This article says,
I had thought radioactive decay produces almost all the earth’s internal heat, with perhaps a very small amount due to gravitational contraction. I guess I was wrong.
What produces the “other half”?
Probably leftover heat from the original accretion of the material of the earth.
Wow, that heat still would not have dissipated after 5B years?!
I’m not any kind of expert on this, or even a particularly well-read enthusiast on the topic of geothermometry except tangentially (with regard to establishing the evolution and age of the planet as a refutation to certain Creationist arguments), but my understanding from my limited reading on the topic of planetary physics is that there are three mechanisms responsible for heating:[list=][li]Radioactive decay.[]Diffusion in the mantle due to gravity (i.e. heavier materials sinking toward the center causing convection currents).[*]Gravitational tides of the Moon and Sun perturbing the mantle.[/list][/li]
The first is generally considered to be the most significant for Earth, though some models dispute this. The second is significant and accounts for some of the specifics of plate techtonics. The third is minimal (but not dismissable) for the Earth, but for a volcanically active body near a much more massive body it is probably very significant; the volcanism of the Jovian moon Io is widely attributed to its proximity to its massive primary.
Note that there is not, AFAIK, one fully established, widely accepted model of techtonophysics and geothermometry, and indeed, plate techtonics (the fact of which is now universally accepted, aside from a few Young Earth Creationists and members of the Flat Earth Society) has been accepted for a shorter period than the Big Bang and relativity. In some ways, we know and have better accepted models of what happened in the far reaches of space twelve billion years ago than we do of what is going on today ten miles under our feet. If that isn’t truely astonishing I don’t know what is.
Stranger
Yes, it would have. That’s one of the reasons people like Darwin had trouble convincing some scientists that the earth was old, as indeed it should have been cold by now. Then the Curies came along and all was clear.
Like you I thought radioactivity was responsible for much more than half. I wonder if the NYT got its numbers wrong, wouldn’t be the first time.
According tothis source only about 4% of the heat that should reach the surface from the interior actually does so. Most of the internal heat is returned back to the interior by convection currents in the mantle and the core.
So it sounds to me as if the earth wouldn’t cool off as fast as Lord Kelvin thought it would. I don’t think he knew about the circulation of the material of the interior.
And this source states that about 2/3 of the interior heat is from radioactive decay and 1/3 from primordial accretion and differentiation of the earth.
All of Kelvin’s calculations assumed that the Earth was (at least currently) solid. That greatly simplified the physics and maths, but ruled out just such mechanisms in advance. Several of his critics explicitly attacked this assumption and came up with alternative models in which the interior was liquid and the planet older, so he should have been aware that this was a weak point.
There was indeed virtually no evidence available at time about conditions inside the Earth and the wiser of those involved in the debate realised this and so, suggesting that there were too many uncertainties involved for anyone to draw firm conclusions about cooling.