Gully Formation Research Articles

Marine geophysical evidence for ice sheet extension and recession on the Malin and Northwest Irish shelves: New evidence for the western limits of the British Irish Ice Sheet

Gullies are a class of geologically recent landform on Mars that show evidence of downslope transport of sedimentary material from recessed alcoves, through incised channels, to distributary fans or aprons. The mechanisms invoked to account for the formation of gullies on Mars range from completely dry, granular flows or landslides, to debris flows that incorporate some component of liquid water, to fluvial erosion and alluvial-fan-like deposition. Each of these processes requires different amounts of liquid water, and produces different characteristic morphologies. We report on the identification of unusual lobate structures present in proximity to gullies in Protonilus Mensae. The lobes are up to ~ 3 m thick and terminate in rounded snouts. These lobate structures are present mostly downslope of gullies, and can be traced upslope through channels, to gully fan termini, and in places, onto gully fan surfaces. Crater dating indicates that the deposits formed recently—potentially within the past ~ 500 ka. We use HRSC digital elevation models to constrain mechanical properties of the lobate deposits, and to compare their formation environment to that of typical martian gullies. The Protonilus Mensae lobate deposits are interpreted to indicate local dominance of wet debris flows in the formation of the observed gullies and lobes. These observations are consistent with 1) top-down melting of the ice component of the latitude-dependent mantle terrain in which the gullies form and 2) initiation of debris flows by mobilization of the dusty lithic component of the mantle. The suite of morphological observations diagnostic of wet debris flow processes suggests the identification of an unusual environment in which the wet debris-flow formational end-member is locally the dominant gully forming process; elsewhere on Mars gully morphology may be more consistent with a range of other water-related sediment transport mechanisms including fluvial erosion, hyperconcentrated flow, and low-strength mudflows (that deposit as fans rather than lobes). These results suggest that 1) gully-forming processes involve liquid water, 2) that the water source is associated with the martian latitude-dependent mantle, and 3) that a range of water-related sediment transport processes is involved in gully formation.

Analysis of a Deep-Seated Landslide in Permafrost, Richardson Highway, South-Central Alaska

A portion of the Richardson Highway near Glennallen, Alaska, is threatened by slope movement in an area where the highway travels along a bluff overlooking the Copper River. Analysis of aerial photographs indicates that portions of the bluff have experienced an average of 1.5 m of retreat per year since 1979, and gully erosion has completely obliterated portions of the pre-1965 highway alignment. The subsurface consists of a 1.5-m-thick surficial silt layer underlain by frozen, ice-rich clayey soils with an average annual temperature of −0.56°C. At 16 m below the ground surface, the soils become ice-poor and coarser grained. In situ measurements, soil creep tests, and slope stability analysis indicate that the ice-rich clayey soils are experiencing movement, with creep velocities of up to 2.5 cm per year. This deep-seated creep results in fracturing of the near-surface soil layer that is manifested at the surface by cracks, scarps, and back-tilted blocks. Groundwater perched above the permafrost table and seeping toward the bluff face facilitates the rapid retreat of the bluff edge through the formation of gullies.

Geomorphology of a thermo-erosion gully, Bylot Island, Nunavut, Canada1This article is one of a series of papers published in this CJES Special Issue on the theme of Fundamental and applied research on permafrost in Canada.2Polar Continental Shelf Project Contribution 043-11.

A thermo-erosion gully has been monitored in the valley of glacier C-79 on Bylot Island since 1999. The main channel of the gully reached 390 m in length a few months after its initiation and grew between 38 and 50 m/year over the following decade, for an overall approximated average of 75 m/year. In 2009, the total gully length and area, including the main and relict channels, were 2500 m and 25 000 m2, respectively. Gullies affect snow accumulation, and therefore ground temperature, local water flow, and drainage. Sinkholes, gully heads, pools, baydzherakhi, tunnels, and collapses were grouped as a function of time since gully formation in that area. Sinkholes and tunnels were formed every year after gully inception, and baydzherakhi were found in 3–10 year old sections of the gully. Stabilization of the gully floor and sides took about a decade.

