, e2020JE006723. [3] Silvestro et al. 2021, GRL, 48, e2020GL091651. [4] Montgomery et al. 2012, JGR, 117, E03005. [5] Hugenholtz et al. 2015, Aeolian Research, 18, 135–144 [6] Day 2021, Geology, 49 (12): 1527–1530. [7] Gou et al. 2022, EPSL. [8] Bourke & Viles, 2016, GRL, 43, 12,356–12,362.

Global circulation models (GCMs) can be used to assess sediment transport pathways on Mars surface (e.g. Rubanenko et al., 2023). This requires the knowledge of an effective shear stress threshold that, when used in conjunction with the GCM outputs, allows the prediction of potential sand fluxes. Flux seasonal variations in Nili Patera were estimated from large ripples’ displacements, allowing Ayoub et al. (2014) to calibrate an effective GCM shear stress threshold. Yet, fluxes derived from ripple migration are not representative of bulk sedimentary fluxes, which can be inferred for instance from dune’s slip face advancements (Chojnacki et al., 2021). In addition, the seasonal impact of ice/frost in the mobility of sand on Mars, which is particularly relevant for polar regions that host the majority of dune fields, remains to be studied. We will report on an ongoing effort that seeks to use dune fluxes derived from long-term HiRISE observations of different sites and a GCM to 1) improve the calibration of a GCM effective shear stress threshold, and 2) estimate an ice thickness threshold needed to prevent sand motion on Mars. ReferencesAyoub, F., Avouac, J.-P., Newman, C. E., Richardson, M. I., Lucas, A., Leprince, S., & Bridges, N. T. (2014). Threshold for sand mobility on Mars calibrated from seasonal variations of sand flux. Nature Communications, 5, 5096. https://doi.org/10.1038/ncomms6096Chojnacki, M., Vaz, D. A., Silvestro, S., & Silva, D. C. A. (2021). Widespread Megaripple Activity Across the North Polar Ergs of Mars. Journal of Geophysical Research: Planets, 1–19. https://doi.org/10.1029/2021je006970Rubanenko, L., Gunn, A., Pérez-López, S., Fenton, L. K., Ewing, R. C., Soto, A., & Lapôtre, M. G. A. (2023). Global Surface Winds and Aeolian Sediment Pathways on Mars from the Morphology of Barchan Dunes. 1–12. https://doi.org/10.1029/2022GL102

We present a systematic assessment of the complex aeolian bedforms across the western rim of Hellas Planitia, the largest impact basin on Mars. Repeat orbital imaging shows crescentic, transverse, and star dunes across the Hellespontus Montes region migrate, converge, and occasionally reverse course. Dune morphology and sand ripple migration identify three effective wind regimes that vary spatially and temporally, correlating with atmospheric modeling. Peak sand transport occurs during southern spring and summer to the west and southwest, driven by daytime anabatic slope winds originating from Hellas. Migration rates decline with waning temperatures, when eastward drainage winds into Hellas dominate nights annually and during the colder seasons. However, these diurnally and seasonally cyclic slope winds can be highly localized, as nearby barchan fields swiftly migrate exclusively west where eastward winds are mitigated by topography. These novel findings highlight the complexity of surface‐atmosphere interactions in shaping aeolian systems on Mars.

