Abstract This book concerns the Martian landscape; that collection of volcanoes, valleys, impact craters and ice caps that recent images reveal both to be strikingly familiar but also strangely alien to the surface of our own planet. The primary aim of studying planetary landscapes is to understand the process(es) by which they formed, with the larger goal of unravelling key questions about the origin, evolution and potential habitability of our solar system. Compared with Earth, Mars' surface erosion rates are extremely low ( Golombek & Bridges 2000 ), so Martian landscapes ranging in age from the very ancient to the recent still remain preserved and amenable to observation. Because so much of the planet's geological history remains visible, Martian geomorphology has the potential to provide even deeper insights into the early evolution of the planet than is the case for terrestrial geomorphology. Furthermore, the lack of precipitation (at least for much of Martian geological history: Craddock & Howard 2002 ), vegetation or human influence have preserved landforms on the surface of Mars that on Earth are obscured, degraded or buried, and only recognizable from interpretation of the sedimentary rock record. These observations, together with the fact that virtually all of the geological processes seen on Earth are believed to have also occurred on Mars, make it a powerful laboratory for comparative studies of geomorphological processes. Like any dominantly remote-sensing approach, studies of the Martian surface must account for in situ data, but outcrop and hand-sample examination is a luxury afforded

Abstract Mars is the fourth planet in our Solar System and orbits roughly 230×10 6 km from the Sun. It has an orbital period of 687 Earth days and a solar day that is approximately 40 min longer than an Earth day. Mars is less dense and has half the radius of the Earth, and so has about one-tenth the mass; hence, the surface gravity of Mars is about four-tenths that of the Earth. Mars has no oceans and its surface area is therefore almost as large as that of Earth's continents. In this chapter, we present a summary of the Martian environment, global geography and geology, and provide some background on the missions and instruments that have played a role in developing our current understanding. Our aim is to provide a broad overview for those unfamiliar with Mars, rather than providing an exhaustive summary of every aspect of the planet's evolution.

Abstract The surface of Phobos, the 27×22×18 km inner moon of Mars, is dominated by several families of parallel grooves. At least seven different groups of hypotheses have been advanced to explain their origin, but studies have always been limited by the fact that, until recently, much of Phobos was imaged at a resolution too low to show grooves. Now, however, the High Resolution Stereo Camera (HRSC) on board the European Mars Express mission has made 134 imaging fly-bys past Phobos. The pictures of the previously poorly imaged regions and much of the rest of the satellite have been returned with resolutions down to a few metres, facilitating the construction of a more complete map of the grooves. Each of the seven hypotheses was tested against the new data on groove morphology, positions and orientations, and it was found that six of the previous hypotheses could be discarded. The only hypothesis to pass all tests was that they are chains of secondary impact craters from primary impacts on Mars. An implication of these results is that previous estimates of an unusually thick Phobos regolith of 100–200 m depth are no longer necessary, and our conclusions place no constraints on the interior of Phobos, so recent evidence that Phobos is a ‘rubble pile’ is consistent with our work. The preferred hypothesis also sheds light on the origin of crater chains on Eros, and on impact processes in the early stages of crater excavation.

Abstract A systematic survey was undertaken and an investigation carried out into the geomorphological characteristics of lobate debris aprons in the Tempe Terra region of Mars. Based on the most recent high-resolution (sub 15 m per pixel) imagery and on new topography data, this study endeavoured to raise and discuss questions regarding their formation (emplacement) and modification (deformation sequence), as well as the role of a mantling deposit found at mid-latitude locations on Mars. Furthermore, a model for the formation of debris aprons in the Tempe Terra–Mareotis Fossae settings is proposed. Image survey, in combination with basic morphometric observations within a geomorphological context, provided additional insights into the source, emplacement and modification of hillslope debris material. Our results imply that lobate debris aprons are not mainly relicts of remnant degradation but are substantially composed of mantling material probably deposited episodically in the course of planetary obliquity changes and over a long timespan, as derived erosion rates suggest. Crater-size frequency statistics and the derivation of absolute ages show ages of sub-recent modification and document earlier resurfacing events.

Abstract Possible periglacial and relict glacial landforms in the ancient mountain range of the Thaumasia Highlands, Mars, are described. The landforms include large-scale mantling, lineated crater and valley-fill materials, debris aprons, protalus lobes and ramparts. The most pristine ice-related landforms appear to be small-scale protalus lobes and ramparts with no visible distinct impact craters at both medium (High Resolution Stereo Camera (HRSC)) and high (Mars Orbiter Camera (MOC) narrow angle (NA), Context Camera (CTX)) spatial resolution. These small landforms are possibly active at present and post-date more extensive features such as crater fills, possibly formed during high obliquity climatic periods. In contrast to the rock glacier-like landforms with distribution preferentially occurring on south-facing slopes, possibly controlled by enhanced exposure to the Sun, older, less pristine lineated fill materials show a less systematic distribution of flow directions, suggesting a more generalized periglacial and possibly glacial environment in the Thaumasia Highlands.

