Article

Modern Mars’ geomorphological activity, driven by wind, frost, and gravity

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Abstract

Extensive evidence of landform-scale martian geomorphic changes has been acquired in the last decade, and the number and range of examples of surface activity have increased as more high-resolution imagery has been acquired. Within the present-day Mars climate, wind and frost/ice are the dominant drivers, resulting in large avalanches of material down icy, rocky, or sandy slopes; sediment transport leading to many scales of aeolian bedforms and erosion; pits of various forms and patterned ground; and substrate material carved out from under subliming ice slabs. Due to the ability to collect correlated observations of surface activity and new landforms with relevant environmental conditions with spacecraft on or around Mars, studies of martian geomorphologic activity are uniquely positioned to directly test surface-atmosphere interaction and landform formation/evolution models outside of Earth. In this paper, we outline currently observed and interpreted surface activity occurring within the modern Mars environment, and tie this activity to wind, seasonal surface CO2 frost/ice, sublimation of subsurface water ice, and/or gravity drivers. Open questions regarding these processes are outlined, and then measurements needed for answering these questions are identified. In the final sections, we discuss how many of these martian processes and landforms may provide useful analogs for conditions and processes active on other planetary surfaces, with an emphasis on those that stretch the bounds of terrestrial-based models or that lack terrestrial analogs. In these ways, modern Mars presents a natural and powerful comparative planetology base case for studies of Solar System surface processes, beyond or instead of Earth.

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Aeolian megaripples, with 5- to 50-m spacing, are abundant on the surface of Mars. These features were repeatedly targeted by high-resolution orbital images, but they have never been observed to move. Thus, aeolian megaripples (especially the bright-toned ones often referred as Transverse Aeolian Ridges-TARs) have been interpreted as relict features of a past climate. In this report, we show evidence for the migration of bright-toned megaripples spaced 1 to 35 m (5 m on average) in two equatorial areas on Mars indicating that megaripples and small TARs can be active today. The moving megaripples display sand fluxes that are 2 orders of magnitudes lower than the surrounding dunes on average and, unlike similar bedforms on Earth, can migrate obliquely and longitudinally. In addition, the active megaripples in the two study areas of Syrtis Major and Mawrth Vallis show very similar flux distributions, echoing the similarities between dune crest fluxes in the two study areas and suggesting the existence of a relationship between dune and megaripple fluxes that can be explored elsewhere. Active megaripples, together with high-sand flux dunes, represent a key indicator of strong winds at the surface of Mars. A past climate with a denser atmosphere is not necessary to explain their accumulation and migration.
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Linear dunes occur on planetary surfaces, including Earth, Mars, and Titan, yet their dynamics are poorly understood. Recent studies of terrestrial linear dunes suggest they migrate by elongation only in supply‐limited environments. Here, we investigate elongating linear dunes in the Hellespontus Montes region of Mars which are morphologically similar to terrestrial systems. Multitemporal, high‐resolution orbital images show these linear dunes migrate by elongation only and that the fixed sediment source of the dunes probably restricts any lateral migration. Some linear dunes maintain their along‐length volume and elongate at rates comparable to adjacent barchans, whereas those which decrease in volume show no elongation, suggesting they are near steady state, matching morphometric predictions. Limited sediment supply may restrict Martian linear dunes to several kilometers, significantly shorter than many terrestrial linear dunes. Our results demonstrate the close similarities in dune dynamics across the two planetary surfaces.
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The stability of the residual carbon dioxide cap near the south pole of Mars is currently not well understood. The cap's survival depends on its radiation budget, controlled by the visible albedo and infrared emissivity. We investigated the role of CO2 snowfall in altering the albedo and emissivity, leading to the observed asymmetry in the net CO2 accumulation at the two poles. Uncontaminated snowfall increases albedo, and lowers emissivity, due to scattering by optically thick clouds and granular surface deposits. Data from the Mars Climate Sounder (MCS) show that fall and winter snowfall is correlated with higher springtime albedo at both poles. For the seasonal CO2 deposits in each polar region >60° latitude, we find mean albedo values of 0.39 in the north and 0.51 in the south, and winter 32‐μm emissivity values of 0.84 in the north and 0.87 in the south. Using a radiative transfer model and the MCS data, we find that the north polar deposits have ∼10× higher dust content than those in the south, explaining the ∼31% lower albedo of the north seasonal cap during spring. Our model shows that greater amounts of snowfall can explain the ∼4% lower emissivity of the north polar seasonal cap. These findings demonstrate that winter snowfall and dust transport affect the composition of Mars' seasonal ice caps and polar energy balance. Snowfall and dust loading are therefore important in modeling the CO2 cycle on Mars, as well as the planet's long‐term climate variations.
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Conditions on Saturn's moon Titan suggest that dust devils, which are convective, dust‐laden plumes, may be active. Although the exact nature of dust on Titan is unclear, previous observations confirm an active aeolian cycle, and dust devils may play an important role in Titan's aeolian cycle, possibly contributing to regional transport of dust and even production of sand grains. The Dragonfly mission to Titan will document dust devil and convective vortex activity and thereby provide a new window into these features, and our analysis shows that associated winds are likely to be modest and pose no hazard to the mission.
