Chang’E-4 and the Moon’s mantle

The spacecraft Chang’E-4 landed on the far side of the Moon in January; something of a triumph for the Peoples’ Republic of China as it was a first. It was more than a power gesture at a time of strained relations between the PRC and the US, for it carried a rover (Yutu2) that deploys a panoramic camera, ground penetrating radar, means of assessing interaction of the solar wind with the lunar surface, and a Visible and Near-infrared Imaging Spectrometer (VNIS). The lander module itself bristles with instrumentation, but Yutu2 (meaning Jade Rabbit) has relayed the first scientific breakthrough.

ChangE
Variation in topography (blue – low to red – high) over the Moon’s South Pole, showing the Aitken Basin and the Chang’E-4 landing site. (Credit: NASA/Goddard)

The landing site is within the largest impact structure on the Moon, the 2500 km-wide Aitken Basin. Unlike the near-side maria, Aitken has only a small masking veneer of flood basalts that formed by internal melting resulting from the mare-forming impacts. Instead it is surrounded by the heavily cratered lunar crust of the Highlands made of calcium-rich plagioclase feldspar, i.e. anorthosite. Within the Aitken Basin lies the 930 km Orientale impact structure. The dark colour of the massive basin contrasts with the highly reflective nature of the Highlands and, in the absence of a basalt veneer, suggests that impacts penetrated the lunar crust to fling mantle material across the surface. The Chang’E-4 landing site therefore offered a chance to examine samples of the Moon’s mantle for the first time – none of the samples returned by the Apollo programme of the 196Os and 70s are of such material.

While Chang’E-4 is not equipped for sample return, the Jade Rabbit’s VNIS is capable of supplying information bearing on the minerals strewn across the basin. The instrument detects reflected radiation in the 450 to 2400 nm wavelength range split into many narrower channels, thereby reconstructing detailed spectra. These can be matched with reference spectra of a large range of minerals. The first results reveal the presence of the minerals olivine ((Mg,Fe)SiO4) and orthopyroxene ((Mg,Fe)Si2O6) in the lunar soil close to the lander, both of which could be from the Moon’s mantle (Li, C. and 16 others 2019. Chang’E-4 initial spectroscopic identification of lunar far-side mantle-derived materials. Nature, v. 569, p. 378–382; DOI: 10.1038/s41586-019-1189-0). Such material may represent the denser, mafic crystalline products of a magma ocean through which they sank, while lower density feldspar floated to the surface to form the Moon’s highly reflective crust.

While the spectral signature of olivine has been detected by similar instruments on satellites in lunar orbit, such results stemmed from broad areas of mixed materials. The Jade Rabbit’s discoveries can be related to actual rock fragments.

Related article: Pinet, P. 2019. The Moon’s mantle unveiled. Nature, v. 569, p. 338-339; DOI: 10.1038/d41586-019-01479-x

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