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 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 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.

ABSTRACT The Great Miami Buried Valley Aquifer system (GMBVAS) is a U.S. EPA–designated sole-source aquifer for most of the ~2.3 million residents of the Great Miami River basin. The aquifer system is similar to many of the glacial outwash–alluvial aquifers found throughout the southern Midwestern United States: a valley deeply incised into bedrock by meltwater from repeated glaciations, subsequently aggraded with glacial outwash, lake clays, tills, and post-glacial alluvium. These aquifers are often immediately adjacent to or below modern streams with which they are hydrologically connected. Because of the exchange of water between the aquifer and surface streams, they must be considered two parts of a single hydrogeologic system; anything impacting one part will impact the other. Because virtually all water is recycled, public water suppliers, agricultural users, and industrial users withdraw water from the GMBVAS, then discharge the used water to the next downstream consumer. For example, agricultural runoff and municipal wastewater enter the Great Miami River. River water recharges the aquifer near well fields through induced infiltration. Municipalities, agriculture, and industry pump groundwater from the aquifer and discharge some of the water back into the river as a return flow. Sound water management requires balancing the needs of these diverse users. Agriculture, particularly in the upper Great Miami basin, is a major source of revenue and jobs but requires heavy application of nutrients and herbicides. Sand and gravel quarrying in the Great Miami basin is critical for construction and infrastructure but also removes vast amounts of aquifer material. Not only is healthy reliable groundwater important for the public water supply, it is also essential to bottling facilities, distilleries, pharmaceutical companies, and other water-dependent industries that have been attracted to the area. This field trip explores the GMBVAS, its uses, its interaction with Great Miami River, and the contrasting needs of the people and businesses that depend on it.