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Pluto's atmosphere is mainly nitrogen and is in solid‐gas equilibrium with the surface nitrogen ice. As a result, the global nitrogen ice distribution and the induced nitrogen condensation‐sublimation flows strongly control the atmospheric circulation. It is therefore essential for Global Climate Models to accurately account for the global nitrogen ice distribution in order to realistically simulate Pluto's atmosphere. Here we present a set of new numerical simulations of Pluto's atmosphere in 2015 performed with a Global Climate Model using a 50‐km horizontal resolution (3.75° × 2.5°) and taking into account the latest topography and ice distribution data, as observed by the New Horizons spacecraft. In order to analyze the seasonal evolution of Pluto's atmosphere dynamics, we also performed simulations at coarser resolution (11.25° × 7.5°) but covering three Pluto years. The model predicts a near‐surface western boundary current inside the Sputnik Planitia basin in 2015, which is consistent with the dark wind streaks observed in this region. We find that this atmospheric current could explain the differences in ice composition and color observed in the northwestern regions of Sputnik Planitia, by significantly impacting the nitrogen ice sublimation rate in these regions through processes possibly involving conductive heat flux from the atmosphere, transport of dark materials by the winds, and surface albedo positive feedbacks. In addition, we find that this current controls Pluto's general atmospheric circulation, which is dominated by a retrorotation, independently of the nitrogen ice distribution outside Sputnik Planitia. This exotic circulation regime could explain many of the geological features and longitudinal asymmetries in ice distribution observed all over Pluto's surface.
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Recurring Slope Lineae (RSL) on Mars have been enigmatic since their discovery; their behavior resembles a seeping liquid but sources of water remain puzzling. This work demonstrates that the properties of RSL are consistent with observed behaviors of Martian and terrestrial aeolian processes. Specifically, RSL are well-explained as flows of sand that remove a thin coating of dust. Observed RSL properties are supportive of or consistent with this model, which requires no liquid water or other exotic processes, but rather indicates seasonal aeolian behavior. These settings and behaviors resemble features observed by rovers and also explain the occurrence of many slope lineae on Mars that do not meet the strict definition of RSL. This indicates that RSL can be explained simply as aeolian features. Other processes may add complexities just as they could modify the behavior of any sand dune.
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The current understanding of the Martian surface indicates that briny environments at the near-surface are temporarily possible, e.g. in the case of the presumably deliquescence-driven Recurring Slope Lineae (RSL). However, whether such dynamic environments are habitable for terrestrial organisms remains poorly understood. This hypothesis was tested by developing a Closed Deliquescence System (CDS) consisting of a mixture of desiccated Martian Regolith Analog (MRA) substrate, salts, and microbial cells, which over the course of days became wetted through deliquescence. The methane produced via metabolic activity for three methanogenic archaea: Methanosarcina mazei, M. barkeri and M. soligelidi, was measured after exposing them to three different MRA substrates using either NaCl or NaClO4 as a hygroscopic salt. Our experiments showed that (1) M. soligelidi rapidly produced methane at 4 °C, (2) M. barkeri produced methane at 28 °C though not at 4 °C, (3) M. mazei was not metabolically reactivated through deliquescence, (4) none of the species produced methane in the presence of perchlorate, and (5) all species were metabolically most active in the phyllosilicate-containing MRA. These results emphasize the importance of the substrate, microbial species, salt, and temperature used in the experiments. Furthermore, we show here for the first time that water provided by deliquescence alone is sufficient to rehydrate methanogenic archaea and to reactivate their metabolism under conditions roughly analogous to the near-subsurface Martian environment.
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Predicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: Is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus-challenging developments in sediment transport experiments, two-phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system-spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle-bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well-known Shields diagram, where measurements of fluid entrainment thresholds could actually correspond to entrainment-independent cessation thresholds.
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A massive CO2 ice deposit overlies1 part of Mars’s primarily H2O ice2–4 south polar cap5. This deposit rivals the mass of Mars’s current, 96% CO2, atmosphere6. Its release could substantially alter Mars’s pressure and climate1. The deposit consists of alternating CO2 and H2O ice layers to a depth of up to approximately 1 km (refs. 1,7,8). The top layer is an enigmatic9–11 1–10 m covering of perennial surface CO2 ice12 called the residual south polar cap. Typical explanations of the layering invoke orbital cycles1,7. Up to now, models assumed that the H2O ice layers insulate and seal in the CO2, allowing it to survive high-obliquity periods7,13. However, these models do not quantitatively predict the deposit’s stratigraphy or explain the residual south polar cap’s existence. Here we present a model in which the deposit’s near-surface CO2 can instead exchange with the atmosphere through permeable H2O ice layers. Using currently observed albedo14,15 and emissivity16 properties of the Martian polar CO2 ice deposits, our model predicts that the present massive CO2 ice deposit is a remnant of larger CO2 ice deposits laid down during periods of decreasing obliquity that are ablated, liberating a residual lag layer of H2O ice, when obliquity increases. Fractions of previous CO2 deposits remain as layers because the amplitudes of the obliquity maxima have been mostly decreasing during the past ~510 kyr (ref. 17). Our model simultaneously explains the observed massive CO2 ice deposit stratigraphy, the residual south polar cap’s existence and the presence of a massive CO2 ice deposit only in the south. We use our model to calculate Mars’s pressure history and determine that the massive CO2 ice deposit is 510 kyr old. The long-term evolution and stratigraphy of the CO2 ice residual southern polar cap of Mars can be explained by a model that includes the active coupling of near-surface CO2 with the atmosphere through the permeable H2O ice layers.
