Martian Gullies Research Articles

The Dynamics of CO2‐Driven Granular Flows in Gullies on Mars

AbstractMartian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid‐latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present‐day gully activity. However, due to a lack of in situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2‐driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2‐driven granular flows shows that they are dynamically similar to terrestrial two‐phase granular flows, that is, debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2‐driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far‐reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation‐driven flow processes.

How, when and where current mass flows in Martian gullies are driven by CO2 sublimation

Martian gullies resemble water-carved gullies on Earth, yet their present-day activity cannot be explained by water-driven processes. The sublimation of CO2 has been proposed as an alternative driver for sediment transport, but how this mechanism works remains unknown. Here we combine laboratory experiments of CO2-driven granular flows under Martian atmospheric pressure with 1D climate simulation modelling to unravel how, where, and when CO2 can drive present-day gully activity. Our work shows that sublimation of CO2 ice, under Martian atmospheric conditions can fluidize sediment and creates morphologies similar to those observed on Mars. Furthermore, the modelled climatic and topographic boundary conditions for this process, align with present-day gully activity. These results have implications for the influence of water versus CO2-driven processes in gully formation and for the interpretation of gully landforms on other planets, as their existence is no longer definitive proof for flowing liquids.

Insights into the interaction between defrosting seasonal ices and gully activity from CaSSIS and HiRISE observations in Sisyphi Cavi, Mars

Martian gullies are surface features, typically composed of an alcove, a channel and a depositional apron. They have been extensively studied since their first observations in the 2000s and were initially attributed to the action of liquid water. Later studies highlighted that their activity is spatially and temporally correlated with the seasonal presence of surface CO2 ice, suggesting a link between seasonal frost and gully activity. However, the exact mechanisms leading to gully formation are still under debate. Establishing whether or not gullies are formed by liquid water has important implications for Mars recent climate and habitability. Here, we study the evolution of seasonal frost and its connection to gully activity in Sisyphi Cavi, located in the southern circumpolar region (68°S - 74°S; 345°E − 5°E). The high latitude of this site allows for detailed temporal monitoring, owing to the high frequency imaging by spacecraft in polar orbits, largely unavailable for other martian gullies, hence this site has the potential to reveal new insights on gullies in general. In particular, we used repeat images from MRO HiRISE (Mars Reconnaissance Orbiter, High Resolution Imaging Science Experiment) and TGO CaSSIS (Trace Gas Orbiter, Colour and Stereo Surface Imaging System). Our findings show that the general timing and characteristics of the defrosting patterns in Sisyphi Cavi follow seasonal patterns that are consistent from year to year. The timing of the start of defrosting and its temporal evolution are controlled by the overall orientation of the host hillslope, with equator-facing slopes defrosting before pole-facing ones and the steepest pole-facing slopes defrosting last. Gully alcoves defrost before channels and aprons located on the same hillslope independent of the orientation of the hillslope likely because the aprons have a lower thermal inertia than the alcoves that counteracts the orientation effect, which on pole-facing slopes would mean the alcoves should defrost last. We observe the presence of seasonal and ephemeral dark spots and flows which are interpreted to be a result of sediment deposition on top of the seasonal ice deposits, triggered by basal sublimation. Our observations suggest that the appearance of dark spots and dark flows can be spatially correlated with surface roughness, including presence of boulders, contrasts in material types, irregular lobes/levees and braiding caused by gully activity, and therefore could be used to help detect recently active gullies on Mars in areas with seasonal frost without the need of repeat imaging. Finally, we propose that presently observed gully activity in Sisyphi Cavi is driven by defrosted material flowing on to a frosted apron. We infer that the presence of a frosted apron could be common precursor for this type gully activity. We note that this activity only involves the mobilisation of loose materials and we have not observed any erosion of the wall rock present in Sisyphi Cavi.

