HR: 10:20h
AN: P22A-01    [Abstracts]
TI: Mars Northern Lowlands: Wind-eroded Versus Sediment-filled, a Comparison of Hypotheses
AU: * Leovy, C
EM: janleovy@yahoo.com
AF: Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640 United States
AU: Catling, D
EM: davidc@atmos.washington.edu
AF: Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640 United States
AU: Wood, S
EM: sewood@atmos.washington.edu
AF: Dept. of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640 United States
AB: The northern lowlands are remarkably smooth at scales from several hundred meters to hundreds of km, are locally covered by obvious layers of sediment that are up to a few tens of meters thick, and lie beyond and below the terminuses of many large outflow channels. Primarily for these reasons, the hypothesis that these lowland areas correspond to the sediment-filled beds of ancient bodies of standing water has been widely accepted (the prevailing hypothesis). On the other hand, atmospheric general circulation model (GCM) simulations incorporating dust lifting and transport consistently indicate that the northern plains are dominated by wind erosion in the present climate regime, and that they have been dominated by wind erosion in most past climate regimes as well. This is a robust result of the simulations that follows from the occurrence of strong mid-latitude storm systems together with the relatively low wind speed threshold for saltation resulting from higher pressure at lower elevation. This model result is supported by orbital and surface observations of frequent storms in this region and widespread seasonally variable surface wind streaks. It is difficult to reconcile this evidence for a robust erosional regime with the prevailing hypothesis of an ancient surface of fine sediments deposited beneath standing water. We develop an alternate hypothesis that the northern plains are erosional rather than depositional. This hypothesis is consistent with widespread areas of high surface thermal inertia, frequent occurrences of rock outcrops and boulders visible in orbital and surface high resolution images, frequent occurrences of multiple rims around craters whose diameters are in the few hundred meter to several kilometer range, absence of dune forms, and relatively infrequent occurrence of small craters. All of these features of the northern plains will be documented in this paper. Very significantly, the alternate hypthosis provides a natural explanation for the properties of the population of very large craters (diameter greater than 100 km) that is otherwise very difficult to understand. Through a side-by-side comparison of the prevailing hypothesis and the alternate hypothesis, we will argue that the alternate hypothesis provides a more satisfactory explanation of all the observations than the prevailing hypothesis. However, it remains to be explained why a much more intense erosional event would have occurred in the northern than in the southern hemisphere, and why dust storms are so frequently observed in the northern plains despite the absence of obvious features associated with saltation.
DE: 5415 Erosion and weathering
DE: 5445 Meteorology (3346)
DE: 3307 Boundary layer processes
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting