Review

The lunar dynamo

Science  05 Dec 2014:
Vol. 346, Issue 6214,
DOI: 10.1126/science.1246753

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Structured Abstract

BACKGROUND

It is unknown whether the Moon has a fully differentiated and melted structure with a metallic core or retains a partially primordial, unmelted interior. The differentiation history of the Moon is manifested by its record of past magnetism (paleomagnetism). Although the Moon today does not have a global magnetic field, the discovery of remanent magnetization in lunar rocks and in the lunar crust demonstrated that there was a substantial lunar surface field billions of years ago. However, the origin, intensity, and lifetime of this field have been uncertain. As a result, it has been unclear whether this magnetization was produced by a dynamo in the Moon’s advecting metallic core or by fields generated externally to the Moon. Establishing whether the Moon formed a core dynamo would have major implications for understanding its interior structure, thermal history, and mechanism of formation, as well for our understanding of the physics of planetary magnetic field generation.

Embedded Image

The interior structure of the Moon and the lunar dynamo. New magnetic measurements of lunar rocks have demonstrated that the ancient Moon generated a dynamo magnetic field in its advecting liquid metallic core (innermost red shell). This dynamo may have been driven by convection, possibly powered by crystallization of the core (innermost red sphere) and/or stirring from the solid mantle (thick green shell). The magnetic field was recorded as magnetization by rocks on the lunar surface. [Image created by Hernán Cañellas] The interior structure of the Moon and the lunar dynamo. New magnetic measurements of lunar rocks have demonstrated that the ancient Moon generated a dynamo magnetic field in its advecting liquid metallic core (innermost red shell). This dynamo may have been driven by convection, possibly powered by crystallization of the core (innermost red sphere) and/or stirring from the solid mantle (thick green shell). The magnetic field was recorded as magnetization by rocks on the lunar surface. [Image created by Hernán Cañellas]

ADVANCES

A new generation of laboratory magnetic studies of lunar rocks and spacecraft measurements of lunar crustal magnetic fields have produced major advances in our understanding of the evolution of ancient magnetic fields on the Moon. It has now been established that a dynamo magnetic field likely existed on the Moon from at least 4.5 billion to 3.56 billion years ago, with an intensity similar to that at the surface of Earth today. The field then declined by at least an order of magnitude by 3.3 billion years ago. The early epoch of high field intensities may require an exceptionally energetic power source such as mechanical stirring from mantle precession. The extended history of the lunar dynamo appears to demand long-lived power sources such as mantle precession and core crystallization.

OUTLOOK

Measurements of the intensity of the ancient lunar dynamo have shown that it was surprisingly intense and long-lived. The next phase of lunar magnetic exploration will be to obtain more accurate measurements of field paleointensities and to determine when the dynamo initiated and finally disappeared. This will be coupled with the continued development of magnetohydrodynamic models for characterizing mechanical and other unusual dynamo mechanisms and further investigations into the thermal, structural, and geodynamical history of the lunar core and mantle. The eventual availability of absolutely oriented samples and in situ spacecraft measurements of bedrock should enable the first measurements of the paleo-orientation of lunar magnetic fields. Such directional data could determine the lunar field’s geometry and reversal frequency, as well as constrain ancient local and global-scale tectonic events.

Abstract

The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization.

Lunar magnetism persisted via dynamo

Today the Moon has no magnetic field, but this was not always the case. Remnant magnetization in lunar rock and crust samples indicates that substantial fields existed billions of years ago. Weiss and Tikoo review how modern magnetic studies have established that these fields were powered by a magnetic dynamo that lasted from 4.2 to 3.56 billion years ago. However, the possible mechanics behind the dynamo, such as mantle precession or core crystallization, remain under investigation. To find out how and when the dynamo came and went now requires improvements in magnetohydrodynamic models and more accurate paleointensity measurements, possibly even those that show the field direction.

Science, this issue 10.1126/science.1246753

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