Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51b1421s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51B-1421
Mathematics
Logic
[5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties
Scientific paper
The formation of “mottled” albedo surfaces in the Martian northern plains have remained enigmatic since their initial observation in Mariner 9 and Viking Orbiter data sets. We re-assess the geologic evolution of these materials in Chryse and Acidalia Planitiae by mapping and analyzing the spatial distribution of their characteristic landforms, including lobate margins and pitted cone fields. Bright lobes are defined by thin, concentric margins that abut, surround, and link decameter high knobs. These lobes range from 5.4 to 129.7 km long (24.3 km mean) and are located marginal to Chryse Planitia between -3291 and -5009 m elevation (-4050 m mean). Dark lobes are defined by thick, digitate margins that are discontinuous, self-overlapping, and often levee-bound. These lobes range from 6.5 to 95.5 km long (28.0 km mean) and are located near the center of Acidalia Planitia between -3602 and -5169 m elevation (-4627 m mean). Lobe orientations are strongly bi-modal, with bright lobes facing west-southwest (245°) and the dark lobes facing northeast (41°). Two populations of pitted cones pervasively occur in smooth to undulating bright plains adjacent to (or northward of) bright lobes. High-density fields are defined by irregularly-shaped, coalesced pitted cones that stand <10 m above surrounding plains, and occur ~1 per km2. Low-density fields are defined by quasi-circular, smooth-sided domes and cones that stand >20 m above surrounding plains, and occur ~0.1 per km2. The latter occur in plains units that embay and bury lower standing, high-density fields. Thermal radiance, morphology, spatial orientation, and cross-cutting relationships clearly discriminate two discrete and overlapping (though unrelated) geologic units that were emplaced through contrasting processes. We interpret dark lobes as vestige digitate terminations of debris flows sourced from circum-Chryse fractures and channels and light lobes as overlapping layers of relatively block-free sediment, perhaps emplaced through the liquefaction of a pre-existing unit. If these features formed through flow-related processes, as suggested by their lobate marginal forms, the corresponding energy source for emplacement was from nearly opposite quadrants: light lobes from inside the northern plains and dark lobes from outside. Light lobes and associated pitted cones have been observed in regions that are wholly un-related to catastrophic outflow events (e.g., southern Utopia Planitia), effectively ruling out outflow as the primary formational mechanism for these features.
Fergason Robin L.
Skinner Andrew J.
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