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Moon Magnetism Mystery

A 40 year old moon mystery may have been solved recently.

It looks like we may have two viable theories why some moon rocks are magnetized even though the moon has no magnetic field. Not only that, up until recently, there was no theoretical mechanism by which the moon could even create a magnetic field.

 It started in the early 70s.

Moon rocks brought back from the Apollo missions caused a surprise when it was discovered that many were magnetized. This meant almost certainly that there had to be a magnetoshpere  around the moon at some point in its past. The problem is , how did it make one?


The moon is just too small to make one of its in the way the earth does.

The earth’s field is caused by the interplay of two things: convection of electric charges (which is key) and rotation. That is the essence of a geo-dynamo. Stars do the same thing.

The large temperature differences of the liquid metallic alloys in the outer core of the earth causes a lava-lamp type of convection of these electrically conducting fluids. This coupled with the rotation of the core creates a magnetic field that extends to the surface of the earth and far into space which totally saves our ass from the solar wind.

The moon though could never make a magnetic field this way. It’s just way too small. The temperature differences near the core just aren’t great enough to support the convection required.

Even earth-sized planets don’t necessarily have to have a magnetic field. It’s a delicate interplay of forces that create it.

You’d think that Venus for example would definitely have a magnetic field since it’s a virtual twin to earth but it doesn’t. That’s because its rotation is just too slow.


So how did the moon magntize its rocks then?

Clearly there must be another way for a geomagnetic field to be created.

This is what these two new theories that are in the Journal Nature are about. 

The first one describes the moon soon after it was formed. In those early years it was much closer to the earth. It appeared 15 times bigger in the sky. The tidal forces would of course have to be equally great. The moon raised mountain sized tides here on earth. Imagine living at that time and seeing a mountainous swell of water scour the land twice a day. That event is on my top-ten list of historical must-sees when I get my time-machine.

Such a dramatic tidal interaction is of course a two-way street. The earth would also have to have a titanic impact on the moon in a similar manner. Earth’s tidal forces caused the moons mantle to rotate around a different axis than the molten outer core was. This caused turbulence within the molten iron giving rise to a magnetic field which in turn imprinted itself on the rocks cooling on the surface of the moon.

Michael Le Bars, author of one of the Nature papers and a scientist at the Research Institute for Out-of-Equilibrium Phenomena in Marseille, France has another take on moon magnetism.

His theory describes a moon being pelted by waves of asteroids. This titanic energy caused the mantle to rotate against the rotation of the core causing turbulence that then gave rise to a magnetic field that could last 10,000 years. In fact, the period of time in the solar system when this was a common occurence, called the late heavy bombardment is precisely when the moons magnetic field seems to have existed.


You may have noticed that these two theories are in fact complementary. The key that they share is that convection is not required. Simple mechanical stirring is all you need.

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