A new view of lunar magnetism
One of the enduring mysteries about the Moon is whether it ever had an internal-generated magnetic field, and if so, when the core dynamo ceased. New research by Tinghong Zhou, John Tarduno, Rory Cottrell, and Eric Blackman at the University of Rochester and collaborators from the University of Notre Dame, UC Santa Cruz, and the University of Arizona, in a study supported by NSF and NASA, have provided new insights into this lunar puzzle, narrowing down the potential lifespan of the Moon’s dynamo to its first ~140 million years. The new study focused on analyzing magnetic field intensity (called paleointensity) recorded in Apollo samples that are between 4.36 to 3.7 billion years old. Using an advanced technique known as single-crystal paleointensity analysis, the researchers were able to obtain accurate measurements of the Moon’s ancient ambient surface magnetic field environment – which indicated negligible field strengths. This evidence for the absence of a dynamo resolves the long-lasting paradox between the previously hypothesized long-lived lunar dynamo and energy considerations, namely that the tiny lunar core would have been unable to power a strong, sustained magnetic field.
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| Figure 1: Astronaut John Young stands on the rim of the Plum Crater on the Moon. Image source: NASA, Apollo Lunar Surface Journal, Apollo Image Library Hasselblad Magazine, 109/G (B&W), AS16-109-17804. |
A key part of this new understanding of lunar magnetic history comes from a focus on magnetic carriers meeting the demanding requirements of paleointensity theory. Single-domain magnetic grains, which are very small, some 20 to 200 nm in size, are required. In contrast, larger magnetic grains are far less reliable because their internal domain walls can move with time and during laboratory treatments, corrupting any original magnetic signal. Magnetic minerals in lunar rocks are dominated by these problematic multidomain grains, making paleointensity analysis very challenging. The single-crystal paleointensity technique used in the study by Zhou and others builds on an earlier study led by the University of Rochester and focuses on silicate crystals that contain single-domain magnetic grain inclusions to meet the paleointensity recording requirement. The authors tested the fidelity of their records by CO2 laser heating in different fields and in the presence or absence of an applied field. These tests exclude thermal alteration and provide a measure of recording efficiency. The authors found high recording efficiencies, indicating that if surface fields had been present, they would have been recorded. Hence, the absence of a paleointensity indicates absence of a surface field.
In addition to the single crystal paleointensity, the study also employed whole rock paleointensity on 3.7-billion-year-old Apollo basalts using a non-thermal technique. Unlike thermal methods that measure magnetization acquired from natural cooling, non-thermal methods rely on additional assumptions and empirical calibrations. The results from the non-thermal technique showed abnormally high and inconsistent paleointensities. These anomalies could indicate shock magnetization from lunar impacts or issues with the multidomain grains and/or the applied non-thermal method. Because non-thermal analysis of whole rocks is the basis for some calls for an episodic lunar dynamo, the researchers conclude there is no robust evidence for such a phenomenon from Apollo samples.
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| Figure 2: Lunar magnetic history indicated by paleointensity data. Single crystals suggest a null lunar magnetic field since 4.36 Ga, while some whole rock data obtained by non-thermal methods yield abnormally high values that might be related to large multidomain magnetic grains and/or impact induced magnetic field. Figure modified from Tarduno et al., 2021 and Zhou et al., 2024. |
If the Moon did not have a dynamo for most of its history, the early Earth’s (for example, during the Archean and Hadean eons) atmosphere can be transferred to the Moon, which would be unshielded by an intrinsic field, and preserved in its regolith. With a smaller Earth-Moon distance and the stronger solar wind in the Archean and Hadean, this transfer would have been enhanced. By studying the volatiles trapped in the lunar regolith, we might have opportunities to better understand the composition of the early Earth’s atmosphere and the conditions that influenced the evolution of life.