Morphological and mechanical characterization of gullies in a periglacial environment: The case of the Russell crater dune (Mars)

Gullies on terrestrial sand dunes are rare, and their presence on Mars, as well as their mechanical properties, and the quantity of fluid required for their formations currently remain poorly understood. This study focuses on gully morphologies on the Russell megadune (54.5°S; 12.7°E) using High Resolution Imaging Science Experiment (HiRISE) images and Digital Terrain Models (DTM). Based on the scenario of ground ice melting in a periglacial environment, we propose to test the hypothesis that Martian gullies on dunes are debris flows. This implies a flow with a significant proportion of liquid water (>10% in volume). We used an original method to study Martian gullies based on empirical equations from terrestrial debris flows in order to calculate the physical properties of Martians flows. We observe a decrease in viscosity induced by the relative increase of fluid concentration (from 28% to 39%) during the flow advance. The total estimated volume of eroded and deposited material range from ∼14850m3 to ∼18890m3. The volume of liquid water required to generate one gully ranges from 4450m3 to 6900m3. The calculated results for Martian gullies are consistent with terrestrial studies on debris flows. Based on a morphological description and on the estimated physical parameters, we propose a model for gully formation on Martian dunes. The melt water from near-surface ground ice is incorporated in the debris flow and water concentration increases during its propagation. The increase of water concentration in the debris flow can be explained by a progressive increase of water/ice content in the permafrost downstream. Consequently, the lack of a final deposit at the front of the gullies tends to demonstrate that the flow became relatively highly concentrated in liquid downstream and all the water could have been lost in the final stage of the flow. This process could explain the lack of terminal lobes at the front of the gullies. We conclude that a process of formation similar to terrestrial debris flows is plausible. The large amount of liquid water involved requires formation of Martian gullies on dunes during a warmer climatic episode.

Post-fire rill and gully formation, Schultz Fire 2010, Arizona, USA

The Schultz Fire burned 6100ha on the eastern slopes of the San Francisco Peaks in northern Arizona. The fire burned between June 20th and 30th, 2010, across moderate to very steep ponderosa pine and mixed conifer watersheds. About 40% of the fire area was classified as high-severity, mostly on mountain slopes greater than 30% and in places exceeding 100%. The upper slopes rise to over 3300m and are the source for high energy water, coarse sediments, and woody material. A series of flood events beginning in mid-July 2010 initiated erosion that removed substantial soil from the upper slopes of the watersheds. The second event was characterized by a peak rainfall of 24mm in 10min. Substantial amounts of soil were eroded out of a newly developed rill and gully system, removing the A horizon and much of the B horizon. Prior to the fire there were no rills or gullies as the soil was protected by a thick O horizon. This protective organic layer burned off during the fire leaving the soil exposed to raindrop impact and erosion. There was widespread occurrence of high severity fire, with some watersheds classified as 70% high severity wildfire. This left most of the soils with moderate to severe water repellency. The development of an extensive rill and gully network fundamentally changed the hydrologic response of the upper portions of every catchment. The intense, short duration rainfall of the 2010 monsoon interacted with slope, water repellency and extensive areas of bare soil to produce flood flows an order of magnitude in excess of flows produced by similar pre-fire rainfall events. The rill networks now cover much of the upper slopes and gullies are beginning to cut to bedrock. Sediment delivery to the channel systems is likely to continue unabated for many years and hydrologic response will continue to be flashy.

Seasonal activity and morphological changes in martian gullies

Recent studies of martian dune and non-dune gullies have suggested a seasonal control on present-day gully activity. The timing of current gully activity, especially activity involving the formation or modification of channels (which commonly have been taken as evidence of fluvial processes), has important implications regarding likely gully formation processes and necessary environmental conditions. In this study, we describe the results of frequent meter-scale monitoring of several active gully sites by the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). The aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non-dune slopes. Our observations indicate that (1) gully formation on Mars is ongoing today and (2) the most significant morphological changes are strongly associated with seasonal frost and defrosting activity. Observed changes include formation of all major components of typical gully landforms, although we have not observed alcove formation in coherent bedrock. These results reduce the need to invoke recent climate change or present-day groundwater seepage to explain the many martian gullies with pristine appearance.