Orbital data from the Messenger spacecraft (1) reveal that part of the Mercury surface is covered by smooth plains, which are interpreted to be flood volcanic material across the planetary surface (2). In this work, we present a detailed geo-structural map of the northern smooth plains between latitudes 29°N and 65°N. Our 1:100.000-scale map is obtained semi-automatically, using an algorithm to map all scarps from a DEM (3,4) followed by visual inspection and classification in ArcGIS. We created a DEM using the raw MLA (Mercury Laser Altimeter) data (1) ,with 500 m/pix, and we used the Mercury Messenger MDIS (Mercury Dual Imaging System) (1,2) base map with 166m per pixel for the classification stage. With this approach, we mapped and characterized 51664 features on Mercury, creating a database with several morphometric attributes (e.g. length, azimuth, scarp height) which we will use to study the tectonic evolution of the smooth plains. In this way, we classified wrinkle ridges’s scarps, ghost craters, rim craters and central peaks. The morphometric parameters of the wrinkle ridges will be quantitatively analyzed, in order to characterizer the possible tectonic process that could have formed them.This map can be considered an enhancement for the north pole of the global geological map of Mercury (1, 5). ReferencesHawkins, S. E., III, et al. (2007), The Mercury Dual Imaging System on the MESSENGER spacecraft, Space Sci. Rev., 131, 247–338.. Denevi, B. W., et al. (2013), The distribution and origin of smooth plains on Mercury, J. Geophys. Res. Planets, 118, 891–907, doi:10.1002/jgre.20075. Alegre Vaz, D. (2011). Analysis of a Thaumasia Planum rift through automatic mapping and strain characterization of normal faults. Planetary and Space Science, 59(11-12), 1210–1221. doi:10.1016/j.pss.2010.07.008 . Vaz, D. A., Spagnuolo, M. G., & Silvestro, S. (2014). Morphometric and geometric characterization of normal faults on Mars. Earth and Planetary Science Letters, 401, 83–94. doi:10.1016/j.epsl.2014.05.022. Kinczyk, M. J., Prockter, L., Byrne, P., Denevi, B., Buczkowski, D., Ostrach, L., & Miller, E. (2019, September). The First Global Geological Map of Mercury. In EPSC-DPS Joint Meeting 2019 (Vol. 2019, pp. EPSC-DPS2019).

The north polar region of Mars, with its greater atmospheric pressure and vast inventory of ever-changing volatiles (e.g., CO2, H2O), hosts arguably the most active and diverse aeolian bedform systems on the planet. Here, we explore how these dune fields evolve spatiotemporally using up to 8 Mars years (16 Earth years) of MRO HiRISE observations to test the impact of various boundary conditions on annual mobility. A high degree of sand flux heterogeneity was observed for some dunes, whereas other sites displayed steady-state migration relative to long-term rates. These large changes in annual migration are attributed to the variable length of the frost-free seasons, sediment availability in relation to the timing of peak katabatic winds, and the influence of global dust storms on seasonal ice thickness. Consistent with our previous work, we continue to observe extremely high transport rates at Olympia Cavi. All stages of aeolian system evolution are observable (sand patch > protodune > dune), along with additional phenomena not previously observed outside of terrestrial settings (e.g., dune calving and collisions, remote transfer). Transitory protodunes may evolve from modest sand mounds to prominent barchans with slipfaces several meters tall within 3–5 Mars years, while adjacent duneforms may suffer slipfaces collapse as they lose sand supply. These Martian protodunes, which appear to be larger than terrestrial equivalents, and mature dunes found downwind are among the swiftest yet reported on Mars. These rapidly evolving cryo-aeolian systems provide a window into longer-term landscape evolution of non-polar dune fields.

Orbital data from the MESSENGER spacecraft show that a significant portion of Mercury's northern hemisphere is covered by smooth plains, which are interpreted to be flood volcanic material and/or impact melt. The smooth plains show pervasive tectonic structures and encompass a broad raised bulge of uncertain geophysical interpretation. In this work, we focus on the mapping of all the morphostructures within the northern smooth plains, aiming at providing a useful dataset for further studies about the mapped area. The structural map is obtained through a twofold process: first with an automatic mapping, using an algorithm to identify all the lineaments from a DEM; and second with a visual inspection and classification of the results of the algorithm in a GIS environment. The final maps are drafted at two different scales, 1:300,000 and 1:600,000. With this approach, we mapped and characterized more than fifty thousand lines marking scarps on the surface, creating a database with several morphometric attributes for each of the identified scarps (e.g. length, azimuth, and height), which can be used for geostatistical study of smooth plains tectonics. Our structural map reveals that: (i) the area is broadly dominated by wrinkle ridges, ghost crater assemblages of lineaments, and scarps related to impact crater processes (e.g. radial faults, secondary crater chains, ejecta emplacement) and that (ii) the amount of strain was not evenly accommodated throughout the northern smooth plains.

Aeolian processes on Mars form a distinct class of meter-scale ripples, whose mechanisms of formation are debated. We present a global morphometric survey of bedforms on Mars, adding relevant observational constraints to the ongoing debate. We show that the bedforms located in the Tharsis region form a distinct group, not akin to the large dark-toned ripples which cover dune fields elsewhere on the planet. The relation between wavelength and atmospheric density derived from the new data is consistent with the predictions of a wind-drag mechanism, favoring the model that uses a saltation saturation length. Regardless of the mechanism that limits the size of bedforms, these results confirm the existence of a robust relationship between the wavelength of large ripples and atmospheric density (ripples spacings increases with decreasing atmospheric density). This provides further support to the interpretation of paleoatmospheric conditions on Mars through the analysis of its aeolian sedimentary record.

