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 Gasa crater has been the most active site observed on Mars to date, making it of particular interest for studying the process(es) behind gully formation and activity. In this study, we investigate whether differences in thermal inertia across different segments of gully systems, combined with morphological and colour observations with High-Resolution Imaging Science Experiment (HiRISE), can provide some constraints on the physical characteristics associated with recent activity within gullies in Gasa. We also investigate thermophysical differences between slopes in Gasa dominated by gully activity compared to those predominantly modified by dry mass-wasting processes. We find that Gasa crater exhibits clear variations in thermal inertia across its walls, controlled by the material properties and the types of dominant mass movement processes occurring on each wall. The youthful gully-fan lobes display thermal inertia values c. 20–40 J m −2 K −1 s −1/2 higher than adjacent older eroded and dust-covered lobes. The talus aprons from mass wasting in Gasa have thermal inertia values c. 60–80 J m −2 K −1 s −1/2 higher than gully aprons. The results of this study thus suggest that thermal imaging can inform us on surface change detection on Mars.

Abstract The Hale impact crater is a large complex crater ( c. 150 × c. 125 km) in the southern hemisphere of Mars. Recurring slope lineae have been observed on its central-peak slopes, as have relatively youthful gully-like landforms; the latter are observed adjacent to or in the midst of the former, as well as on all of the rim-material slopes. Three of the gullied slopes on the northern-rim materials exhibit landscape features that, on Earth, are synonymous with wet periglaciation, i.e. landscape modification by the freeze–thaw cycling of water. These features include: (1) gelifluction-like lobes; (2) patches of surface polygonization, possibly underlain by ice wedges and formed by thermal-contraction cracking; and (3) shallow, rimless and polygonized basins morphologically akin to terrestrial alases. Here, we use the spatial association of the gully-like landforms together with the putatively wet periglacial assemblages or complexes to deduce and ascribe, albeit indirectly, a wet origin to the former.

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

We compared the morphology of gully sedimentary fans on Svalbard as possible analogs to gullies on Mars in order to constrain whether fluvial and/or debris-flow processes are predominantly responsible for the formation of Martian gullies. Our analysis is based on high-resolution imagery (High Resolution Stereo Camera [HRSC-AX], ~20 cm/pixel) acquired through a flight campaign in summer 2008 and ground truth during two expeditions in the summers of 2008 and 2009 in Svalbard, compared to high-resolution satellite imagery (High Resolution Imaging Science Experiment [HiRISE], ~25 cm/pixel) from Mars. On Svalbard, fluvial and debris-flow processes are evident in the formation of gullies, but the morphological characteristics clearly show that the transport and sedimentation of eroded material are predominated by debris flows. Most investigated gullies on Mars lack clear evidence for debris-flow processes. The Martian gully fan morphology is more consistent with the deposition of small overlapping fans by multiple fluvial flow events. Clear evidence for debris flows on Mars was only found in one new location, in addition to a few previously published examples. The occurrence of debris-flow processes in the formation of Martian gullies seems to be rare and locally limited. If predominantly fluvial processes caused the formation of gullies on Mars, then large amounts of water would have been required for their formation because of the relatively low sediment supply in stream and/or hyperconcentrated flows. Repeated seasonal or episodic snow deposition and melting during periods of higher obliquity in the recent past on Mars can best explain the formation of the gullies.

We present landforms on Svalbard (Norway) as terrestrial analogs for possible Martian periglacial surface features. While there are closer climatic analogs for Mars, e.g., the Antarctic Dry Valleys, Svalbard has unique advantages that make it a very useful study area. Svalbard is easily accessible and offers a periglacial landscape where many different landforms can be encountered in close spatial proximity. These landforms include thermal contraction cracks, slope stripes, rock glaciers, protalus ramparts, and pingos, all of which have close morphological analogs on Mars. The combination of remote-sensing data, in particular images and digital elevation models, with field work is a promising approach in analog studies and facilitates acquisition of first-hand experience with permafrost environments. Based on the morphological ambiguity of certain landforms such as pingos, we recommend that Martian cold-climate landforms should not be investigated in isolation, but as part of a landscape system in a geological context.