Gasa impact crater, Mars: Very young gullies formed from impact into latitude-dependent mantle and debris-covered glacier deposits?

The mode of formation of gullies on Mars, very young erosional–depositional landforms consisting of an alcove, channel, and fan, is one of the most enigmatic problems in martian geomorphology. Major questions center on their ages, geographic and stratigraphic associations, relation to recent ice ages, and, if formed by flowing water, the sources of the water to cause the observed erosion/deposition. Gasa (35.72°S, 129.45°E), a very fresh 7-km diameter impact crater and its environment, offer a unique opportunity to explore these questions. We show that Gasa crater formed during the most recent glacial epoch (2.1–0.4Ma), producing secondary crater clusters on top of the latitude-dependent mantle (LDM), interpreted to be a layered ice-dust-rich deposit emplaced during this glacial epoch. High-resolution images of a pre-Gasa impact crater ∼100km northeast of Gasa reveal that portions of the secondary-crater-covered LDM have been removed from pole-facing slopes in crater interiors near Gasa; gullies are preferentially located in these areas and channels feeding alcoves and fans can be seen to emerge from the eroding LDM layers to produce multiple generations of channel incision and fan lobes. We interpret these data to mean that these gullies formed extremely recently in the post-Gasa-impact time-period by melting of the ice-rich LDM. Stratigraphic and topographic relationships are interpreted to mean that under favorable illumination geometry (steep pole-facing slopes) and insolation conditions, melting of the debris-covered ice-rich mantle took place in multiple stages, most likely related to variations in spin-axis/orbital conditions. Closer to Gasa, in the interior of the ∼18km diameter LDM-covered host crater in which Gasa formed, the pole-facing slopes display two generations of gullies. Early, somewhat degraded gullies, have been modified by proximity to Gasa ejecta emplacement, and later, fresh appearing gullies are clearly superposed, cross-cut the earlier phase, and show multiple channels and fans, interpreted to be derived from continued melting of the LDM on steep pole-facing slopes. Thus, we conclude that melting of the ice-rich LDM is a major source of gully activity both pre-Gasa crater and post-Gasa crater formation. The lack of obscuration of Gasa secondary clusters formed on top of the LDM is interpreted to mean that the Gasa impact occurred following emplacement of the last significant LDM layers at these low latitudes, and thus near the end of the ice ages. This interpretation is corroborated by the lack of LDM within Gasa. However, Gasa crater contains a robustly developed set of gullies on its steep, pole-facing slopes, unlike other very young post-LDM craters in the region. How can the gullies inside Gasa form in the absence of an ice-rich LDM that is interpreted to be the source of water for the other adjacent and partly contemporaneous gullies? Analysis of the interior (floor and walls) of the host crater suggest that prior to the Gasa impact, the pole-facing walls and floor were occupied by remnant debris-covered glaciers formed earlier in the Amazonian, which are relatively common in crater interiors in this latitude band. We suggest that the Gasa impact cratering event penetrated into the southern portion of this debris-covered glacier, emplaced ejecta on top of the debris layer covering the ice, and caused extensive melting of the buried ice and flow of water and debris slurries on the host crater floor. Inside Gasa, the impact crater rim crest and wall intersected the debris-covered glacier deposits around the northern, pole-facing part of the Gasa interior. We interpret this exposure of ice-rich debris-covered glacial material in the crater wall to be the source of meltwater that formed the very well-developed gullies along the northern, pole-facing slopes of Gasa crater.