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Ancient lacustrine and aeolian sediments have been separately identified in a variety of locations on Mars. In this work, we interpret the depositional history of Barth crater and its surrounding area in Arabia Terra where exposed geologic units preserve a record of lacustrine and aeolian interaction. Aeolian sandstone in the study area overlies lacustrine strata and is interbedded with easily eroded interdune deposits. The aeolian sandstone preserves dune strata with structures that indicate paleo‐sediment transport toward the northwest. The aeolian unit also preserves a transition up‐section from separated barchan dunes to continuous transverse dunes, capturing the development of the ancient dune field from sediment limited to sediment rich. Inverted fluvial channels provide evidence that water was delivered to the area at the same time the dune field was present, providing a simple mechanism, via wetting and cementation, for aeolian strata preservation. This example of wet‐system aeolian accumulation preserves an upward drying sequence in the sedimentary record that may have been coincident with the widely hypothesized global climate transition. The terrains described in this work provide a framework for interpreting similar aeolian units in elsewhere on Mars, even where cross‐strata cannot be easily resolved.
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We derive the depth of the water ice table on Mars by fitting seasonal surface temperature trends acquired by the Mars Climate Sounder and Thermal Emission Imaging System with a two‐layer regolith model assuming frozen H2O as the lower material. Our results are consistent with widespread water ice at latitudes as low as 35°N/45°S buried sometimes a few centimeters below sand‐like material, with high lateral ice depth variability, and correlated with periglacial features. While several investigations have already predicted, identified, and characterized some properties of near‐surface ice on Mars, our results constitute a significant advance in the context of the upcoming crewed exploration because (1) they focus on very shallow depths accessible with limited equipment, (2) they provide continuous regional coverage including the midlatitudes, and (3) they yield moderate spatial resolution maps (3 ppd) relevant to landing site selection studies.
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Plain Language Summary Robotic lander and satellite images show sand movement by wind frequently occurs on Mars. Martian wind speeds, however, rarely exceed the minimum wind speed thought to be required to move sand. To resolve this dilemma, we conducted wind tunnel experiments to determine if sand could move at slower wind speeds than previously predicted. Previous experiments used the onset of continuous motion throughout a wind tunnel to define the minimum wind speeds required to move sand on Mars and ignored sporadic transport that can occur at slower wind speeds. Here, we use the onset of sporadic bursts of sand movement in a wind tunnel to define the minimum wind speed for sand motion on Mars. These bursts can induce a cascade of motion that develops from discrete patches and grows exponentially into continuous transport, which has the potential to produce significant landform change on Mars. We find the minimum speeds necessary to initiate sporadic bursts of motion are slower than previous model estimates by a factor of 1.6 to 2.5. Our results offer one explanation for abundant ripple and dune movement and dust emission under current thin‐atmosphere climate conditions and suggest winds have sculpted the Martian landscape over billions of years.
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In situ measurements of relative humidity (RH) on Mars have only been performed by the Phoenix (PHX) and Mars Science Laboratory (MSL) missions. Here we present results of our recalibration of the PHX thermal and electrical conductivity probe (TECP) RH sensor. This recalibration was conducted using a TECP engineering model subjected to the full range of environmental conditions at the PHX landing site in the Michigan Mars Environmental Chamber. The experiments focused on the warmest and driest conditions (daytime) because they were not covered in the original calibration (Zent et al., 2010, https://doi.org/10.1029/2009JE003420) and previous recalibration (Zent et al., 2016, https://doi.org/10.1002/2015JE004933). In nighttime conditions, our results are in excellent agreement with the previous 2016 recalibration, while in daytime conditions, our results show larger water vapor pressure values. We obtain vapor pressure values in the range ~0.005-1.4 Pa, while Zent et al. (2016, https://doi.org/10.1002/2015JE004933) obtain values in the range ~0.004-0.4 Pa. Our higher daytime values are in better agreement with independent estimates from the ground by the PHX Surface Stereo Imager instrument and from orbit by Compact Reconnaissance Imaging Spectrometer for Mars. Our results imply larger day-to-night ratios of water vapor pressure at PHX compared to MSL, suggesting a stronger atmosphere-regolith interchange in the Martian arctic than at lower latitudes. Further, they indicate that brine formation at the PHX landing site via deliquescence can be achieved only temporarily between midnight and 6 a.m. on a few sols. The results from our recalibration are important because they shed light on the near-surface humidity environment on Mars.
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A thermophysical model for rough terrain is developed that is capable of processing spatial domains of megapixel size. This computational advance makes it possible to characterize thermal environments on Mars at unprecedented scale and at a resolution of 1 m per pixel. The model is applied to Palikir Crater, Mars, where many recurring slope lineae (RSL) are located, often in bedrock alcoves. In areas with RSL, subsurface water ice is not stable, that is, any subsurface ice is lost to the atmosphere in the long term. On large portions of the craters walls, water frost accumulates continuously for up to hundreds of sols each Mars year, but no relation is found between the location of RSL and seasonal water frost accumulation. Some RSL do not have access to even 1 m² of water frost, at any time of the year. Where water frost is present, it stops accumulating in early southern spring at the latest, long before major RSL activity. Based on the model results, neither CO2 frost, perennial subsurface ice, nor seasonal water frost patches (>1 m²) are connected with RSL activity.
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The reflectance of water ice and dust mixtures depends, amongst other parameters, on how the components are mixed (e.g. intimate mixture, areal mixture or coating). Therefore, when inverting the reflectance spectra measured from planetary surfaces to derive the amount of water ice present at the surface, it is critical to distinguish between different mixing modes of ice and dust. However, the distinction between mixing modes from reflectance spectra remains ambiguous. Here we show how to identify some water ice/soil mixing modes from the study of defined spectral criteria and colour analysis of laboratory mixtures. We have recreated ice and dust mixtures and found that the appearance of frost on a surface increases its reflectance and flattens its spectral slopes, whereas the increasing presence of water ice in intimate mixtures mainly impacts the absorption bands. In particular, we provide laboratory data and a spectral analysis to help interpret ice and soil reflectance spectra from the Martian surface.