Morphologic and morphometric differences between gullies formed in different substrates on Mars: new insights into the gully formation processes

Abstract. Martian gullies are kilometer-scale, geologically young features with a source alcove, transportation channel, and depositional fan. On the walls of impact craters, these gullies typically incise into bedrock or surfaces modified by the latitude-dependent mantle (LDM; inferred as consisting of ice and admixed dust) and glaciation. To better understand the differences in the alcoves and fans of gullies formed in different substrates and infer the flow types that led to their formation, we have analyzed the morphology and morphometry of 167 gully systems in 29 craters distributed between 30 and 75∘ S. Specifically we measured length, width, gradient, area, relief, and relief ratio of the gully alcoves and fans; Melton ratio, relative concavity index, and perimeter; and form factor, elongation ratio, and circularity ratio of the gully alcoves. Our study reveals that gully alcoves formed in LDM/glacial deposits are more elongated than the gully alcoves formed in bedrock, and they possess a distinctive V-shaped cross section. We have found that the mean gradient of fans formed by gullies sourced in bedrock is steeper than the mean gradient of fans of gullies sourced in LDM/glacial deposits. These differences between gullies were found to be statistically significant and discriminant analysis has confirmed that alcove perimeter, alcove relief, and fan gradient are the most important variables for differentiating gullies according to their source substrates. The comparison between the Melton ratio, alcove length, and fan gradient of Martian and terrestrial gullies reveals that Martian gully systems were likely formed by terrestrial debris-flow-like processes. Present-day sublimation of CO2 ice on Mars may have provided the adequate flow fluidization for the formation of deposits akin to terrestrial debris-flow-like deposits.

Gullies on Mars could have formed by melting of water ice during periods of high obliquity.

Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars' rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.

Martian gully activity and the gully sediment transport system

The formation process for Martian gullies is a critical unknown for understanding recent climate conditions. Leading hypotheses include formation by snowmelt in a past climate, or formation via currently active CO2 frost processes. This paper presents an expanded catalog of >300 recent flows in gullies. The results indicate that sediment transport in current gully flows moves the full range of materials needed for gully formation. New flows are more likely to transport boulders in gullies that have pre-existing boulder-covered aprons, indicating that current flows are transporting the same materials required for gully formation overall. The distribution of gully activity frequencies can be described by a power law and indicates that the recurrence intervals for flows in individual gullies are commonly tens to hundreds of Mars years. Over the last ~300 kyr, climate variations have been modest but individual gullies have had tens to thousands of flow events. This could be sufficient to account for the entirety of gully formation in some cases, although the same processes are likely to have occurred further in the past. For any gullies that may have initiated under higher-obliquity conditions, this level of recent activity indicates that the observable morphology has been shaped by CO2-driven flows. These observations of sediment transport and the tempo of gully activity are consistent with gully formation entirely by CO2 frost processes, likely with spatial and temporal variability, but with no role required for liquid water.

Geomorphologic observations and physical hypothesis on Martian gullies

We propose a hypothesis to explain the erosive phenomenon of Martian gullies under current climatic conditions. This model is consistent with the morphology and finds support in the characteristic geographic diffusion of the gullies, which seem to avoid exposure to the sun and the hottest latitudes. We hypothesize a transient flow of spring water fed by melting permafrost which can occur only in certain latitudes and altitudes, with seasonal variability and for a very short time. Furthermore, this phenomenon can only occur in certain ranges of temperature and pressure, therefore when particular and sporadic weather conditions allow it. For this reason, we propose to call it weather-springing water (WSW).

