Computer Science
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...41l&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 41
Computer Science
Iron Oxides, Lunar Geology, Lunar Maria, Lunar Topography, Lunar Composition, Structural Basins, Clementine Spacecraft, Laser Altimeters, Lunar Prospector
Scientific paper
The South Pole Aitken (SPA) Basin is an immense structure that dominates the geology of much of the farside of the Moon. Its floor is composed mostly of impact deposits, though it also has numerous relatively small regions of mare basalt. The basin floor exhibits a lower albedo and higher mafic mineral abundance than the surrounding highlands [ I ]. The origin of this mafic anomaly is a major question in lunar geology. Hypotheses for the presence of the mafic anomaly were briefly reviewed in [2] and include mare deposits mixed and obscured by basin or crater ejecta (cryptomaria), a large impact melt sheet that may have differentiated, exposed lower crustal material, and a significant component of excavated mantle. A study of mineralogy as revealed in Clementine UV-VIS imagery for limited portions of the basin found a predominantly low-Capyroxene (noritic) character [2], ruling out cryptomaria as an important contributor to the mafic enhancement. A few small cryptomaria, revealed by dark-halo impact craters and light plains units with high-FeO contents, have been found in SPA; however, it appears that extensive cryptomaria are lacking in this basin. The uniformly noritic lithology within SPA led to favor exposed lower crust or a homogenized melt sheet as the explanation for the mafic anomaly. Models of basin formation predict that a basin the size of SPA should have excavated through the entire lunar crust (assuming nonoblique impact), potentially exposing or mixing a large component of material from the mantle. Comparison of SPA floor FeO and Ti02 (derived from Clementine UV-VIS observations) and also Th (from Lunar Prospector) with model-mantle chemistries appears to be consistent with a mixture of approximately equal proportions of lower-crust and mantle material. In the present study, we examine the relationship between the basin's topography and composition in order to provide further insight on the origin of the basin floor material. Data: Clementine UV-VIS data at 1-km/pixel spatial resolution were placed in an orthographic projection centered on the basin (55S, 180E). Maps of Fe and Ti abundance were produced using the spectral algorithms. A map of topography derived from Clementine laser altimeter measurements was retrieved from the Planetary Data System Web site [61 and reprojected to match the UV-VIS image and elemental maps. Findings and Discussion: We find an inverse correlation between topography and FeO content for the basin interior. The correlation coefficient between elevation and [1/(FeO)] is aboutO.7. A typical east-west compositional profile across the basin is shown compared to a model profile generated by a linear fit to the data. The occurrence of greater FeO contents at greater depths could be explained in several ways. The magma ocean model for the formation of the lunar crust predicts a crust that becomes more Fe rich with depth, so in SPA we may be seeing an exposed crustal column. An undifferentiated melt sheet composed of such a stratified crustal material is lower in FeO. The excavation of the SPA cavity would have been a massive unroofing event that could have triggered significant mafic igneous activity through pressure-release melting. The melts that were generated would tend to erupt and or collect in the low-elevation central portion of the basin. At higher elevations, progressively smaller quantities of magma and greater impact dilution with underlying more-felspathic rocks might lead to the observed correlation between elevation and Fe content. It is interesting to note that KREEP basalts, which may have originated in a similar manner in the Imbrium Basin, possess FeO of roughly 12 wt%, comparable to much of the floor of SPA.
Blewett Dave T.
Hawke Bernard Ray
Holtzmann Jon
Jeffrey Taylor G.
Lucey Paul G.
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