AGNPS GIS-Based Tool for Watershed-Scale Identification and Mapping of Cropland Potential Ephemeral Gullies

The formation of ephemeral gullies in agricultural fields has been recognized as an important source of sediment contributing to environmental degradation and compromising crop productivity. Methodologies are being developed for assessing gully formation and gully sediment yield. The Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model is an important tool for multi-temporal watershed-scale simulations because it contains the necessary components for ephemeral gully investigation, making AnnAGNPS a commonly used tool for evaluations of agricultural conservation and operation practices. AnnAGNPS requires the user to define the location of ephemeral gullies throughout the watershed, what often constitutes a time consuming task where users may not accurately locate and describe all ephemeral gully locations. Alternatively, herein a GIS-based graphical user interface is described for the automated identification of areas with high probability of forming ephemeral gullies, referred to as potential ephemeral gullies (PEGs), based on the modified Compound Topographic Index (CTI). Through the aid of a study case, PEG mouth locations along with their attributes are generated through an iterative procedure by varying different CTI threshold values (99.9%, 99.5%, 99.0%, 98.5%, and 98%). Three sets of yielded PEG mouth locations and corresponding attributes were then integrated with AnnAGNPS for assessment of the impact of potential ephemeral gullies in the watershed sediment erosion. Analysis of the spatial distribution of estimates of annual average of gully erosion identifies the sub-watersheds prone to ephemeral gully erosion, thus enhancing the applicability of AnnAGNPS to evaluate conservation practices and/or targeted interventions designed to address ephemeral gully erosion at the watershed-scale.

Quantifying interrill and ephemeral gully erosion in a small Sicilian basin interrill and ephemeral gully erosion in a small Sicilian basin

applied to document the contribution of rill-interrill erosion over the same area. Ephemeral gully formation occurred seven hydrologic years out of ten, with a frequency corresponding to 70 % of the years covered occurred seven hydrologic years out of ten, with a frequency corresponding to 70 % of the years covered by the survey and to a return period of almost 1.4 years. The mean soil loss in the seven years when EG by the survey and to a return period of almost 1.4 years. The mean soil loss in the seven years when EG was active was equal to 37.88 t ha was active was equal to 37.88 t ha –1 year –1 . The ratio between soil loss attributable to EG and total soil loss . The ratio between soil loss attributable to EG and total soil loss (SEDD + EG) resulted quite variable between erosion events accounting for 23 % to 98 % of total soil lost. EG) resulted quite variable between erosion events accounting for 23 % to 98 % of total soil lost. The overall results indicate that there are signifi cant and important differences between these two types of The overall results indicate that there are signifi cant and important differences between these two types of degradation processes elucidating the importance of EG erosion in semiarid regions. Also, the contribution degradation processes elucidating the importance of EG erosion in semiarid regions. Also, the contribution due to EG erosion emphasizes the need to extend the existing data set of soil loss by appropriate measuredue to EG erosion emphasizes the need to extend the existing data set of soil loss by appropriate measurements and fi eld campaigns. ments and fi eld campaigns.

Pedological perspective of gully erosion sites within Kendu escarpment-Sondu Miriu Region, West Kenya

Gully erosion is a common feature in western Kenya, rendering large expanses of otherwise arable land uncultivable and uninhabitable. Gully erosion in the area was classified into two types: the Awach-type and the Sondu-type. The current study aimed at providing insight into physical and chemical properties of soil that promote soil erosion and determine the gully formation type. Field studies were conducted at 4 sites (3 eroded and 1 non- eroded). Physical (particle size distribution, structure stability, infiltration rate, dispersibility, and soil hardness), and chemical (pH, exchangeable cations, total-C, and total-N) properties of soil samples collected from the sites were analyzed in the laboratory. The results showed alkaline pH; high exchangeable sodium (Na) percentage (ESP); fragile soil structure; high dispersibility; and low infiltration rate are major factors contributing to the formation of Awach-type of gully. In contrast, impermeable top layer of hard crust and coarse-textured soil layers beneath it in addition to high ESP facilitated the formation of Sondu-type of gully. Soils of the eroded sites and the non-eroded sites differed mainly in the ESP, which influenced the soil structure stability, water infiltration rate, and soil dispersibility. Susceptibility of soil layers to erosion depended on the magnitude of ESP and sand content. Key words: Exchangeable sodium percentage, gully erosion, infiltration rate, Kenya, soil dispersion, soil structure

