Geologists Discover Hidden Magmatism at Chang’e-6 Lunar Landing Site

Lunar igneous activities, including intrusive and extrusive magmatism, and their products contain important information about the lunar interior and its thermal state. Their distribution is asymmetric on the nearside and farside, reflecting the global lunar dichotomy.

In addition to previously reported lunar samples all from the near side (Apollo, Luna, and Chang’e-5), samples from the South Pole-Aitken (SPA) basin on the far side have long been considered to hold the key to rebalancing asymmetric understandings of the Moon and revealing the enigma of the lunar dichotomy.

Earlier this year, the Chang’e-6 mission of China’s lunar exploration program, successfully launched on May 3, landed on the lunar surface on June 2 and returned to Earth on June 25 with a total of 1,935.3 g of lunar soil. It is the world’s first lunar far-side sample return mission, which landed south of the Apollo Basin in the SPA Basin on the far side.

These valuable samples could provide a window into the lunar dichotomy, and reshape human knowledge of our closest neighbor. However, compared to the well-known marine volcanism surrounding the Chang’e-6 landing site, the intrusive magmatic activities have a much more obscure presence and origin, hampering future sample analyses when they become available for application.

In a research article published in Letters from the Astrophysical JournalDr Yuqi Qian, Professor Joseph Michalski and Professor Guochun Zhao from the Department of Earth Sciences, The University of Hong Kong (HKU) and their national and international collaborators have comprehensively studied the intrusive magmatism of the Chang’e-6 landing site and its surroundings based on remote sensing data.

The study revealed their wide distribution and obscure nature with important implications for the petrogenesis of lunar plutonic rocks and the Chang’e-6 mission, which will facilitate scientists’ further study of the far side of the Moon.

Main conclusions

The study revealed that intrusive magmatism is widespread in the ASP basin. It occurs in a variety of forms, including sills beneath modified-floor craters, linear and annular dikes evidenced by gravity data, and Mg-suite intrusions with characteristic spectral absorptions.

These observations are consistent with the intermediate-thick crust of the SPA where intrusion is favored. By landing in the SPA basin, Chang’e-6 likely collected plutonic rocks, excavated and transported by adjacent impact craters to the sampling site, which could be examined by ongoing sampling studies.

They discovered two craters with heavily degraded ground, prompting them to identify other similar features on the Moon. All of these indicate that intrusive magmatism is abundant in the Chang’e-6 sample region.

This study traced potential plutonic materials in the Chang’e-6 samples and found that Mg suite materials most likely exist, mainly from the western peak ring of the Apollo Basin delivered by Chaffee S crater.

These Mg-rich materials contain crucial information on the origin of the mysterious rocks of the Mg-poor KREEP suite. Samples of the intrusive and extrusive magmatism of the never-sampled far side, particularly the mysterious Mg suite, will provide additional insights into solving the lunar dichotomy puzzle and a series of fundamental scientific questions related to the formation of the secondary crust and the early evolution of the Moon.

Professor Xianhua LI, an academician of the Chinese Academy of Sciences (CAS) and head of China’s lunar sample studies at the Institute of Geology and Geophysics (CAS), said: “The results of this research establish an important geological framework for the study of plutonic rocks in Chang’e-6 samples, especially the Mg suite rocks.”

Professor Li pointed out: “Their petrogenesis and chronology are unclear, and this research would greatly help to understand their origin mechanism.”

“This research is an excellent example of HKU’s deep involvement in China’s lunar exploration program,” said Professor Guochun Zhao, academician of the Chinese Academy of Sciences and chair professor of Earth Sciences (HKU).

“The lunar and space exploration programmes are an important part of China’s goal of becoming a science and technology powerhouse, and HKU’s proactive involvement in these programmes will provide additional resources for Hong Kong to become an international centre of science and innovation,” he continued.

The University of Hong Kong is the first university in Hong Kong to have lunar samples obtained by the Chang’e-5 mission. Building on this work, geologists at the University of Hong Kong will also seek to study samples from Chang’e-6 in the future and become more involved in China’s lunar exploration program.