Wind-formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment [1-4]. Here we show that the landing site experienced multiple climatic changes recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs) [5, 6]. Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Superposition relationships of the PBRs with a dark-toned geological unit [4] indicate that such a change in the main wind condition likely occurred during the Amazonian. Active bedform migration from nearby craters (McLaughlin and Oyama) show winds coming from the North, matching the orientation of the wind streaks visible in the putative landing ellipse. Our results provide constrains on the wind regime in Oxia Planum and offer indications on present and past winds that will be crucial for understanding the landing site geology.For full details, see [1].[1] Silvestro, S. et al. 2021. Periodic Bedrock Ridges at the ExoMars 2022 landing site: Evidence for a Changing Wind Regime. GRL, 48, 4.[2] Favaro, E. et al. 2021. The Aeolian Environment of the Landing Site for the ExoMars Rosalind Franklin Rover in Oxia Planum, Mars. JGR, 126, 4.[3] Balme, M. et al. 2017. Surface-based 3D measurements of small aeolian bedforms on Mars and implications for estimating ExoMars rover traversability hazards. PSS, 153, 39-53.[4] Quantin, C. et al. Oxia Planum: The Landing Site for the ExoMars ‘‘Rosalind Franklin’’ Rover Mission: Geological Context and Prelanding Interpretation. Astrobiology, 21, 3.[5] Montgomery, D. R. et al. 2012. Periodic bedrock ridges on Mars. JGR, 117, E03005.[6] Hugenholtz, C. H. et al. 2015. Formation of periodic bedrock ridges on Earth. Aeolian Research, 18, 135–144.

Morphometric database of martian meter-scale eolian bedforms. For details refer to: David A. Vaz, Simone Silvestro, Matthew Chojnacki, David C. A. Silva (2022?). Constraining the mode of eolian transport on Mars through a global morphometric survey of bedforms. Submitted to Geology (9/7/2022).

Martian deltaic deposits are good indicators of water distribution and provide insight into the climate evolution of Mars [1]. A notable case is the fan deposit located in Jezero crater, currently being studied by the Perseverance rover. Hydrated silica-bearing deposits were identified in this area [2, 3, 4], and the overall stratigraphy and morphology point to an evolution passing through a progradation and a transgression phase, to an erosion episode [5]. All these evidences suggest that this deposit was formed by fluvio-deltaic activity in an ancient lake basin [6] during the late Noachian or early Hesperian epochs [7]. After its formation, the deposit likely suffered a complex and profound exhumation [8] which we will try to constrain through a mass balance analysis. Here, we present the first results of a project seeking to understand the depositional and post-depositional evolution of the Jezero delta, including the study of its hydrographic basin [9]. We will focus on a mass-balance survey, analyzing and comparing the volume of sediments that were eroded from the basin and deposited into the crater. We integrate HRSC, CTX and HiRISE data to create a regional DTM and orthophoto map covering the Jezero crater and its drainage basin. These datasets enable us to estimate the volume of sediments eroded from the drainage basin, as well as the present-day volume of the fan deposit. Finally, we will apply the mass balance model introduced by [10], aiming at evaluating putative sediment offshore loss and post-depositional fan erosion. This will allow to infer the initial volume of the deltaic deposit and test different lake water levels. [1] Di Achille, G. and Hynek, B. M. (2010). Nature Geoscience, Vol. 3 (7),459-463. [2] Pan, L., et al, (2021). Planet. Sci. J., 2(2), 65., [3] Horgan, B., et al, (2020). Icarus, 339. [4] Mangold, N., (2007). J. Geophys. Res. 112(8). [5] Goudge, T. A., et al. (2018). Icarus, 301 58–75. [6] Fassett, C. I., & Head, J. W. (2005). Geophys. Res. Let., 32(14), 1–5., [7] Mangold, N., et al, (2021). Science 10.1126, [8] Quantin-Nataf, C., et al (2021). 52nd LPSC, [9] Goudge, T. A., et al, (2015). J. Geophys. Res. Planets 120., [10] Vaz, D. A., et al, (2020). Earth Planet. Sci. Lett., Vol. 533.

