Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.c21c0825s&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #C21C-0825
Mathematics
Logic
1823 Frozen Ground, 1824 Geomorphology (1625), 6207 Comparative Planetology, 6225 Mars
Scientific paper
Pingos are ice-cored, perennial frost mounds of which there are two main types: closed or open. Along with ice wedge polygons they are benchmarks of periglacial processes in cold-climate, non-glacial landscapes. The genesis and development of both pingo types is contingent upon three limiting factors: 1. the availability of water in liquid form; 2. an impermeable and constraining boundary layer in the ground; and, 3. surface deformation driven by hydrostatic pressure (closed pingo) or hydraulic pressure (open pingo). In the case of a closed pingo, ground temperatures also must be above the freezing point of water for pingo genesis to be initiated. Ice-wedge polygons are the product of a different set of periglacial processes: thermal contraction, the infiltration of liquid water into surficial cracks, ice wedge-development and surficial temperatures supportive of freeze-thaw cycles. The existence of a Martian periglacial landscape highlighted by pingos and ice-wedge polygons would point to: 1. liquid water having been available prior to landscape genesis; and, 2. surface temperatures having been high enough to sustain water-freezing processes and repeated freeze-thaw cycles. As of yet, active periglacial landscapes on Mars have not been identified. By contrast, the existence of artefactual periglacial landscapes comprising pingos and ice wedge polygons has been hypothesised in a number of instances. However, poorly resolved Viking imagery and hypothesised periglacial landscapes not consistent with terrestrial analogues have left some key questions unanswered. On the basis of MOC2-357 we have identified a possible periglacial landscape comprising 15 pingo-like mounds arrayed in a field of small-scale, ice-wedge like polygons. The mounds, bracketed by the polygons, sit on the floor of an isolated, medium-sized, impact crater in the northwest corner of Utopia Planitia (64.8° N / 292.7° W). If the formation of the northern plains dates back to the late Hesperian or early Amazonian period, then the age of the impact crater and of the landforms residing within it must be younger still. Using the data generated by the Mars Orbiter we conducted image enhancement and photoclinometric analyses of the crater basin. Morphometric measurements of the mounds and polygons are consistent with the dimensions of terrestrial pingos and ice-wedge polygons. Equally, the location of these mounds and of the surrounding polygons on the crater floor, possibly the floor of an ancient ice covered basin or paleo-lake, is consistent with the location of closed pingos and ice-wedge polygons on the floor of drained arctic lakes. Adjacency to a region of hydrogen abundance, possibly ice-rich sediments or ground ice, has been mapped. On Mars as on Earth these sediments could be geocryological artefacts of surficial liquid water that percolated down through the regolith and froze in situ. The availability of liquid water is a requirement of pingo and ice-wedge genesis. Equally, the presence of ice-rich sediments would meet the ground impermeability requirement of pingo genesis. No less suggestive of periglacial processes are the hundreds of small pits and depressions that are ubiquitous in Utopia Planitia. The morphology of these pits and depressions is reminiscent of terrestrial thermokarst and differential frost heave. We argue that the disparate but interconnected strands of surficial and sub-surficial geomorphological data sustain the hypothesised presence of a crater-based and water-fed periglacial landscape in northwest Utopia Planitia.
Peloquin C.
Soare Richard J.
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