Evaluating the role of volatiles in bedrock chute formation on the Moon and Mars

Steep channel-like landforms—referred to here as bedrock chutes—line the rocky walls of some craters on the Moon and Mars. The role of volatiles, such as H₂O or CO₂, or dry rockfall in the formation of bedrock chutes is unknown on either planetary body. To test whether bedrock chute formation on Mars involved volatile activity, we used digital elevation models of Mars generated from HiRISE and CTX stereo-imagery to survey 4–9 km diameter craters globally and measure chute morphology as a function of latitude and orientation—properties that might co-vary with volatile activity. We also analyzed bedrock chutes on the Moon, which is presumably devoid of significant erosion due to volatile activity, using LROC NAC data, to serve as a volatile-free endmember for comparison with Mars. Martian bedrock chutes occur at all latitudes and have median values of chute spacing (wavelength) of ~300 m, relief of ~15 m, and slope of ~33o. Chutes on the Moon are less common and are generally steeper (~41o) with less relief (~5 m) as compared to Mars. While dry rockfall might have formed bedrock chutes on both the Moon and Mars, martian chutes are systematically deeper on pole-facing slopes between 10°S- 30°S indicating a likely role for volatile activity in chute formation. Bedrock chutes are also deeper where they co-occur with well-incised martian gully channels. The latitude-dependence for deeper bedrock chutes on pole-facing slopes extends to lower latitudes than gullies—within the contemporary tropics—indicating the potential for volatile-related activity closer to the equator than documented for gullies or other ice-related features on Mars. Chutes carved into bedrock likely form slowly compared to gully channels, which are incised into more erodible ice-cemented sediment, and therefore might provide a longer record of environmental conditions over larger swaths of Mars.

The Distribution of Frosts on Mars: Links to Present‐Day Gully Activity

AbstractNumerous types of activity in mid latitude martian gullies have been observed over the last decade. Some activity has been constrained to occur in the coldest times of year, suggesting that surficial frosts that form seasonally and diurnally might play a key role in this activity. Here we use thermal infrared data to explore the global, spatial and temporal variation of temperatures conducive to CO2 and H2O frost formation on Mars, and assess their distribution relative to gully landforms. CO2 frost detections are observed at all latitudes and are strongly correlated with dusty, low thermal inertia regions near the equator. While it is difficult to accurately detect the formation of H2O frost, the global H2O frost point distribution generally follows water vapor column abundance, and is weakly correlated with surface pressure. Most global CO2 frost detections do not contain gullies, but 47% of all gullies, and 73% of active gullies (76% in the south, and 25% in the north) do overlap with CO2 frost detections. We predict that the conditions necessary for significant present‐day gully activity include a few centimeters of CO2 frost within loose, unconsolidated sediments (I ∼ 300 ) on relatively steep (>20°) slopes. Additionally, it could be possible for small amounts of H2O frosts to play a role in present‐day equatorial mass wasting events. However, whether present‐day gully activity is representative of gully formation is still open to debate, because it seems unlikely that frosts can erode channels into rocky substrates–even considering geologic timescales.

Evidence of Exposed Dusty Water Ice within Martian Gullies

AbstractMidlatitude slopes on Mars are mantled by deposits proposed to contain H2O ice and dust, overlaid by a desiccated lag. However, direct evidence of their volatile content is lacking. Here we present novel evidence of light‐toned materials within midlatitude gully alcoves eroded into these mantles. The appearance and Lambert albedo of these materials suggests that they are either dust or H2O ice. We interpret them to be H2O ice because it is unlikely for a short‐term, localized dust deposit to form only within the mantle walls. The temperatures are generally too warm (>∼240 K) for the ice to be a frost in equilibrium. Therefore, this ice is likely similar to the dusty ice documented within midlatitude scarps, but with more dust, and exposed in smaller patches by slumping. It has been proposed that CO2 frosts remove the overlying lag, causing the exposed H2O ice to sublimate, liberate dust within the ice for transport, and erode gullies in the mantle. But we observe gullies eroded in wall rock that continue into the mantle, implying that the same process erodes both substrates. H2O ice melt can explain gullies eroded in the wall rock and the mantle. Numerical models show that relatively dense H2O snow on Mars melts only when it contains small amounts of dust. The observed exposure of dusty ice provides a mechanism for it to melt under some conditions and form some gullies. Access to liquid water within this ice could provide potential abodes for any extant life.

