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An early lunar core dynamo driven by thermochemical mantle convection


Although the Moon currently has no internally generated magnetic field, palaeomagnetic data, combined with radiometric ages of Apollo samples, provide evidence for such a magnetic field from 3.9 to 3.6 billion years (Gyr) ago1, possibly owing to an ancient lunar dynamo1,2. But the presence of a lunar dynamo during this time period is difficult to explain1,2,3,4, because thermal evolution models for the Moon5 yield insufficient core heat flux to power a dynamo after 4.2 Gyr ago. Here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo. Using a three-dimensional spherical convection model6, we show that a dense layer, enriched in radioactive elements (a ‘thermal blanket’), at the base of the lunar mantle can initially prevent core cooling, thereby inhibiting core convection and magnetic field generation. Subsequent radioactive heating progressively increases the buoyancy of the thermal blanket, ultimately causing it to rise back into the mantle. The removal of the thermal blanket, proposed to explain the eruption of thorium- and titanium-rich lunar mare basalts7, plausibly results in a core heat flux sufficient to power a short-lived lunar dynamo.

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Figure 1: Thermochemical evolution models in stable (ac) and unstable (dh) thermal blanket regimes as seen in temperature (a, d, g), composition (b, e, h) and core heat flux (c, f) compared with palaeomagnetic intensity data (i).

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We thank B. Buffett for helpful discussions; C. Johnson, R. Jeanloz, M. Manga and H.-P. Bunge for comments; and D. Stevenson and M. Zuber for reviews that improved this manuscript. We dedicate this work to the memory of our co-author Stephen Zatman, who inspired us by his interest in this subject. This work was supported by IGPP LANL, NASA CT project, the National Science Foundation, and the Miller Institute for Basic Research in Science.

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Correspondence to Dave R. Stegman.

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Stegman, D., Jellinek, A., Zatman, S. et al. An early lunar core dynamo driven by thermochemical mantle convection. Nature 421, 143–146 (2003).

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