Abstract With the advent of the space age, planetary geomorphology has become a stand-alone discipline. This contribution provides a summary of the different processes that have been identified to form landscapes and landforms on planetary bodies in our Solar System, including rocky planets, icy planets and moons, dwarf planets, comets and asteroids. I highlight the insights these landforms have provided into the workings of these bodies and how what has been learnt in space has often taught us new lessons about the Earth. Finally, I conclude that despite the limitations imposed by remote sensing, planetary geomorphology has a bright future in planning future missions to explore our Solar System as well as understanding the data that will be returned.

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.

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.

Abstract We describe in detail an annual seasonal process that occurs on the surface of the Russell Crater megadune on Mars. We give these features the name ‘perennial rills’, because their surface topographical expression persists from year-to-year and they form a distinctive, downstream-branching network of small channels, or rills. We used time-series images, elevation data from stereophotogrammetry and spectral data to characterize the evolution of these features over 6 Mars years. Growth and modification of these networks occurs abruptly in spring (at a solar longitude of c. 200°) after most of the seasonal CO 2 ice has sublimated. We find that the peculiar morphology of perennial rills seems to be the only aspect that sets them apart from active linear dune gullies. By comparison to terrestrial analogues, we identified two conditions favouring the production of such a network: (a) the presence of an impermeable layer; and (b) the repeated formation of obstacles in front of propagating channels. We find that the most plausible formation mechanisms that can explain the formation of both the perennial rills and the active linear dune gullies are levitating CO 2 blocks or liquid debris flows of water/brine, but neither can completely satisfy all the observational evidence.

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.

Abstract Gasa crater has been the most active site observed on Mars to date, making it of particular interest for studying the process(es) behind gully formation and activity. In this study, we investigate whether differences in thermal inertia across different segments of gully systems, combined with morphological and colour observations with High-Resolution Imaging Science Experiment (HiRISE), can provide some constraints on the physical characteristics associated with recent activity within gullies in Gasa. We also investigate thermophysical differences between slopes in Gasa dominated by gully activity compared to those predominantly modified by dry mass-wasting processes. We find that Gasa crater exhibits clear variations in thermal inertia across its walls, controlled by the material properties and the types of dominant mass movement processes occurring on each wall. The youthful gully-fan lobes display thermal inertia values c. 20–40 J m −2 K −1 s −1/2 higher than adjacent older eroded and dust-covered lobes. The talus aprons from mass wasting in Gasa have thermal inertia values c. 60–80 J m −2 K −1 s −1/2 higher than gully aprons. The results of this study thus suggest that thermal imaging can inform us on surface change detection on Mars.

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. Supplementary material: Video clips depicting sediment transport types are available at https://doi.org/10.6084/m9.figshare.5208847

Abstract Gullies are widespread morphological features on Mars for which current changes have been observed. Liquid water has been one of the potential mechanisms to explain their formation and activity. However, under present-day Martian conditions, liquid water is unstable and should only be transiently present in small amounts at the surface. Yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure. Here we present the results of laboratory experiments studying the interaction between liquid water flowing over a sand bed under Mars-like atmospheric pressure ( c. 9 mbar). The experiments were performed in a Mars Simulation Chamber (at the Open University, UK), in which we placed a test bed of fine sand at a 25° slope. We chose to investigate the influence of two parameters: the temperature of the water and the temperature of the sand. We performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 K. Under all experimental conditions, the water was boiling. We investigated and compared the types and timing of sediment transport events, and the shapes, characteristics and volumes of the resulting morphologies. In agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water. We found four main types of sediment transport: entrainment by overland flow; grain ejection; grain avalanches; and levitation of saturated sand pellets. Our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism. The temperature of the water played a negligible or minor role, apart from the duration of sand ejection and avalanches, which lasted longer at low water temperature. At low sand temperature the majority of the sand was transported by overland flow of the liquid water. At higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water. At the highest temperatures, sediment transport was dominated by the formation of levitating pellets, dry avalanches and ejection of the sand grains. This resulted in a transport volume about nine times greater at a sand temperature of 297 K compared with 278 K. Our heat transfer scaling shows that the boiling behaviour will be enhanced under Martian low gravity, resulting in more efficient transport of sediment by levitating sand pellets even at temperatures close to the triple point. Our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water. This implied that the amount of water required to produce morphological changes at the surface of Mars could be lower than previously estimated by assuming stable liquid water. Boiling is a critical process to be considered when assessing gully formation and modification mechanisms mobilized by liquid water. Our work could have similar implications for any water-formed landform on Mars, which could include recurring slope lineae, dark dune flows and slope streaks. Supplementary material: Videos of the experiments are available at https://doi.org/10.6084/m9.figshare.c.3990330

Abstract The formation process of recent gullies on Mars is currently under debate. This study aims to discriminate between the proposed formation processes – pure water flow, debris flow and dry mass wasting – through the application of geomorphological indices commonly used in terrestrial geomorphology. High-resolution digital elevation models (DEMs) of Earth and Mars were used to evaluate the drainage characteristics of small slope sections. Data from Earth were used to validate the hillslope, debris-flow and alluvial process domains previously found for large fluvial catchments on Earth, and these domains were applied to gullied and ungullied slopes on Mars. In accordance with other studies, our results indicate that debris flow is one of the main processes forming the Martian gullies that were being examined. The source of the water is predominantly distributed surface melting, not an underground aquifer. Evidence is also presented indicating that other processes may have shaped Martian crater slopes, such as ice-assisted creep and solifluction, in agreement with the proposed recent Martian glacial and periglacial climate. Our results suggest that, within impact craters, different processes are acting on differently oriented slopes, but further work is needed to investigate the potential link between these observations and changes in Martian climate.