Post contributed by David P. Mayer, Department of Geophysical Science, the University of Chicago
Debris-covered glaciers are glaciers whose ablation zones are at least partially covered by supraglacial debris. On Earth, debris-covered glaciers are found in all major mountain glacier systems. The debris itself is primarily derived from rockfall above the accumulation zone. This material becomes entrained in the accumulating ice and is carried englacially before emerging in the ablation zone. On Mars, numerous mid-latitude landforms have been interpreted as debris-covered glaciers based on their geomorphic similarity to nearby ice-rich landforms such as lobate debris aprons (LDA), as well as their similarity to terrestrial debris-covered alpine glaciers (Head et al., 2010 and refs. therein).
LDA extend up to ~15 km from rocky massifs and display convex up topographic profiles with steep termini, consistent with the expected profiles for flowing ice. Holt et al. (2008) used data collected by the subsurface sounding radar, SHARAD, onboard the Mars Reconnaissance Orbiter to identify nearly pure buried ice deposits beneath several LDAs in the southern mid-latitudes of Mars.
Image 1 shows an aerial photo of Mullins Glacier, a debris-covered glacier in Antarctica. This is an example of a cold-based based glacier, so-called because the temperature at the base of the ice remains below the pressure melting point of water, thereby causing it to flow by internal creep alone rather than also sliding over the landscape on a lamina of liquid water at its base. Ice-rich landforms on Mars are inferred to have existed under similar cold-based regimes for at least the last several million years (Head et al., 2003). In this color photograph, a nearly-continuous layer of brown debris can be seen across the surface of the glacier. Polygonally-patterned ground caused by thermal contraction of the debris cover is highlighted by snow in the cracks, particularly in the lowest reaches of the glacier where the ice is effectively stagnant (Rignot et al., 2002).
Image 2 shows one of several dozen tongue-shaped features on the north interior wall of Greg crater on Mars that display features similar to terrestrial debris-covered glaciers such as Mullins Glacier. In particular, this landform contains a series of concentric arcuate ridges along its outer margin that extend downslope in a lobe-like pattern. Whereas LDA tend to extend uniformly from a central massif into flat plains, features such as the one shown in Image 1 emanate from a single alcove along a cliff face (in this case, an interior crater wall) and appear to deflect around topographic obstacles, much like terrestrial debris-covered glaciers. The spacing between individual arcuate ridges narrows where the feature encounters confining slopes but then opens in areas without confining topography (such as near the downslope tip).
The surface texture of the feature is dominated by hummocky polygonally-patterned ground, mostly in the form of rounded hexagons ~10 meters wide. The interior slopes of the tallest arcuate ridges display a more knobby terrain but with the polygonal pattern sometimes superposed. This knobby terrain has been interpreted to be the result of differential sublimation of ice from areas with locally-thin debris cover (Hartmann et al., 2014 and refs therein). Taken together, these morphological similarities to terrestrial terrestrial debris covered glaciers and geographic proximity to ice-rich features on Mars support the interpretation that these tongue-like features are debris-covered glaciers or at least the remnants thereof (Brough et al., 2016).
Water ice is not stable at the surface of Mars under present climate conditions. Could the remnant debris-covered glaciers inside Greg crater still contain buried ice? It is not currently feasible to identify buried ice deposits in such small landforms on relatively steep slopes with SHARAD as was possible for nearby LDA, however models of ice loss from Mullins Glacier in Antarctica suggest that it is possible to preserve glacial ice beneath a protective layer of debris for potentially millions of years (Kowalewski et al., 2011). In environments such as Beacon Valley (or Mars) where sublimation (rather than melting) is the dominant process by which ice is removed from a glacier, debris cover acts as an important control on the rate of ice loss. As ice sublimates by vapor diffusion, the overlying debris layer cover becomes thicker, insulating the ice from incoming solar radiation, thereby slowing the rate of diffusion. If a future space mission were to extract a sediment core from this feature and discover buried ice, it would not only support the geomorphic interpretation of it as a debris-covered glacier, but also provide scientists with a valuable sample for probing the past climate on Mars.
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