CO2gas fluidization in the initiation and formation of Martian polar gullies

[1] Martian gully landforms, indicative of rapid flow of sediment down steep slopes, have been cited as evidence of the action of near-surface liquid water in Mars' recent past. Gullies in polar regions cannot involve pure liquid water because ambient temperatures are too low. Here, we show that polar gullies could be initiated by fluidization of sediment over a subliming seasonal deposit of CO2 frost, as has been proposed previously. For gullies in sand, the gas speed and CO2 sublimation rate required for fluidization is calculated with the semi-empirical Ergun relation (as validated in industrial applications). For gullies in dust, the gas speed and sublimation rate required for fluidization are estimated from laboratory experiments with comparable materials. To determine if those rates can be achieved, we compute diurnal and seasonal surface (and subsurface) temperatures through a Martian year (including CO2 condensation and sublimation) using the MARSTHERM one-dimensional finite difference thermal model. Models were run without sediment, and with layers of sand or dust 10−4–10−1 m thick deposited over a seasonal layer of CO2 frost. The simulations show that, in the spring, sufficient heat reaches the CO2 frost, underlying the surface sediment layer, to fluidize them. This result confirms that Martian gullies may have diverse origins – and that polar and mid-latitude gullies can be initiated on steep slopes by the fluidization of loose sediment on a sublimating seasonal deposit of CO2 gas.

Morphodynamics of a gully and floodout system in the Sneeuberg Mountains of the semi-arid Karoo, South Africa: Implications for local landscape connectivity

Abstract The aim of this study was to investigate sedimentary processes in a discontinuous gully system, and to establish how they impacted upon overland flow and erosion in the short-term, while also considering their impact on long-term landscape connectivity. This manuscript documents the effect that a series of discontinuous gullies, located in the semi-arid Karoo (South Africa), had on local valley morphology and landscape evolution. The longitudinal and cross-sectional morphology of two discontinuous gully systems were accurately surveyed. In addition, the sedimentology of eight cores was described and sediment particle size and percentage organics determined, while the activity of the radioactive element 210 Pb was measured in three of the cores. In contrast to previous studies, floodouts on the Karoo gullies formed progressively downstream, as avulsion occurred on the floodout more quickly than upstream backfilling. 210 Pb geochronology suggests that the most recent avulsion occurred following one of the wettest seasons on record, where double the mean annual rainfall fell in just a few months. A series of palaeo-floodouts along the length of the gullies suggests that the process of gullying and depositional floodout formation is not restricted to the recent past, and is a long-term landscape process. Thus, despite the lack of characteristic cut-and-fill features, such as terraces, the long-term evolution of the Karoo landscape may be related to episodic gullying. In the short term, palaeo- and current floodouts alter the overland flow hydrology of the landscape, and prevent the local formation of badlands by diverting overland flow away from gully thalwegs. This results in the formation of amphitheatre-type gullies, with laterally eroded areas draining into narrow outlet channels incised into palaeo-floodouts.

Resolving the gully erosion problem in Southeastern Nigeria: Innovation through public awareness and community-based approaches

The formation of gullies is one of the greatest environmental disasters in Southeastern Nigeria. Large areas of agricultural lands are lost or have become unsuitable for cultivation due to gully erosion. There have been numerous attempts to curb gully erosion in the region; especially through large-scale engineering projects, however, little has been discussed about ways to prevent their onset or the use of community-based low-technology approaches to mitigate their development. We reviewed pertinent literature, interviewed key stakeholders, observed gully erosion sites and had informal discussions with key experts in the field on the causes of gully erosion and community-based approaches in tackling the problem. Based on these discussions, interviews and observations, we showed that many eroding gullies in the region are human-induced and therefore can be prevented. The results also reveal that many of the causes of gully erosion can be traced to poor land management practices and to a lack of innovation and awareness measures. We conclude that community-based, low-technology land management practices and public awareness programs through workshops could halt the development of many gullies in the Southeastern region of Nigeria. Key words: Erosion, gully, environmental management, soil, Southeastern Nigeria, workshop, public awareness.

