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.

When Authority Preaches Nonviolence

Ta-Nehisi CoatesWhen nonviolence is preached as an attempt to evade the repercussions of political brutality, it betrays itself. When nonviolence begins halfway through the war with the aggressor calling time out, it exposes itself as a ruse. When nonviolence is preached by the representatives of the state, while the state doles out heaps of violence to its citizens, it reveals itself to be a con. And none of this can mean that rioting or violence is “correct” or “wise,” any more than a forest fire can be “correct” or “wise.” Wisdom isn’t the point tonight. Disrespect is. In this case, disrespect for the hollow law and failed order that so regularly disrespects the community.

—Ta-Nehisi Coates
Nonviolence as Compliance

Kerry Was an Excellent Candidate — Except for…

John KerryI want to make one thing clear: I never thought John Kerry was a bad presidential candidate. I was a very early supporter of his. I never much liked Howard Dean. Now, of course, I would have been a supporter of Dick Gephardt or Dennis Kucinich. But then, I really liked Kerry. I even read his stupid campaign biography! And since then, of course, I’ve done my own research. And by my calculations, Bush had a built-in advantage as big as the one that his father had when he ran against Dukakis in 1988. And it was almost as big as the one Clinton had over Dole in 1996 and Obama had over Romney in 2012. Just the same, there is one way that Kerry was a weak candidate — which I will get to in a moment.

But according to Matt Yglesias, a lot of people remember Kerry as a bad candidate, If Hillary Clinton Is the Next John Kerry, That’s Good News for Democrats. It’s an interesting article, but kind of shallow. All it talks about is how Kerry actually did somewhat better than would be expected given the economic fundamentals. I was expecting some information on demographics. I suspect that just the change in the composition of the American electorate would have changed the election. If John Kerry had received just 120,000 more votes in Ohio, he would have become president. (This has made me start to rethink my position on economic fundamentals, which I plan to write about soon.)

Yglesias thinks that there are three reasons why many people think that Kerry was a bad candidate: (1) he lost; (2) people think Bush should have been easy to beat; and (3) most people didn’t like Kerry that much — they picked him as the “electable” candidate. I don’t think those second two make any sense at all. Back then, Karl Rove had this reputation for being some kind of political magician. People may have thought Bush was an idiot — but it was an idiot savant. We might not have thought he could run a country, but we knew he could campaign. As for the second point, it is almost never the case that partisans are really excited by their candidates. The last time that happened was in 1980 with the Republican Party.

I think people remember Kerry as a poor candidate because he lost. And that’s the only reason. What happens is that all the bad things stick out. There was his style of speech, to start with. Had he been a two term president, that style of speech would be remembered today as the very definition of “presidential.” But he didn’t even win one term, so it is remembered as awkward and pompous. There was the sailboarding. That is a clear sign of virility, but because he lost, now people remember it the way Republicans tried to spin it — as something strange. And there were actual mistakes that would be forgotten had he won. For example, there was the strange decision to not mention George W Bush at the convention. I still don’t know what that was about. On the other side of things, people don’t remember his great performance in the first presidential debate.

But there was a way to win that campaign. (Given what happened in 2008, it is probably best the Democrats did not win.) According to Lynn Vavreck’s book, The Message Matters, the only way for the Democratic Party to have won in 2004 was to change the subject from the economy. (I said this same thing about Romney in 2012.) When the economy is improving, the out-party just can’t make the case that the economy would grow even faster if only we “threw the bum out!” If the Democrats had nominated Dean, he could have made the campaign about the Iraq War. And he could have become president. The problem is that Dean really wasn’t a very good candidate. We needed Kerry — but without his vote for the Authorization for Use of Military Force Against Iraq.

Otherwise, Yglesias is right: Kerry was a strong candidate. I will be happy if Clinton is as good.

Bruce Jenner and the Death of LGBT

Bruce JennerI don’t much care what Bruce Jenner does or says. But his recent interview does allow me to discuss something that I care about: why people vote the way they do. To many people, the big thing in the interview was that Jenner is a Republican. At The Fix, Hunter Schwarz wrote, Bruce Jenner Is a Republican. Here’s the Reason That Shouldn’t Surprise You. Most of that article has to do with the fact that most athletes are conservative.

There is sample bias in this claim. Schwarz talks about professional athletes and people at Brown University. So we aren’t talking about just any people who are interested in sports, but people who have chosen to take it to a very high level. But okay, I’ll yield the point. I’m sure there is something to athletes being conservative, but not in the way that Schwarz thinks. He wrote, “[A]thletes inhabit a world of meritocracy and hierarchy.” Yeah, but that correlation probably runs the other way. One thing I’ve been harping on a while is the fact that at the highest levels of sports, there is basically no difference between people. My bet is that the kind of people who find working out to be such a fantastic thing are people with higher than average testosterone levels.

But the real reason that we shouldn’t be surprised that Bruce Jenner is a Republican is much simpler: he’s rich. The rich vote Republican, as I discussed almost three years ago, It’s the Poor, Stupid. So there really isn’t much to deconstruct about this. We should applaud him for doing something that is good for the nation generally and the transgender community specifically. But the fact that he is just another rich conservative who says he’s a Republican because, “I believe in the Constitution”? It’s worth a yawn.

What is more interesting is what I wrote about two years ago, Farewell Gay Liberals. I made a more general argument, but said much the same thing back then. Overall, the gay and lesbian community is richer than the nation overall. And gay rights are very quickly going away as an issue. Is there any question about where this leads?

That’s as it should be. But I don’t think that someone being in favor of gay rights makes them a liberal. And as we move into the future, we will see increasing numbers of gay pundits slip away from liberal causes as they become unmoored from gay rights.

Jenner’s revelation highlights this. There is little doubt that eventually, LGBT rights will not be a political issue at all. The LGBT community has the advantage that its members are born randomly. So its status as a minority group is likely to be very short indeed, whereas the status of African Americans as a minority group will go on and on and on. What we’ll see going forward with regard to the LGBT community is that those in that group will act politically just like the population as a whole. The poor and smart members of the community will be liberal. The rich and dumb members of the community will be conservative. And most will not show up to vote anyway.

Morning Music: Luciano Pavarotti

O Sole Mio - Luciano Pavarotti Based upon how often I sing it, “‘O Sole Mio” is one of my very favorite songs. Of course, other than those first three syllables (“my own sun”), I don’t know any of the words. So I just make up nonsense syllables or repeat the three I know. It is a powerful and, as it turns out, cheery song. It’s a love song. It is about how the singer’s own sun lights up the face of his loved one. But who cares? It’s the music that really matters.

At this point, the best known version of the song is by Luciano Pavarotti. I must admit that when I hear his name, I think of John Candy’s not terribly flattering impersonation of him. Just the same, he has the look and mannerisms down. But Pavarotti was an amazing talent, as you can see in this live version of the song back in 1987 at Madison Square Garden: