Destinations for Sampling Impact Melt Produced by the South Pole – Aitken Basin Impact Event

Content Provider	Semantic Scholar
Author	Hurwitz, Debra M. Kring, David Allen
Copyright Year	2014
Abstract	Introduction: The intensity of impact activity during the earliest history of the Solar System is poorly constrained due to the lack of samples collected from ancient planetary terrains. The South Pole – Aitken (SPA) basin is the oldest basin identified on the Moon based on stratigraphic superposition and, thus, represents a key target for characterizing this earliest impact record [1,2]. To determine the absolute age of SPA, rocks that formed as a result of the impact, such as impact melt, must be identified, collected, and analyzed. In this paper, we use high-resolution images obtained by the Lunar Reconnaissance Orbiter Narrow Angle Camera (LROC NAC) to explore locations that potentially contain SPA impact melt. These observations are integrated with spectral analyses of surface compositions and models of melt sheet differentiation to identify destinations where SPA impact melt samples can be collected. Identifying SPA Impact Melt: Petrological modeling suggests that the melt sheet generated in the transient crater during the impact event would have differentiated, forming a shallow layer of low-Ca pyroxene (pyx) + plagioclase (plag) beneath a layer of quenched melt [3-5]. This quenched melt would have the same bulk composition as both the initial melt within the melt sheet and the melt ejected from the transient cavity. Modeling suggests this initial impact melt composition would have been dominated by lowand high-Ca pyx with plag [4,5]. These results are consistent with spectral observations of the SPA interior. Clementine, Lunar Prospector (LP), Kaguya Spectral Profiler, and Moon Mineralogy Mapper (M) spectral data (e.g., [6-9]) indicate a vast region of elevated FeO and Th content within the SPA interior (Fig. 1). SPA melt may also be found in material ejected from the basin, but the anomalous signatures shown in Fig. 1 likely correspond to the highest concentration of this melted material. Spectra indicate the dominant mafic mineral throughout SPA is low-Ca pyx (e.g., [6-9]), with occurrences of high-Ca pyx and non-mafic minerals increasing with distance from the basin center (e.g., [8]) outside the extent of the predicted transient crater (e.g., [10]). Exposures of crystalline low-Ca-bearing materials have been identified in the walls and central peaks or peak rings of several craters (e.g., Bose, Bhabha, and Finsen [7,8,11]) and basins (e.g., Leibnitz, Apollo, and Schrödinger [7,11,12]) within SPA. Anomalous high-Ca pyx materials have been identified near the basin center in Mafic Mound [7]; this composition might be consistent with a rafted portion of the quenched melt layer from the SPA melt sheet (e.g., [4,5,13]). These outcrops of potential impact melt represent crucial destinations for collecting and returning samples that can refine our knowledge of the earliest impact record. Analyses: Using LROC NAC images, we investigated areas within craters and basins that might contain SPA impact melt (labeled in Fig. 1). We specifically looked for outcrops and fallen rocks that are both characterized
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Language	English
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Resource Type	Article