Ground penetrating radar (GPR) was used to investigate the internal structure and development of an active parabolic sand dune in the Bigstick Sand Hills of southwestern Saskatchewan, Canada. The radar survey was conducted in a grid configuration using 250 MHz antennas. The radar frequency and the properties of the aeolian sands limited the penetration of the radar signal to the uppermost 4 m. Radar profiles parallel to the prevailing westerly wind reveal three zones with differing structural arrangements that are interpreted to represent three phases in the development of the dune: (1) underlying low-angle reflections representing preexisting aeolian strata associated with sand sheet or dune marginal deposition; (2) high-angle reflections representing downwind migration by grainflow; and (3) a variety of high- and moderate-to-low-angle reflections representing a more complex pattern of migration involving grainflow, grainfall, and ripple deposition. Radar profiles perpendicular to the prevailing wind are characterized by convex-up and concaveup reflections along the dune head and are interpreted as spur and trough structures, respectively. Radar profiles over the wings reveal an arrangement of high-angle reflections radiating away from the center of the dune. The main structural features from the radar profiles are summarized into two radar surfaces; three radar packages; and three radar facies, one of which has two subfacies. Observations of exposed surface stratigraphy following extensive wind erosion lend support to the interpretations made from the GPR data.

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 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.