Wind-formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment [1-4]. Here we show that the landing site experienced multiple climatic changes recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs) [5, 6]. Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Superposition relationships of the PBRs with a dark-toned geological unit [4] indicate that such a change in the main wind condition likely occurred during the Amazonian. Active bedform migration from nearby craters (McLaughlin and Oyama) show winds coming from the North, matching the orientation of the wind streaks visible in the putative landing ellipse. Our results provide constrains on the wind regime in Oxia Planum and offer indications on present and past winds that will be crucial for understanding the landing site geology. For full details, see [1]. [1] Silvestro, S. et al. 2021. Periodic Bedrock Ridges at the ExoMars 2022 landing site: Evidence for a Changing Wind Regime. GRL, 48, 4. [2] Favaro, E. et al. 2021. The Aeolian Environment of the Landing Site for the ExoMars Rosalind Franklin Rover in Oxia Planum, Mars. JGR, 126, 4. [3] Balme, M. et al. 2017. Surface-based 3D measurements of small aeolian bedforms on Mars and implications for estimating ExoMars rover traversability hazards. PSS, 153, 39-53. [4] Quantin, C. et al. Oxia Planum: The Landing Site for the ExoMars ‘‘Rosalind Franklin’’ Rover Mission: Geological Context and Prelanding Interpretation. Astrobiology, 21, 3. [5] Montgomery, D. R. et al. 2012. Periodic bedrock ridges on Mars. JGR, 117, E03005. [6] Hugenholtz, C. H. et al. 2015. Formation of periodic bedrock ridges on Earth. Aeolian Research, 18, 135–144.

The mechanism/s responsible for sediment entrainment by wind and bedform migration on Mars are a matter of debate [1]. Martian large ripples (LRs) migrate under present-day low pressure conditions and have been interpreted has fluid/wind drag ripples [2] or as bedforms formed by aeolian saltation [3]. An important constraint to this debate is the relation between bedform wavelength and atmospheric density (as a function of elevation). This dataset was later complemented by the measurement of bedform wavelengths in other 11 areas [2]. Lapotre el al. [2] proposed that the fluid drag theory fits the measured wavelength vs. atmospheric density relation, a view not shared by Lorenz [1, Fig. 2].To try to address this divergence, we will present a new method that allows the automatic mapping and morphometric characterization of bedforms (LRs to TARs) using HiRISE imagery. It consists in a windowed multiscale spectral approach, followed by a supervised classification stage using neural networks. This method can accurately identify the bedforms (overall accuracy of 94%) and provide precise wavelength measurements within a ±12% confidence interval. The surveyed bedforms have crests spaced between 1 and 100 m, and include large ripples, megaripples and TARs.We will review and compare previous datasets and studies with our measurements. The main objective is to re-evaluate how well the wind drag hypothesis can predict bedforms’ spacing on Mars, and for this purpose we employ an improved measurement approach that allows the mapping of entire dune fields. Furthermore, we significantly increased the number of mapped areas and extended the range of sampled elevations.Preliminary results of this ongoing effort will be presented at the conference.[1] Lorenz, R.D. (2020). Martian Ripples Making a Splash. J. Geophys. Res. Planets 125, 12–15.[2] Lapotre, M.G.A., Ewing, R.C., Lamb, M.P., Fischer, W.W., Grotzinger, J.P., Rubin, D.M., Lewis, K.W., Ballard, M.J., Day, M., Gupta, S., et al. (2016). Large wind ripples on Mars: A record of atmospheric evolution. Science (80). 353, 55–58.[3] Sullivan, R., Kok, J.F., Katra, I., and Yizhaq, H. (2020). A Broad Continuum of Aeolian Impact Ripple Morphologies on Mars is Enabled by Low Wind Dynamic Pressures. J. Geophys. Res. Planets 125, 1–39.[4] Lorenz, R.D., Bridges, N.T., Rosenthal, A.A., and Donkor, E. (2014). Elevation dependence of bedform wavelength on Tharsis Montes, Mars: Atmospheric density as a controlling parameter. Icarus 230, 77–80.

