Earth's magnetic field

Why is our planet magnetic? Have we always had the same magnetic field, and is it true that some animals can detect it and use it to find their way around? This week we’re going to try and find out. And with us to answer some of those questions is Kathy Whaler, from the University of Edinburgh. Kathy maps the Earth’s magnetic field, and she spoke to Chris Smith. First of all, Chris wanted to find out more about the asteroid name after her!

Kathy - When I was president of the Royal Astronomical Society, which also covers geophysics, I was awarded that at the end of my service.

Chris - What’s it called? Has it got a good name?

Kathy - It’s called Kathy Whaler. Very uninteresting I’m afraid. It sits in the asteroid belt.

Chris - An imaginative bunch these astronomers aren't they?

Good to have you with us Kathy. I’ve got a compass sitting here and when I go walking I use this to find my way around. It’s a small magnet which senses the planet’s magnetic field, responds to it, and tells me when I’m going in the right direction.

Why has the Earth got the ability to make a magnetic field and how’s it do it?

Kathy - We all sit and walk about and drive over the Earth’s crust, which is a brittle layer 7 to 60 kilometres thick depending on where you are - thinner over the oceans. And that’s made of silicon, of rocks and beneath that it’s also another rocky layer, the mantle. Slightly different in that it’s able to creep on rather slow timescales. So that’s the top half of the Earth. And when you get about  halfway down there’s a real major change, so we go from silica dominated material to iron dominated material and the iron is where we get our magnetic field from. We get it specifically from the outer part of what we call the core, the area towards the centre of the Earth divided into a liquid outer part and a solid inner part.

Chris - Which of those two bits: the liquid outer part and the more solid inner part is responsible for making the magnetism?

Kathy - It’s the outer part because that undergoes a process we call convection. Basically it churns around and as that liquid moves then it’s able to self-generate a magnetic field.

Chris - Now given that this is going on thousands of kilometres beneath our feet, how do you, as a group of geologists, know that what you’ve told me is true?

Kathy - Well, we measure the field on different timescales by different mechanisms and that enables us to see the changes in the field, and the changes tell us that we have to have some active dynamic process that’s responsible for its generation. It’s not a residual field from when the Earth was formed, for instance or anything like that.

Chris - It has to be originating from that mobile core?

Kathy - It does.

Chris - Given that the Earth is losing heat all the time, Kathy, and cooling off, that includes the core doesn’t it? So if the core were to go solid, what would happen to our field?

Kathy - Well, if the core was to freeze out altogether then we would actually lose our magnetic field, and then we’d lose this protective shield that we’ve just been talking about that protects us from solar wind particles and things like that.

For instance, if you think about our near neighbour Mars, Mars used to have a magnetic field but it stopped being generated and, as a result, Mars lost its atmosphere.