An Impact crater serves as a bore hole to probe the moon’s interior, says S.Ananthanarayanan.

The moon is a proximate and still truly celestial body. Research on and about the moon is preparation for study of more distant regions of space, and the moon is within easy reach. Its thin atmosphere allows a large part of meteors, or shooting stars, to make it down to the surface and the moon is a good place to study the composition of meteoric material too.

An area that is challenging is the study of the interior of the moon. While there are conjectures of how the moon was formed, the studies that have been possible are restricted to the surface. Chunlai Li, Dawei Liu, Bin Liu, Xin Ren, Jianjun Liu, Zhiping He, Wei Zuo, Xingguo Zeng, Rui Xu, Xu Tan, Xiaoxia Zhang, Wangli Chen, Rong Shu, Weibin Wen, Yan Su, Hongbo Zhang and Ziyuan Ouyang, from the Chinese Academy of Sciences, Beijing, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai and Chinese Academy of Sciences, Guiyang, describe in the journal, Nature, a first study of the inner composition of the moon by a Chinese spacecraft that landed in a deep impact crater on the ‘far side of the moon’.

The usual model of the formation of the planets, and satellites, is the gravitational accretion of solid material. This process, in the formation of large objects, is so energetic that the temperatures are high and the object formed is almost all in the liquid state, like the spherical drop of the liquid. While the object loses heat by radiation and cools, the heavier, metal-rich components sink to the bottom and the core that forms is richest in these elements. The lightest materials, such as the silicates, float to the surface, as the crust, while the heavier part, metallic ores, forms the intervening mantle. A section of the earth is shown in the picture.

The crust, on the earth, is around 100 km deep. As our deepest bore-hole, to date, is just over 12 km, we have no direct information about the mantle. Our information is from the material that is brought to the surface through volcanoes, traces of minerals, like diamond, which originate within the mantle or with the help of seismic probes. This last method is a kind of CT scan of the earth, with the help of compression waves from disturbances like earthquakes. The waves that travel inwards return to the surface when they reflect off layers separating the crust and parts of the mantle, and this reveals their structure. However, with such limited information about even on the earth, clearly, there is very little we know about the internals of the moon. But the information is important to understand the evolution of stars, planets and comets as well as earth itself.

The information about the moon is primarily from visual observation, and after the missions to the moon, by analyses of moon rock. The analyses are here on the earth as well as by robotic methods on the surface of the moon. Information about the interior has come from seismic studies carried out by lunar rovers or other devices, and by recording of the moon’s gravitational field, by sensors in orbit. But direct studies are limited to the near surface, as even bore-holes are only tens of meters deep, with a plan for 100m in 2024.

There has hence been interest in studying the material at the bottom of the naturally occurring deep cavities on the moon’s surface. These are typically the craters caused by large meteor impacts and it is believed that larger impacts would have reached the mantle. The craters have been extensively mapped and we know that the largest crater is the approximately 2,500-kilometre-diameter depression, known as the South Pole-Aitken (SPA) basin, on the lunar far side.

Back door entry

When we say the lunar far side, we mean the part of the moon that we, on the earth, never see. Tidal forces have slowed the rotation of the moon, to the extent that it turns once round on its axis in the same time that it takes to go once around the earth. The moon thus keeps turning its face as it goes around the earth, and observers on the earth never see one half of the moon’s surface. The ‘other side’, however, is bathed in sunlight when we on the earth see the ‘new moon’ and the lunar day is as long as the lunar month.

Spacecraft orbiting the moon have mapped the physical features, including the meteor craters, on the ‘far side’ and the SPA depression is well documented. The paper in Nature describes the Chinese mission of landing an exploratory station on the floor of the SPA basin, to look for signs of material that could have come from the mantle. China’s spacecraft, Chang’E-4, the ‘ far side lander’ touched down in a 180 km-wide crater, the Von Kármán crater, adjoining Finsen, a 72 km wide and youngest crater, within the SPA basin, to explore the basin using Yutu-2, its robotic lunar rover.

Mantle material expected is mainly the mineral olivine, a silicate (like sand) which contains magnesium and iron, and pyroxenes, metallic minerals found in volcanic lavas. As the surface samples sent back by the US Apollo missions and the Russian Luna probes did not contain significant traces, there is specific interest is seeing if these minerals are there in samples that arise from the moon’s mantle.

The paper reports that the material found in the Von Kármán crater, as identified by the Visible and Near Infrared Spectrometer on board Yutu-2, is indeed different from what has been found elsewhere. This is particularly in its content of magnesium and iron compounds, dominated by olivine and low calcium pyroxenes, the kind expected in the mantle. This strongly suggests that the moon’s upper mantle may be dominated by these materials, which narrows the range of speculation about the moon’s interior.

While Yutu-2 is to continue its investigations, the first results are a window to improved understanding of the processes in the early period of the moon’s formation. These would be the dimensions of the sea of molten minerals in the moon’s early history and how the molten magma cooled, with stratification and distribution of its content. Study of the geology of the moon, our neighbor, how it formed and evolved, would help us understand the same processes in other planets.

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