Olivine-norite rock detected by Yutu-2 likely crystallized from the SPA impact melt pool

Olivine-norite rock detected by Yutu-2 likely crystallized from the SPA impact melt pool
VNIS spectra of the rock and lunar regolith measured by Yutu-2 rover. The image resolution is ~0.6 mm/pixel. Credit: Science China Press

The South Pole-Aitken (SPA) is the largest and deepest basin on the Moon, theoretically opening a window into the lunar lower crust and likely into the upper mantle. However, compositional information of the SPA basin was mainly obtained from orbital remote sensing. Chang'E-4 landed in the SPA Basin, providing a unique chance for in situ probing the composition of the lunar interior. The landing site is located on ejecta strips radiating from Finsen crater, which lies ~135 km to the northeast. The lunar surface at the landing site consists of a very homogenous regolith overlain by few scattered rocks.

A surface rock and the at 10 sites along the rover Yutu-2 track were measured by the onboard Visible and Near-Infrared Imaging Spectrometer in the first three lunar days of mission operations. In-situ spectra of the regolith have peak band positions at 1 and 2 μm, similar to the spectral data of Finsen materials from the Moon Mineralogy Mapper, which confirms that the regolith's composition of the landing area is mostly similar to that of Finsen ejecta. The estimated modal composition of the lunar regolith is dominated by agglutinates, plagioclase and pyroxenes with more low-calcium than high-calcium, inconsistent with mare basalts. Thus, the surface materials at the are predominantly ejecta from neighboring craters, with little contribution from the underlying mare basalt. Our observations are also supported by topographic features.

The rock spectrum shows similar band peak positions, but stronger absorptions, suggesting relatively fresh exposure. This rock is likely representative of the original bedrock in the Mg-Pyroxene Annulus of SPA basin. This rock is >20 cm in size, sitting on the surface. No grains can be unambiguously recognized on the surface based on the image with spatial resolution of 0.6mm/pixel, suggesting a fine- or medium-grain-size texture (<3 mm). Our modeling results suggest the rock is composed of 38.1 ± 5.4% low calcium pyroxene, 13.9 ± 5.1% olivine and 48.0 ± 3.1% plagioclase, referred to as olivine-norite. The plagioclase-abundant and olivine-poor modal composition of the rock is inconsistent with the origin of the mantle, but representative of the lunar lower crust. Alternatively, the rock crystallized from the impact-derived melt pool formed by the SPA impact event via mixing the lunar crust and mantle materials. This scenario is consistent with fast cooling thermal conditions of a shallow melt pool, indicated by the fine- to medium-sized texture of the , and the SPA impact melting model.

Olivine-norite rock detected by Yutu-2 likely crystallized from the SPA impact melt pool
The spectral features of Yutu-2 measurements and comparison with that of craters near the landing region. Credit: Science China Press

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More information: Honglei Lin et al, Olivine-norite rock detected by the lunar rover Yutu-2 likely crystallized from the SPA impact melt pool, National Science Review (2019). DOI: 10.1093/nsr/nwz183
Citation: Olivine-norite rock detected by Yutu-2 likely crystallized from the SPA impact melt pool (2019, November 19) retrieved 12 December 2019 from https://phys.org/news/2019-11-olivine-norite-yutu-crystallized-spa-impact.html
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