Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p42a0547w&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P42A-0547
Mathematics
Logic
5417 Gravitational Fields (1227), 5430 Interiors (8147), 5475 Tectonics (8149)
Scientific paper
Recent mapping expeditions from Mars Global Surveyor have shown distinct differences in the elevation between the northern and southern hemispheres of Mars. The northern hemisphere is a sparsely cratered plains region well below the Martian mean surface elevation. The current state of the northern plains implies a geologically younger surface that emerged after the heavy bombardment of the southern hemisphere. The southern hemisphere or southern highlands region is a heavily cratered surface between 1 and 4 km above the mean surface elevation. The region between the two vastly different hemispheres where a large change in elevation takes place is known as the crustal dichotomy. Theories for the origin of the crustal dichotomy include impacts, plate tectonics, and the previous existence of an ocean. The crustal dichotomy itself is evidence that the lithosphere on Mars is able to support substantial stresses from masses above and below the surface. Currently, there are two competing crustal models to explain the difference in elevation between the north and south hemispheres. Smith and Zuber suggest that the crustal thickness varies gradually from the northern plains to the southern highlands. This gradual difference may be the result of a large impact event in the northern hemisphere, northern crustal thinning by mantle convection, or plate tectonics. The competing model by Frey speculates that the crustal dichotomy is a region where the crustal thickness varies sharply, similar to the transition zone between passive continents and oceans on Earth. Changes in crustal properties such as composition and thickness can support a significant change in crustal thickness over a short distance. The topographically low northern plains have younger crustal features that reveal a region highly marked with gravity anomalies. The southern highlands have much older crustal features with the exception of some volcanoes and impact basins that produce relatively smooth gravity anomalies. The crustal dichotomy itself is a region lacking gravity anomalies, suggesting it has been a long-lived feature of Mars. An isostatic geoid (areoid) approximation can be used to evaluate the competing crustal thickness models. Our preliminary model has a sharp change in crustal thickness at the dichotomy with a thick crust (i.e., 80-100 km average crustal thickness). In addition, our model predicts a small density difference between the Martian crust and mantle (i.e., 100-300 kg/m3). A sensitivity analysis will be presented to address the robustness of the model.
King David S.
Whitesell K. E.
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