Lobate Debris Aprons (and friends): Martian Debris-Covered Glaciers

Lobate debris aprons, lineated valley fill, and concentric crater fill are all Martian landforms that exhibit marks of possible glacier flow on their surface. These include compressional ridges (aligned concentric in the interior of craters) and lineations that resemble glacier moraines (Squyres, 1978). Their surfaces are mantled by debris however, with no evidence for water ice down to ~0.5 m depth (Boynton et al., 2002; Feldman et al., 2004). For this reason they have been compared to rock glaciers and debris-covered glaciers and have been earmarked as landforms with an interior likely to be rich in water ice. With thicknesses typically up to ~400 meters and horizontal dimensions of tens of kilometers, they are likely to contain vast reservoirs of ice.

Shallow Radar Sounding (SHARAD) allows us to penetrate the interior of these features; Holt et al., 2008 and Plaut et al., 2009 showed for targeted lobate debris aprons that the radar penetrates to their basal contact with bedrock and that their interior is made of high purity water ice. No contact was imaged between the surface debris layer and the ice-rich interior, with the favored interpretation being that the surface debris layer is thinner than SHARAD’s resolution—this constrains its thickness to between ~0.5–10 m. Petersen et al., 2018 extended this work to all lobate debris aprons, lineated valley fill, and concentric crater fill in the regions of Deuteronilus and Protonilus Mensae, confirming that these features are indeed debris-covered glaciers.

Figure 1: Viscous flow features on a lobate debris apron in eastern Deuteronilus Mensae, Mars. Note the compressional ridges at the tongue of the feature, the flow lines along it’s length, and the crevasses at its edge. Image rendered from DTMs produced from Stereo HiRISE and CTX data using the Ames Stereo Pipeline.

In the SWIM project we will continue to extend SHARAD surveying of candidate debris covered glaciers across all of the broad survey swaths in the northern hemisphere. In terms of regions of interest for candidate glaciers these include the regions of Deuteronilus, Protonilus, and Nylosirtis Mensae, the Phlegra Montes, and the band of latitudes containing a large population of concentric crater fill (Levy et al., 2014).

Figure 2: Sourdough Rock Glacier in the Wrangell Mountains, Alaska. Note the compressional ridges near its toe and flow lines along the length, similar to the morphology on the Martian feature. Sourdough is thought to be a debris-covered glacier cored by high purity ice that flows at a rate of up to 2 m/yr, producing the observed viscous flow morphology. The ice-rich interior is buried under a surface layer of ~2.5 meters of “debris”—boulders, cobbles, and gravel derived from rock falling from the mountain slopes above. Photograph taken by Eric Petersen.

Off-Nadir Clutter: The Bane of Orbital Radar Sounding

Because the Shallow Radar sounder (SHARAD) is operated on a spacecraft orbiting ~300 km above the surface of Mars, there is the possibility of strong surface reflections from planar surfaces far from the spacecraft’s nadir ground track. The increase in time delay from these reflections means they can appear in the data to be similar to reflections in the subsurface. We call such off-nadir surface reflections “clutter,” as they can clutter up the data, obscuring or masquerading as subsurface information. Clutter is not typically a problem in broad, flat plains but can be ubiquitous in high topography areas such as the dichotomy boundary regions. To disambiguate clutter from nadir subsurface returns we employ a clutter simulator which reproduces all surface returns SHARAD may see based on the MOLA (Mars Orbiter Laser Altimeter) global topography dataset (Choudhary et al., 2016). When interpreting SHARAD data for possible subsurface returns, the observed radar is compared to simulated clutter before mapping the subsurface (Figure 3).

For the SWIM project, clutter simulations have been produced for every radar observation in our study regions. These are called upon when needed for visual inspection of the radar data.

Figure 3: Example of SHARAD data exhibiting clutter, indicated by the white arrows in each panel. Top panels: radar data acquired by the SHARAD instrument over lobate debris aprons. The surface reflections are seen with convex-up profiles; candidate subsurface reflections are indicated by the white arrows. Middle panels: clutter simulations showing all nadir and off-nadir reflections predicted using MOLA data. The candidate subsurface reflections are in fact off-nadir surface reflections (white arrows). Bottom panels: echo power maps produced from the clutter simulator. The horizontal green line at center is the nadir ground track of the spacecraft flight. The blue dots indicate the locations of first radar echo returned to the spacecraft. In both images the highland massif is to the right and the lowland plains are to the left with the aprons hugging the edge of the massif. The white arrows indicate possible sources of clutter, including (left panel) the edge of the apron, oriented sub-parallel to the nadir track, and (right panel) a mesa located far off-nadir.

References and Additional Reading

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