Global documentation of overlapping lobate deposits in martian gullies

Martian gullies are landforms comprising an alcove, channel and debris apron. In some gullies lobate deposits have been observed, implying a debris-flow-like process, but uncertainty remains as to the importance of this process in the gully-population and whether lobate deposits are related to a specific context. We document evidence of overlapping lobate deposits on gully-fan surfaces within craters emplaced between 30°-75° in both hemispheres. We have identified 26 craters in which lobate deposits occur in gullies, of which 6 were previously reported. This corresponds to 3.39% of the 765 gullied craters studied using 1004 HiRISE images. We show that gullies with lobate deposits (1) do not show any location preference, (2) are poleward-facing, (3) are found in craters with and without latitude dependent mantle (LDM) and/or glacier-like-forms, (4) are emplaced at average slopes of 22° and 15° at the foot of fans in the northern and southern hemispheres, respectively, and (5) form in craters of all ages. We infer that preservation of lobate deposits in gullies is the main factor leading to their relative paucity within the gully-population. Additionally, we find that the size of the clasts embedded in the lobes does not seem to vary with host-crater age and the presence of LDM and/or glacial landforms. Further, we observe morphological features associated with terrestrial wet-debris flows including: overlapping tongue-shaped terminal lobes, levees, channel backfilling, plug formation and avulsion. We have not observed any significant present-day changes in the morphology and topography of gullies and/or lobes. We conclude that a debris-flow-like process is likely responsible for the majority of sediment transport in gully-landforms, but whether the fluidising agent is or was liquid water or CO2 ice sublimation remains unknown.

Initiation and Flow Conditions of Contemporary Flows in Martian Gullies.

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0–1.5 m and initial release volumes of 100–200 m3. Entrainment leads to final flow volumes that are ∼2.5–5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross‐channel flow velocities decrease from 3–4 m/s to ∼1 m/s from release area to flow terminus, while flow depths generally decrease from 0.5–1 to 0.1–0.2 m. The mean cross‐channel erosion depth and deposition thicknesses are ∼0.1–0.3 m. Back‐calculated dry‐Coulomb friction ranges from 0.1 to 0.25 and viscous‐turbulent friction between 100 and 200 m/s2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of ≪1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility.

Present-day development of gully-channel sinuosity by carbon dioxide gas supported flows on Mars

Martian gullies have been widely studied, but their formation mechanism is still under debate. Their channels generally trend straight downslope, but some display sinuosity. Seasonally active gullies are common on sand dunes and their channels have been reported to develop sinuosity. Here, we perform a detailed analysis of a gully on a dune within Matara Crater (49.5°S; 34.9°E) where development of channel sinuosity has taken place over 5 martian years (MY29-33) of observation. This study was performed using HiRISE images, HiRISE elevation data, spectroscopic CRISM data and a 1D GCM for surface temperature modelling. The morphological evolution of the gully suggests a significant seasonal contribution of fluid. Each year we observed material collapse and accumulation in the alcove, followed by transport events during which lateral migration and extension of the channel occur together with growth of the debris apron. Over one martian year, the debris apron propagated by almost 140 m from an initial length of 800 m. These transport events occur in the middle of winter when CO2 frost is still present and are contemporaneous with the beginning of the defrosting. We propose that the activity and the sinuosity development in the gully could be explained by: 1) a flow composed of sand and CO2 gas, producing morphologies similar to those in hyper-concentrated flows on Earth and 2) contribution of material from alternating of the alcove source location.

About this title - Martian Gullies and their Earth Analogues

Gullies on Mars resemble terrestrial gullies involved in the transport of abundant material down steep slopes by liquid water. However, liquid water should not be stable at the Martian surface. The articles in this volume present the two main opposing theories for Martian gully formation: climate-driven melting of surficial water-ice deposits and seasonal dry-ice sublimation. The evidence presented ranges from remote-sensing observations, to experimental simulations, to comparison with Earth analogues. The opposing hypotheses imply either that Mars has been unusually wet in the last few million years or that it has remained a cold dry desert – both with profound implications for understanding the water budget of Mars and its habitability. The debate questions the limits of remote-sensing data and how we interpret active processes on extra-terrestrial planetary surfaces, even beyond those on Mars, as summarized by the review paper at the beginning of the book.

