Radioactivity as a heat source for planets

[Ken G]

I doubt a pulsar planet would have an abnormally radioactive interior.

[neilzero]

Hi publiusr. If 20% of Earth's internal heat is due to radioactive substances, then we only need 5 times more radioactive substances to get all the heat from radioactivity. A small world would have proportionally more heat loss, but tectonics might be possible in a world as small as Earth's moon, if it was mostly dense radio active isotopes.
I'm assuming your mentioning pulsar means the super nova that formed the pulsar striped most of the lighter elements away leaving a Moon size planet made mostly of radio active isotopes. If the average density was 18, a moon size body could have almost one g surface gravity, which could hold an atmosphere. Neil

[George]

Even that depends on which "M" you consider-- the geologists were almost unanimously inclined to take Perry's explanation as the right one, it was only the astrophysicists, for whom Kelvin was a superstar, who would see Perry's position as ATM.

Good point, that late in the century, geology was mainstream with uniformitarinism, which favored Darwin's theory (still ATM for him, however).

But on the physics side of the fence, it seems Kelvin held the upper hand. So much so that Perry, apparently, elected not to pursue his view too much with further publications and presentations since he could not convince Kelvin himself. At least, that's I how see it based only on limited reading, admittedly. Physicists seemed to hold much higher ground, at least in prestige (Lyell was a lawyer, after all ). Of course, physicists are all humble and gracious today. Shoot, when was the last time you saw a physicist or astronomer toss a cute lick at geologists? Heck, it's been at least 4 or 5 days .

Another hiccup impeding the heat issue is that conductivity is normally far superior in transferring heat compared to convection.

So the hiccup list that has kept this in confusion might be:

1) Radioactivity was a quick and exciting answer to the assumed need for additional heat to buy more time for the Earth. It had both sparkle and sizzle, so was, and is, an appetizing answer.
2) Radioactivity was introduced at a critical moment, too, and by an up-coming big dog in physics (Rutherford). It also did not conflict with Kelvin who had assumed no new energy source for the Earth. [Kelvin, apparently, privately admitted his age estimate had been overturned by this.]
3) Convection within the Earth was not understood (believed), and no direct evidence exists for it (I assume).
4) On small scales and within [viscous] liquefied material, convection is a poor heat transfer mode compared to conduction.

Is this close?

[Ken G]

1) Radioactivity was a quick and exciting answer to the assumed need for additional heat to buy more time for the Earth. It had both sparkle and sizzle, so was, and is, an appetizing answer.
2) Radioactivity was introduced at a critical moment, too, and by an up-coming big dog in physics (Rutherford). It also did not conflict with Kelvin who had assumed no new energy source for the Earth. [Kelvin, apparently, privately admitted his age estimate had been overturned by this.]
3) Convection within the Earth was not understood (believed), and no direct evidence exists for it (I assume).
4) On small scales and within [viscous] liquefied material, convection is a poor heat transfer mode compared to conduction.

Is this close?

I'm good with all of them but #4, I just don't know about that one. Convection transports heat on the timescale of the flow, conduction transports it on the timescale we've experienced-- how long would you hold a rock that had the other side in a fire? Pretty long, I should think. I don't know why convection wasn't given more attention by those other than Perry.

[George]

...how long would you hold a rock that had the other side in a fire? Pretty long, I should think.

Yes, but the thought here is only of conduction. [The irony here is that radiation and possibly convection, in your example, will be burning the holder much sooner. ]

It is this very mindset that I suspect was maintained. Perhaps there are two possibilities: very viscous magma; thermal isotropy (even gradient is what I mean; what do you call this?) of the interior.

If the magma were to be seen as too viscous, no convection would be possible, as in your rock example. If it was assumed there were no hot pockets deep within the interior, how could convection form?

I don't know why convection wasn't given more attention by those other than Perry.

This is such a good question that I'm intrigued why it wasn't given more recognition.

I do think we have to throw Darwin's theory in the mix to help us. As respected has his ideas were (natural selection and branching), Darwin was unable to get past any ad-hoc mechanism to explain more fully his theory. [He eventually came up with one, but it was seen as ad-hoc.] Further, Kelvin wasn't shy about showing that Darwin must be wrong due to his age calculations. Darwin called him an "odious spectre" and Kelvin may not have enjoyed this pejorative near as much as I.

[Quickly added: Obviously I am arguing that Kelvin was the one allowed to hold his big stick on this topic. Why he didn't consider convection may be the key question? Simple pride could be the reason, but perhaps there were other circumstances he had to defend his position.]

[geonuc]

Having fumbled the initial question, I'll venture back in. Is it not true that the very notion of flowing rock in the mantle is fairly new? For scientists back in Lord Kelvin's day to suppose that convection cells existed, well, that would have been quite radical, I'd think.

[Ken G]

Having fumbled the initial question, I'll venture back in. Is it not true that the very notion of flowing rock in the mantle is fairly new? For scientists back in Lord Kelvin's day to suppose that convection cells existed, well, that would have been quite radical, I'd think.

You may be right, there must have been some reason why Lord Kelvin didn't feel he needed to imagine convection. It's always an interesting puzzle in science to decide which direction a solution should flow when it gets trapped between obstacles-- should the geologists' estimates have been taken as reliable, forcing Kelvin to look for potential flaws in his analysis (such as the possibility that rock can flow when hot enough, which was known from lava but maybe only the slow sticky type of lava was known at that time, not the fast slippery type?), or should the Kelvin calculation have been taken as reliable, and all the geologists had missed the boat somehow? The history of science is full of cases where we tried to remove the wrong obstacle. I wonder which wrong obstacles we're hard at work on at the moment?

[EricFD]

Well, by the early 1900s, Norman L. Bowen had established how various kinds of minerals were transformed to other types of minerals in his reaction series. He had established the difference and properties of ultramafic, mafic, intermediate and felsic magmas and it was pretty well established that the ultramafic and mafic magmas that make up the lower and upper mantle have a very low viscosity and flow very readily. Perry was well aware of Bowen's work and this might be the reason why Perry realized that convection was an important piece of the puzzle. I'm just speculating here. Only Perry knew what was going through his own mind. But it seems like a reasonable assumption.

Eric

[Ken G]

So that might be it, maybe Perry was just better informed about more recent developments in interior viscosity and thermal expansion.

[EricFD]

So that might be it, maybe Perry was just better informed about more recent developments in interior viscosity and thermal expansion.

Yes, I think that could very well have been the reason.

[George]

Here's a more thorough article on the topic.

Kelvin's view did have several lines of evidence to support his claim. Apparently, the heat flux was known at the time and his calculations (Fourier's work on heat diffusion through solids) were close to this. Another line of evidence was Kelvin's own calculations for the Sun of about 100 million years, which was a little more than his Earth age.

Kelvin also was convinced the Earth was solid for at least the outer 1500km due to some tide information he seemed to of had. He was not open to the coblers' wax analogy Perry used to win him to a more fluid interior.

Some other tid bits include:
> Perry's work was a decade or so before Rutherford's radioactivity explanation.
> Rutherford initially used radium as the radioactive source, but the main isotope has only a 1600 year half-life (the others are only a few years)! Did he not know this very limited half-life? If he did, why use radioactivity as an explanation at all?
> Perry was an assitant to Kelvin at some time.
> Kelvin viewed geologists as limited in their ability to appreciate the math in science models.
> Geologists then seemed to think physicist's models too simple to be accurate or useful. (Maybe this claim from the link is true)
> Perry reasoned that iron and other elements below would increase the rate of conductive heat transfer, but convection was his real and correct solution, of course.