Why doesn’t the moon have a magnetic field?

Chris Smith answered this question...

Chris - That's a very good question. It's all to do with where the moon comes from.

If you look at the composition of the moon - and we know what the moon's made of because one of the people we just heard mentioned, Neil Armstrong, was there and scientists have brought back bits of the moon's surface - it's largely made of the same stuff as the Earth's crust.

Where did it come from? Scientists have pieced back together a theoretical model of how the moon could have been made. The moon's relatively big relative to the Earth. It's much bigger than most moons are. What scientists think is that around about the time when the Earth was first forming, about 4.5 billion years ago when the solar system was very young, two planets - the Earth and a second planet, which is notionally called Thea, ended up on a planetary collision course.

They had the planetary equivalent of an RTA. The two ran into each other and the massive catastrophic collision that ensued meant that a lot of debris from the crust of those planets got ejected like a cocoon around the Earth and this other planet, and the cores of those two planets merged together.

What you ended up with is one bigger planet with a very dense iron core and this cocoon of very "crusty" material round the outside, which then slowly settled just like the planets formed in the first place to form a moon, which was all that debris aggregating in orbit around what was then the Earth.

The core of the Earth has got a lot of iron in it. It's a mobile iron core and we think that's the ingredient you need to create a magnetic field. The moon being a bit smaller, colder, smaller and made principally of crust material doesn't have that iron core that's liquid, spinning around making that magnetic dynamo effect, and therefore doesn't have that magnetic field.

If we didn't have that magnetic field on Earth we would largely resemble Mars: a dried out prune of a planet because our magnetic field helps to deflect off the solar wind.

Dave - Also the moon being so much smaller than the Earth lost its heat a lot quicker. It's entirely solid all the way through, so you don't get this molten conducting metallic core which you need to create a strong magnetic field like the Earth has.