Chang'e-5 lunar soil reveals a big mystery!

Based on the study of Chang'e-5 lunar soil, Chinese scientists proposed a new lunar thermal evolution model, revealing a major mystery that has plagued the academic community: why the moon still had volcanic activity 2 billion years ago .

This is a schematic diagram of the magma and thermal evolution of the moon at different times (drawn on October 20, 2022). Image source: Institute of Geology and Geophysics, Chinese Academy of Sciences

Chen Yi, a researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, introduced that the moon was formed about 4.5 billion years ago and its mass is only about 1% of the Earth. For such a small celestial body, in theory it should have cooled down quickly, stopped volcanic activity very early, and become a "dead" planet.

The return of the lunar soil samples from Chang'e-5 has triggered a new wave of lunar research. In October 2021, Chinese scientists published three articles in the journal Nature, revealing that lunar volcanic activity could have continued until 2 billion years ago, refreshing human understanding of the lunar magma activity and thermal evolution history.

Experts said that lunar basalt is a rock formed by the partial melting of the lunar mantle (equivalent to the Earth's mantle) and the cooling and crystallization of magma on the lunar surface through volcanic eruptions. International scholars have proposed two hypotheses for the partial melting of the continuously cooling lunar mantle: one is that radioactive elements generate heat to cause the lunar mantle to heat up; the other hypothesis is that if the water content is high, it will lower the melting point of the lunar mantle.

However, Chinese scientists' research on Chang'e 5 basalt revealed that the lunar mantle source region is not rich in radioactive heat-generating elements and is very "dry", ruling out the above two hypotheses . Therefore, why lunar volcanic activity has lasted for so long has become an unsolved mystery in the new round of lunar research.

Yuan Jiangyan, an engineer at the Scanning Electron Microscope and Electron Probe Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, is analyzing the composition of the lunar soil of Chang'e 5 (photo taken on October 14, 2022). Image source: Institute of Geology and Geophysics, Chinese Academy of Sciences

To address this issue, Chen Yi led the scientific research team to select 27 representative Chang'e-5 basalt rock chips, and used the latest scanning electron microscope energy spectrum quantitative scanning technology to analyze the main components of the whole rock of the rock chips. Combined with a series of petrological and thermodynamic simulation calculations, they successfully restored the initial magma composition of the Chang'e-5 basalt, and compared it with the initial magma of the Apollo samples to infer their origin depth and temperature.

The study found that compared with the Apollo samples, the initial magma of the young Chang'e-5 basalt contains more calcium and titanium, which indicates that the lunar mantle source of the Chang'e-5 basalt contains more calcium-rich and titanium-rich materials. This part of the material is precisely the product of the late crystallization of the lunar magma ocean and has the property of being fusible. Its addition will significantly reduce the melting point of the lunar mantle, inducing partial melting of the lunar mantle to form young lunar basalt. Further simulation calculation results show that the temperature of the interior of the moon has only dropped by about 80 degrees Celsius after more than a billion years of continuous cooling.

Chen Yi said that the study shows that although the interior of the moon continues to cool slowly, the fusible components crystallized in the late stage of the lunar magma ocean are continuously added to the deep lunar mantle, which not only "replenishes calcium and titanium" for the lunar mantle, but also lowers the melting point of the lunar mantle, thereby overcoming the slowly cooling interior environment of the moon and triggering long-term continuous lunar volcanic activity .

The results were published in the latest issue of the American journal Science Advances.

Source: Xinhua News Agency

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