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Reactor at Earth's Core?
An anonymous submitter sent in this story suggesting that uranium in the Earth's core may be acting as a giant breeder reactor, generating a large amount of heat and perhaps being responsible for the Earth's magnetic field.
So the earth is sort of like a giant shed in your mother's backyard?
"And like that
The question is just how much of a chain reaction there is. I guess traditionally, the assumption was "not much". But it seems quite plausible that uranium might concentrate and actually start a significant chain reaction.
Anyone want to take bets on how long until some boy scout tries to make a planet in his mother's potting shed?
--Dan
Concerns about global warming aside, the Earth's external temperature has fluctuated *wildly* during the 4 or 5 billion years our planet has been around. At times, such as when the atmosphere wasn't made primarily of oxygen for example, it has been much colder or hotter. Assuming that our planet's core does indeed act like a breeder reactor, something so slight as global warming is not going to significantly affect it.
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Man, is this article bad.
The article proposes a 5-mile sphere of uranium at the Earth's core as the source of Earth's geothermal energy.
Now, the idea of radioactive decay powering the Earth's geothermal heat generation isn't a new one - in fact, it's one of the more plausible models, as it works quite well over a very long time frame and explains the presence of helium in the mantle. However, the uranium, thorium, and other elements involved would be diffused through the core material (i.e. in solution in the liquid outer core, or as dopants in the iron crystal of the inner core). The absolute concentration of uranium in the Earth is very, very low. Without a really strong mechanism operating to separate it out and concentrate it, it's going to remain a trace impurity in other ores, and not a ball at the center of the Earth.
Secondly, I'm pretty sure that a 5-mile ball would be over the critical mass/volume envelope for an uncontrolled chain reaction of the U-235 and U-233 present in uranium ore. The fact that the Earth hasn't exploded suggests that uranium is not concentrated into a ball. Anyone with the fast-neutron cross section data care to work this out?
Now, on to other questionables.
The Earth's magnetic field is created by movement of the conducting material in the Earth's outer core. The polarity changes in the field are adequately explained by the idea that turbulence destabilizes the dynamo fluid currents every so often. A fascinating article was published about this a while back, but the citation escapes me.
Turbulence happens; it's a known and expected phenomenon. However, the article authours propose no mechanism for their magical solid-state fission reactor to turn on and off every so often to reset the dynamo currents. Thus, I consider the turbulence conjecture the more plausible.
Now, on to Jupiter. While Jupiter undoubtedly also has heavy element fission contributing to its heat, the majority of its heat is expected to come from it essentially continuing to slowly compact itself. The idea is that as hydrogen progresses from gas to liquid to metallic liquid to metallic solid state, it undergoes several exothermic phase transitions (analagous to the heat of condensation for more common substances). This provides a feedback loop that limits the rate of conversion - for example, if a lot of liquid hydrogen starts converting to liquid metallic hydrogen, the boundary layer heats up, which makes it less favourable for the conversion to continue. The rate of conversion for some of these phases is expected to be slow enough for Jupiter not to have reached its final equilibrium composition. If the conversion is still going on, then as it's exothermic, it could indeed explain heat generation in Jupiter.
A similar mechanism involving crystallization of iron was proposed as a source of Earth's geothermal energy, though this is less convincing as the amounts of matter involved are small enough that Earth should have reached equilibrium long ago.
In summary, the "mysteries" that the article attempts to invoke compact reactors to solve are already adequately explained without the need for such an implausible mechanism.
As usual, the pop science publications do this little justice. The primary literature is published by the Proceedings of the National Academy of Sciences and is published online at: http://www.pnas.org/cgi/content/full/98/20/11085 and certainly makes the case sound more convincing.
The article at EV world doesn't didn't, to my dismay, discuss the electro-magnetic implications at all. It sounded like a stretch to me to conflate geomagnetism with a nuclear process.
Yes, they do: Also, re Without a really strong mechanism operating to separate it out and concentrate it, it's going to remain a trace impurity in other ores, and not a ball at the center of the Earth.
There is such a mechanism. Everyone agrees that the core is iron. How did that happen, when the Earth coalesced from random rocky materials? The same way. If iron, why not uranium? From the article:
Without a really strong mechanism operating to separate it out and concentrate it, it's going to remain a trace impurity in other ores, and not a ball at the center of the Earth.
There is such a mechanism. Everyone agrees that the core is iron. How did that happen, when the Earth coalesced from random rocky materials? The same way. If iron, why not uranium?
Iron is plentiful. Enough of it was present for gross gravitational effects to be enough to separate it from lighter ores within solution.
Uranium is far, far less plentiful, and so would tend to remain dissolved.
There are no "grains" to migrate, as you suggest - uranium would be mixed in as impurities on an atomic level, literally dissolved in other metals and metal oxides.
That would kind of put a new variable into the Drake equation.
Either that, or give plot ideas to Isaac Asimov. Damn Spacers.
True, but the pressure of the material above the center, which is attracted towards the core, is very strong. That's what causes the earth's core to be dense - gravity creates pressure.
Is there a point just outside the center of the earth where gravity would actually pull you *UP*, since the bulk of the earth's mass lays above you?
No. The Earth is roughly spheroid, remember? The "center" of the Earth for the purposes of this discussion is the center of gravity, which is the net gravitational effect of all of Earth's atoms at any point. At the center of gravity, those forces cancel out. But anywhere other than that, the net force is towards the center of gravity.