Chang’E-4 offers clues to the inside of the Moon

The moon’s mantle – the geological layer that lies between its surface crust and central core – may comprise the minerals low-calcium pyroxene and olivine, results from China’s Chang’E-4 lunar mission suggest.

In January 2019, the Chang’E-4 lander touched down successfully on the dark side of the moon, inside the satellite’s largest impact crater, known as Von Kármán. It subsequently deployed a rover, Yutu-2, to sample and analyse the crater floor.

In a paper published in the journal Nature, researchers led by Chunlai Li from the Chinese Academy of Science in Beijing deliver the first tranche of results.

The choice of the crater as the mission landing spot was driven by the primarily geophysical nature of the Chang’E-4 project. The scientists behind the mission are intent on resolving a long-standing mystery concerning the composition of the moon’s mantle.

The general mechanics of how the moon coalesced around its dense iron core are thought to be well understood and uncontroversial.

During a late stage of the moon’s formation, energy from incoming meteorites caused its outer layers to heat up and become a magma ocean. The liquid gradually separated with a comparatively light mineral, a member of the feldspar group called plagioclase, floating to the surface. 

Plagioclase remains the dominant component of the crust. 

Meanwhile, heavier iron-rich minerals in the magma-gloop sank. Eventually, the ocean cooled and solidified, stabilising both the dense mantle and the lighter top layer.

In the absence of a very large drilling rig, determining the exact composition of the mantle presents a challenge for lunar geophysicists. 

Information previously obtained by orbiting spacecraft indicated the presence of fragmentary iron- and magnesium-rich material scattered on the surface in some areas, and identified them as pyroxene – a mineral class that includes jade – and olivine, which is common on Earth, greenish in colour and sometimes known as the gemstone peridot. 

It was not possible to definitively establish their mantle provenance, leaving open the possibility that they may be exogenous – perhaps debris from a meteor strike.

However, scientists working on the Chinese project reasoned that an incoming meteor large enough to create a massive crater may well have possessed enough momentum to punch right through the crust and into the mantle – throwing up bits of it onto the surface.

Hence the choice of Von Kármán.

Li and colleagues report that observations made by the rover’s onboard Visible and Near Infrared Spectrometer (VNIS) instrument indicate the presence of the same two key minerals. 

More research will be needed to absolutely confirm their underground origin. However, the researchers suggest that if they are indeed mantle components, they did not actually originate in the Von Kármán crater itself.

Rather, they suggest, they were likely ejected as a result of the impact that caused a neighbouring hole – the 72-kilometre-wide Finsen crater – and propelled through the air to their final resting place.

In a related editorial in the same issue of Nature, Patrick Pinet from Research Institute in Astrophysics and Planetology (IRAP) in France describes the Chinese results as “thrilling”.

The findings, he writes, “could have considerable implications for characterising the composition of the Moon’s upper mantle, and for establishing constraints on characteristics of the lunar magma ocean that would have varied with time”.

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