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Sublimation waves are periodic, linear and transverse bedforms that grow by sublimation of icy substrates under turbulent winds. They occur in different environments of the Earth and other planets, where the climate is favorable to sublimation. Their morphological and kinematic characteristics (wavelength, migration velocity, characteristic time of formation) depend on the environmental conditions in which they develop (atmosphere viscosity, wind speed, sublimation rate). To highlight these relations, we designed a theoretical model for their formation, based on models previously designed for dissolution bedforms. From a linear stability analysis of the model, we derived a dispersion relation and three scaling laws. These are validated by comparison with measurements available for two natural terrestrial examples (Blue Ice Areas of the Antarctic ice sheets and ice caves) and with new measurements in a laboratory experiment. They are applied to linear bedforms, of hitherto unknown origin, that we identified on the Martian North Polar Cap. We thus demonstrate that sublimation waves are convenient geomorphic markers to constrain surface compositions, atmospheric properties, climates, and winds on terrestrial ice sheets and other icy planetary surfaces. Full-text: https://authors.elsevier.com/a/1c0wR2weQiqnQ
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The Mars Global Digital Dune Database (MGD³) archives the location of large dark dunes on Mars, including several morphological, mineralogical, and thermophysical characteristics; it has been widely used by the Mars scientific community as a resource for aeolian studies (Hayward, R. K., Mullins, K. F., Fenton, L. K., Hare, T. M., Titus, T. N., Bourke, M. C., Colaprete, A., and Christensen, P. R. (2007a), Mars Global Digital Dune Database: MC2 - MC29: U.S. Geological Survey Open-File Report 2007-1158, doi:https://doi.org/10.3133/20071158). This work presents the results of a new global dune field survey using high resolution orbital images of Mars that have been acquired in the years since the MGD³ was first compiled. More than 2000 new dune fields have been identified and mapped, more than doubling the previous number known. The new dune fields are mostly small (<~5 km²), collectively spanning ~52,000 km². The total areal coverage of dune fields on Mars is now estimated to be 1.042 × 10⁶ km² (or 0.577 × 10⁶ km² when weighted for dune density). Many of the new dune fields are located in regions previously considered largely devoid of dune fields, such as the northern lowlands, as well as Hellas and Argyre Planitiae. Intended to supplement and update the original MGD³, this survey highlights the need for inventories and databases capable of handling dynamic data in the rapidly changing planetary exploration environment.
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Mars yardang fields were mapped using the newly available high-resolution Context Camera (CTX) mosaics (30°N to 30°S) through visual interpretation and manual digitization. We classified the Martian yardangs into three types according to their shapes: long ridge, inverted hull and curvilinear shapes, and found that the number of long ridge yardang fields account for 77.97% of all 394 yardang fields. The mapping results show that the total area of yardang fields is about 734,115 km2, within which the area of the Medusae Fossae Formation (MFF) yardangs is about 562,640 km2. We also identified and digitized individual yardang bodies wider than 500 m in the CTX mosaics. The resultant 466 yardang bodies are subsequently used as samples for statistical analysis of the morphometric parameters. Among all of the orientations, N–S is the dominant orientation of the yardang samples, and the average length and width are 5626 m and 993 m respectively. The length to width ratios vary from 1.2:1 to 46.9:1 within (30°N to 30°S) and vary from 1.5:1 to 46.9:1 in the most studied MFF. The mapping and statistical results provide important information about the spatial distribution of Mars yardangs and their morphological characteristics, which will be valuable to further study of the formation and evolution of Mars yardangs, and the aeolian environment.
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The continuously increasing number of multi-temporal high-resolution images from the surface of Mars offers the possibility for detailed studies of present-day surface activity. In this study we investigated all gullies in the Sisyphi Cavi region (355° E, 71° S) of the south polar region of Mars. This region is influenced by the seasonal deposition of a decimeters-thick translucent slab ice in late autumn/winter and its subsequent sublimation in spring. We mapped all gullies (n = 17.760) and measured their orientations. We also identified gullies with contemporary activity (n = 35) using multi-temporal HiRISE images for martian years (MY) 28 to 34. We observed two different kinds of activity: (1) dark flow-like features, and (2) movement of blocks. For both, sediment was transported from the source region (alcove and/or flanks of channels) down the gully. Using image data from HRSC, CTX, and HiRISE, we monitored the general defrosting of the study region. We also analyzed the maximum daytime surface temperatures of the complete study region based on TES data from MYs 24 to 26. To identify the origin and triggering mechanism of the observed activity, we used: (1) detailed topographic investigations (e.g., slope angles) of two extensively gullied slopes based on two HiRISE-DTMs, (2) identification of small scale displacements with Digital Image Correlation (DIC), and (3) orientation measurements of active gullies and comparison to non-active gullies. We found that for the active gullies studied, activity happens at the end of spring between LS ~ 225° and ~250°. This is consistent with the timing of final stages of defrosting in the region. At this time, some surfaces are already defrosted while others still host the seasonal slab ice cover. For the surfaces with slab ice, dark defrosting spots (and flows, if the surface is inclined) are observed on dark dunes as well as on gully aprons and in gully channels. These spots form when, triggered by basal sublimation generated overpressure, sediment entrained in CO2-gas is transported through cracks within the ice and redeposited onto the frosted surface. We compared and linked both morphologic features (dark dune spots and dark flow-like features) and concluded that these features have comparable or even the same triggering mechanisms. Based on this extensive study, the most plausible mechanism for ongoing gully activity can be divided into two steps: 1) accumulation of material within gully channels via small dry flows on top of the slab ice (comparable to dark dune spots/flows), 2) when a critical mass is reached, the sediment flows down the still frosted gully on top of the sublimating ice or as a mixture of dry material and ice in a catastrophic flow. The triggering factor for the movement of blocks remains unclear, as their timing could not be constrained with the available data. We identified headwall erosion in one gully in the study region, whereas a discrete source could not be identified for the other sites, suggesting multiple failure mechanisms could be active in such gullies. Finally, through volume balance calculations we show that active gullies in Sisyphi Cavi could have been formed within decades to several tens of thousands of MY, but gully-morphology indicates a much-longer period for formation of the entire gully-landform.