Martian gullies and their Earth analogues: introduction

Abstract Martian gullies are widespread landforms in the mid-latitudes of Mars. When the first reports of these kilometre-scale features were published in 2000, they were controversially hailed as a sign of recent flows of liquid water on the surface of Mars. This supposition was contrary to our understanding of recent environmental conditions on Mars, under which water should not exist in its liquid form. In response to their discovery, researchers proposed a wide range of scenarios to explain this apparent paradox, including scenarios driven by CO 2 , climate change or the presence of a liquid water aquifer. This Special Publication is a collection of papers arising from the topics discussed at the Second International Workshop on Martian Gullies held at the Geological Society, London. A review paper opens the Special Publication and thereafter the papers are presented under three themes: Martian remote sensing, Earth analogues and laboratory simulations. This Special Publication establishes the state of the art in Martian gully research, presents the latest observations and interpretations of the present-day activity and long-term evolution of Martian gullies, explores the role of Earth analogues, highlights novel experimental work and identifies future avenues of research. The importance of gullies as a potential marker of habitable environments on Mars underlines their importance in framing space exploration programmes.

Geomorphological analysis of gullies on the central peak of Lyot Crater, Mars

Abstract The central peak region of Lyot Crater provides an intriguing case study of Martian gully formation on local topographic highs. To better understand how these gullies formed, we carried out a detailed morphological analysis using a HiRISE stereo image pair and a digital terrain model. Gully lengths range from c. 2.2 to 4.3 km, with maximum depths from 17 to 54 m. Alcove slopes range from c. 20 to 22°, channel slopes range from 12 to 16° and apron slopes range from 10 to 14°. In general, these slopes are much lower than both the angle of repose ( c. 33°) required to initiate dry flows and the apex slope required to keep these flows from depositing (>21°) under Mars gravity. We find that the observed gully morphology and spatial associations are consistent with an origin by liquid flow. Apron volumes are c. 10–40% of the gully volume, which we suggest indicates significant volatile loss (water and/or CO 2 ) in the slope materials in which they formed because apron volumes emplaced by dry gravitational flows would equal or exceed the gully volumes due to their lower packing density. These observations, coupled with the gullies’ unique micro-environmental setting, lead us to favour a fluvial origin. In addition, we find that the gullies formed almost exclusively on the western slope region, which suggests a possible orographic component. Potential water sources may have been supplied locally in the recent geological past by dry winds blowing across an ice-covered lake situated in a low elevation area just west of the central peak or more globally during periods of higher obliquity. In either case, the integration of morphometric and morphological investigations suggests that these gullies probably formed by surface runoff and through-flow from snow- or icepack melting on the central peak of Lyot Crater.

Martian gullies: a comprehensive review of observations, mechanisms and insights from Earth analogues

Abstract Upon their discovery in 2000, Martian gullies were hailed as the first proof of recent (i.e. less than a few million years) flowing liquid water on the surface of a dry desert planet. Many processes have been proposed to have formed Martian gullies, ranging from liquid-water seepage from aquifers, melting of snow, ice and frost, to dry granular flows, potentially lubricated by CO 2 . Terrestrial analogues have played a pivotal role in the conception and validation of gully-formation mechanisms. Comparison with the terrestrial landscape argues for gully formation by liquid-water debris flows originating from surface melting. However, limited knowledge of sediment transport by sublimation is a critical factor in impeding progress on the CO 2 -sublimation hypothesis. We propose avenues towards resolving the debate: (a) laboratory simulations targeting variables that can be measured from orbit; (b) applications of landscape-evolution models; (c) incorporation of the concept of sediment connectivity; (d) using 3D fluid-dynamic models to link deposit morphology and flow rheology; and (e) a more intense exchange of techniques between terrestrial and planetary geomorphology, including quantitative and temporal approaches. Finally, we emphasize that the present may not accurately represent the past and that Martian gullies likely formed by a combination of processes.

