Abstract Repeat, high-resolution imaging of dunes within the Martian north polar erg have shown that these dune slopes are very active, with alcoves forming along the dune brink each Mars year. In some areas, a few hundred cubic metres of downslope sand movement have been observed, sometimes moving the dune brink ‘backwards’. Based on morphological and activity-timing similarities of these north polar features to southern dune gullies, identifying the processes forming these features is likely to have relevance for understanding the general evolution/modification of dune gullies. To determine alcove-formation model constraints, we have surveyed seven dune fields, each over 1–4 Mars winters. Consistent with earlier reports, we found that alcove-formation activity occurs during the autumn–winter seasons, before or while the stable seasonal frost layer is deposited. We propose a new model in which alcove formation occurs during the autumn, and springtime sublimation activity then enhances the feature. Summertime winds blow sand into the new alcoves, erasing small alcoves over a few Mars years. Based on the observed rate of alcove erasure, we estimated the effective aeolian sand transport flux. From this, we proposed that alcove formation may account for 2–20% of the total sand movement within these dune fields. Supplementary material: A full listing of the HiRISE images used within this study and supplementary images, and analysis descriptions are available at https://doi.org/10.6084/m9.figshare.c.3936919

Abstract We reanalyse the global distribution of gullies in order to provide a set of observational constraints that models of gully formation must explain. We validate our results derived from the global data with four detailed case studies. We show that the availability of steep slopes is an essential factor to consider when assessing the spatial distribution and abundance of gullies. When the availability of steep slopes is taken into account, it reveals, with a few exceptions, that gullies are found almost uniformly across the whole 30°–90° latitude band. Our analysis also reveals that massive ice deposits are anti-correlated with gullies, and that the undulations in the equatorwards limits of the gully distribution could be explained by longitudinal variations in maximum surface temperatures (controlled by variations in surface properties, including thermal inertia and albedo). We find a sharp transition in both hemispheres between pole-facing gullies, which extend from 30° to 40°, to a more mixed, but dominantly equator-facing orientation of gullies polewards of 40°. We have no definitive explanation for this transition but, based on previous studies, we suggest it could be linked to the availability of near-surface ice deposits.

Abstract We conducted comparative morphological analysis of gullies within two high-latitude Martian craters (Domoni and Maricourt) in the northern hemisphere of Mars with (1) the debris-flow gully systems in the Ladakh Himalaya and (2) Istok Crater in the southern mid-latitudes of Mars where water-bearing debris-flow deposits have been previously reported. Our findings suggest that the debris-flow landforms preserved on gully and alluvial fans in the Ladakh Himalaya are potential analogues for the deposits preserved over the equator-facing slopes of Domoni and Maricourt Craters. Further, we found that the morphological attributes of channels and deposits (including overlapping terminal lobes, levées, tongue-shaped/lobate deposits and broad/small depositional deposits) within both the study craters and Istok Crater are similar. As a result, the studied craters emerge as additional sites in which possible evidence of water-bearing debris-flows are preserved on Mars. By comparison to our Earth analogue, we further propose that episodic melting of snow accumulated within the sheltered alcoves is the most likely source of water for the formation of such gullies. Taken together, our findings suggest that debris-flow may not be a rare process in gully formation on Mars and evidence may be preserved in other unexplored areas.

Abstract To understand the potential applicability of the process of ‘slush flow’ (a sediment-carrying flow with water and ice) to gullies on Mars, we undertook a series of flume experiments at Earth surface temperatures and pressures, which we subsequently scaled for Mars. Experiments were conducted in a 3 × 0.5 m hinged flume filled with medium-grain-size sand. The experiments were performed over a slope angle range of 10°–30°, corresponding to the slope range for gullies observed on Mars. A water ice mix, or ‘slush’, was flowed through a 19 mm-diameter silicone hose and released onto the surface at the top of the slope. A variety of morphometric parameters were measured on each form produced in the flume. The forms produced in our experiments developed the three principal morphological components of Martian gullies: alcove, channel and apron, and had a diversity of planimetric forms. The forms produced during simulations compared well to slush flows in terrestrial arctic climates, and open up the possibility that some Martian gullies may result from slush flows associated with the thawing of permafrost-active layers or surface frost under favourable thermal regimes. Supplementary material: A complete spreadsheet of all measurements summarized in Figures 5 and 6 is available at https://doi.org/10.6084/m9.figshare.c.3930613