A schematic representation of the suggested mechanism: tridimensional motions are generated by elliptical instability in the Moon’s core following the combined effect of an impact induced desynchronisation of the Moon and of the tidal distortion of its core-mantle boundary.
Credit: M.-H. Deproost, ORB, Belgique
LONDON: Scientists have proposed that the Moon's ancient magnetic field was powered by mechanically driven motions within its fluid core that may have occurred continuously or been triggered by asteroid impacts.
Two studies have presented solutions as to how the ancient magnetic field persisted for more than 400 million years, and also describe a new way to generate planetary magnetism that could be relevant to other celestial bodies.
"Our work means that smaller bodies can have a dynamo than previously thought, and might help explain why some asteroids seem to be magnetised," said Christina Dwyer from the University of California, Santa Cruz, lead author of the first of two papers published this week in Nature.
"These mechanisms could superimpose to the convective motions in lots of planets, and they could even be the dominant mechanism in others such as Mercury and early Earth," added geophysicist Michael Le Bars from France's national science organisation CNRS and the Université Aix-Marseille, who led the second study.
An ancient lunar dynamo
Although the present day Moon has no internally generated magnetic field, evidence from lunar rocks indicates that Earth's satellite had a long-lasting global magnetism present between 3.9 and 3.6 billion years ago.
Previous theories have assumed this to be the result of a thermally driven dynamo similar to that on Earth, where the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core.
However, scientists have struggled to explain how this could generate a magnetic field of sufficient strength and duration, because the Moon's core is too small to sustain the convective currents required.
Continual stirring of the core
Two new studies now present the possibility that the magnetic field of the Moon was powered by a mechanical dynamo, caused by the differential rotation of internal layers in the early planetary body.
The scientists propose that the movement of the Moon's solid rock mantle past its iron core resulted in a stirring of the liquid centre, generating fluid motions sufficient to drive a lunar dynamo. However, they present alternative models to explain how this movement came about.
Dwyer and colleagues consider that the rotational axes of the core and mantle are misaligned, as they have been throughout history, resulting in the continual generation of fluid motions. However, the strength of the stirring movement depends upon the size of the angle between the rotational axes of the core and mantle, and the distance of the Moon from the Earth, both of which have altered over time.
"Both of these were more favourable earlier in time when the Moon was closer to the Earth. At some point the Moon got far enough away that the stirring wasn't vigorous enough to support a dynamo," explained Dwyer.
Lunar magnetism compared with Earth's
Is it possible that the combined magnetic field of the moon and Earth could interfere with each other. Or are they to small at those distances?
Lunar magnetism
Could a large crash, Like a comet/asteroid turn the load stone atoms one way generating a N/S magnet?