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An expression for saltation threshold – the minimum wind speed required to initially saltate particles – is necessary for modeling aeolian processes on Earth and other bodies. Analysis of a compilation of experimental data led to the conclusion that this threshold is a function of the ratio of the density of the particle to that of the entraining fluid (ρp/ρ), and to a curve for the dimensionless threshold parameter, A(ρp/ρ). Whereas data of low-density ratio and of high-density ratio conditions show constant A values, the single dataset used to define the transitional region of the curve shows a range of values. To revisit this transitional region, we collect new freestream threshold data at 1–20 bars (1–20 × 10⁵ Pa) with particles 150–1000 µm in diameter and having densities 400–3300 kg/m³ using the Titan Wind Tunnel. From these new data spanning a range of intermediate density ratios, we calculate friction wind speeds and values for A(ρp/ρ). We filter our threshold data for the same conditions (particle diameter > 200 µm, particle Reynolds number > 10) as in previous work and combine them with previously published data to derive a new density ratio curve with the same form as the previous expression. This new curve of A(ρp/ρ), with different parameter values and including uncertainties, confirms the slope in the transitional region between low- and high-density ratios, though giving slightly higher values for A. This work offers improved prediction of threshold wind speeds under thicker-than-terrestrial atmospheres on other solar or extrasolar planets, while also suggesting current challenges to accurate experimental simulation of aeolian transport under such conditions.
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The dominant topographic features on two-lobed Kuiper Belt object (486958) Arrokoth (provisionally designated 2014 MU69) are scattered, small, circular depressions or pits up to ~1.0 km across and curvilinear troughs observed near the terminator during the New Horizons encounter of 01 January 2019. With important exceptions, evidence for an endogenic origin for pits is lacking and impact remains the most likely origin. Pit depths relative to the local surface are shallower than hypervelocity simple craters on icy moons and consistent with low velocity (~300 m/s) impacts in an icy target (the role of porosity and cohesion in cratering on Arrokoth being uncertain). There is, however, a large scatter in observed d/D, with some pits too shallow to measure reliably. The range of preservation states observed for both pits and troughs is consistent with slow but persistent degradation on the surface of this (and perhaps other) small primitive planetesimal(s) in the Kuiper Belt by agents such as micrometeorite bombardment or volatile loss, indicating that degradation does indeed occur on such bodies. Arrokoth pits could also be related to circular depressions on comet 9P/Tempel (though comparisons of surface features on Arrokoth and comets are intriguing they are limited by resolution differences and the evolved state of cometary surfaces). Two of the curvilinear troughs occur along the terminator of the Large Lobe. One trough is a shallow depression devoid of resolvable structures, the other features several linear scarps a few 10s of meters high, suggesting that coherent failure can occur on the surfaces of small Kuiper Belt objects such as Arrokoth. Several of the deepest small pits are also clustered within this walled trough, similar to tectonically associated pit chains on other planets. These pits and an obliquely viewed linear chain of elongate depressions remain the best candidates for endogenic features on Arrokoth. Subtle undulations and linear features no more than a few 10's of meters in amplitude (and even a few candidate positive relief features) are also evident in the terminator regions, indicating that the long-term evolution of this body may have been complex.
Article
Recurring Slope Lineae (RSL) are narrow, dark features that typically source from rocky outcrops, incrementally lengthen down Martian steep slopes in warm seasons, fade in cold seasons and recur annually. In this study we report the first observations of RSL at Hale crater, Mars, during late southern summer by the Color and Surface Science Imaging System (CaSSIS) on board ESA's ExoMars Trace Gas Orbiter (TGO). For the first time, we analyze images of RSL acquired during morning solar local times and compare them with High Resolution Imaging Science Experiment (HiRISE) observations taken in the afternoon. We find that RSL activity is correlated with the presence of steep slopes. Our thermal analysis establishes that local temperatures are high enough to allow either the melting of brines or deliquescence of salts during the observation period, but the slope and aspect distributions of RSL activity predicted by these processes are not consistent with our observations. We do not find any significant relative albedo difference between morning and afternoon RSL. Differences above 11% would have been detected by our methodology, if present. This instead suggests that RSL at Hale crater are not caused by seeping water that reaches the surface, but are best explained as dry flows of granular material.
Article
Progress in the geosciences has often followed the same fundamental paradigm for about two centuries: Earth’s present is the key to understanding its past and its future. This concept is at the root of most of what is known about the Earth. Similarly, knowledge of Earth’s geological and atmospheric processes can be, and has been, applied when studying the history of other planetary bodies. More recently, however, observations from other planets have fed back into our understanding of Earth. In this Perspective, we argue that many scientific mysteries about the Earth can be solved only by looking beyond it, and describe instances where other bodies, such as Mars, Venus and the Moon, have or could augment our understanding of processes on Earth. Future space missions offer the opportunity to probe the rich diversity of planetary environments and compositions, and further explore how they might serve as analogues, experiments and archives.
