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Understanding Earth's Magnetic Field
neutron_p writes "Researchers from the University of Maryland's nonlinear dynamics and chaos research group are seeking to solve a major scientific mystery: How is the Earth's magnetic field formed and what causes changes in the field? To find answers, they are recreating on a small scale the forces that produce Earth's own magnetic field. Scientists have constructed a series of "geodynamos" - metal spheres filled with liquid sodium that emulate conditions of the Earth's spinning, churning molten iron core. This project involves more than 14 tons of sodium metal and a 10-foot stainless steel sphere."
I hope the sprinkler system doesn't go off.
I've had this sig for three days.
I just so happen to be taking a Geology course, this semester. As I understand it, while Geologists are rather certain that radioactive materials provide the majority of the Earth's internal heat, they are equally certain that the core consists mostly of iron. The "liquid" outer sphere of iron produces the magnetic field through its motion.
As for the study itself: Wouldn't the Earth's own magnetic field interfere with the experiment, somehow? I saw nothing about this in the article, but I'm assuming that the Earth's magnetic field would either fail to significantly effect the results or the scientists are countering for it somehow, either in the experiment itself or in their calculations.
At any rate, I wish them the best of luck.
~UP
Eat the Path.
Radioactive materials aren't the same as a nuclear reactor. Unstable elements (like uraniam) or isotopes will break down regardless. A reactor has nearby radioative atoms triggering fission in their neighbors. For this, you need a fairly high density of radioative elements in close proximity. That's why they have to purify uranium ore, for example, to make fuel rods. (And further purify the isotopes to make weapons-grade uranium.)
The interiorof the Earth is almost certainly not a reactor. That theory has a lot of holes (we've argued this before on Slashdot, I know). The Earth's interior is more like an RTG on a spacecraft: you let the atoms decay at their own pace and use the heat rather than trigger a chain-reaction.
That theory is treated with serious skepticism for a reason. (Well, lots of reasons, really.) I can recall having this discussion here before and I forget the details of the theory, but I recall that the author showed an accute lack of understanding of planetary science among other things. (Jupiter is unlikely to have much more of a given metal than the Earth, oddly enough. The core is only at most about 10 Earth masses, and that's mostly ices. Also, the reactor theory doesn't explain field reversals or why the Sun has a field while dynamo theory explains both fairly naturally. Mind you, no one pretends to understand the details of theory since it's wickedly non-linear, but the basics of the theory seem to be fairly solid.)
Just in your post, I can say that it's unlikely that the field would stop because of build-up of wastes. For one thing, the wastes would either build up or they'd continually be lost. If they *did* build up, they'd slow the reactor down which would cool the system, leading to more sluggish convection and less mobile atoms. That would tend to freeze the wastes in place, not remove them.
I don't see how measuring the effects on a 10-foot diameter sphere (filled with Na) can be equivalent to the effects on the central core of a 6400km-radius ball of rock (filled with many different elements). If they want to figure out more about the Earth's magnetic field, I suggest they take measurements, etc... on the EARTH.
The nice thing about building our own sphere of molten metal is that we a) know its structure and composition in detail, b) can put sensors inside, and c) can alter parameters (temperature gradient, rate of spin) and see what happens. None of these are practical for Earth, though we do have a reasonably good idea of what its composition and large-scale internal structure are.
The patterns of motion they're setting up are common to a very wide range of fluid systems - you don't need something as big as Earth to generate them. It's very hard to measure fluid flows and magnetic fields deep within the earth (all that's easy is density change boundaries), and the Earth's field isn't likely to flip within our lifetimes (or the next several centuries, minimum, even if the wierdness we're seeing _does_ represent the start of a flip). A small-scale mock-up run in the same turbulence modes that the core has will flip many times during the course of observation, and tell us a _lot_ about how the flipping occurs.
In short, we'll learn a lot more about the geomagnetic field from this experiment than we would from more studies of the Earth itself.
That theory is treated with serious skepticism for a reason. (Well, lots of reasons, really.) I can recall having this discussion here before and I forget the details of the theory, but I recall that the author showed an accute lack of understanding of planetary science among other things. (Jupiter is unlikely to have much more of a given metal than the Earth, oddly enough. The core is only at most about 10 Earth masses, and that's mostly ices. Also, the reactor theory doesn't explain field reversals or why the Sun has a field while dynamo theory explains both fairly naturally.
Actually, while the original poster's linked model is indeed bunk, it turns out that many of these objections aren't entirely accurate.
For one, the model doesn't dispute that the field arises from a dynamo. All it disputes is the nature of the heat source driving it (near-critical ball of uranium vs. a mixture of radioactives in far subcritical spontaneous decay mode). The mechanism for setting up the field is the same.
For another, if the model's tenets are accepted, field reversals aren't mysterious. The dynamo is shut down and restarted; there's no reason for it to restart with the same field orientation as before. All of the core material is far past the Curie point for holding a residual field, so I'd expect the restarted orientation to be random (constrained only by how the earth's rotation axis affects dynamo flow patterns).
What I find dubious about the model are the claims that a) lithophyllic elements like uranium would be concentrated in the core material, and b) material would diffuse preferentially towards the core strongly enough to result in fractioning, as opposed to just slightly increased concentrations. kT is big, and gravitational potential energy change with location is small down there, so I'd expect material to diffuse anywhere it pleased.
