We consider the early thermo-chemical history of the Moon and specifically address the question of how the Moon could have had an internally generated magnetic field suddenly 'switch-on' somewhat late in its evolution and then just as quickly 'switch-off'. It is commonly assumed the Moon never underwent mantle convection, given the majority of the surface geology is likely the original crust formed nearly simultaneous with the Moon. This can partially be explained by the fact that thermal evolution may well occur under the regime of stagnant-lid convection. Furthermore, there are a few tantalizing clues that perhaps the Moon possessed a brief, internally generated magnetic field (Cisowski et al., 1983) and by implication, that the interior of the Moon was once convecting. Lunar samples returned from the Apollo missions provide a few which may contain a remnant thermal magnetism, possibly acquired during the Moon's 'magnetic era' (Cisowski et al., 1983). These samples also reveal the near side of the Moon contains large areas flooded with volcanic material, the lunar mare. These mare (Latin for sea), erupted during a pulse of magmatism beginning 0.5 billion years after the Moon had formed and mostly ended after 1 billion years of activity. We have recently shown (Stegman et al., 2003) that chemical overturn models suggested to explain the eruption of the Maria basalts may also account for the hitherto unexplained existence of a lunar magnetic field (core dynamo) at about the same time ( 3-4 billion years ago). We have chosen the approach of parallel computing to solve governing equations using the 3-D spherical finite element model (for which a considerable amount of effort was spent implementing a Lagrangian tracer algorithm). Our convection models bring together the main features of early lunar post-magma-ocean history, and carry an important testable prediction - that further analysis of lunar samples may yield a definite onset time at 4 Ga for the lunar core dynamo.
The Moon presently has no internally-generated magnetic field (i.e. core dynamo). However, paleomagnetic data combined with radiometric ages of Apollo samples record the existence of a magnetic field from approximately 3.9 to 3.6 Ga ('magnetic era') possibly due to an ancient lunar dynamo (Cisowski et al., 1983; Collinson, 1993). A dynamo during this time period is difficult to explain(Collinson, 1993; Stevenson, 1983), because current thermal evolution models for the Moon (Konrad and Spohn, 1997) yield insufficient core heat flux to power a dynamo after 4.2 Ga. In Figure 38.1, we show that a transient increase in core heat flux following an overturn of an initially stratified lunar mantle may explain the existence and timing of an early lunar dynamo. Using a 3-D spherical convection model (Baumgardner, 1985), we show that a dense layer, enriched in radioactive elements ("thermal blanket"), at the base of the lunar mantle initially prevents 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 basalts Hess and Parmentier, 1995), plausibly results in a core heat flux sufficient to power a short-lived lunar dynamo.
We thank B. Buffett, C. Johnson, R. Jeanloz, M. Manga, and H-P. Bunge for helpful discussions. This work was supported by IGPP LANL, NASA CT project, NSF, Miller Institute for Basic Research. We dedicate this work to the memory of the late Stephen Zatman.
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Zhong, S., E.M. Parmentier, and M.T. Zuber, A dynamic origin for the global asymmetry of lunar mare basalts, Earth Planet. Sci. Lett., 177, 131-140, 2000.
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