Recent scientific investigations, significantly aided by samples retrieved during China's Chang'e-6 mission and new atmospheric research, have provided insights into the Moon's geological evolution, its distinctive hemispheric differences, and its long-standing interaction with Earth. These studies collectively address long-held questions about lunar formation, impact history, and the origins of volatile elements found on its surface.
Lunar Hemispheric Asymmetry Attributed to Ancient Impact
New research, based on the first samples collected from the Moon's far side by China's Chang'e-6 mission, indicates that a massive ancient impact event is responsible for the compositional differences between the Moon's two hemispheres. The Earth-facing near side is characterized by dark basaltic plains, while the far side is lighter-hued, mountainous, and heavily cratered, a distinction observed since 1959.
A team led by planetary scientist Heng-Ci Tian from the Chinese Academy of Sciences analyzed potassium and iron isotopes in basalt samples from the South Pole-Aitken (SPA) basin on the far side. They found that these samples contained a higher proportion of heavier isotopes of potassium and iron compared to near-side samples obtained during the Apollo program and China's Chang'e-5 mission, which showed a prevalence of lighter isotopes.
Researchers propose that the impact event approximately 4.25 billion years ago, which formed the SPA basin, penetrated deep into the Moon. The intense heat generated during this event caused material in the lunar mantle to vaporize, with lighter volatile elements such as potassium-39, zinc, and sulfur evaporating more readily than heavier isotopes. This loss of volatile elements is believed to have significantly increased the melting point of rocks in the far side's interior, thereby suppressing the formation of magma necessary for widespread volcanic activity. This mechanism offers an explanation for the rugged, non-volcanic nature of the lunar far side compared to the near side's extensive basalt plains. The study was published in the Proceedings of the National Academy of Sciences.
Unified Lunar Impact Chronology Established
Scientists have confirmed that impact cratering rates are consistent across both the near and far sides of the Moon, establishing a basis for a globally unified lunar chronology system. This finding, derived from a revised model by a research team from the Chinese Academy of Sciences' Institute of Geology and Geophysics, challenges previous hypotheses, including the concept of a "Late Heavy Bombardment," by suggesting that early lunar impact events followed a gradual decline rather than dramatic fluctuations.
Historically, dating lunar surface regions has relied on counting impact craters, with higher densities indicating older surfaces. However, previous methods were limited by their reliance on near-side samples, the oldest of which were no more than 4 billion years old.
The Chang'e-6 mission played a pivotal role by returning 1,935 grams of samples from the Apollo Basin, located within the South Pole-Aitken Basin on the Moon's far side. Analysis of these samples identified two main rock types: basalt aged 2.807 billion years and norite formed 4.25 billion years ago. The ancient norite, originating from magma that crystallized after the formation of the SPA basin, provided a critical anchor point for reconstructing early lunar history.
Researchers systematically mapped crater densities in the Chang'e-6 landing area and the broader South Pole-Aitken Basin using high-resolution imagery. This new density data was integrated with historical sample data from the Apollo, Luna, and Chang'e-5 missions to construct a more comprehensive lunar impact chronology model. The alignment of far-side crater density data with the near-side-derived model indicates a homogeneous impact flux across the entire Moon, providing a reliable foundation for future lunar studies and the dating of other planetary bodies.
Earth Continuously Transfers Atmospheric Particles to the Moon
A separate study has revealed that particles from Earth’s atmosphere are continuously carried into space by solar wind and have been landing on the Moon for billions of years, mixing into the lunar soil. This research addresses a long-standing question regarding the presence of substances like water, carbon dioxide, helium, and nitrogen in lunar samples returned by Apollo missions.
Earlier theories had suggested the Sun as a primary source for some of these substances or proposed contributions from a young Earth's atmosphere prior to the development of its magnetic field. However, the new research suggests that Earth's magnetic field may facilitate, rather than block, the transfer of atmospheric particles to the Moon, a process believed to continue to this day.
Computer simulations tested scenarios representing both ancient Earth (strong solar wind, no magnetic field) and modern Earth (weaker solar wind, strong magnetic field). The modern Earth scenario proved more effective in transferring atmospheric fragments. These simulated results were validated against data from lunar soil analysis from Apollo 14 and 17 missions.
Earth's magnetic field forms a magnetosphere that generally deflects solar wind, but it also creates a comet-like tail. Researchers suggest that while the magnetic field is largely protective, its pressure can inflate Earth's atmosphere, increasing solar wind access. Furthermore, when the Moon passes through Earth's magnetosphere, particularly during its full-moon phase, atmospheric particles can be guided to its surface. This ongoing transfer implies a continuous supply of volatile gases, such as oxygen and nitrogen, to the lunar soil, which could be relevant for future lunar exploration and resource utilization.