As far as Jupiter is concerned, I can't find references that say that the "icy" chemicals are in the core. As jupiter is expected to be molten throughout (as far as I can find), I'd expect them to diffuse out. Most sources say that carbon and nitrogen are mostly bound as methane and ammonia above the layers of liquid hydrogen. Some of the oxygen is bound as water in the atmosphere, and some of it as silicates in the "rocky" part of the core (which is presumably fractioned into silicates on top of a [molten] iron inner core, as in Earth).
Moot point re. the original article, of course, as you are definitely correct about the rocky core's mass.
Just in your post, I can say that it's unlikely that the field would stop because of build-up of wastes. For one thing, the wastes would either build up or they'd continually be lost. If they *did* build up, they'd slow the reactor down which would cool the system, leading to more sluggish convection and less mobile atoms. That would tend to freeze the wastes in place, not remove them.
My understanding was that the model proposed that they built up, shut down the core, froze in place, kept the core shut down until they decayed enough for the core to be near-critical again, and then dispersed as the hotter core allowed for faster diffusion away from the reactor area. Still pretty dubious, but I'd want to see a fairly detailed model of temperature, reaction rate, and mobility changes before writing that aspect off as outright impossible.
That the Earth's core is not molten sodium, nor is it made of stainless steel, this is a pretty poor experimental model, but, hey, I'll bet the grant money was pretty good.
Considering that magnetic field generation depends only on the pattern of fluid flow and the fact that the fluid is conducting, I'd say this is actually a pretty good experimental model. See my previous post about why it's really handy to have a dynamo you can change the parameters of.
As I understand it, the universe has only one megnetic field and the Earth (and other masses) merely distorts that field. Same goes for gravity. Is this not true? I realize this doesn't change the sense of the article at all, but it always bothers me to hear people talk of the "Earth's" magnetic field like it is somehow unconnected to anything else.
The answer to this is "sort of".
The short answer is, the Earth's magnetic field is best thought of as belonging to Earth, as opposed to being a disturbance in a larger universal field. ObCaveats about the interaction between the Earth's field and the Sun's field and the Milky Way's field giving important effects; all of these can still be considered fields local to the objects generating them.
The long answer is that the electromagnetic force can be thought of as charges disturbing a field of virtual photons that does indeed fill the universe. However, saying that there's a magnetic field pervading the universe doesn't really make sense, as the measured magnetic and electric field strengths without charges and currents disturbing the virtual photon field will be zero, and these disturbances for unchanging fields have a very limited range of effect (or rather, an unlimited range but a strength that drops off very fast with distance).
In summary, "Earth's magnetic field" is probably the best description.
Yeah, I considered that the restarted dynamo would be randomly oriented. That was actually the objection, although I didn't want to go on about it in the original post. You'd expect, in reactor case, to see the new field aligned the same way as before the (dipole!) fieldless period as often as anti-aligned. You don't see that, however. It actually reverses. That requires explanation in the model and the reactor doesn't manage that. A simple, continuous dynamo does it well. (You can get a dynamo with feedback to do this, never mind one as complex as Earth's. And besides, the Sun does it. And we know that the Sun's energy source doesn't shut off every 11 years ;-)
I'd like to see your sources on the core of Jupiter. I can cite a lot of sources to back up my statement, if you like. "The New Solar System" is an easily accessable book that covers the topic adequately. If you want something more detailed, "Protostars and Planets IV" has a nice discussion of this. I'd bet that the new Jupiter book from Cambridge University Press covers it, but my copy hasn't arrived yet.
If you're not finding references that say that the core is mainly ice, I'm curious where you're looking. (No, really: I'm curious.)
That said, no, there probably ices there if there is, in fact, a core. (We don't know for certain that there is as the data are sketchy. Oddly, it's easier to tell at the other giant planets.) Under the kinds of pressures at the center of Jupiter, rock and ice would be slushy, we think. We really don't understand the physics all that well for those pressures and temperatures, alas. (Which are, obviously, difficult to reproduce and to model since we have no good equations of state.)
Even if there aren't any ices, you're right that it's a moot point: there's not that much uranium in the planet unless our cosmochemistry is seriously wacked. (That is to say, unless there's a lot MORE uranium in Jupiter than in the Earth and in the galaxy at large. Which then requires an explanation as to where it all came from and how it got enriched in the giant planets.)
What's worse is that you need a lot more uranium than Earth has to generate your heat that way. Jupiter puts significantly more heat that it takes in from the Sun. (Earth takes in about 1360 W/m^2 and adds an additional 0.01 W/m^2 to the outgoing flux due to internal heat. Jupiter's internal heat is of order the same as what it takes in from the Sun. The latter being about 1/30 of what the Earth recieves.)
And you can't restart the reactor by letting the uranium daughter istopes decay. What do you think that they decay into? Lead, mainly. If thorium stops the reactions, I'm pretty sure that lead will, too.
If you want other objects, I gots 'em. Like the fact that you need a LOT of uranium to make this work. (Again, where is it coming from?) And that the primorial Earth would have been wickedly active. (Take the heat for formation, heat of differentiation, and add in not the radioactive decay buy a nuclear generator with a LOT more fuel and therefore a much more vigorous reactor. Basically, what the reactor model does is speed the burn rate. Which means, since we know the present heating rate of the Earth pretty well, you have to make it a lot hotter in the past with the reactor model than with pure decay. One would need to look at the model to see how hot, but I wouldn't be surprised, say, 3 billion years ago there would be too much heating to leave solid rock lying around.
You raised another of mine, how the uranium headed downward rather than sticking around with the silicates.
stuff