If uranium-238 is formed in a star (supernova), how can it be used to date the age of the earth? Aren’t you dating the age of the supernova? What about earth’s formation creates a marker that can be dated with isotopes?

Question

So how do you get 4.5 billion years by dating isotopes that existed long before the formation of the earth?

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Edit: I’m not creationist trolling. I believe the #, just trying to learn about the sicence.

In: 180

Answer 1

“Rock” is the short answer. “Rock and meteorites” is the longer answer. “Rock and meteorites and radioactive decay” is the longest short answer.

U-238 decays at a predictable rate. But you’re correct in that simply knowing that doesn’t tell you much – you have to know how much you started off with. So how do we know how much U-238 we started off with?

Well, U-238 will pretty much always decay into the same elements at the same rate. So what you do is you look at a sample that contains both U-238 and its decay elements (lead is the most reliable one) and look at the ratio of uranium to lead. That ratio lets you date the sample.

The way that you can use it to date a sample and not simply the age of the supernova is that you’re checking crystalline solid samples. When U-238 is created in a supernova, it’s a gaseous/dust form. So when it decays, the decay products are free to float off into the vacuum of space. When the U-238 is locked into a solid structure (such as a rock), however, the decay products can no longer escape, and so now you can start counting back until the creation of that crystalline solid.

You get 4.5 billion years by following a long, self-referencing, self-consistent chain of similar samples, some of which are young enough that you can use other evidence (such as the fossil record, ice-core samples or C-14 dating) to verify your math. Ultimately, you check your final math by comparing the decay ratios of your oldest Earth rocks with those of meteorites (which should be relatively close to each other). And you use as many samples as you can to correct for random drifts and sample corruption.

ETA: It’s a fun side note that the hunt for the age of the Earth is also what led to the eventual banning of leaded gasoline. See if you can figure out why without looking it up!