Conference Paper
We provide an overview of quality characteristics of the regional data products of the HRSC Mars Chart (HMC-30) series, and present new data on coordinate accuracy relative to the MOLA global reference, which were not available from previous HRSC DTMs. Finally, details of the HMC-30 global map layout and tiling scheme are reported.
Article
In its polar layered deposits (PLD), Mars possesses a record of its recent climate, analogous to terrestrial ice sheets containing climate records on Earth. Each PLD is greater than 2 km thick and contains thousands of layers, each containing information on the climatic and atmospheric state during its deposition, creating a climate archive. With detailed measurements of layer composition, it may be possible to extract age, accumulation rates, atmospheric conditions, and surface activity at the time of deposition, among other important parameters; gaining the information would allow us to “read” the climate record. Because Mars has fewer complicating factors than Earth (e.g. oceans, biology, and human-modified climate), the planet offers a unique opportunity to study the history of a terrestrial planet's climate, which in turn can teach us about our own planet and the thousands of terrestrial exoplanets waiting to be discovered. During a two-part workshop, the Keck Institute for Space Studies (KISS) hosted 38 Mars scientists and engineers who focused on determining the measurements needed to extract the climate record contained in the PLD. The group converged on four fundamental questions that must be answered with the goal of interpreting the climate record and finding its history based on the climate drivers. The group then proposed numerous measurements in order to answer these questions and detailed a sequence of missions and architecture to complete the measurements. In all, several missions are required, including an orbiter that can characterize the present climate and volatile reservoirs; a static reconnaissance lander capable of characterizing near surface atmospheric processes, annual accumulation, surface properties, and layer formation mechanism in the upper 50 cm of the PLD; a network of SmallSat landers focused on meteorology for ground truth of the low-altitude orbiter data; and finally, a second landed platform to access ∼500 m of layers to measure layer variability through time. This mission architecture, with two landers, would meet the science goals and is designed to save costs compared to a single very capable landed mission. The rationale for this plan is presented below. In this paper we discuss numerous aspects, including our motivation, background of polar science, the climate science that drives polar layer formation, modeling of the atmosphere and climate to create hypotheses for what the layers mean, and terrestrial analogs to climatological studies. Finally, we present a list of measurements and missions required to answer the four major questions and read the climate record. 1. What are present and past fluxes of volatiles, dust, and other materials into and out of the polar regions? 2. How do orbital forcing and exchange with other reservoirs affect those fluxes? 3. What chemical and physical processes form and modify layers? 4. What is the timespan, completeness, and temporal resolution of the climate history recorded in the PLD?
Article
Araneiforms, or “Spiders” (spider-like surface modifications) are suggested to form through basal sublimation of a seasonal translucent CO2 ice slab layer and subsequent gas jetting, and are so far only known to occur in the Martian south polar region. Their spatial configuration characteristics remain incompletely understood. We observe a non-random spatial distribution in seven study regions with significant regional variation of average spacing. The non-randomness results from the pressure release by jetting from one spider, inhibiting ice rupture in the vicinity to initiate a new spider. Rose diagrams constructed from our trough orientation mapping show that spider troughs do not have preferred orientations and appear randomly distributed. We suggest that substrate properties such as permeability, cohesion and porosity regulate and modify the spatial configuration of a spider population on a regional scale, and that insolation and obliquity determine whether general conditions are favorable for spider formation. The limited areal distribution of spiders may be due to the strict formation constraints on the thickness of the seasonal translucent CO2 ice layer. This work provides insight into the regional variability of gas jetting and spider formation.
Article
Aeolian transport of sand is abundant on modern-day Mars, as revealed by remote sensing measurements of the motion of dunes, and of the meter-scale ripples that mantle them. We study a large-scale natural sand trap within the Meroe Patera dune field: a 1.8-km diameter crater which features a dune-free “shadow” in its lee. We compare the volume of sand trapped within this crater to the sand volume that would be expected to cover the area of the crater and its dune-free shadow behind it if the crater were not present. We find that the crater holds less sand than this “missing” volume would predict, implying that sand escapes from the crater over time. Modern day imagery shows an apparent lack of sand escaping from the Meroe crater, however, suggesting that changes in the wind regime at the site may have allowed sand to escape in the past. The persistence of an altered dune morphology all the way to the far downwind edge of the dune field suggests consistent wind conditions over the time of the crater-dune field interaction.
Article
We used Phoenix Surface Stereo Imager data, constrained by other Phoenix and MRO data taken via a planned coordinated measurement campaign, along with radiative transfer modeling to assess the vertical water vapor profile at the Phoenix arctic location during its spring and summer mission. We examined 16 mid-afternoon observations spanning Ls = 97.5°–148°. We developed a 2-layer model of vapor distribution which reproduces the water vapor band depth. Using the results of the 2-layer model, we retrieved the mass mixing ratios in each layer and the implied surface vapor pressure. We found that near surface water vapor was enhanced relative to higher layers, resulting in a large percentage of the water column (>25% and up to nearly 100%) confined below ~2.5 km.
