MadSci Network: Astronomy
Query:

Re: INFORMATION ABOUT MAGNETISM OF PLANETS

Date: Fri Apr 19 22:58:22 2002
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Astronomy
ID: 1019123819.As
Message:

An excellent question, and you've already suggested some excellent points as well! We think that planetary magnetic fields require three things:

  1. A conducting, liquid interior,

  2. A rapidly rotating planet, and

  3. Convection in the interior.

To explain a bit more about each:

  1. To make a magnetic field, we need electrical currents. This is true in electromagnetics (remember that they just have wires carrying currents around and around) and ferromagnets (like in a compass; these currents are very, very small scale). Planets turn out to be more like electromagnets than permanent metallic magnets (ferromagnets). We know that this is true of Earth because we've seen evidence in rocks that Earth's field changes in direction and strength over time. Metallic magnetics can't do this. Thus, we need something which can carry a charge, a conducting material. And it needs to be able to move, hence a fluid. In Earth, this is molten iron in its core. In Jupiter and Saturn, this is probably metallic hydrogen. Metallic hydrogen is hydrogen which has taken on the properties of a metal (mainly electrical conductivity) because of high pressures.

  2. In order to get the electrical charges moving in the right way, a planet needs to rotate pretty quickly, just like we have to drive electrons through loops of wire to make an electromagnet.

  3. Convection, the process where hot material rises and cold material sinks around in great loops, is thought needed to amplify the otherwise weak magnetic field of the planets.

The details of how planetary magnetic fields work are actually poorly understood right now. (This would be a great area to start researching!) We have seen evidence that we are on the right track, however. Jupiter, the largest (and with the most metallic hydrogen in its interior, or metallic anything, actually) and most rapidly rotating planet has by far the strongest field. Venus, the slowest rotating planet has no measurable field. Mars apparently used to have a field, but no longer does. However, we also think Mars's interior has cooled and solidified in the past billion years or so. So this actually fits well. The really odd planet is Mercury, which rotates slowly (every 59 days) and ought to be frozen inside (like our Moon), but has a reasonably strong field anyway. There are theories out there explaining this, but it is still the subject of a lot of discussion. There is an interesting way of plotting magnetic fields as a function of the angular momentum of planets (angular momentum takes both mass and spin rate into account). There is a version of this in Figure 6 of an essay that I have written. You will notice that the planets which are biggest and spin fastest indeed have the strongest field. (This also holds for the Sun, which is intriguing.)

What happens as you move into a planet? Does the field get stronger? Weaker? Well, for a while, it will get stronger as you get nearer the part of the planet which is making the field. Similarly, as we move away from Earth, the field gets weaker. However, if you dig far enough down, right into Earth's core, you'll find that the field becomes jumbled and starts weakening. This is because you start getting into the outer core where the field is created, and that whole area is very messy.

You might find this Ask an Astronomer question interesting if you want to know more. Also, there is some information and a nice model at this Generating Magnetism site. The figure at the upper left of that page is a computer generated model of the magnetic field of Earth. The colored lines are field lines and the jumble of lines in the middle are what happens at the core of Earth, where the field is being produced. (The physics of magnetic fields is so complex that computers are required to really understand what's happening.)


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