Lo, and 16 co-authors, report that over 60 years of spacecraft exploration has revealed that the Earth’s Moon is characterized by a lunar crust dominated by the mineral plagioclase, overlying a more mafic (richer in iron and magnesium) mantle of uncertain composition. Both crust and mantle formed during the earliest stages of lunar evolution when late-stage accretional energy caused a molten rock (magma) ocean, flotation of the light plagioclase, sinking of the denser iron-rich minerals, such as olivine and pyroxene, and eventually solidification.
Very large impact craters can potentially penetrate through the crust and sample the lunar mantle. The largest of these craters is the approximately 2,500-kilometer-diameter South Pole-Aitken (SPA) basin on the lunar far side. Evidence obtained from orbiting spacecraft shows that the floor of the SPA basin is rich in mafic minerals, but their mantle origin is controversial and their in situ geologic settings are poorly known. China’s Chang’E-4 lunar far-side lander recently touched down in the Von Kármán crater to explore the floor of the huge SPA basin and deployed its rover, Yutu-2.
Here the authors report on the initial spectral observations of the Visible and Near Infrared Spectrometer (VNIS) onboard Yutu-2, which the authors interpret to represent the presence of low-calcium (ortho)pyroxene and olivine, materials that may originate from the lunar mantle. Geological context suggests that these materials were excavated from below the SPA floor by the nearby 72-km-diameter Finsen impact crater event, and transported to the landing site. Continued exploration by Yutu-2 will target these materials on the floor of the Von Kármán crater to understand their geologic context, origin and abundance, and to assess the possibility of sample-return scenarios. The models show that the SPA event should have penetrated to depths of several hundred kilometres, well below the crustal thickness estimated by the GRAIL data, and would have clearly excavated mantle material and redistributed it on the surface within and in the vicinity of the SPA basin. However, the absence of abundant olivine in the SPA interior remains a conundrum
But in summary, the authors found that materials at the Chang’E-4 landing site are obviously different from most sampled mare basalts and possess mafic components that are interpreted to be dominated by LCP and olivine, with very small amounts of HCP. These may represent deep-seated materials potentially from the lunar mantle, delivered to the Chang’E-4 landing site by ejecta from the Finsen crater. Moreover, our results suggest that the predictions of an olivine-rich lunar mantle cannot be ruled out. The lunar upper mantle could be composed predominantly of both LCP and olivine, rather than simply dominated by LCP with minor olivine content. Future operations of the Chang’E-4, Yutu-2 rover will focus on validating these initial results, understanding the origin of the olivine and analyzing the distribution, geologic settings, characteristics and origin of these mafic candidate mantle materials on the floor of the Von Kármán crater.
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