Article
The Residual South Polar Cap (RSPC) of Mars is a thin covering of CO2 ice resting on water-ice rich deposits. As such, it is a likely indicator of the net effects of recent polar climate. This covering has had minor changes in outline for the period of spacecraft observation (Piqueux and Christensen, 2008a) and estimates of its recent mass balance suggest fractionally small changes in its volume (Thomas et al., 2016). Pit growth by scarp retreat (Malin et al., 2001; Byrne and Ingersoll, 2003a; Thomas et al., 2005, 2013) is a major, relatively easily measured component of the cap's mass balance. This scarp retreat is only the beginning of a process: fracturing, slumping, and sublimation (Byrne et al., 2008; Thomas et al., 2009) lead to production of a trail of debris. This study focuses on that debris using spacecraft imaging data. The rough, relatively dark debris forms ubiquitous ramps, typically tens of m wide, around scarps within pits or on perimeters of mesas. Much wider accumulations of debris, “debris fields,” mimic distinctive scalloped outlines of mesas and are observed to originate by merging of expanding pits within a mesa. The subsequent evolution of debris fields includes repeated year-to-year local relief inversions that involve trapping and retention of seasonal ice in low areas that effectively reduce the vertical loss rates. Complete loss of the CO2 debris from a surface of water-ice rich Polar Layered Deposits (PLD) can initiate new net accumulation of CO2 ice. The longevity of some debris fields, essentially instances of slow downwasting, appear to facilitate relief inversion on large scales by allowing the surrounding areas to accumulate new CO2 ice and to thicken relative to the debris fields.
Article
We developed a change detection method for the identification of ice block falls using NASA’s HiRISE images of the north polar scarps on Mars. Our method is based on a Support Vector Machine (SVM), trained using Histograms of Oriented Gradients (HOG), and blob detection. The SVM detects potential new blocks between a set of images; the blob detection, then, confirms the identification of a block inside the area indicated by the SVM and derives the shape of the block. The results from the automatic analysis were compared with block statistics from visual inspection. We tested our method in 6 areas each consisting of 1000 × 1000 pixels, where several hundreds of blocks were identified. The results for the given test areas produced a true positive rate of ~75% for blocks with sizes larger than 0.5 m2 (i.e., approx. 3 times the available ground pixel size) and a false discovery rate of ~8.5%. Using blob detection, we were also able to recover the size of each block within 3 pixels of their actual size.
Article
The Mars Workshop on Amazonian and Present-Day Climate was held in June 2018. In this paper, we describe the context and motivations for the Workshop and summarize the proceedings. In particular, we identify 37 high-priority Mars Amazonian and present-day climate questions that arose from presentations and discussion at the workshop, which we group under Key Questions on (1) interpreting the record in the Polar Layered Deposits, (2) identifying and characterizing off-polar climate records (such as large subsurface mid-latitude ice deposits and signs of aqueous alteration), (3) the global budget of dust/sand, salts, and ice, and (4) the global movement of these materials. These 37 questions pertain to processes in the atmosphere, surface, and subsurface active in the present and past, as well as the record they leave of the historical climate. As many of the processes of interest are interconnected, the questions we present are often intertwined; some questions are identified as being of highest-priority because of their larger scientific value and/or that addressing it would contribute important information towards answering many other questions. Related to the high-priority questions, we identify key measurements in the areas of PLD formation and evolution, surface change/interactions, state of global reservoirs, and environmental (surface/atmospheric) conditions. Our objective is for these lists of high-priority questions and measurements to contribute towards discussion of future Mars scientific exploration, and to record the state of Mars Amazonian climate knowledge as of the time of our workshop.
Article
Present-day topographic changes are observed on steep slopes in equatorial regions of Mars that are associated with sulfate-rich sediments. Hydrated sulfates are known to be present in many sedimentary deposits on Mars. We document volume changes in the form of mass movements and gullies over these regions. We have estimated erosion rates of ~12 mm/yr (or ~1.2–120 mm/yr with uncertainties) over steep slopes on sulfate-rich mounds in Ganges Chasma, much higher than Mars average erosion rate near a few μm/yr. At this rate, the mounds would have shrunk in diameter by ~18,000 km over 3 b.y., which greatly exceeds the width of the canyon, supporting suggestions that these sediments once filled the canyons. Due to the soft nature of typical sulfate-rich sediment, it is susceptible to mass wasting, and active eolian processes may remove loose material to maintain steep slopes. The water in hydrated sulfates could potentially be extracted and used as a resource for future humans on Mars, and our results suggest that such deposits would be mechanically weak.
Article
Recurring slope lineae (RSL) are dark linear features on the surface of Mars that advance incrementally downslope, fading and re-growing annually. Numerous hypotheses have been proposed to explain RSL formation, including “wet” models that involve liquid water or brines and “dry” mechanisms involving liquid-free mass-wasting or solid-vapor phase changes that trigger granular flow. In part, hypotheses to explain RSL formation rely on observations of the physical characteristics of the slopes on which RSL are present. To determine if RSL exhibit physical characteristics typical of one particular formation mechanism, we examined the initiation and termination points of over 10,000 RSL across 16 confirmed RSL sites to determine their slope, elevation, orientation, and thermal inertia. RSL typically form within an envelope of high elevations at each mapping site, are oriented equatorward and west, and have hillslope thermal inertia values consistent with sand and gravel. Notably, the RSL begin and end at slopes above, within, and below the angle of repose required to trigger and sustain dry granular flows. Sensitivity testing indicates that these slope values are not a result of pixel-scale noise. Discontinuous RSL are observed at several study sites. The slope and thermal inertia values of the RSL, as well as these additional geomorphic observations of discontinuous dark features, are not consistent with the characteristics of dry flows in which continuous dust avalanches begin within the dynamic angle of repose and terminate within the static angle of repose. Although RSL are found to be common on coarse sandy and rocky slopes, not all sandy slopes in RSL regions that are close to the angle of repose contain RSL. We conclude that mechanisms other than dry mass wasting are needed to explain all of the observed geomorphic characteristics of RSL source and termination points.
