Why Mercury Has a Substantial Magnetic Field

Mercury and Earth CoresBack when I was teaching planetary astronomy, one thing really bugged me: Mercury’s magnetic field. Even at that time, we knew that the planet’s field was far larger than we would expect. And with the MESSENGER spacecraft about to crash on Mercury, I thought it would be a good time to think about this issue. So let’s start by getting a little basic planetary astronomy out of the way so that you can understand my confusion.

It is not obvious that planets and moons should have magnetic fields — or at least stable ones that exist for billions of years. And despite the fact that the earth has quite a powerful one (Critically important to protecting we organic creatures!) it has only been in the last century that scientists have gotten a handle on why we have one. The way it works here on earth is not always the way it works elsewhere. But it is thought to be the way it works on Mercury.

When talking about the earth’s magnetic field, people think of a bar magnet. But I think that confuses the issue. The explation for the earth’s notable magnetic field is the dynamo theory. The outer core of the earth consists of liquid iron and nickel. So it is a conductor — a very good one. And since the earth is spinning, this causes the free electrons to circle around due to the Coriolis effect. (Note: the Coriolis effect does not control water swirling around your bathtub drain!) So what we have is a natural current. And as you may know because you took a physics course some time: a magnetic field is created by a moving charge (current). Thus: magnetic field!

If you’ve been paying attention, you should have noticed that the solid core should also create some magnetic field. It has a couple of problems. First is the fact that it isn’t liquid, so it doesn’t get any Coriolis effect movement. Second, it is much closer to the earth’s center of mass, so the direct rotational current is much smaller. So it doesn’t matter on the earth. But it could matter, as we shall soon see.

Clearly, the faster that a planet rotates, the larger its magnetic field should be (all else equal). Consider, for example, our moon — which has a very small magnetic field. And as far as we can tell, it doesn’t have any magnetic field due to the dynamo theory action. It has a tiny liquid core (so little potential current) and an extremely slow rotation rate (so very little current even if there had been a large liquid core). So: few electrons, moving slowly. Not a recipe for much of a magnetic field.

Mercury has the same problems: slow rotation rate and small liquid core. But there is a difference! First, as I indicated above, a solid core creates a magnetic field. In the earth, this solid core is very small. But in Mercury, it is quite large. But in addition to the direct effect, this also means that Mercury’s liquid outer core is further from the center of mass. That means there is a greater Coriolis effect than there is in the moon where the liquid core is way down toward the center. Also: Mercury’s liquid core is substantial.

All of this adds up to Mercury having a larger magnetic effect than I originally thought it should. It is still a weak field: only about one percent that of the earth. But we know why that is: larger outer core and and much faster spin — although Mercury has a larger solid core. Regardless, I feel better. I don’t like it when things don’t make sense. Although it does give scientists something to do with their time other than hanging out at Comic-Con International.


That was a joke about Comic-Con. In my experience, actual scientists really aren’t especially into the nerd culture that lay people always associate with them. I’ve always minded The Big Bang Theory for that. I find the scientists on Better Off Ted far more realistic.

Update (30 April 2015 7:39 pm)

I want to thank RJ for the excellent question about the electric flux of the positively charged atoms and the free electrons. Unfortunately, my astronomy related sources have not really been that up on the issue. This is probably because very few people deal with planetary astronomy. It is of more interest to scientists of the earth. And let’s be honest: the weak magnetic field of Mercury is not nearly as spectacular as the accretion disk of a binary pulsar. But I appreciate the help I got from my old boss Lynn Cominsky. Lynn, ever the high energy astrophysicist sent out some cheeky email to her group, “Any ideas here? I think Mercury is a planet…” But this comment from Kevin was too depressingly true, “Mostly people just wave their hands, and if you press them they run away screaming.” Indeed.

But I’m going to tell you what I think I know. The most important thing, as I mentioned in the comments, is that this current that is created is not the usual kind of current that we talk about in E&M class. We are used to copper wires with free electrons that move through them. But in this case, all the free electrons move to the surface of the core — to get as far away from the positively charge atoms that — if you anthropomorphize objects like I do — menace them. So they are out of the picture and the magnetic field really is created by the positively charged atoms that are moving about inside the liquid core.

