I was humbled today in a discussion with a co-worker. I though that the earth’s interior was hot because of radioactive decay. He said it was residual heat from when it was formed, compression of the interior from gravity , and friction. Damned of when I Googled it seems he was at least partially right.
I am still somewhat confused. I know that if you compress something it will get hotter, but if you leave it alone it will not continue to create heat, right? The friction through me for a loop as well, as I didn’t see how you can have friction add heat to a closed system. It seems to be a result of the movement of rock from convection, which is caused by heat in the first place. I know there are tidal effects from the moon and sun, but I assume those are relatively small. I finally grasped that the friction may be the result of the slow migration of heavier material towards the core and the upheaval of lighter material. In that case you are trading potential energy for heat.
In any case, is it settled as to what portion of the earth’s internal heat is the result of compression, friction, latent heat, and radioactive decay?
My geology experience consists of reading a bunch of books on the subject and, although I’m sure I’ve forgotten 99% of what I read, I don’t think I’ve ever heard your co-workers explanation. This wiki page says 20% planetary accretion and 80% radioactive decay and looks to be based on a Geodynamics book.
In the late 19th century, one of the arguments used against Darwin’s evolutionary theory was that if the Earth was as old as Darwin (and uniformitarian geologists such as his his mentor, Lyell) believed, then, by Newton’s law of cooling, even if it had started off as completely molten, it would long since have lost all its internal heat. Evolution by Natural Selection would not have had enough time to produce the complex and diverse biota that we observe.
This was not an issue of religious fanaticism or creationism, but a genuine scientific problem for the theory. Serious physicists such as Lord Kelvin (he of the temperature scale) made this argument, and Darwin himself began, partly for this reason, to de-emphasize the role of Natural Selection and give more credence to the possibility of Lamarkian-style mechanisms. (At the time the science of genetics was sufficiently developed for it to be possible to rule this out.)
One of the key factors in the revival of Natural Selection theory in the early 20th century was the discovery of radioactivity. It was realized that this provided a natural mechanism in the rocks that could keep the interior hot, even as heat was continually being lost into space.
So basically, I think you were right the first time. Radioactivity is by far the most important mechanism for keeping the core molten, although I guess it is true that tidal forces will make some contribution. (What you say about convection seems right.)
I don’t know, but that looks unlikely to me. If the Earth has had enough time (and much, much more) to cool and solidify if it were not for radioactivity, then I think it should have had enough time for nearly the heavy stuff to have reached the core by now. That is why it is mostly iron down there, isn’t it?
ETA: I believe there is quite a bit of uranium and other radioactive elements in common igneous rocks such as granite (though not enough to mine), and presumably down in the magma too.
I read an article in Discover or Scientific American a few years ago about a guy claiming that the earth is undergoing nuclear fission in its core and/or mantle, and it wasn’t just radioactive decay keeping us warm. Any clue what became of this guy and his theory?
As a fan of Kelvin I must point out that the beauty of the kelvin, the SI unit of thermodynamic temperature, is that it isn’t part of a scale. You can add kelvins and multiply them and take their fourth power and their log and so forth. That isn’t true of scale temperatures like Celsius and Fahrenheit. Kelvin (and Clausius and Joule and Carnot and others, in a somewhat mixed process) had the insight to be able to separate the stuff of temperature from the scales traditionally used to express it. Admittedly, the development of units and scales as separate and partly exclusive ideas is itself still a little rocky, and you can find plenty of precident for referring to the “kelvin scale”. Nevertheless, the possibility of having a true temperature unit, which is a much more versatile idea than a scale measurement, is so beautiful that it got Kelvin an SI base unit named after him (he’s one of only two such people).
Of course, the posting I quote does, quite rightly, pit Kelvin on the losing side of a famous argument with Darwin et al. Even Kelvin fans can’t be fans of ALL his exploits.
There may be residual heat still leaking out from gravitational compression of the debris field into the mass that is now Earth. However, I suspect (based on some back of envelope calculations) that this heat is long since decayed to insignificant levels.
Tidal forces definitely influence the Earth, but the amount of energy that is “lost” into the Earth as irreversible flexure and other losses that will eventually convert to heat (easily measured from the periodic change in kinetic energy) don’t account for more than a few percent of heating at best. On the other hand, we know that radioactive decay occurs in the mantle due to the large quantities of alpha decay products like [sup]3[/sup]He that are emitted during volcanic eruptions. The reason we don’t know more about processes in the lower mantle and core is because, outside of really bad sci-fi disaster films, there isn’t any way to sample or probe the core; we have to infer from geology and vulcanic activity.