Do Planets' Magnetic Fields Depend on Their Composition?

[CaptainQuasar]

I know this is technically a planetary science question rather than astronomy but this still seemed like the best forum to ask it in.

Do we know why planets have magnetic fields? Does it have anything to do with conductive matter content or ferrous / metallic content? Do purely silicate asteroids have magnetic fields? Would a large body of water afloat in space have a magnetic field? Does it have anything to do with proximity to solar/stellar radiation?

 

[g33kski11z]

Yes, we know that planets have magnetic fields.. its what causes the northern lights here on earth. I'm not an expert like some in this forum, but I seem to remember hearing that its Earth's spinning iron core that gives us our magnetic field, but Mercury {I think} has an iron core that is not spinning so its field is much lower/smaller. Also, I think Jupiter's field comes from its hydrogen. So I guess there are different ways for a planet, given its content, to create a magnetic field.

Did that answer your question?
(and please someone correct me if I'm wrong)

 

[Kurdt]

The magnetic field of the planets is basically explained through dynamo theory (or magnetohydrodynamic). Basically what happens is the motion of conductors (ie molten iron or plasmas) in the presence of a small magnetic field can induce currents that reinforce the original magnetic field.

Chapter 18 of the following course by Kip Thorne is about MHD and has a short section on the Earth's magnetic field.

http://www.pma.caltech.edu/Courses/ph136/yr2004/

 

[CaptainQuasar]

Yeah, thanks guys, those responses seem to basically answer my question. And thanks for the link to the Kip Thorne course Kurdt, that's a great resource.

So it seems from what you guys say and the stuff in the online course, it is due to the presence of conductive matter within the planets, to the best of our knowledge. And the “solar wind” interacts with planetary magnetic fields because it's composed of plasma, which is also a hydromagnetic substance (and its magnetic field causes it to rotate as it moves away from the sun? Fascinating).

So a purely silicate asteroid should not have a magnetic field, nor would a mass of water, unless it was so great a mass of water that by compression it developed a core of metallic hydrogen or something, right? Or would there be a weak magnetic field in those cases because silicate and water are conductive at some level, which might be increased to a material strength in the presence of sufficiently intense solar wind?

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