Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Age of Earth as discussed in Cosmos

I just watched Cosmos episode 5. The age of the earth was estimated by the ratio of uranium to lead in the Arizona meteorite. The premise seemed to be that the “clock” started when the rocks of our solar system were formed, became solid.

My thought: uranium and lead were created in a supernova. The uranium decay began at that time. Would not the ratio of uranium to lead found in a meteorite indicate the original ratio of uranium to lead, adjusted for the decay of uranium from the time of its (and that of the lead’s) creation? If so, this the age of the supernova not the age of the earth.

Am I missing something?

sorry if this is the second post - I am having difficulty figuring out this forum.

Hi, WMikeC,
Welcome to the forum. I hope you don't have too much trouble figuring us out. If you need help you can PM me or Kate from your profile page. We're here to help.

Congratulations for thinking and asking questions. Keep on doing that and it will lead you to a lot of interesting things.

In the Cosmos description they use the short form description, just that it was done with Uranium-Lead dating. That leaves out a lot of the stuff that was actually used in the process. They can be excused on the basis that they only had 35 to 40 minutes to get the whole story in. The following quote from the Wikipedia article on Uranium-Lead dating gives a good idea why they could use the zircons to determine the Uranium/Lead ratios and then calculate the age of the Earth based on that.



For more information about the Uranium-Lead dating methods visit the Wikipedia article.

Bill Gill

As Bill said, there is a lot more to the scenario than they could fit into that sort of program.

This discussion from PF may be a bit off topic, but it gives a flavour of the complexity.

http://www.physicsforums.com/showthread.php?t=722594

BTW. I would really appreciate some comments on this series, as I can't get it, so it would be valuable to know if it is worth watching for when/if it is issued on DVD.

John Atwell Rasmussen, PhD. AJP kindly supplied the following information:

“In the study of uranium decay, the original ratio of lead isotopes can be determined at the formation of the mineral crystal that is being age dated, then the change in lead isotopes ratio can be used to determine the age of that crystal.”

@ Bill S - I get it - the crystal forms when the solids of the earth accrete, this crystal carrying only Uranium. Decay products - lead - accumulate in the crystal from then on.

Yes, that would have been too arcane for the Cosmos series. Which I loved.

MikeC

That's a good explanation, Mike. Naturally there are more complications as one looks closer, but perhaps "Cosmos" should have included an explanation at that level.

I rather enjoyed Cosmos, but one source that has been giving some comments on it sort of said that one of the problems was that they kept giving explanations that were rather simplistic. He would have liked for them to provide some explanations that were a little more in depth. I think that he might be right. A lot of the time their explanations were kind of shallow.

Bill Gill

I don't think more depth would have been appropriate to the intentions of the producers. After all, I guess the target audience is not the scientifically literate minority, but rather the remaining majority of viewers. I suspect the series was a great success, just as Carl Sagan's was. An inspiration. Those who are confident that they already know all that stuff, and want to learn more, have access wealth of relevant videos on YouTube.