Microscopic magnetic-field imaging of a single lunar dust grain

Did the Moon, Earth's companion for billions of years, once sustained a global magnetic field? If so, when and by what mechanisms it ceased? The answers are critical to understanding the Moon's internal structure and thermal evolution history.

In recent years, orbital measurements have revealed spatially heterogeneous lunar magnetic anomalies, yet their origin remains debated: are they remnants of an ancient core dynamo, or magnetizations caused by localized events such as meteorite impacts? Resolving this question hinges on identifying the magnetic carriers within lunar regolith and their magnetic characteristics.

Conventional methods, however, face limitations. Orbital magnetometers lack sufficient spatial resolution (kilometer-scale), while macroscopic bulk rock-magnetic measurements (e.g., VSM) provide only volume-averaged information. Consequently, resolving magnetic distributions and origins at the single-particle or even sub-micron scale has become critical.

In a recent study published in Fundamental Research, Prof. Jiandong Feng's team at Zhejiang University, in collaboration with Prof. Jinhua Li's team at the Institute of Geology and Geophysics, Chinese Academy of Sciences, used a custom-designed magnetic imaging microscopy to conduct high-resolution magnetic imaging on Chang'e-5 lunar soils, realizing the direct observation of magnetic field distributions on single lunar regolith grains (Fig. 1).

The core instrument of this study is the NV quantum sensing based microscopy. This technique utilizes quantum defects in diamond as "super-probes" to achieve precise detection of weak magnetic fields. By optimizing the optical system and sensor design, the research team achieved the following advantages:

1. High Spatial Resolution: Achieving a spatial resolution of 2.2 μm, equivalent to precisely mapping a magnetic field on the cross-section of a hair strand, enabling detailed observation of the magnetic structures of lunar soils (Fig. 2).
2. High Detection Sensitivity: The magnetic field detection precision reaches one-thousandth of the geomagnetic field, capable of capturing extremely weak magnetic signals.
3. Three-dimensional vector magnetic field reconstruction：Beyond two-dimensional magnetic mapping, the workflow integrates vector-field reconstruction and correlative 3D X-ray microscopy to constrain magnetic carrier morphology and infer complex magnetic-domain structures.

By observing basalt and breccia particles from Chang'e-5 lunar soils, the research team "saw" the specific carriers and origins of lunar particle's magnetism at the microscopic scale. The results indicate that lunar particle's magnetism is inherently heterogeneous and reflects multiple remanence acquisition processes:

1. Records of Magmatic Evolution: In basaltic grains, magnetic signals are primarily associated with native iron (potentially coexisting with or embedded within troilite-bearing regions). Their relatively uniform direction suggests they may preserve information about the lunar paleomagnetic field generated by early internal dynamo activity.
2. Impacts and Space Weathering Modification: In breccias, magnetic signals are stronger and more complexly distributed, showing high spatial correlation with magnetic carriers such as nanophase iron (np-Fe), Fe-Ni alloys, and crack features. This indicates that their magnetic origins are closely related to subsequent modification processes, including meteorite impacts and space weathering.

Looking ahead, the teams plan to further optimize microscopy performance—improving spatial

resolution and sensitivity to access even smaller magnetic carriers—and to expand analyses to a broader suite of Chang'e-5 and Chang'e-6 returned samples. Systematic cross-sample comparisons are expected to provide stronger constraints on the spatiotemporal evolution of the lunar magnetic field, refining models of lunar interior evolution and supporting the continued advancement of deep-space exploration program.

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Contact the author: 

Jiandong Feng, Laboratory of Experimental Physical Biology, Department of Chemistry, Zhejiang University, Hangzhou 310058, China, jiandong.feng@zju.edu.cn;

Jinhua Li, Key Laboratory of Deep Petroleum Intelligent Exploration and Development, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China, lijinhua@mail.iggcas.ac.cn

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