Active Mass Wasting of Ices in the North Polar Region of Mars

Details Active Mass Wasting of Ices in the North Polar Region of Mars Active Mass Wasting of Ices in the North Polar Region of Mars

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21d..08r&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P21D-08

Other

[0700] Cryosphere, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions

Scientific paper

The HiRISE camera on the Mars Reconnaissance Orbiter has discovered two forms of mass-wasting in the north-polar region. One is the scarpward retreat of bright layers of the north polar basal unit (BU, immediately underlying the north polar layered deposits, NPLD) and the overlying NPLD by fracture-controlled and undercutting-assisted piecewise failure of layer edges, resulting in rockfalls and rockslides, called blockfalls here. This is important as it suggests an alternative, significant mode of erosion in addition to sublimation which is traditionally held account-able for erosion of polar surfaces. The other form, caught in-action during imaging, comprises falls and avalanches of frost and dust over a few steep NPLD scarps during early spring. Here we report the latest findings in both of these dramatic and currently active processes and assess their role in the evolution and history of polar deposits. NPLD and BU mass wasting: Typical evidence of recent blockfall activity is in the form of blocks, flows, and debris on BU slopes. In a few cases, the appearance of new blocks in subsequent images proves this process is ongo-ing. In general, [likely] active outcrops have a higher overall MOLA-derived slope than [likely] quiescent outcrops (those with a lack of debris and abundance of sand on the BU). Furthermore, none of the NPLD peripheral scarps without BU exposure have slopes above 40°. This suggests BU exposure and this type of mass wasting play a role in maintaining and possibly creating over-steepened NPLD scarps (often 45°-65°, with sections approaching vertical). The lower-sloped NPLD scarps with no BU present also do not display the severe fracturing that characterizes steep NPLD cliffs directly above BU exposures. This may be due to fracture propagation from undercutting by the BU, thermal stresses induced by rapid and intense heating on steepened slopes with high solar incidence angles, or rapid uncovering of ice that was under compressive stresses. Feedback mechanisms between these mechanisms, steepness, fracturing, and BU erosion likely exist. CO2 frost-dust falls and avalanches: In Mars Year 29, 9 of these events were observed at Ls 27° - 39°, all at one scarp. In Mars Year 30, 27 events were observed, ceasing by Ls 50°, several at additional scarps between 90° - 270° E. All scarps are steep, >40°, and exhibit fractured NPLD. Events appear to originate on the scarp face. Loosely coincident timing with regional CO2 frost sublimation suggests a causal relationship. Thermal modeling suggests CO2 may be expected to disappear from steep sections of an idealized scarp before cessation of this activ-ity. Observational constraints on CO2 presence are provided by seasonal hyperspectral CRISM data. The fractured scarps are far from smooth, with ubiquitous crevasses and recesses, providing the potential for CO2 to persist later into the spring. As of yet, no missing NPLD blocks on the scarp face or new blocks below in subsequent images have been unequivocally attributed to these events. One observation suggests that a passing avalanche cloud can disrupt blocks of BU bright layers, which would tend to accelerate the type of blockfall mass-wasting discussed above.

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