Are different Martian gully morphologies due to different processes on the Kaiser dune field?

Abstract We describe and compare the morphology and activity of two types of gullies with different orientations collocated on the Kaiser dune field in the southern hemisphere of Mars: large apron gullies and linear dune gullies. The activity of large apron gullies follows an annual cycle: (i) material collapse into the alcove (mid-autumn/late winter) as CO 2 condenses; (ii) remobilization by mass flows (late winter); and (iii) continuous appearance of hundreds of ‘digitate flows’ on the fan (autumn/winter). We find that large apron gullies could form in hundreds of Martian years. In contrast, linear dune gullies are active briefly in late winter, when the CO 2 frost disappears. Their activity is characterized by the extension of channels, the creation of pits and the darkening of the surface. Linear dune gullies are likely to form within one to tens of Martian years. We infer that insolation, which influences the depth to ground ice and the amount of volatile deposited, may be the factor differentiating large apron gullies and linear dune gullies. Sediment transport by CO 2 sublimation is a good candidate for the activity observed in all of these features. However, linear gullies could also be formed by brine release when the temperature rises abruptly after the removal of the CO 2 ice.

Debris flows and water tracks in northern Victoria Land, continental East Antarctica: a new terrestrial analogue site for gullies and recurrent slope lineae on Mars

Abstract Although the present environmental conditions on Mars prohibit the generation of significant volumes of liquid water, observations of several very young landforms, such as gullies and recurrent slope lineae, have been interpreted as signals for aqueous processes. To explore the range of conditions under which such features can be formed on Earth, a field site in northern Victoria Land, East Antarctica, was geomorphologically investigated. Despite the small size of the ice-free area, the site displays gullies, water tracks and other traces of liquid water. The gullies show clear evidence of sediment transport by debris flows, and are typical of paraglacial processes on steep slopes in a recently deglaciated area. Water tracks appear in different forms, and seem to recur seasonally in the austral summer. Melting of snow and surface glacier ice is the major water source for both debris flows and water tracks. The observations presented here highlight the potential for hyperarid polar deserts to generate morphogenetically significant amounts of meltwater. The gullies are morphologically analogous to Martian gullies, and water tracks on steep slopes appear very similar to recurrent slope lineae. The observations suggest that even small ice-free sites in continental Antarctica may enable observations which can serve as a basis for working hypotheses in Mars analogue studies, and future field work should consider more areas in Antarctica in addition to the McMurdo Dry Valleys to search for Mars analogue landforms.

CO 2 sublimation in Martian gullies: laboratory experiments at varied slope angle and regolith grain sizes

Abstract Martian gullies were initially hypothesized to be carved by liquid water, due to their resemblance to gullies on Earth. Recent observations have highlighted significant sediment transport events occurring in Martian gullies at times and places where CO 2 ice should be actively sublimating. Here we explore the role of CO 2 sublimation in mobilizing sediment through laboratory simulation. In our previous experimental work, we reported the first observations of sediment slope movement triggered by the sublimation of CO 2 frost. We used a Mars regolith simulant near the angle of repose. The current study extends our previous work by including two additional substrates, fine and coarse sand, and by testing slope angles down to 10°. We find that the Mars regolith simulant is active down to 17°, the fine sand is active only near the angle of repose and the coarse sand shows negligible movement. Using an analytical model, we show that under Martian gravity motion should be possible at even lower slope angles. We conclude that these mass-wasting processes could be involved in shaping Martian gullies at the present day and intriguingly the newly reported CO 2 -creep process could provide an alternative explanation for putative solifluction lobes on Mars.