A widely accepted perspective suggests that the far side of the moon exhibits a more rugged and cratered landscape because it has historically acted as a protective "shield," absorbing meteorite impacts that might have otherwise struck Earth. Yet, recent findings stemming from soil samples collected during the Chang'e 6 mission call this traditional view into question. New evidence indicates that the rates and impacts of meteorite collisions are strikingly similar on both the near and far sides of the moon.
In light of this revelation, researchers in China have introduced an innovative model for lunar crater chronology. This model allows scientists to accurately estimate the ages of unexamined areas of the moon by analyzing the density of craters present, without the need for direct sampling. This significant advancement was detailed in a study published on Thursday in the journal Science Advances, offering a comprehensive framework that could revolutionize the examination of lunar science.
Yue Zongyu, the lead author of this study and a professor at the Institute of Geology and Geophysics under the Chinese Academy of Sciences, remarked, "The moon functions as a historical record for impact events across our solar system. Understanding the age of its surface is crucial for piecing together its evolutionary timeline."
Yue elaborated that when considering unsampled regions of the moon, scientists typically assess the density of craters to gauge their age; essentially, the older a lunar surface is, the more craters it will contain. This methodology hinges upon establishing a correlation between the accurately measured radiometric ages of known soil samples and the estimated ages derived from crater analysis.
Previous models of lunar chronology were largely based on samples retrieved from the moon's near side, which were all from surfaces younger than 4 billion years. This limitation has sparked debates regarding the reliability of such models. However, the recent collection of 1,935 grams of samples from the far side of the moon during the Chang'e 6 mission has significantly altered the discourse. Among these samples, scientists found norites estimated to be around 4.25 billion years old, likely corresponding to the formation of the South Pole-Aitken basin, which is recognized as the largest and oldest impact crater on the moon.
The research team undertook an analysis of the radiometric ages of these far-side samples while also integrating high-resolution remote sensing data. They drew upon historical information from previous lunar exploration missions, including those conducted by the United States' Apollo program, the Soviet Union's Luna missions, and China's own Chang'e missions, to construct their new lunar chronology model.
The findings revealed a consistent rate of impacts between the near and far sides of the moon, suggesting that the frequency of crater formation per unit area over the same time period has remained nearly identical on both sides.
This new model presents a challenge to the long-standing "Late Heavy Bombardment" hypothesis, which posits that a significant wave of impacts took place approximately 3.9 billion years ago—an idea supported by the clustering of many Apollo samples around that epoch. Instead, the new research indicates that these samples may only represent localized events rather than a global cataclysm. The early impact history of the moon appears to reveal a gradual decrease in impact frequency over time.
