Scientists were shocked by a few rock samples, brought back by the Apollo astronauts – not literally of course but by the fact that they were magnetic. That required the rocks to solidify in the presence of a magnetic field at the time they formed but the researchers had no idea where that field would com from as the Moon was too small to have one. Not one but two new proposals appeared in the last few weeks however about possible mechanisms to drive a magnetic dynamo in the Moon.

Let's stir up things

In the case of the Earth, the magnetic field is generated in the iron core of the planet: though the inner core is solid, it is surrounded with a molten outer core too. The liquid, strongly conductive molten iron is constantly in motion between the hot inner core and the cooler mantle, just as boiling water circulates between the bottom and top of the pot on the stove. The flowing, conductive iron generates a magnetic field: that's the geodynamo. The core of the Moon was too small however to allow convective flows to arise. What did propel it then sufficiently to generate a magnetic field?

Actually, both papers envisage the same process to power the dynamo: the bottom of the mantle, which is normally in a fairly peaceful relationship with the top of the core, stirs up the layer of molten iron through some external influence. However, the authors of the two papers have different opinions on just what that influence might was.

Lunar sample No. 76535, collected by Apollo-17: this piece of rock was apparently magnetised 4.2 billion years ago.

Tides and axes

The Nature paper of three Californian states that the rotation axes of the core and the mantle had pointed at different angles at first, caused by the precession created by tidal forces of Earth that affected the two layers differently. The differences in rotation resulted in a continuous stirring of the molten iron core below the mantle. The scenario also explains why did the magnetic field cease to exist quite quickly: as the Moon backed away from the Earth over one billion years, the angle differences slowly vanished as the tidal forces decreased. (For math-lovers: tidal forces diminish much faster with distance, by 1/r³, compared to gravity's 1/r². )

Magnetizing impacts

If we turn one more page in the Nature however, we'll meet the paper by French and Belgian authors that link the core stirring to intense events: giant impacts. We know those exist as the Moon itself was formed that way. The authors calculated that an event capable to create several hundred km wide impact basins was also able to change the rotation period of the mantle sufficiently enough to start the stirring process of the core material, as described above. Then thousands to tens of thousand years passed until the difference vanished and the two parts of the Moon synchronized to each other. In that case the magnetic field didn't existed and decreased continuously, but was present only episodically, capable to produce much larger field strengths.

Both proposals are very interesting and they complement each other, actually. If this stirring method is really operable, the young Moon likely had a continuously weakening magnetic field which was boosted by impacts for short periods, just like flicking the nitro switch. Then, after about one billion years, it run out of gas for good...

Topography and magnetic field strength of Mare Crisium, a ~550 km wide impact basin. Several old, large basins feature similar magnetic anomalies, suggesting a lunar magnetic field that magnetized the impact melt several billion years ago.

László Molnár

Image sources:

1.) M.-H. Deproost, ORB, Belgium

2.) NASA  / JSC / LPI

3.) NASA

4.) Nature