Showing papers by "Mark A. Wieczorek published in 2001"
TL;DR: In this paper, the authors propose that buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead, and they find that mare basaltic magmas are, in fact, less dense than the Moon's upper anorthositic crust.
87 citations
TL;DR: In this paper, the authors correlate the depths of origin of materials that make up central peaks with a geophysi-cally derived dual-layered crustal thickness model and find that the peak compositions are consistent with this strati- ed model.
Abstract: Spectral-reflectance data of lunar central peaks haverevealedthattheMoon'scrustvariesbothlaterallyand vertically in composition. We correlate the depths of origin of materials that make up central peaks with a geophysi- cally derived dual-layered crustal thickness model and nd that the peak compositions are consistent with this strati- edmodel. Specically,peakscomposedexclusivelyofrocks containing more than 85% plagioclase (by volume) come from this model's upper crust, whereas peaks that contain some norite or gabbro-norite come from the model's lower crust. Extrapolating these data we nd that the Moon's upper crust is composed of 884% plagioclase, correspond- ingto29to32wt.%Al 2O3 .T hemost-maclowerportion of the crust is composed of 658% plagioclase, having an Al2O3 content that lies between 18 and 25 wt.%. We show that the lower portion of the crust is consistent with having formed by cumulateflotation in a lunar magma ocean.
82 citations
TL;DR: In this article, the authors investigated whether the convergence of Imbrium's ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly.
Abstract: The northwest corner of the Moon's South Pole-Aitken (SPA) basin contains a high abundance of thorium and a unique Imbrian aged geomorphologic unit that consists of “grooves and mounds” (referred to here as the Imbrian grooves). Because the location of these features are almost antipodal to the Imbrium basin, where high-thorium ejecta and seismic energy are expected to have converged, an Imbrium origin for these units has long seemed certain. By modeling the deposition of impact ejecta on the Moon, we have investigated whether the convergence of Imbrium's ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly. As a result of the Moon's rotation, our results show that ejecta from this basin should converge more than 12° west of its antipode. Both the Imbrian grooves and thorium anomaly within SPA, however, reside slightly to the east of Imbrium's antipode. In an attempt to reconcile this disparity, the effects of a putative oblique Imbrium impact have been qualitatively investigated. While this model can distribute ejecta in the general vicinity of the Imbrian grooves, the planform of our modeled antipodal ejecta is distinctly different from that which is observed. As an alternative explanation for the origin of these features, we find that the modeled distribution of ejecta from an oblique Serenitatis impact is surprisingly similar to the planform of the Imbrian grooves, with the exception that it is offset directly to the east. This eastward offset is likely to be an artifact of our not being able to properly include the effects of the Moon's rotation in our oblique impact models. We conclude that the Imbrium grooves and SPA thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the Serenitatis basin. If this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined, then this implies that (1) the Serenitatis target contained a high abundance of thorium and (2) the convergence of seismic energy at the antipodes of either the Imbrium or Serenitatis basin was not sufficient to cause substantial surface modification. Extrapolating this result to Mercury suggests that the “hilly and lineated” terrain antipodal to the Caloris basin was formed by the convergence of ejecta, and not seismic waves.
53 citations
01 Mar 2001
TL;DR: In this paper, the authors investigated whether the convergence of ImbriumOs ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly.
Abstract: The northwest corner of the MoonOs South Pole-Aitken (SPA) basin contains a high abundance of thorium and a unique Imbrian aged geomorphologic unit that consists of Ogrooves and moundsO (referred to here as the Imbrian grooves). Because the location of these features are almost antipodal to the Imbrium basin, where high-thorium ejecta and seismic energy are expected to have converged, an Imbrium origin for these units has long seemed certain. By modeling the deposition of impact ejecta on the Moon, we have investigated whether the convergence of ImbriumOs ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly. As a result of the MoonOs rotation, our results show that ejecta from this basin should converge more than 12° west of its antipode. Both the Imbrian grooves and thorium anomaly within SPA, however, reside slightly to the east of ImbriumOs antipode. In an attempt to reconcile this disparity, the effects of a putative oblique Imbrium impact have been qualitatively investigated. While this model can distribute ejecta in the general vicinity of the Imbrian grooves, the planform of our modeled antipodal ejecta is distinctly different from that which is observed. As an alternative explanation for the origin of these features, we find that the modeled distribution of ejecta from an oblique Serenitatis impact is surprisingly similar to the planform of the Imbrian grooves, with the exception that it is offset directly to the east. This eastward offset is likely to be an artifact of our not being able to properly include the effects of the MoonOs rotation in our oblique impact models. We conclude that the Imbrium grooves and SPA thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the Serenitatis basin. If this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined, then this implies that (1) the Serenitatis target contained a high abundance of thorium and (2) the convergence of seismic energy at the antipodes of either the Imbrium or Serenitatis basin was not sufficient to cause substantial surface modification. Extrapolating this result to Mercury suggests that the Ohilly and lineatedO terrain antipodal to the Caloris basin was formed by the convergence of ejecta, and not seismic waves.