Introduction: Martian fan-shaped deposits have been the subject of study for their presumed deltaic origin, which is thought as a good indicator on how the atmospheric conditions and climate evolved on Mars [1-3]. Therefore, the study of these deposits constitutes one of the few ways to constrain the amount of water that existed on the surface, providing valuable indications regarding the paleoenvironmental conditions. However, these deposits present a large morphological diversity suggesting different depositional settings. In [4] is argued that two different types of depositional fans are present on the Martian surface. These two different sets of fans were identified by measuring and correlating the volume of sediments eroded from the valleys and deposited into the fanshaped deposits. Type I fans were linked with limited fluvial-deltaic activity, probably developed through alluvial processes, glacial processes, landslides or other gravity driven flows. In contrast, Type II fans have relatively less sediments accumulated near the valley mouths, suggesting basin dispersion of sediments probably linked with deltaic environments. In this abstract we discuss the improvements we are making to the existing valley-fan volumetric database [4] (https://ars.els-cdn.com/content/image/1s2.0-S0012821X19307447-mmc2.zip). Namely, we are improving the quality of the data used for areas already included in the database (replacing HRSC data with newly available CTX DTMs) and completing the database with new areas (Fig. 1). In addition, in order to better understand the sedimentary conditions in which these deposits were formed we discuss our plans to integrate in the database other sources of information, such as deposition ages estimated from crater counts and mineralogical information. With this effort we seek to improve the deposits’ classification and perform a more robust mass balance analysis in the future.

Introduction and motivation: Dune fields across Mars host a variety of aeolian bedform classes, which reflect differences in boundary conditions (e.g., grain size, wind energy, sediment supply) (1, 2). The smallest class observed from orbit (1-5 m spacing and ~40 cm tall) are dark-toned ripples (DTRs) found migrating atop dunes or within isolated patches (3–5). The larger (10100 m spacing and 1-14 m tall), light-toned Transverse Aeolian Ridges (TARs) can occur in association with dunes or as large TAR fields (6–8), but often lack unambiguous signs of activity. The size range in between these commonly cited bedform populations (5-20 m spacing, ~1-2 m tall) have been largely unexplored and generally assumed to be inactive like TARs (9). We term these intermediate-scale bedforms as “megaripples” based on their greater dimensions and brighter crests than DTRs, where we infer the latter is due to a coarser grain size component (10). Recent analysis using images acquired by the High Resolution Imaging Science Experiment (HiRISE) camera (0.25–1 m/pix) (11) have shown certain locations host migrating megaripples, which are most apparent among high flux ripples and dunes (2, 12). However, it is not clear how frequent this mobility is or even their broader occurrence. The goal of this project is to: 1) survey aeolian sites across Mars for the presence of intermediate-scale bedforms, 2) assess any migration or changes over multiple Mars years, and 3) quantify relative sand flux contributions of bedforms. The latter part of this abstract will focus on regional trends of the northern circum-polar ergs and any implications for aeolian science. Data sets and methods: Along with bedform morphology HiRISE stereo data were used to characterize bedform topography (1 m/pix). Orthoimages were used with COSI-Corr software to derive megaripple fluxes using the method of (12). Dune migration vectors were converted to flux vectors by multiplying the migration rates to the dune heights derived from DTMs – see the method of (13). Global morphologic survey results: We surveyed dune fields across all latitudes of Mars to assess the presence or absence of intermediate-scaled bedforms. TARs were designated for light-toned, transverse bedforms, which were interpreted as being stratigraphically below dark dunes and meter-scale ripples. In contrast, megaripples were noted to be present for typically smaller, variable-albedo bedforms which were stratigraphically above neighboring features. Of the 214 locations surveyed, 80.4% had megaripples, 49.5% had TARs, 41.1% had both, and 11.2% had neither class of intermediate-sized bedforms (Table 1). With minor exceptions (e.g., sand sheets) DTRs were found at most locations. Whereas the megaripple distribution is quite uniform across all latitudes, TARs are generally absent in both polar regions, similar to earlier reports (14). Likewise, the highest proportion of HiRISE images lacking either megaripples or TARs was in these high latitude areas. The remainder of this abstract will focus on these polar regions. Polar megaripple activity results: We examined various polar dune fields with HiRISE DTMs and longbaseline orthos (up to 6 Mars years) for evidence of megaripple migration (Fig. 1). Of the 14 aeolian system monitoring sites in the North Polar ergs: 9 had significant megaripple changes (75%), 2 had minor changes (17%), 1 have no megaripple changes (8%), while the remaining 2 had no megaripples present. In many cases, unambiguous megaripple migration is evident in

