question about planetary magnetism

[Spacehog]

Hello there, I'm trying to get a few answers for a short story I'm working on. I'm brand new to this forum, so if I'm posting in the wrong place I apologize. Anyway, I had a question about planetary magnetic fields. From what I remember in college, Earth has a magnetic field but Mars no longer does. I think that Mars did have a magnetic field, but it was lost when the core of the planet hardened or something like that. I guess I'm not totally clear why one planet's magnetic field would be stronger than another. I thought that it was purely based on size, but again I remember hearing in a lecture that Mars' magnetic field used to be far stronger than earth's despite Mars being a smaller planet.

So, here is my question: What makes a planet's magnetic field stronger or weaker? Is it the composition of the core? The speed of the core's rotation? Speed of the planet's rotation? All of the above? None of the above?

Thanks in advance!

 

[Vertigo]

Hi and welcome Spacehog, This is probably as good a place as any but I'm sure a mod will move it should they feel it necessary

On your question, I'm not an expert on this, but there's sure to be one along shortly.

However my understanding is that there are a number of factors to consider. The composition of the core and it's movement. As I recall it is the circulation of our molten iron core that creates the magnetic field and I think any planet with a molten metallic core would behave in much the same way. Mars is significantly smaller than us and farther out from the sun which I guess is why it has cooled faster, solidified and so lost it's magnetic field. I don't think we actually know Mars' core is solid yet but that is our current assumption.

There are also many other implications arising from that. Without the magnetosphere the planet's atmosphere gets stripped away surprisingly rapidly by the solar wind. Without the magnetosphere and with a thinner atmosphere the planet's surface is bathed in massively more radiation. I'm on much thinner ground here but I think: the average annual radiation received on Earth from space is around 0.39mSv per year, Curiosity has measured the radiation on the surface of Mars at 0.67mSv per day (on route to Mars in the spacecraft it measured an average of 1.8mSv per day). Those are some figures worth thinking about when you have your humans ambling around on the surface of Mars.

 

[Spacehog]

Yes indeed, very helpful Vertigo. Thanks! The points you make all seem right on and they help me remember some of the stuff I studied almost ten years ago at this point. The thing that surprised me at the time was the assertion that Mars' magnetic field used to be stronger than earth's. The Prof. didn't really say why Mars had a stronger magnetic field. I wish I'd have asked him in hindsight. He did say that the magnetic signature from rock samples told them that Mars' magnetic field was so strong at some point in the past.

Incidentally, Mars has nothing to do with the short story I'm interested in. I'm more interested in planets with very strong magnetic fields. I imagine a planet that has a very powerful magnetic field, but I want to understand more about magnetic fields before I just start writing a bunch of crazy stuff that isn't accurate.

Your point about the composition of metals in the core is really interesting though. I wonder what would happen if a planet had a high concentration of Neodymium in its core. That would probably make for a powerful magnetic field.

 

[chrispenycate]

In the QI terms - nobody knows. Jupiter has a massive field, and no core as such. Venus doesn't have much, but doesn't rotate much - but the thought that the core is molten and rotating doesn't explain away the inversion of the field every so often (fortunately infrequently). There are lots of theories, but none that are guaranteed right.

 

[Spacehog]

Yeah, that's also a good point chrispenycate. The reversing fields are really interesting too. I also didn't know that Venus' magnetic field was relatively weak, that is also interesting. I am just trying to find a reasonable explanation to why a planet would have an abnormally strong magnetic field relative to the planet's size. I guess in my mind the fact that Jupiter (or the sun for that matter) has a strong magnetic field seems easy to believe because there is so much mass in flux. But having a massive magnetic field on a planet like earth is what I am imagining. Now I just need to find out a way for that to work even if it is theoretical.

 

[Vertigo]

Sorry I can't help any more than that really. Not my strongest area. Chrispy is generally much more knowledgeable than me

 

[Mirannan]

Spacehog - Nice idea about neodymium, but in fact it wouldn't work.

The reason is that the special properties of rare earths only come into play when making permanent magnets, that rely on ferromagnetism to keep their magnetic fields. But that isn't how planetary magnetic fields work. Planetary (and stellar) magnetic fields are generated by electric currents flowing in a fluid, conducting core - which are in turn generated (in some way not so far understood, AFAIK) by convection currents turned into swirling by the Coriolis effect.

Thus, the Earth's magnetic field is generated by currents in the outer core, which is mostly liquid iron. Jupiter's is generated in the liquid metallic hydrogen of Jupiter's core, and stellar magnetic fields are generated in the highly conductive plasma that makes them up.

There are two reasons why Jupiter's field is so strong. One is the sheer mass of liquid metal involved, and the other is that Jupiter spins very fast. A day of about 9 hours, I believe.

 

[Vertigo]

Excellent and eloquent answer Mirannan! I guess for Spacehog that means the only way he'll get his bigger magnetic field is either more mass or faster spin. The latter is probably the more realistic if it's to be inhabited. Actually that would offer a neat way of introducing the concept since a fast spin would, I think, also mean more turbulent weather.

 

[Spacehog]

Oh that is VERY helpful Mirannan and Vertigo! It's perfect actually, just what I was looking for. So to summarize, if I had a planet that was a bit bigger than earth, that had a fast rotation and a liquid core, then I could theoretically have my planet with a strong magnetic field. True?

 

[Spacehog]

I also really like the more turbulent weather and maybe also more turbulent plate tectonics. That will work perfectly for what I have in mind. Thank you all a thousand times. This is exactly why I joined this forum!

 

[Mirannan]

Oh that is VERY helpful Mirannan and Vertigo! It's perfect actually, just what I was looking for. So to summarize, if I had a planet that was a bit bigger than earth, that had a fast rotation and a liquid core, then I could theoretically have my planet with a strong magnetic field. True?

Yup. There's even possibly a good excuse for it. The theory at the moment (AFAIK) is that Earth was "spun up" by the impact that created the Moon, and way back when (I mean WAY back, maybe a couple of billion years) Earth rotated significantly faster than it does now; day of maybe 12 hours. Connected to that is the fact that the Moon was closer then as well, which makes for bigger tides and (because of tidal flexing of the Earth's rocks) more volcanic activity as well.

So make the place a young planet?

 

[Spacehog]

Yes, an excellent idea. I believe that should work quite well for what I have in mind. I'll let you know how it all works out. Thanks again for all of your guys' contributions!

 

[Vertigo]

And give it a large moon that will churn it up internally!

 

[Spacehog]

Yeah, I like that! A large moon in closer orbit so that it can have some violent tides as well as everything else stated.

Now all I have to do is finish writing a story to go with this world and I'll be set. I'll certainly let you guys know how things progress. I'm sure I'll be bugging people all over these forums for more information and ideas too. Thanks for this though! I joined this forum to get this question and ones like it answered. So far I am more than pleased with this community. I'm probably going to get my buddy to join here as well since he is also looking for something like this.

 

[Vertigo]

It is an excellent forum. And, which seems so rare these days, a very well mannered one. Flaming is virtually unheard of, though some of us might get a little hot under the collar occasionally and there are a lot of smart folk including a number of published and self-published writers. The writing community on here is very very active and do a lot of critiquing of each others work (though you'll need a few more posts before you can submit any work for critiquing).