ALBERT

Description: The confirmation of near-surface ground ice and perchlorates at the Phoenix landing site suggest that high-latitude ground-ice thaw may be more easily achieved than previously envisaged, providing the potential to drive significant, distinctive morphogenesis. We describe the results of a survey of 23 High Resolution Imaging Science Experiment (HiRISE) images covering 337° of longitude between latitudes 59°N and 79°N in which such morphogenesis is apparent, confirming that thaw has been a regionally important morphological agent. Some of the strongest geomorphological indicators of cyclical ground-ice thaw described are assemblages of sorted landforms, including clastic patterned ground resulting from cryoturbation of ice-rich regolith and lobate forms reflecting solifluction. Also described are braided gully-fan systems sourced at thermokarst pits and channels that have evolved from enlarged thermal contraction cracks. Not only are these landforms indicative of thaw and flowing liquid but the incision of solifluction lobes by thermokarst gullies demonstrates that thaw has been responsible for polycyclic morphogenesis. The presence of these landforms across the high northern latitudes of Mars indicates that the regional importance of thaw has been underestimated. This in turn has important implications for the development of better climate models and the search for life on Mars.

Description: 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.

Description: Lethe Vallis is an approximately 230 km-long and 1.5 km-wide channel connecting several shallow basins in the Elysium Planitia region of Mars. It sits within a distinctive morphological unit defined by a platy-ridged-polygonized texture. We have documented the geomorphology of the system, and constructed topographical long profiles of the channel thalweg and the contacts of the platy-ridged-polygonized material.The Lethe thalweg is shallow (with a slope of about 0.0001) but contains steeper sections that match the locations of observed cataract systems. The contact profiles suggest that the small basins linked by Lethe progressively ponded and over-spilled as the system developed, the cataracts being associated with this over-spill. Other landforms observed in the system include streamlined islands, anastomosing distributary systems, fluvial hanging channels and terraces on the channel margins. There are also possible dunes and/or antidunes within the channel. These all point to catastrophic fluvial flooding. Estimates of formative discharge are of the order of 1×104–5×104 m3 s-1, similar to the discharge of the Mississippi River.We infer that Lethe Vallis formed as a fluvial ‘fill and spill’ catastrophic flood system. This demonstrates that the main Western Elysium Basin, the upstream source of Lethe Vallis, contained a substantial transient lake.

Description: To understand Martian palaeoclimatic conditions and the role of volatiles therein, the spatiotemporal evolution of gullies must be deciphered. While the spatial distribution of gullies has been extensively studied, their temporal evolution is poorly understood. We show that gully size is similar in very young and old craters. Gullies on the walls of very young impact craters (less than a few myr) typically cut into bedrock and are free of latitude-dependent mantle (LDM) and glacial deposits, while such deposits become increasingly evident in older craters. These observations suggest that gullies go through obliquity-driven degradation–accumulation cycles over time, controlled by: (1) LDM emplacement and degradation; and (2) glacial emplacement and removal. In glacially-influenced craters, the distribution of gullies on crater walls coincides with the extent of glacial deposits, which suggests that the melting of snow and ice played a role in the formation of these gullies. Yet, present-day activity is observed in some gullies on formerly glaciated crater walls. Moreover, in very young craters, extensive gullies have formed in the absence of LDM and glacial deposits, showing that gully formation can also be unrelated to these deposits. The Martian climate varied substantially over time, and the gully-forming mechanisms are likely to have varied accordingly.

Description: 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.

Description: Liquid water is generally only meta-stable on Mars today; it quickly freezes, evaporates or boils in the cold, dry, thin atmosphere (surface pressure is about 200 times lower than on Earth). Nevertheless, there is morphological evidence that surface water was extensive in more ancient times, including the Noachian Epoch (~4.1 Ga to ~3.7 Ga bp), when large lakes existed and river-like channel networks were incised, and early in the Hesperian Epoch (~3.7 Ga to ~2.9 Ga bp), when megafloods carved enormous channels and smaller fluvial networks developed in association with crater-lakes. However, by the Amazonian Epoch (~3.0 Ga to present), most surface morphogenesis associated with liquid water had ceased, with long periods of water sequestration as ice in the near-surface and polar regions. However, inferences from observations using imaging data with sub-metre pixel sizes indicate that periglacial landscapes, involving morphogenesis associated with ground-ice and/or surface-ice thaw and liquid flows, has been active within the last few million years. In this paper, three such landform assemblages are described: a high-latitude assemblage comprising features interpreted to be sorted clastic stripes, circles and polygons, non-sorted polygonally patterned ground, fluvial gullies, and solifluction lobes; a mid-latitude assemblage comprising gullies, patterned ground, debris-covered glaciers and hillslope stripes; and an equatorial assemblage of linked basins, patterned ground, possible pingos, and channel-and-scarp features interpreted to be retrogressive thaw-slumps. Hypotheses to explain these observations are explored, including recent climate change, and hydrated minerals in the regolith ‘thawing’ to form liquid brines at very low temperatures. The use of terrestrial analogue field sites is also discussed.