Object-based gully feature extraction using high spatial resolution imagery

Gully erosion is responsible for a substantial amount of soil loss and is generally considered an indicator of desertification. Hence, mapping these gully features provides essential information needed on sediment production, identification of vulnerable areas for gully formation, land degradation, and environmental and socio-economical effects. This paper investigates the use of object-oriented image analysis (OOA) to extract gully erosion features from satellite imagery, using a combination of topographic, spectral, shape (geometric) and contextual information obtained from IKONOS and GEOEYE-1 data. A rule-set was developed and tested for a semi-arid to sub-humid region in Morocco. The percentage of gully system area indicated negligible overestimations between the reference area and the OOA area in two sub-watersheds (0.03% and 1.77%). We also observed that finer gully-related edges within the complex gully systems were better identified semi-automatically than was possible by manual digitization, suggesting higher detection accuracy. OOA-based gully mapping is quicker and more objective than traditional methods, and is thus better suited to provide essential information for land managers to support their decision making processes, and for the erosion research community.

Holocene geomorphological processes and soil development as indicator for environmental change around Karakorum, Upper Orkhon Valley (Central Mongolia)

Mantles of silt- and sand-size particles, paleosols and fluvial deposits preserve valuable information on Holocene environmental change. These archives were used to reconstruct the landscape history in the upper Orkhon Valley close to the former capitals of the Uighurs (Kharbalgasin Tuur) and the Mongolian Empire (Karakorum) near the recent town of Kharkhorin, Central Mongolia. A holistic approach involving the use of high spatial resolution geomorphological mapping, sedimentological and geochemical analysis, palynology, and geochronology shows several phases of landscape activity and stability in the region. This includes phases of fluvial erosion, aeolian sedimentation, and soil formation. By using luminescence and radiocarbon dating, phases of landscape change, indicated by soil formation, occurred mainly at around 6.5–6 ka. Pollen data and a weakly humic horizon at around 1.0 ka probably indicate enhanced moisture supply in the region and a reduction of human activity between the time of the reigns of the Uighurs (8th–9th century AD) and the Mongols (1220–1388 AD). Since 3 ka, especially within the last two millennia of Historical Time (300 B.C.–present), a more intensified human occupation in the Upper Orkhon Valley occurred in this region. This included a more densely grazing of cattle to supply the growing population demands of the Uighurs and Mongols. This overgrazing caused an increase in erosion and the formation and deepening of fluvial gullies, together with soil deflation and subsequent deposition of aeolian sediments. Human activity, in addition to climate, has been dominant in driving landscape evolution of this region since the late Holocene.

Prediction of spatial patterns of collapsed pipes in loess-derived soils in a temperate humid climate using logistic regression

Soil piping (tunnel erosion) has been recognised as an important erosion process in collapsible loess-derived soils of temperate humid climates, which can cause collapse of the topsoil and formation of discontinuous gullies. Information about the spatial patterns of collapsed pipes and regional models describing these patterns is still limited. Therefore, this study aims at better understanding the factors controlling the spatial distribution and predicting pipe collapse. A dataset with parcels suffering from collapsed pipes (n=560) and parcels without collapsed pipes was obtained through a regional survey in a 236km² study area in the Flemish Ardennes (Belgium). Logistic regression was applied to find the best model describing the relationship between the presence/absence of a collapsed pipe and a set of independent explanatory variables (i.e. slope gradient, drainage area, distance-to-thalweg, curvature, aspect, soil type and lithology). Special attention was paid to the selection procedure of the grid cells without collapsed pipes. Apart from the first piping susceptibility map created by logistic regression modelling, a second map was made based on topographical thresholds of slope gradient and upslope drainage area. The logistic regression model allowed identification of the most important factors controlling pipe collapse. Pipes are much more likely to occur when a topographical threshold depending on both slope gradient and upslope area is exceeded in zones with a sufficient water supply (due to topographical convergence and/or the presence of a clay-rich lithology). On the other hand, the use of slope-area thresholds only results in reasonable predictions of piping susceptibility, with minimum information.