Dust devils have been proposed as a tool to investigate martian near-ground wind conditions. However, further studies are needed in order to fully understand how background atmospheric conditions affect dust devil forward motion. One of the main issues is related to the lack of synchronous acquisition of the dust devils forward speed and ambient wind measurements. This work aims to present an effective methodology to retrieve the dust devil translational velocity using the horizontal wind time series acquired by a single stationary anemometer, and utilize this method to deeply investigate its relation with the ambient velocity. For this purpose, we first tested the reliability of our method using the data acquired during an intensive weeklong dust devil survey, during which we deployed a meteorological station coupled with a camera in a Sahara desert site. After confirming the technique by comparing the results with the ones obtained with the camera, we applied the method to a meteorological data set of 338 dust devil events we acquired in a previous Saharan campaign. We studied the characteristics of the forward velocity, observing how it closely matches the ambient wind regime, with similar to 70% of the events lying in a range of 20 degrees and 1 m/s from the ambient velocity measured at 4.5 m. Our results indicate how the vortex forward speed follows a vertical profile similar to the boundary layer wind, confirming the effectiveness of the dust devils monitoring for the study of the surface winds.

The most expansive dune fields on Mars surround the northern polar cap where various aeolian bedform classes are modified by wind and ice. The morphology and dynamics of these ripples, intermediate-scale bedforms (termed megaripples and Transverse Aeolian Ridges [TARs]), and sand dunes reflect information regarding regional boundary conditions. We found that populations of polar megaripples and larger TARs are distinct in terms of their morphology, spatial distribution, and mobility. Whereas regionally restricted TARs appeared degraded and static in long-baseline observations, polar megaripples were not only widespread but migrating at relatively high rates (0.13 ± 0.03 m/Earth year) and possibly more active than other regions on Mars. This high level of activity is somewhat surprising since there is limited seasonality for aeolian transport due to surficial frost and ice during the latter half of the martian year. A comprehensive analysis of an Olympia Cavi dune field estimated that the advancement of megaripples, ripples, and dunes avalanches accounted for ~1%, ~10%, and ~100%, respectively, of the total aeolian system's sand fluxes. This included dark-toned ripples that migrated the average equivalent of 9.6 ± 6 m/yr over just 22 days in northern summer-unprecedented rates for Mars. While bedform transport rates are some of the highest yet reported on Mars, the sand flux contribution between the different bedforms does not substantially vary from equatorial sites with lower rates. Seasonal off-cap sublimation winds and summer-time polar storms are attributed as the cause for the elevated activity, rather than cryospheric processes.

Wind‐formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment. Studies of aeolian bedforms at the landing site were focused on assessing the risk for rover trafficability, however their potential in recording climatic fluctuations has not been explored. Here we show that the landing site experienced multiple climatic changes in the Amazonian, which are recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs). Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Our results provide constrains on PBR formation mechanisms and offer indications on paleo winds that will be crucial for understanding the landing site geology.

The ESA/ROSCOSMOS ExoMars 2022 will land in Oxia Planum an area that shows outcrops of clay-rich Noachian-aged phyllosilicates overlaid by an Early Amazonian volcanic dark resistant unit (Adru) [1]. Using HiRISE images, we identified NE-SW (53.9 ± 13.2°) oriented TARs overlying an enigmatic ~EW (95.4 ± 10°) oriented ridge pattern that we interpreted as periodic bedrock ridges (PBRs) [2]. Ridges (~ 38 m spaced) display Y-junctions, show cross-cutting fractures and share the same blocky texture of the bedrock they are associated with. Ridge crestlines are locally found in continuity outside and inside heavily eroded impact craters around the dark upstanding material (Adru) exposed in the center of many craters. These stratigraphic relationships suggest that the ridges (PBRs) formed after the event(s) that eroded the crater rims and thus after deposition of the Adru (2.6 Ga). Ridges are even visible in association with impact crater ejecta and are superimposed by 10-25 m craters and boulders, so they pre-date these impact events. When associated with crater ejecta, ridges locally show two different crests. Both crests are truncated by craters suggesting they were emplaced before the impacts. We interpret this double crest arrangement as megaripples detaching from PBRs. The ejecta deposited over the megaripple-PBRs favored the preservation of the megaripple crests from a subsequent episode/s of erosion that led to the complete exposure of the PBRs on the plain. Because the preserved megaripples are locally visible on the southern edges of the PBRs, the wind that formed the megaripple-PBR system should have blown from N-NNE because the megaripples are located at the downwind side of PBRs [3]. To better understand the relative age of the ridges, we mapped their occurrence on 316 craters in the study area that we qualitatively classified as relatively degraded/old and pristine/young. Results show that ridges are only found in degraded/old craters but are never found inside pristine/young craters. Thus, the ridge forming process was only active in-between the formation of degraded/old and pristine/young craters. A major change in the wind regime occurred during or after the event that exposed the PBRs: N-NNE winds that shaped the PBRs changed into dominant SE winds that led to the deposition of the TARs above the PBR/megaripples. This work unveils a complex history of aeolian erosion and deposition in Oxia Planum during the Amazonian. By visiting PBRs for the first time, the ExoMars 2022 mission will provide further constraints on PBR formation and paleo-winds, shedding light on a past Amazonian environment. This work is a summary of a manuscript that is currently in press on Geophysical Research Letters: Silvestro et al. 2021, Periodic Bedrock Ridges at the ExoMars 2022 Landing Site: Evidence for a Changing Wind Regime. DOI: 10.1029/2020GL091651. [1] Quantin-Nataf C. et al. (2021). Astrobiology, 21, N.3. [2] Silvestro S. et al. (2020). 6th Int. Planet. Dunes Work. 12-15 May, 2020. LPI No. 2188, id.3009. [3] Hugenholtz C. H. et al. (2015). Aeolian Res. 18, 135–144.

