Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p53d1553g&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P53D-1553
Physics
[5400] Planetary Sciences: Solid Surface Planets, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
The South Pole-Aitken (SP-A) basin is remarkable because of its great age, size, elliptical structure, and asymmetrical depth of excavation. Recent studies have elucidated its general shape (e.g. [1]), but questions still remain about what processes influenced the distribution of topography and crustal thickness inside the basin. There are emerging indications that SP-A may have impacted into a unique terrain, and that there is a remaining signature of that terrain imbedded in the details of the SP-A topography and crustal thickness. A recent study of the lunar farside highlands north of the basin [2] demonstrated that their crustal structure and shape can be described by a degree-2 harmonic that extends over about a quarter of the lunar surface. The process that generated this terrain would have likely once extended into the southern half of the farside and its characteristic degree-2 shape would have been obscured by the South Pole-Aitken impact. If so, the low topography in the southern central part of the basin, which is the lowest on the Moon, would be partially the result of the superposition of the SP-A excavation cavity over a preexisting degree-2 terrain that decreases in elevation and crustal thickness towards the south pole. We tested this hypothesis in several ways, including fitting degree-2 harmonics to the farside highlands and extrapolating them across the basin, “removing” the expected degree-2 signal from SP-A, and measuring the degree-2 character of the interior basin topography and crustal thickness. In all cases, there is evidence that the preexisting terrain was degree-2 in character, and that it controlled portions of the basin topography and crustal structure we see now. There are three implications of these results. The first is that the north-south asymmetry in the depth of SP-A [1] is not likely due to some asymmetry in the impact process that develops at very large scales. The second is that knowledge of the preexisting crustal thickness is useful in assessing how and where SP-A might have excavated mantle and lower crustal materials. The third is that the degree-2 character of the lunar farside crust may have once extended over most of the farside, possibly comprising up to 40% of the lunar surface. This result yields further insight into the mechanisms that built the lunar crust. [1] Garrick-Bethell, I. and Zuber, M. T. (2009), Icarus 204, 399. [2] Garrick-Bethell, I., Nimmo, F., and Wieczorek, M.A. (2010), submitted.
Garrick-Bethell Ian
Nimmo Francis
Wieczorek Mark A.
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