UK Mars research and priorities in the Aurora programme

John Bridges and Axel Hagermann summarize an RAS Special Discussion Meeting in January 2011, which looked at the prospects for the UK exploring Mars. High-resolution imaging and near-infrared mapping of the martian surface, together with detailed studies of martian meteorites and exobiology research, have revealed an increasing amount of detail about how climate and surface processes have changed over the history of Mars. The exploration of Mars is now being planned through a joint NASA–ESA programme. New missions such as the Mars Science Laboratory in 2012 and the Trace Gas Orbiter in 2016 will also improve our understanding about the evolution of Mars. Mission planning is being guided by recent reports of methane in the Mars atmosphere and the identification of minerals and landforms associated with water such as the Eberswalde Delta. Mars research is also acting as a spur for the development of mission concepts and analytical techniques such as new XRD techniques, microRaman and a Life Marker Chip. The second UK in the Aurora Programme meeting was held on 14 January 2011 at Burlington House, London. The aim was to bring together researchers, particularly in the field of Mars research and instrumentation, in order to provide a platform to show our research and to discuss what the priorities for future missions within Aurora should be. The Aurora programme encapsulates Europe's current plans, aiming to send unmanned spacecraft to explore our neighbouring world to examine its climate, search for evidence of past or present life, and learn how conditions there relate to those on Earth. The meeting was introduced by Frederick Taylor (Oxford). High-resolution imagery of some of the surface of Mars using the HiRise camera on the Mars Reconnaissance Orbiter (MRO) at 25 cm/pixel has changed our view of the evolution of its surface and climate. One of the ways our view of Mars has changed is the recognition of the influence of glacial and periglacial activity in shaping landscapes. Matt Balme (Open University) and Peter Gallagher (Trinity College Dublin) presented new observations of periglacial landforms that appear to have been formed by water as part of freeze–thaw cycles: self-organized patterns of stone stripes, polygons, circles and clastic solifluction lobes. The low density of impact craters suggests these terrains formed in a geologically recent era. Kate Goddard et al. (Imperial College) presented a study of youthful gullies and depositional fans on Mars in Mojave Crater. Understanding the role of water or other processes, including CO 2 ice sublimation, in the formation of such gullies is a crucial way of reaching a more accurate understanding of martian climate variations within the last few million years. Balme and Gallagher speculated that because perchlorates (identified at the Phoenix landing site) have eutectic temperatures below 240 K and can remain liquid at temperatures far below the freezing point of water, perchlorate brines may explain freeze–thaw processes and could cause geomorphological changes.

Keys to gully formation processes on Mars: Relation to climate cycles and sources of meltwater

Advances in dating gullies on Mars using superposition relationships and a stratigraphic marker horizon link gully chronostratigraphy to recent climate cycles. New observations of gully morphology show the close association of gully source regions, channels, and fan deposits with well-documented ice-rich latitude-dependent mantle deposits, the deposition of which is interpreted to be coincident with recent ice ages. On the basis of these close correlations, we interpret the formative processes for mid-latitude gullies to involve melting of these ice-rich mantling deposits and the generation of an aqueous phase leading to fluvial activity. Continued monitoring of gullies by spacecraft in the current “interglacial” climate period (∼0.4 Ma to the present) will permit assessment of changing rates and styles of gully activity in the now largely depleted source areas.

Erosion of Cinder Cones in Tenerife by Gully Formation, Canary Islands, Spain

Cinder cones were produced by the main basaltic eruptive episodes during the last 1 Ma in Tenerife, Canary Islands. The erosion of cinder cones is a natural process that begins immediately after an eruption. Factors controlling the erosion of monogenetic volcanoes include morphology, age, topographic and emplacement characteristics of the morphoclimatic environment, and local runoff depth. The aim of this paper is to document the evolution of gully erosion on 43 dated cinder cones over time and determine whether age is a critical factor in the degradation level of volcanoes, independent of other factors. The methodology employed is based on geomorphological and morphometric analyses. Only 39.5% of the Tenerife volcanoes have gullies; nevertheless, more than 84.6% of the Pleistocene cones present gullies, compared to only 20% of the Holocene volcanoes. This means that 80% of the Holocene cones have no gullies, whereas only 15.4% of the Pleistocene cones are gully-free. Therefore, the older volcanoes appear to have been more affected by torrential rains. Although the age of a volcano is an essential factor in the degree of erosion, other factors intervene as well. The rate and evolution of these cinder volcanoes depend on a combination of these factors.

Identifying Martian gully evolution

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.