All of the radioactive elements that made up the early Earth started out in the hot ash of ancient supernova explosions. This means we鈥檝e been working through their half-lives for at least 5 billion years since our planet was born. How much hotter was Earth鈥檚 interior then? What would the heat from nuclear reactions mean for tectonic activity and the evolution of life in our world鈥檚 feverish youth?
鈥� It would take a major study to calculate details of the isotopic make-up of the planet 5 billion years ago. I suspect that internal temperature would have been dictated by gravity making the planet contract and impactors hitting its surface, rather than isotopic decay or nuclear fission.
Our present concerns with nuclear decay vanish over such timescales. Consider that plutonium-242 has a half-life of more than 370,000 years. There are several thousand half-lives in a billion years, so even if our galaxy were made of solid plutonium-242 at the start, none would now remain. With a half-life of over 710 million years even uranium-235 would by now be reduced to less than 1 per cent of the original. Strontium-90 would vanish in less than 1 million years and would be gone before the planet had clumped together from the remnants of supernovae.
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In short, we need not consider isotopes with lives much shorter than uranium-235鈥檚. Having said that, self-sustaining nuclear chain reactions are possible. Natural nuclear fission reactions took place around 1.7 billion years ago in uranium-235 deposits in Oklo, Gabon. They ran for a few hundred thousand years, with an average power output of 100 kilowatts during that time.
We don鈥檛 know exactly when life started, but it was a good 500,000 years after Earth formed, when tectonic activity had calmed to something less than the free convection of molten rock.
鈥淲e don鈥檛 know exactly when life started, but it was a good 500,000 years after Earth formed鈥�
Jon Richfield, Somerset West, South Africa
If nuclear decay didn鈥檛 heat the planet, does anyone know what did? 鈥� Ed