Geologists reveal mysterious and diverse volcanism in the moon’s Apollo Basin, the landing site of Chang’e-6

The far side of the moon is a mysterious place that is never visible from Earth. The most remarkable feature of the moon is the asymmetry between the near and far sides of the moon in terms of composition, crustal thickness, and mare volcanism. Scientists have not yet reached a consensus on the origin of the lunar asymmetry due to the lack of samples on the far side, which is one of the most important remaining questions of lunar science.

The Chang’e-6 mission, launched on May 3, 2024 and currently en route to the moon, is the world’s first sample return mission to the far side of the moon. The goal is to return approximately 2 kg of lunar soil to Earth from the southern mare plain of the Apollo Basin in the South Pole-Aitken Basin, the largest known impact feature in the solar system. These monsters hold enormous scientific potential that could be used to solve the lunar dichotomy riddle and even reshape our knowledge of our nearest neighbor.

As shown in a recent article published in Earth and planetary science lettersDr. Yuqi Qian, Professors Joseph Michalski and Guochun Zhao from the Department of Earth Sciences of the University of Hong Kong (HKU) and their international collaborators have extensively studied the volcanism of the Apollo Basin and its surroundings, revealing the mysterious and diverse volcanism of the Chang’ e-6 landing site with significant implications for the Chang’e-6 sample analysis and the origin of the lunar dichotomy.

The study found that the Apollo Basin has extensive volcanic activities ranging from the Nectarian period (~4.05 billion years ago) to the Eratosthenian period (~1.79 billion years ago). The volcanic activity in the region was significantly influenced by the thickness of the Earth’s crust. Dikes in a medium-thickness crust tend to jam beneath the crater floor and spread laterally to form a sill and fractured crater.

Dikes under the crust that were thinned by the Apollo Basin event reached directly to the surface and erupted to form widespread lava flows, and dikes in thick crust lingered before they could reach the surface, forming basaltic dike intrusions. “This fundamental finding indicates that the difference in thickness of the Earth’s crust between the near and far sides may be the main cause of asymmetric volcanism on the moon,” said Dr Qian. “This can be tested from the returned Chang’e-6 samples.”

Off the southern Mare Plain in the Apollo Basin, where Chang’e-6 is set to land, there are at least two eruptions. The first erupted ~3.34 billion years ago with a low Ti composition and covered the entire topographically low region between the Apollo peak ring and the basin edge. The later eruption occurred ~3.07 billion years ago with a high Ti composition near Chaffee S crater and flowed eastward with decreasing thickness until encountering proto-ripple ridges.

The authors suggested that the high-Ti basalts in the West have the most abundant scientific significance. Sampling it would recover high-Ti basalts, underlying low-Ti basalts, and exotic nonmare materials transported by impact events. Professor Michalski emphasized: “Diverse sample sources could provide important insights in solving a series of scientific moon questions hidden in the Apollo Basin.”

“The result of our research is a great contribution to the Chang’e-6 lunar mission. It provides a geological framework for a complete understanding of the soon-to-be-returned Chang’e-6 samples and will be an important reference for the upcoming sample analysis for Chinese scientists,” said Professor Guochun Zhao, chairman professor of the Department of Earth Sciences at HKU and co-author of the article. “It is a big step for HKU, where the university is pursuing excellence in planetary sciences and increased participation in the national space program.”

HKU is the only university in Hong Kong to possess lunar samples obtained by the close-range Chang’e-5 mission. Building on the foundation of this work, HKU’s geology team will also take advantage of the opportunity to acquire Chang’e-6 samples. This initiative aims to enable HKU to possess lunar samples representing both the near and far sides, opening up a new window of scientific research into the study of two lunar hemispheres.