The 84 elements found on Earth occur as 339 isotopes. Only 269 of these are stable, and the other 70 are radioactive. An additional 1650 radioactive isotopes have been created in nuclear reactors and in particle accelerators.
The following is a table of all 29 known radioactive isotopes that have a half-life of one million years or more, and that are not being continually produced by natural nuclear reactions. It has been sorted in order of half-life. For each isotope, the table shows whether it is one of the ones found on Earth.
| Isotope | Half-Life (megayears) |
Found on Earth? |
| Vanadium 50 | 6,000,000,000 | yes |
| Neodymium 144 | 2,400,000,000 | yes |
| Hafnium 174 | 2,000,000,000 | yes |
| Platinum 192 | 1,000,000,000 | yes |
| Indium 115 | 600,000,000 | yes |
| Gadolinium 152 | 110,000,000 | yes |
| Tellurium 123 | 12,000,000 | yes |
| Platinum 190 | 690,000 | yes |
| Lanthanum 138 | 112,000 | yes |
| Samarium 147 | 106,000 | yes |
| Rubidium 87 | 48,800 | yes |
| Rhenium 187 | 43,000 | yes |
| Lutetium 176 | 35,000 | yes |
| Thorium 232 | 14,000 | yes |
| Uranium 238 | 4,470 | yes |
| Potassium 40 | 1,250 | yes |
| Uranium 235 | 704 | yes |
| Samarium 146 | 103 | no |
| Plutonium 244 | 82 | by extreme effort |
| Curium 247 | 16 | no |
| Lead 205 | 15 | no |
| Hafnium 182 | 9 | no |
| Palladium 107 | 7 | no |
| Cesium 135 | 3 | no |
| Technetium 97 | 3 | no |
| Gadolinium 150 | 2 | no |
| Zirconium 93 | 2 | no |
| Technetium 98 | 2 | no |
| Dysprosium 154 | 1 | no |
The thing to notice is that this list falls naturally into two halves. Short-lived radioactives are suspiciously absent from the Earth. If we had carried this list all the way down to 1,000 year half-lives, the block of no's would be 37 long instead of 10 long.
The most obvious explanation for the above is that all these elements were present when the Earth was formed, but by now the short-lived ones have decayed away. This explanation is compatible with the age scientists accept for the Earth.
Of course, nothing about this list really proves that the Earth is old. But the list is exactly what we would expect if the Earth is old, and it is a very puzzling list if the Earth is young.
Footnotes:
- The list is of isotopes not being continually produced on Earth. I
left out four isotopes because of this rule.
- Manganese 53 and Beryllium 10 are produced by cosmic-ray radiation hitting dust in the upper atmosphere.
- Uranium 236 is produced in uranium ores by neutrons from other radioactives.
- Iodine 129 is produced from Tellurium 130 by cosmic-ray muons.
Radioactives with half-lives shorter than one million years are also produced: for example, Carbon 14 with a half life of 5730 years. (return)
- The missing isotopes could have been present when the Earth was formed. It is reasonable to ask if they are missing because they were somehow never created in the first place. The answer is that they are not particularly difficult to produce "artificially", and current scientific theories about stars and supernovas say that these elements should have been produced in fairly large quantities. For example, Technetium 97 is in the no list above, but it has been detected in stars. One recent scientific theory about stars proposes how they manufacture Technetium 97 and also how Supernova 1987a manufactured Cobalt-56. (Supernova 1987a was special because it was not very far away. Theory predicts that such a supernova would create about 0.1 solar masses of nickel-56, which is radioactive. Nickel-56 decays with a half-life of 6.1 days into cobalt-56, which in turn decays with a half-life of 77.1 days. Both kinds of decay give off very distinctive gamma rays. Analysis of the gamma rays from SN1987a showed mostly cobalt-56, exactly as predicted. And, the amount of those gamma rays died away with exactly the half-life of cobalt-56.) (return)
- The list is essentially compatible with the age many scientists propose for the
earth. That age is 4.55 billion years. For most practical purposes, a
radioactive material is no longer present after 10 or 20 of its half-lives.
This is because 210 is about a thousand, and 220 is
about a million. So, after 20 half-lives, only one millionth of the original
amount remains.
Uranium 235's half life is 704 million years, so 4.55 billion years is just a bit over six half-lives. It's reasonable for Uranium 235 to still be around in small quantities after that amount of time. And, in fact, it makes up about one percent of the Uranium now on Earth. The amounts of Uranium 235 and Uranium 238 would have been about equal, 4.55 billion years ago. (return)
- Finding Plutonium 244. Its half life is 82 million years, so 4.55
billion years is 55 half lives. You might reasonably ask how come Plutonium
244 isn't listed as no. The answer is that someone made a very serious
effort to find it: their article is referenced below. Eighty five kilograms of
molybdenum ore were chemically concentrated, and then the lot was tediously
run through a mass spectrometer. The amount of Plutonium 244 they found,
10-14 grams, was so small that it would have averaged one single
radioactive decay every six years. Clearly, they could not have detected this
Plutonium 244 with a geiger counter. However, 55 half lives ago, it would have
been about one kilogram of plutonium metal. That's believable in 85 kilograms
of metal ore.
Samarium 146's half life is 103 million years, so 4.55 billion years is 44 half lives. This means that Samarium 146 could be 200 billion times rarer than Uranium 235, but could be a thousand times commoner than Plutonium 244. I predict that if anyone tries very very hard to find Samarium 146, they will succeed. Curium 247, at almost 300 half lives, is completely out of the question." (return)
Conclusion: In short, the cutoff point in the list of isotopes is consistent with a old earth.