The ESA/ROSCOSMOS ExoMars 2022 will land in Oxia Planum an area that shows outcrops of clay-rich Noachian-aged phyllosilicates overlaid by an Early Amazonian volcanic dark resistant unit (Adru) [1]. Using HiRISE images, we identified NE-SW (53.9 ± 13.2°) oriented TARs overlying an enigmatic ~EW (95.4 ± 10°) oriented ridge pattern that we interpreted as periodic bedrock ridges (PBRs) [2]. Ridges (~ 38 m spaced) display Y-junctions, show cross-cutting fractures and share the same blocky texture of the bedrock they are associated with. Ridge crestlines are locally found in continuity outside and inside heavily eroded impact craters around the dark upstanding material (Adru) exposed in the center of many craters. These stratigraphic relationships suggest that the ridges (PBRs) formed after the event(s) that eroded the crater rims and thus after deposition of the Adru (2.6 Ga). Ridges are even visible in association with impact crater ejecta and are superimposed by 10-25 m craters and boulders, so they pre-date these impact events. When associated with crater ejecta, ridges locally show two different crests. Both crests are truncated by craters suggesting they were emplaced before the impacts. We interpret this double crest arrangement as megaripples detaching from PBRs. The ejecta deposited over the megaripple-PBRs favored the preservation of the megaripple crests from a subsequent episode/s of erosion that led to the complete exposure of the PBRs on the plain. Because the preserved megaripples are locally visible on the southern edges of the PBRs, the wind that formed the megaripple-PBR system should have blown from N-NNE because the megaripples are located at the downwind side of PBRs [3]. To better understand the relative age of the ridges, we mapped their occurrence on 316 craters in the study area that we qualitatively classified as relatively degraded/old and pristine/young. Results show that ridges are only found in degraded/old craters but are never found inside pristine/young craters. Thus, the ridge forming process was only active in-between the formation of degraded/old and pristine/young craters. A major change in the wind regime occurred during or after the event that exposed the PBRs: N-NNE winds that shaped the PBRs changed into dominant SE winds that led to the deposition of the TARs above the PBR/megaripples. This work unveils a complex history of aeolian erosion and deposition in Oxia Planum during the Amazonian. By visiting PBRs for the first time, the ExoMars 2022 mission will provide further constraints on PBR formation and paleo-winds, shedding light on a past Amazonian environment. This work is a summary of a manuscript that is currently in press on Geophysical Research Letters: Silvestro et al. 2021, Periodic Bedrock Ridges at the ExoMars 2022 Landing Site: Evidence for a Changing Wind Regime. DOI: 10.1029/2020GL091651. [1] Quantin-Nataf C. et al. (2021). Astrobiology, 21, N.3. [2] Silvestro S. et al. (2020). 6th Int. Planet. Dunes Work. 12-15 May, 2020. LPI No. 2188, id.3009. [3] Hugenholtz C. H. et al. (2015). Aeolian Res. 18, 135–144.