Sampling The Youngest Lunar Basalts

Samuel Lawrence
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Sampling The Youngest Lunar Basalts
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Introduction: We advocate an automated sample return mission to the mare basalts south of the Aristarchus plateau to address fundamental questions about the composition of the lunar crust and the time-stratigraphy of lunar volcanic processes, with implications for all of the rocky bodies in the Solar System. Rationale: Lunar mare basalts formed through partial melting of the mantle and are probes of the structure and composition of the Moon’s interior. Mare basalts cover ~17% of the lunar surface [1]. However, the full range of mare basalt compositions and ages has not yet been sampled [2], [3]. Knowledge of the duration of mare volcanism comes from (a) radiometric dating of lunar samples and (b) crater-size frequency distributions (CSFDs) of mare surfaces from remote sensing data calibrated by correlation with returned samples. According to present understanding, mare volcanism reached its maximum volumetric output between 3.8 and 3.2 Ga ago [4] but began as early as 4.3 Ga ago [5-7] and may have persisted until as recently as 1.2 Ga ago [3, 8] and possibly to even more recent times [9]. This uncertainty can be addressed by targeted sample return. Proposed Landing Site: CSFDs suggest that some lunar maria are significantly younger than the youngest Apollo basalts [8]. [10] determined that five basalt units in Oceanus Procellarum have model ages ranging from ~1.5-2.0 Ga. Unit P60 (Figure 1), south of the Aristarchus Plateau, has the youngest model age (1.2 Ga +0.32/-0.35 b. y.). Recent work independently confirmed P60 model ages of 1.03-2.81 Ga [11]. There are now sufficient data from the NASA Lunar Reconnaissance Orbiter to certify a safe P60 landing site (yellow box, Fig. 1). The analysis of returned samples from the P60 region would increase knowledge about major element, trace-element, and isotopic variations in lunar basalts. These new data would aid in constraining the Moon’s thermal history, help distinguish differences in basalt source regions/reservoirs and eruption rates over time, and significantly improve knowledge of the Moon’s absolute chronology and the relative chronology of other rocky Solar System bodies. At this sampling location, Aristarchus crater ejecta would provide additional information about the material excavated by the crater and its age; similarly, the returned sample could yield information about the pyroclastic resources deposit on the plateau. Traceability: Sampling the youngest lunar basalt unit is directly responsive to science goals outlined in [12], especially Goal 5b: Determine the age of the youngest and oldest mare basalts. Mission Strategy: An automated sample return mission similar to the proposed MoonRise mission, lasting no longer than a lunar day, can meet the mission requirements. Sample return is required: Apollo demonstrated the importance of returned samples [13]. Detailed analysis of compositions, mineralogy, rock textures, and physical properties in addition to laboratory-determined radiometric ages are needed. Measurements could be made using in-situ instrumentation, but terrestrial laboratories offer more capability for the foreseeable future, and to date, represent the only means for determining the ages of the youngest lunar basalts with sufficient precision. Samples become resources, so new measurements can be made as analytical techniques improve. References: [1] J. W. Head, Rev. Geophys., (1976) 14, 2, 265–300, 1976. [2] T. A. Giguere et al. (2000) MAPS, 35, 1, 193–200. [3] H. Hiesinger et al. (2011) GSA S. P. 477, 1–51. [4] C. K. Shearer and J. J. Papike, (1999) Am. Mineral. 84, 10, 1469–1494. [5] L. A. Taylor et al., (1983) EPSL, 66, 33–47. [6] E. . Dasch et al. (1987) GCA 51, 12, 3241–3254. [7] C.-Y. Shih and L. E. Nyquist, (1989) LPSC 20, p. 1002. [8] P. H. Schultz and P. D. Spudis, (1983) Nature, 302, 5905, 233–236, [9] S. E. Braden et al. (2014), Nat. Geosci., 7, 11, 787–791. [10] H. Hiesinger et al. (2000) JGR-Planets, 105, E12, 29239–29275. [11] A. Stadermann, et al. (2017) Icarus, Submitted. [12] N. R. C. (2007) The Scientific Context for Exploration of the Moon: Final Report. [13] G. J. Taylor et al. (1991) “Lunar Rocks,” in Lunar Sourcebook, 183–284.
D. S. Draper, J. D. Stopar, B. L. Jolliff, C. R. Neal, J. E. Gruener