Posted: Apr 05, 2010 12:11 pm
Thanks Calilasseia,
The problem is, that the argument for an old Earth is precisely that we don't find isotopes with half-lives shorter than ~100 million years - therefore the Earth must be older than, say, 20 half-lives.
But the isotopes in question were not created when the Earth formed, but when the 'progenitor' supernova(e) exploded. If this event took place billions of years beforehand, then these same isotopes would have long decayed before the Earth even started to form - hence, whether we consider an old or a young Earth, these isotopes would still be missing (except, of course, for those that are continuously generated by the decay of much longer-lived isotopes (as you mention) or cosmic-ray events (14C))
Now, all is not lost, as we could measure the relative abundance of these continuously generated isotopes to their own daughter products - this would give a minimum time for the age of the particular sample.
Let's say that a lump of primordial 238U, say, produced a quantity of 59Ni, this then decays to 59Co with a half-life of 76,000 years - this decay chain could then be used to place a minimum limit to the age of this sample.
Actually, checking up on this example seems to show that the only detectable amounts of 59Ni, are those found in nuclear reactors - but I'm sure other chains could be found?
(My use of the term 'Isotope' is a bit sloppy, - All forms of an element are, of course, isotopes. Please take the term here to mean the unstable, radioactive, nuclides of an element)
The problem is, that the argument for an old Earth is precisely that we don't find isotopes with half-lives shorter than ~100 million years - therefore the Earth must be older than, say, 20 half-lives.
But the isotopes in question were not created when the Earth formed, but when the 'progenitor' supernova(e) exploded. If this event took place billions of years beforehand, then these same isotopes would have long decayed before the Earth even started to form - hence, whether we consider an old or a young Earth, these isotopes would still be missing (except, of course, for those that are continuously generated by the decay of much longer-lived isotopes (as you mention) or cosmic-ray events (14C))
Now, all is not lost, as we could measure the relative abundance of these continuously generated isotopes to their own daughter products - this would give a minimum time for the age of the particular sample.
Let's say that a lump of primordial 238U, say, produced a quantity of 59Ni, this then decays to 59Co with a half-life of 76,000 years - this decay chain could then be used to place a minimum limit to the age of this sample.
Actually, checking up on this example seems to show that the only detectable amounts of 59Ni, are those found in nuclear reactors - but I'm sure other chains could be found?
(My use of the term 'Isotope' is a bit sloppy, - All forms of an element are, of course, isotopes. Please take the term here to mean the unstable, radioactive, nuclides of an element)