🌕 A lunar anomaly, revealed by the Apollo missions, finds an explanation
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For decades, planetary scientists have debated the intensity of this field in the Moon's past. A recent study provides an answer.
During the Apollo missions, the landers touched down in flat, dark regions called lunar maria. These areas are rich in specific volcanic basalts, which recorded magnetic signals. Researchers realized that this limited sampling created a bias, as it does not represent the entire lunar surface. Consequently, presumed episodes with a strong magnetic field were overestimated in previous interpretations.
Chemical analysis of these rocks reveals a link between the presence of titanium and magnetic intensity. Samples with high levels of titanium show traces of a powerful field, while those with little titanium correspond to a weak field. This correlation indicates that the melting of titanium-rich rocks at the boundary between the core and mantle caused intense but brief magnetic bursts.
According to computer models, if the Moon had been sampled randomly, these exceptional events would have been observed less frequently. In reality, for most of its history, between 3.5 and 4 billion years ago, the lunar magnetic field was likely weak. This view aligns with dynamo theory, which explains how planetary cores generate magnetic fields.
a ) Structure of the Moon at the end of the magma ocean solidification. Dense cumulates containing ilmenite and KREEP materials crystallize at the top of the lunar mantle. These cumulates, gravitationally unstable, sink to the core-mantle boundary (CMB), carrying with them a portion of the KREEP materials.
b ) Lunar regime during the Isotopic High-energy Event (IHIE). The radiogenic heat produced by the KREEP materials sufficiently heats the base of the mantle to initiate mantle convection and the melting of ilmenite-bearing cumulates. The melting of these cumulates at the CMB temporarily increases the heat flux out of the core, generating a short-lived, high-intensity dynamo. Simultaneously, titanium-rich basalts erupt at the surface, capturing the rare phenomenon of an intense lunar magnetic field.
Understanding the Moon's magnetic past contributes to studying the evolution of planetary interiors. It provides clues about the cooling of its core and the end of its geological activity. Furthermore, it offers a comparison point to explain why Earth's magnetic field persists while that of the Moon has vanished.
Future missions, like NASA's Artemis program, will allow testing these predictions by exploring new lunar regions. By collecting more diverse samples, scientists hope to refine our knowledge of our satellite's magnetic history.
How a planetary dynamo works
A planetary dynamo is a mechanism that generates a global magnetic field around a celestial body. It relies on convection motion within a molten metallic core, often composed of iron and nickel. This motion, combined with the planet's rotation, creates electrical currents that produce the magnetic field.
On Earth, this process is active and maintains a stable magnetic field, protecting the surface from harmful radiation. In the Moon's case, its small size limited the duration and intensity of its dynamo. The lunar core cooled more quickly, reducing the convection necessary to sustain a strong magnetic field over the long term.
Dynamo theory helps explain why some bodies like Mars have lost their magnetic field, while others like Jupiter possess a powerful one. It depends on factors such as the size, composition, and age of the core. For the Moon, brief episodes of a strong field could be linked to specific geological events, like the melting of particular materials.
Understanding this mechanism is important for studying planetary habitability. A magnetic field can influence the retention of an atmosphere and protection against the solar wind. Thus, research on the lunar dynamo teaches us about the conditions necessary for life on other worlds.