Abstract The confirmation of near-surface ground ice and perchlorates at the Phoenix landing site suggest that high-latitude ground-ice thaw may be more easily achieved than previously envisaged, providing the potential to drive significant, distinctive morphogenesis. We describe the results of a survey of 23 High Resolution Imaging Science Experiment (HiRISE) images covering 337° of longitude between latitudes 59°N and 79°N in which such morphogenesis is apparent, confirming that thaw has been a regionally important morphological agent. Some of the strongest geomorphological indicators of cyclical ground-ice thaw described are assemblages of sorted landforms, including clastic patterned ground resulting from cryoturbation of ice-rich regolith and lobate forms reflecting solifluction. Also described are braided gully-fan systems sourced at thermokarst pits and channels that have evolved from enlarged thermal contraction cracks. Not only are these landforms indicative of thaw and flowing liquid but the incision of solifluction lobes by thermokarst gullies demonstrates that thaw has been responsible for polycyclic morphogenesis. The presence of these landforms across the high northern latitudes of Mars indicates that the regional importance of thaw has been underestimated. This in turn has important implications for the development of better climate models and the search for life on Mars.

Abstract Periglacial landforms on Spitsbergen (Svalbard, Norway) are morphologically similar to landforms on Mars that are probably related to the past and/or present existence of ice at or near the surface. Many of these landforms, such as gullies, debris-flow fans, polygonal terrain, fractured mounds and rock-glacier-like features, are observed in close spatial proximity in mid-latitude craters on Mars. On Svalbard, analogous landforms occur in strikingly similar proximity, which makes them useful study cases to infer the spatial and chronological evolution of Martian cold-climate surface processes. The analysis of the morphological inventory of analogous landforms on Svalbard and Mars allows the processes operating on Mars to be constrained. Different qualitative scenarios of landscape evolution on Mars help to better understand the action of periglacial processes on Mars in the recent past.

Abstract Sublimation-related landforms are ubiquitous on Mars, especially at mid to high latitudes. This paper reviews the main landforms interpreted to form due to sublimation of subsurface ice on Mars. Pits, knobs and dissected terrains are classical landforms thought to form due to subsurface ice sublimation as observed with high-resolution imagery. Sublimation-related processes on Mars are strongly latitude dependent, with sublimation being increasingly important from high (>60°) to low latitudes (down to 25°) due to correspondingly higher mean annual temperatures. Equatorial regions (within 25° latitude) are mainly devoid of any sublimation-related landforms, reflecting an ice-free shallow subsurface. Mean temperatures and water vapour pressure strongly control the sublimation rate, but diffusion and water adsorption are fundamental and vary depending on the regolith porosity and composition, leading to variations in the theoretical depth at which water ice becomes stable. From a geomorphological point of view, this review highlights the importance of subsurface structure (fractures, layering) in the shaping of landforms and in the control of sublimation rates, in addition to usual physicochemical parameters.

Abstract Martian gullies are small-scale, geologically recent features characterized by the alcove-channel-apron morphology associated with flows with a component of liquid water. Theories advanced to explain Martian gully formation include groundwater processes and melting of near-surface ice due to climate variation. Gullies are often associated with ‘mantling terrain’ that drapes topography at mid to high latitudes and which has been proposed to be ice-rich. We have morphologically classified Martian gullies into four groupings according to whether they form solely within the mantle (Type A), erode into ‘bedrock’ (Type B), and by how well developed they appear (1 or 2). Orientation, length, geological setting and latitude were also recorded, as well as whether more than one generation of gullies formed on a given slope (labelled ‘reactivated’). About 25% of gullies form solely within the mantle; these are generally shorter than gullies that erode bedrock and the morphologically simplest gullies (A1) are the shortest. We present latitude and orientation trends for the most recent episode of gully formation. We suggest that this recent activity is probably controlled by either deposition of ice-rich material or degradation of pre-existing ice-rich material.

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.

Abstract Lethe Vallis is an approximately 230 km-long and 1.5 km-wide channel connecting several shallow basins in the Elysium Planitia region of Mars. It sits within a distinctive morphological unit defined by a platy-ridged-polygonized texture. We have documented the geomorphology of the system, and constructed topographical long profiles of the channel thalweg and the contacts of the platy-ridged-polygonized material. The Lethe thalweg is shallow (with a slope of about 0.0001) but contains steeper sections that match the locations of observed cataract systems. The contact profiles suggest that the small basins linked by Lethe progressively ponded and over-spilled as the system developed, the cataracts being associated with this over-spill. Other landforms observed in the system include streamlined islands, anastomosing distributary systems, fluvial hanging channels and terraces on the channel margins. There are also possible dunes and/or antidunes within the channel. These all point to catastrophic fluvial flooding. Estimates of formative discharge are of the order of 1×10 4 –5×10 4 m 3 s −1 , similar to the discharge of the Mississippi River. We infer that Lethe Vallis formed as a fluvial ‘fill and spill’ catastrophic flood system. This demonstrates that the main Western Elysium Basin, the upstream source of Lethe Vallis, contained a substantial transient lake.

Abstract A series of fluid-carved channels in the Sulci Gordii region of Mars were investigated. Numerous channel networks exist in Sulci Gordii, part of the Olympus Mons aureole, and this area comprises some of the youngest volcanic terrain on Mars. The channels ranged in length from 43 to 155 km, with widths of 128–288 m. The morphology of the channels was analysed assuming both lava and water as possible agents. For three of the four channels studied, water appears to be the likely agent, while one channel is probably lava-formed. For the water-formed channels, discharge rates were estimated at 8000–36 000 m 3 s −1 . The lava channel was probably formed from short-lived episodic activity by a low-viscosity lava. The age of the channels and surrounding area was estimated using crater counting to be 100 Ma. Water has appeared to have flowed for almost 150 km under the climatic conditions at this time. There is some evidence for later tectonic activity, possibly as recent as 10 Ma, but crater-dating accuracy was limited by the lack of high-resolution images of some areas. Sulci Gordii is therefore a dynamic site with evidence of hydrological and volcanic activity extending into the recent geological past.