The next thing we must understand is that the Coriolis effect is only part of what’s going on. The primary mechanism is the convection cells that are created by the heating of the core (as things get pushed together — think: the sun) and that heat gets dissipated into the mantle. The Coriolis effect then causes the ions to swirl around those convection cells. And that is as far as I’m going to take this.

But I could be wrong. If an actual planetary astronomer happens by and has some insight into this — or if I’m wrong on the physics (and I am drinking a nice red wine right now) — please straighten me out. I would love, for example, to see an actual model of this process. If I were 20 years younger, I would create my own, because this stuff is amazingly cool! But I only have the energy now to talk about some young genius’ model. Anyway, I have movies to over analyze. I have a stack of Tom DiCillo films sitting here and they aren’t going to watch themselves…

Update (2 May 2015 12:30 am)

My discussion above about the electrons fleeing to the surface is wrong. I was thinking of excess charge. There is no electric force that would cause a neutral metal core to send its electrons to the surface. If it did, then an iron ball would have a charge on it. So I’ve done more research and it seems to be moving me in the opposite direction of enlightenment. I’m seeing a lot of Maxwell equations, but without context. (So u is the velocity? Of what?!) I will look into the problem in a week or two and see if I can figure it out after ridding my mind of it. But the bottom line about this article was always that Mercury has a larger magnetic field than we would expect because it has a much larger liquid core than we would expect. As for the dynamo theory: stay tuned.

8 thoughts on “Why Mercury Has a Substantial Magnetic Field

  1. Dr. H, could you answer a question for me? I’ve never really been able to figure this out. Regarding the magnetic fields of planets – not just Mercury, but others also:

    OK, so I teach physics. And the book tells me that the magnetic field of the earth primarily is due to charged particles rotating the earth in the outer core. So far, so good. But isn’t the flux of positive ions equal to the flux of negative? So it seems like their magnetic fields should cancel out. I am similarly puzzled by the means of generating other planetary magnetic fields.

    I have the sense that I’m missing something easy and obvious. Have you ever got this question from anyone else? I know it’s a bit strange to ask you a science question!

    • I’ve never gotten that question and it is an excellent one! I don’t know for sure, I will check with a good source. My assumption is that it is because the highest energy electrons can go wherever they want. As a result, they will push outward. I would assume that they aren’t effected by the rotation of the earth because the electric field is so strong and steady and the gravitational field is so weak. Thus, the magnetic field is due to the the positive ions.

  2. OK well I was formulating another hypothesis, that there are lots of free electrons (as you mention) in molten metal, moving much faster than the metal cations and therefore it is primarily electrons that give a magnetic field rather than cations. But you seem to be saying more or less the opposite now!

    This god-damned spellcheck is flagging ‘cations’!!! Philistines.

    • Hey, I can’t blame the spell checker. Even after all these years, when I see the word, I see “ca-tions” as though it were “caissons.”

      The mechanism you are talking about would still have to be based on a gravitational force. Thus, it would be no greater than the force acting on the ions. (Ha! Spell check is fine with that!) Just the same, I don’t know about the resistant forces. That’s what makes this question so unbelievably cool! But I think that I’m basically right. See the update above.

      • Again, as someone nowhere near expert on this subject, I would think that there would be some very definite limits on how much negative charge could separate out before the electrostatic force started to pull back. Large charge separations are unstable (that’s what they taught me in chemistry anyway) so a process of electrons sort of floating to the top seems like it could not continue for long.

        Your degree is in atmospheric physics, right? Still ABD in philosophy here, studied chemistry first.

        • You’re making my brain hurt! I’ve never liked E&M. Would the electrons all move to the surface? Actually, I don’t know. I was thinking of excess charge — like a Faraday cage. So I will have to think about this some more.

          But it wouldn’t be the case that convection or the Coriolis effect would affect electrons any more strongly than the nuclei. I will spend some more time thinking about it and get back to you.

          • Just in case you felt like researching more stuff, I similarly am confused about radiation from stars in near-orbit around a black hole. Again it seems like equal positive and negative ion flow, so…

  3. I think that is just accretion. In a close binary system, the matter is charged and so it flows along the magnetic field lines onto the poles of the neutron star. The collision is so intense that it releases x-rays. I believe the reason that the particles are charged is because they are already so energetic that they’ve lost their electrons. Of course, here we are talking mostly hydrogen atoms. Maybe if I ever figure out the earth’s dynamo, I’ll look into it.

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