Chunlai Li1, and 16 co-authors reported 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, our team reports on the initial spectral observations of the Visible and Near Infrared Spectrometer (VNIS) onboard Yutu-2, which we 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 detailed nature of the lunar mantle has eluded investigators since the Moon was first found to be differentiated and the crust to have formed by late-stage accretional energy and plagioclase flotation—a process different from the one that formed Earth’s crust. However, the initial composition and structure of the lunar mantle and how these constrain models of the formation and evolution of the Moon remain unknown. No direct evidence (such as unequivocal samples of the lunar mantle) was found in material returned by the Apollo and Luna missions. Indirect information has come from Apollo seismic experiments and the nature of mare basalts sourced from the lunar mantle, but fundamental questions remain. Predictions ranged from a mantle dominated by olivine, an orthopyroxenite with trace amounts of olivine, or some other combination of these component minerals.
After recognizing that impact craters form ‘drill holes’ into the lunar subsurface, investigators turned to orbital elemental and mineralogical measurements to probe the nature of the lunar crust and mantle, finding evidence of stratification in the lunar crust. Evidence of the presence of olivine—a known major component of planetary mantles—was surprisingly sparse on the lunar surface; the mineral was found mostly around impact basins of moderate to large size, but even there it was not abundant or widespread. Attention turned to the largest and oldest impact basin, the about 2,500-km-diameter SPA basin, which is located on the lunar far side. A basin this large would be the best candidate to have penetrated below the crust and deep into the lunar interior, excavating lunar mantle and distributing it on the surface in the form of ejecta and impact melt deposits.
In summary, the Chinese team 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 low-Ca pyroxene (LCP) and olivine, with very small amounts of high-Ca pyroxene (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.
China's Chang'e-4 lunar rover has also discovered an unusually colored, 'gel-like' substance during its exploration activities on the far side of the moon. The mission's rover, Yutu-2, rolled up to that surprise during lunar day 8. The discovery prompted scientists on the mission to postpone other driving plans for the rover, and instead focus its instruments on trying to figure out what the strange material is.
Day 8 started on July 25; Yutu-2 began navigating a path through an area littered with various small impact craters, with the help and planning of drivers at the Beijing Aerospace Control Center, according to a Yutu-2 'drive diary' published on Aug. 17 by the government-sanctioned Chinese-language publication Our Space, which focuses on space and science communication. So far, mission scientists haven't offered any indication as to the nature of the colored substance and have said only that it is "gel-like" and has an "unusual color." One possible explanation, outside researchers suggested, is that the substance is melt glass created from meteorites striking the surface of the moon.
For the announcement of this discovery, see: https://www.space.com/china-far-side-moon-rover-strange-substance.html
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