The Aristarchus Plateau: Surface Analysis And Sample Return Of Pyroclastic And Silicic Lithologies

Timothy Glotch
Abstract Title: 
The Aristarchus Plateau: Surface Analysis And Sample Return Of Pyroclastic And Silicic Lithologies
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The Aristarchus region has one of the most diverse assemblages of rock types on the Moon. The Aristar-chus plateau is covered by a thick (~20 m) blanket of pyroclastic fragments, portions of which are rich in indigenous water (up to 300 ppm H2O; Milliken and Li, 2017). The Aristarchus pyroclastics are the largest such deposit on the Moon and consists of high-priority samples of the Moon’s mantle that may be unlike anything in the Apollo collection, as they are likely enriched incompatible elements and H2O. Data returned by the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter spacecraft reveal that the late Copernican-aged Aristarchus crater central peak, floor, and ejecta include a highly silicic lithology that is relatively in the Apollo sample collection but occurs in several locations on the Moon (Glotch et al., 2010) (Figure 1). This interpretation is supported by photometric modeling of Lunar Reconnaissance Orbiter Narrow Angle Camera data. This lithology is potentially enriched in important in situ resources, e.g., OH and other useful components. The crater appears to have exposed a subsurface silicic lithology, possibly a granitic pluton. A similar lithology may also be ex-posed in the walls and ejecta of Väisälä crater, ~65 km to the north of Aristarchus (red arrow in Figure 1). Apparently extrusive silicic rocks are exposed in the “Cobra Head,” the source region of the Vallis Schröteri rille (white arrow in Figure 1) and Herodotus Mons, a small silicic dome approximately 180 km NW of Aristarchus (black arrow in Figure 1). The observed sizes and distribution of these silicic surface features implies the presence of a larger regional source of highly evolved rocks (Figure 1). A landed mission to the Aristarchus region would provide a unique opportunity to sample multiple lithologies that are poorly represented or unrepresented in the Apollo sample suite. A landing site near or in the southwestern portion of the ejecta blanket of Aristarchus crater (black box in Figure 1) would provide access to silicic ejecta, including, perhaps, impact melt that would provide a precise age for the crater. This landing site would also provide access to some of the youngest (~1Ga) basalts on the Moon (Hiesinger et al., 2003). Pyroclastics in this region are likely thin and M3 data do not show a strong hydration signature. Nevertheless, their sampling at this site would provide sampling of a mantle source beneath Oceanus Procellarum that would be important for understanding the lunar mantle volatile composition. Another potential landing site is the small silicic dome northwest of Aristarchus (white arrow in Figure 1). Landing at this site would provide access to an extrusive silicic lithology, likely similar to those found at larger lunar volcanic sites including the Compton-Belkovich Volcanic Complex, Hansteen Alpha, and the Gruithuisen Domes. In addition, this dome is surrounded by thick blankets of pyroclastics, which, based on M3 data have up to 300 ppm of H2O, making this another intriguing site for both scientific analysis and in situ resource utilization potential. Because of the diversity of lithologies present in the Aristarchus region, roving capability would be desired, although not strictly required. Desired surface instruments include, but are not limited to high resolution cameras, infrared spectrometers for mineral/rock identification, x-ray and gamma-ray spectrometers for elemental identification, and a mass spectrometer capable of deriving radiometric ages for different lithologies. Furthermore, any landing site in this region should be considered a prime candidate for future sample return, either by robots or humans. Determining the mode(s) of origin of silicic lithologies likely requires micro-scale analyses of samples in terrestrial laboratories. Similarly, detailed inventories of the volatile contents of the pyroclastics and determination of the most precise radiometric dates for all sampled lithologies would require analysis on Earth. The size and complexity of this region of the Moon would lend itself well to a dedicated exploration pro-gram consisting of multiple landed missions. A series of missions of increasing sophistication, beginning with a lander or rover, followed by robotic sample return, and eventually a human exploration mission would maximize both the scientific return from this site and provide valuable experience in designing and implementing human exploration strategies Glotch, T. D. et al. (2010), Science, 329, 1510-1513. Hiesinger, H. et al. (2003), JGR, 108, 5065 Milliken, R. E., and S. Li (2017), Nat. Geosci., 10, 561-565
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