Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p31a0543t&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P31A-0543
Mathematics
Logic
5462 Polar Regions, 5475 Tectonics (8149), 6225 Mars
Scientific paper
Geologic mapping of the north polar plains of Mars (>60° N.), aka Vastitas Borealis, based on Mars Orbiter Laser Altimeter (MOLA) topography, reveals new insights into resurfacing events in the polar region during the Amazonian Period. Deposits making up the Vastitas Borealis Formation (VBF) were emplaced at the end of the Hesperian Period and covered this entire region except for lavas of northernmost Alba Patera. No evidence for a Hesperian polar plateau has been found, although Planum Boreum could bury such evidence. A plains deposit, the Scandia unit, ranges from 20 to 200 m thick and overlies the VBF between Alba Patera and Olympia Planitia. This unit is apparently friable, because it is preserved only as eroded remnants forming knobs and mesas, including those of Scandia Colles. The VBF and Scandia units both appear to be deformed by radial tilting and concentric folding of Alba Patera. Another deposit, the Boreum unit, forms the base of Planum Boreum and possibly high-standing knobs and mesas south of Chasma Boreale and underlies the evenly bedded polar layered deposits. MOC images and MOLA data reveal that this unit has irregular bedding, locally steep scarps, and a dark color and is interpreted by S. Byrne and B. Murray (personal commun.) as a frozen sand sea. The unit may be up to a kilometer thick along the margins of Chasma Boreale and thins out away from there, possibly underlying the northern, southward-sloping ejecta of a 24 km crater at 79° N., 299° W. Potentially, the Scandia and Boreum units could be remnants of a single deposit. Within Chasma Boreale, the chasma unit forms a tongue-shaped plateau as much as 350 m thick. This unit could be a lower section of the Boreum unit, but a lack of relict knobs or mesas on its surface suggests that the unit embays the Boreum unit instead. South of Olympia Planitia, the Scandia unit becomes difficult to trace, and volcanic-like forms possibly related to eruption of material and subsequent collapse and degradation occur. A north-facing, 800-km-long scarp cuts the VBF northeast of Alba Patera. A scenario that may explain these features is as follows. A polar plateau did not exist prior to the Amazonian. Northward tilting of Alba Patera, perhaps induced by lithospheric loading due to deposition of the VBF, caused shallow crustal volatiles (H2O and CO2) to migrate northward. The volatiles erupted out into the north polar basin, perhaps over multiple episodes, causing dispersal of VBF material that redeposited as the Scandia, Boreum, and chasma units. When this activity ceased, mantling of the Boreum unit and surrounding plains by the polar layered deposits and dune fields ensued. If the Boreum unit is volatile poor, then the release of substantial volumes of subsurface volatiles to the surface to carve the outflow channels and deposit the VBF was followed by a period in which polar ice-cap deposition did not occur. During this Early Amazonian episode, a warmer climate may have been supported by a thicker atmosphere.
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