Radioactive Dating of Asteroid Elements: How Do Scientists Know?

[Albertgauss]

TL;DR Summary

When scientists radioactively date elements in the asteroids to determine the age of the solar system, how do they know what the original amount of the radioactive element was?

When scientists use radioactive dating of elements in the asteroids to determine the age of the solar system, how do they know what the original amount of the radioactive element that was in the rock was? Do they need to know what the original amount of the radioactive element was in the rock?

Just looking for a general answer here and if you know of a good webpage that talks about this, go ahead and post it here.

My guess is that there is some ratio of decay products to parent nuclei that can be measured, is this correct?

So for example, let's say that If I look in the asteroid rock and see that 25% of the rock sample is potassium–40 and the other 75% is calcium–40 and Argon–40, and I know the half-life of potassium–40 is 1.3 billion years, then the age of this rock must be about 2.6 billion years (that is, two half-lives have passed by the time and measure the age of this asteroid). Is this along the right logic?

 

[Drakkith]

When scientists use radioactive dating of elements in the asteroids to determine the age of the solar system, how do they know what the original amount of the radioactive element that was in the rock was? Do they need to know what the original amount of the radioactive element was in the rock?

Take, for example, uranium-lead dating. This is often done on the mineral zircon, because zircon strongly rejects lead but will incorporate uranium into it. So we know that all the lead found in the zircon must have come from the radioactive decay of uranium since it couldn't have been formed with lead inside it. Then it's a matter of determining the ratio of lead to uranium and using the decay rate of uranium (which is known) to find out how much time had to have passed to get that ratio.

Other decay dating methods use similar methods.

 

[Albertgauss]

Oh, I see. You really have to know some detailed chemistry of the kind in the example you described to deduce what the original amount must have been. What you wrote makes sense to me. Not a one-chemistry-fits-all but finding out some way of what the original amount or ratio must have been will be a unique method to each radioactive process. I think I get it.