Article
Our research focuses on the detailed study of the aeolian deposits within Moreux crater using multi-resolution imaging and spectral data from the Mars Reconnaissance Orbiter. The morphometric analysis on the dune slip faces and wind streak orientations allowed us to reconstruct the sand transport pathways and the changes of the transport pattern. We used a new automatic procedure based on the Line Detection algorithm in PCI Geomatics’ Geomatica software to characterize small scale aeolian structures as ripples within Moreux crater from HiRISE images. After validation on a previously studied area in Herschel crater, we apply this method to reconstruct the wind regime at high spatial resolution in Moreux and reconstruct the wind circulation forming the aeolian bedforms. Moreover, we used three pairs of CTX images to perform a multi-temporal analysis of the wind streaks. We mapped more than 500 features with data acquisition spanning over four Earth years and the observed wind streak changes may reflect present day atmospheric variations due to local winds. CRISM datasets show an olivine and clinopyroxene mixture characterizing most of the dunes within Moreux crater, while the dark dunes in the northern sector of the crater showed an enrichment in Mg-olivine. This composition is similar to that detected in the central peak bedrock suggesting that central peak erosion contributes to the formation of the northly dunes meanwhile recent northeast wind flows and Moreux topography influence the wind circulation and determine the formation of the sand transport pathways within the crater. These results are consistent with the hypothesis that aeolian sands are generally sourced locally on Mars.
Article
Here we study rocks falling from exposed outcrops of bedrock, which have left tracks on the slope over which they have bounced and/or rolled, in fresh impact craters (1–10 km in diameter) on Mars. The presence of these tracks shows that these rocks have fallen relatively recently because aeolian processes are known to infill topographic lows over time. Mapping of rockfall tracks indicate trends in frequency with orientation, which in turn depend on the latitudinal position of the crater. Craters in the equatorial belt (between 15°N and 15°S) exhibit higher frequencies of rockfall on their north-south oriented slopes compared to their east-west ones. Craters >15° N/S have notably higher frequencies on their equator-facing slopes as opposed to the other orientations. We computed solar radiation on the surface of crater slopes to compare insolation patterns with the spatial distribution of rockfalls, and found statistically significant correlations between maximum diurnal insolation and rockfall frequency. Our results indicate that solar-induced thermal stress plays a more important role under relatively recent climate conditions in rock breakdown and preconditioning slopes for rockfalls than phase transitions of H2O or CO2, at mid- and equatorial-latitudes. Thermal stress should thus be considered as an important factor in promoting mass-wasting process on impact crater walls and other steep slopes on Mars.
Article
At the margins of the north polar deposits of dusty water ice on Mars, steep scarps reveal layered deposits that preserve a record of the planet's recent climate changes. Some of these scarps are currently the most active places on Mars with annual avalanches and block falls. We estimated the erosion rate of a steep north polar scarp by automatically identifying newly apparent blocks throughout a time series of HiRISE images. The total depositional volume over three Mars years corresponds to a minimum erosion rate of ~0.3 m3 per Mars year per meter along the scarp, or a minimum average scarp retreat rate of ~0.2 m/kyr. This rate cannot balance published 0.01 m/yr viscous flow rates, implying that either lower rates of the latter occur or that other processes contribute more than block falls to the slopes' steepness.
Article
Recurring slope lineae (RSL) are narrow (0.5–5 m) low-albedo features that incrementally lengthen down steep slopes during warm seasons, fade in colder seasons, and recur each Mars year. To reduce the effort involved in manually mapping and analyzing each RSL in Garni crater, we developed Mapping and Automated Analysis of RSL (MAARSL) to analyze a set of orthorectified High Resolution Imaging Science Experiment (HiRISE) images and a digital elevation map. MAARSL along with manual mapping allowed us to detect RSL, compute descriptive statistics, and characterize changes over time. We mapped 2910 RSL in 22 orthoimages, from Mars Year (MY) 31 solar longitude (Ls) 133.0° to MY32 Ls 323.7°. The MAARSL results confirmed that RSL lengthening and fading occur concurrently on slopes with the same orientation and many times within some individual RSL. Slope angles of RSL and a slope slump show that some RSL start, stop, and have mean slope angles that are below the angle of repose. Our analysis shows that RSL are actively lengthening on at least one slope-facing direction in all HiRISE observations of the crater. We also found that NE-, N-, and NW-facing RSL in Garni crater lengthened during times of increasing shortwave insolation, while S- and SW-facing RSL lengthened during increasing and decreasing shortwave insolation. A (non-orthorectified) HiRISE image acquired shortly after the MY34 dust storm and shows RSL on every slope-facing direction, which is anomalous with respect to observations prior to the dust storm. We find that dust removal and deposition could explain the darkening and fading (respectively) of RSL, and could also explain the apparent lack of material being transported. Observations of RSL lengthening and fading occur concurrently on slopes could suggest overprinting of dry granular flows. Dry flows could also explain the significant lengthening activity of every slope-facing direction after a fresh layer of dust was deposited via the MY34 dust storm. Alternatively, briny shallow subsurface flows are consistent with observations of RSL on slopes below the angle of repose and those that exhibit concurrent lengthening and fading. However, the most significant problem with briny RSL flows is accessing a source of briny water and removing excess salt from the regolith.