Magnetizations within lunar rocks indicate that the ancient Moon produced an internally generated magnetic field1,2,3,4. Yet the long-lived field intensities of 40–120 μT inferred for the ancient Moon1 are unsustainably high and cannot be reconciled with the small lunar core size, requiring three orders of magnitude more energy than deemed possible5. Here, we report that a 1 Gyr protracted foundering of cool, dense Ti-rich material initially emplaced at shallow depths can generate the high-intensity magnetic fields (≥50 μT) preserved within returned lunar samples through temporary episodes of elevated superadiabatic heat flow (≥50 mW m−3) out of the lunar core. Our results can simultaneously explain both the high magnitude and large-scale variability in field intensities before at least 3.5 Gyr ago1 while still permitting lower fields to have persisted thereafter.
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The code used to produce the numerical results discussed in the text is available from the corresponding author on reasonable request.
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We acknowledge J.T. Keane and J.I. Schneiderman for insightful discussions. A.J.E. was supported by NASA grant 80NSSC20K0861.
The authors declare no competing interests.
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Extended Data Fig. 1 Inferred paleointensities of the core dynamo throughout lunar history as recorded by returned samples.
Modern, high-fidelity measurements (grey) are shown alongside Apollo-era paleomagnetic data (orange). Circles indicate paleointensity determinations and triangles indicate upper limit values for paleointensities. Modern data indicate a lunar dynamo capable of strong surface magnetic fields of approximately 40–100 μT prior to 3.56 Ga (cyan) and weak surface fields of less than 5 μT after ~3.2 Ga (magenta), with a transition period in between (indicated by dashed vertical lines). For reference, the average present-day magnetic field intensity for Earth is 30 μT.
Solid lines indicate the timescale of lateral spreading of a single foundered diapir (d=1) with melt viscosities of 102 Pa s (black) and 104 Pa s (magenta). Dashed line shows lateral spreading timescale for d equidistant foundered diapirs with a viscosity of 102 Pa s and of total volume equivalent to a foundered diapir of radius 100 km.
Magnetic field intensity (μT) as a function of diapir radius (km) for Fig. 1.
Delineations of regions shown in Fig. 2.
Palaeomagnetic data for Extended Data Fig. 1.
Lateral spreading timescales shown in Extended Data Fig. 2.
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Evans, A.J., Tikoo, S.M. An episodic high-intensity lunar core dynamo. Nat Astron 6, 325–330 (2022). https://doi.org/10.1038/s41550-021-01574-y
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