Where do you start?
POSTED BY: citizen
UPDATED: Wednesday, January 8, 2025 02:57
VIEWED: 8803
PAGE 3 of 8
Quote:
Originally posted by AnthonyT:
What I am interested in knowing is how you date a moon rock. Any scientists in the crowd please explain the process.
Radioactive dating is used for Moon rocks. The idea is to look at isotopes with long half-lives (not carbon). One method uses the decay of 87-rubidium (half life is 47 billion years) into 87-strontium. There are a number of techniques using different isotopes. If you understand the idea of nuclear decay you can get the rest of the story at a hyperphysics site...
Check out http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/clkroc.html
Sorry to lead you to another site, but I started typing things out, and the post was getting painfully long.
cf
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Whoa there, Mac. I'm not following you.
Carbon dating involves using material from a once-living thing to determine the age of something found near the once-living thing. Like the carbon in the campfire dating the village, and the village dating the pottery fragment.
This works, presumably, because we can estimate how much carbon a living thing should have absorbed in its lifespan, and compare it to how much carbon it's got right now. (At least, that's what I gathered from the article I copied.)
A moon rock can't work that way, because there's no living things or campfires or anything to compare it to.
I give you a five pound rock. What happens next?
--Amthony
"Liberty must not be purchased at the cost of Humanity." --Captain Robert Henner
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Quote:
Originally posted by AnthonyT:
What I am interested in knowing is how you date a moon rock. Any scientists in the crowd please explain the process.
Some radioactive elements can only exist after other elements have decayed into them. To use a very simplified example Uranium-238 can decay into Lead-206 with a half life of 4.5 Billion years. Thus if there is equal amounts of Uranium-238 and Lead-206 present in the sample the sample is approximately 4.5 Billion years old.
It is more accurate than Carbon-14 dating, as it doesn't depend on assumptions about the Carbon content of the Earths environment through history, just that some quantity of Uranium-238 (or whatever parent isotope being tested) exists within the sample.
More insane ramblings by the people who brought you beeeer milkshakes!
No beast so fierce but knows some touch of pity. But I know none, and therefore am no beast.
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Quote:Well, I’ve never actually radio-dated a rock so I can’t give you a play by play analysis, but basically the question you seem to be asking is ‘what is the baseline?’ In Carbon-14 dating the baseline is perhaps the amount of Carbon absorbed by a living thing, and a rock, as you surmise, is not likely to absorb much carbon at any kind of predictable rate. But what is done in this case is to compare two different decay series. Decay series one should decay linearly compared to decay series two. So that if you evaluate the ratio of the parent and daughter isotopes of each decay series and then match them you should get a straight line and the slope of that line will be the relative concentration of one decay series with the other which will be directly related to the date of the rock.
Originally posted by AnthonyT:
I give you a five pound rock. What happens next?
Nihil est incertius vulgo, nihil obscurius voluntate hominum, nihil fallacius ratione tota comitiorum.
Nothing is more unpredictable than the mob, nothing more obscure than public opinion, nothing more deceptive than the whole political system.
-- Cicero
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Quote:
Originally posted by Finn mac Cumhal:Quote:For the most part it is the same with Carbon-14 dating, just using different, longer lived species. Uranium-235, for instance will decay until it eventually becomes lead-207 with a half life of something like ~billion years. Most rocks contain minerals that have at least one decay series. I think maybe Rubidium-87/Strontium-87 decay series may be used for moon rocks, which has a half life of maybe ~50 billion years. Moon rocks tend to be old.
Originally posted by AnthonyT:
What I am interested in knowing is how you date a moon rock. Any scientists in the crowd please explain the process.
Hey, sorry Finn, I just posted pretty much the same thing you just said. I get a bit too excited about half-lives and such. I could talk forever about decay chains
.
NOTIFY: N | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
So the idea for dating rocks seems to be this...
It's got 5 units of uranium and 5 units of lead, so we can surmise that X amount of time ago it had 10 units of uranium and 0 units of lead?
--Anthony
"Liberty must not be purchased at the cost of Humanity." --Captain Robert Henner
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Yes, except it's not ordinary lead. It's not a lead that exists on it's own anywhere. It's a kind of lead that only exists because the Uranium decays to form it.
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
That's pretty cool.
I guess the only question left is...
Is there anything that can affect the decay rate of these isotopes?
--Anthony
"Liberty must not be purchased at the cost of Humanity." --Captain Robert Henner
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
As in the case of almost all science, there are several assumptions. For starters, Fletch2 gave you one, which is that the stable daughter end product is unique, or more generally that the concentrations of parent and daughter isotopes are not effected by external processes. Another assumption is that the decay rate is constant. And paradoxically, the age of the rock itself must be assumed in order to choose a correct dating method.
For some reason I seem to have answered a different question then the one you asked. 
Not sure why, but oh well.
Quote:It’s all good.
Originally posted by gammaraygirl:
Hey, sorry Finn, I just posted pretty much the same thing you just said. I get a bit too excited about half-lives and such. I could talk forever about decay chains
Nihil est incertius vulgo, nihil obscurius voluntate hominum, nihil fallacius ratione tota comitiorum.
Nothing is more unpredictable than the mob, nothing more obscure than public opinion, nothing more deceptive than the whole political system.
-- Cicero
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME
Quote:
Originally posted by AnthonyT:
That's pretty cool.
I guess the only question left is...
Is there anything that can affect the decay rate of these isotopes?
--Anthony
"Liberty must not be purchased at the cost of Humanity." --Captain Robert Henner
Not as far as anyone knows. If a mechanism could be found that would be good news since it would allow rapid disposal of nuclear waste.
NOTIFY: Y | REPLY | REPLY WITH QUOTE | PERMALINK | TOP | HOME