In a significant development, the recent analysis of lunar soil samples collected by China’s Chang’e 6 mission has unveiled two distinct phases of volcanic activity on the far side of the Moon, occurring approximately 4.2 billion years ago and 2.8 billion years ago. This groundbreaking research was published on November 15, 2024, in the scientific journal Nature, under the title “Chang’e 6 Basalt Reveals Volcanic Activity on the Far Side at 2.8 Billion Years Ago.”
Understanding Lunar Volcanism
Lunar scientists have long observed the Moon’s surface through telescopes, noting its distinct black-and-white regions. The lighter areas, known as “highlands,” consist of the Moon’s earliest crustal materials, while the darker areas, referred to as “maria,” are vast plains formed by the cooling of basaltic lava flows. The basalt found in these maria results from partial melting of the lunar mantle, providing important insights into the Moon’s internal composition and thermal evolution.
Historical Context of Volcanic Activity
Previous lunar missions, including those by the United States (Apollo), the former Soviet Union (Luna), and China’s Chang’e 5, have indicated that volcanic activity on the near side of the Moon dates back at least 4 billion years, with evidence of continued activity until about 2 billion years ago. Notably, volcanic glass beads from Chang’e 5 samples suggested small-scale volcanic activity on the near side as recently as 120 million years ago.
However, until the Chang’e 6 mission, there was limited knowledge about volcanic activity on the Moon’s far side due to the lack of samples from that region.
The Chang’e 6 Mission’s Contributions
The Chang’e 6 mission successfully collected 1935.3 grams of lunar soil from the Apollo crater, located in the South Pole-Aitken basin on the far side, an area known for its thin crust and prominent basaltic composition. The ability to sample this region provides vital data that allows scientists to better understand the far side’s volcanic history.
Radiometric Dating Techniques
To determine the ages of the volcanic eruptions on the Moon, the research team employed radiometric dating techniques, similar to carbon-14 dating in archaeology. Instead of carbon, the scientists used other isotopic systems, primarily the uranium-lead (U-Pb) dating method.
Using a secondary ion mass spectrometry (SIMS) technique developed by the Chinese Academy of Sciences, the researchers analyzed 108 basaltic rock fragments from the Chang’e 6 samples. By measuring the ratios of lead isotopes, they constructed isochrones to derive the absolute ages of the samples. This method enabled them to discern whether the basalt fragments originated from different volcanic events or were from the same eruption.
Findings from the Research
The analysis revealed that 107 of the basalt fragments shared consistent lead isotope ratios, suggesting they likely originated from the same volcanic eruption approximately 2.807 billion years ago. The mantle source of these basalts was characterized by a low μ-value (~360), indicating a depletion of incompatible elements.
Additionally, a single high-alumina basalt fragment dated to approximately 4.203 billion years ago was also identified, with a source characterized by a much higher μ-value (~1620), indicating a potential origin from a more enriched mantle source.
Implications of the Discoveries
These findings suggest that volcanic activity on the far side of the Moon persisted for at least 1.4 billion years. The research indicates that, even after the massive impact that formed the South Pole-Aitken basin, a relatively enriched mantle source existed 4.2 billion years ago. By 2.8 billion years ago, the mantle source had become significantly depleted, yet it was still capable of producing magma. This indicates that the smaller-scale volcanic activity observed on the far side is not directly related to the depleted KREEP (Potassium, Rare Earth Elements, and Phosphorus) material, but rather that the thicker lunar crust may be a primary reason for the limited distribution of basaltic volcanic activity.
These insights not only enhance our understanding of the Moon’s volcanic history but also contribute to broader discussions about planetary evolution and the geological processes that shape celestial bodies.