# B Earth age vs Universe age

1. Mar 20, 2017

### geraud

From my understanding:
• The age of the universe is ~13.8 billion yrs.
• The age of earth is ~4.5 billion yrs.
• The age of the earth is based on radiometric dating.
I understand that the earth was probably formed by gas and dust clouds ~9.3 billion yrs after the universe was born, but if the gas and dust matter was formed during the birth of the universe, why doesn't radiometric dating show that the earth is 13.8 instead of 4.5?

2. Mar 20, 2017

### Comeback City

I would think it is because the radiometric dating is done for certain substances that they believe resulted from the forming of the Earth. Anyone correct me if I'm wrong though.

3. Mar 20, 2017

### mathman

Radiometric dating is based on concentrations (in rock) of radioactive element and daughter.

4. Mar 20, 2017

### Chalnoth

First, we don't actually measure the age of the Earth directly. The rocks on Earth are recycled too frequently. I don't think any have been dated much past 4 billion years.

Rather, we get the age of the Earth from measuring the age of asteroids, as the asteroids all formed at about the same time as the Earth, but don't have a pesky active crust and mantle to reset the ages of the rocks every few hundred million years.

As for actually measuring the ages themselves, that's typically performed using isochron dating (https://en.wikipedia.org/wiki/Isochron_dating). The basis of the idea is that when the rock forms from some kind of molten substrate, it gets all mixed up, but does so in a way that follows the laws of chemistry.

For example, Rubidium has an isotope 87 that decays into Strontium 87. However, Rubidium and Strontium both have very different chemical properties, and don't form in the same locations. So a given rock will start with lots of Strontium and not much Rubidium in certain places, and the reverse in others. Measuring the ratios of the isotopes of Strontium and Rubidium throughout the rock will tell you how much of the Rb87 has decayed into Sr87.

This measures the rock's age to the last time it was in a liquid form. So you can use it to date lava rocks on Earth, or asteroids back to the time they originally formed, which measures the age of our Solar System, and therefore the Earth.

5. Mar 20, 2017

### geraud

Okay, now this makes a lot more sense to me. Non-resetting asteroids + Isochron dating are the missing links I was looking for.

6. Mar 21, 2017

### Staff: Mentor

Somewhat of a side issue, but the universe before stars contained only hydrogen. All of the dust and all of the non-hydrogen gas in the solar sytem was created by the life and supernova of a star (or more than one) that lived and died before our solar system was born.

7. Mar 21, 2017

### Staff: Mentor

Yes, but what always astonishes me in this picture is, that those stars only could have had a rather short life to fit into the time window. Why? One rarely hears something about our sun's predecessors. And shouldn't heavy elements on earth like gold or uranium reveal a much older age then, because they have been formed far earlier? At least some which haven't been exploded to single atoms.

8. Mar 21, 2017

Staff Emeritus
Sherman, set the wayback machine for 13 billion years BC...

OK, now take a look at the sky. All the stars are young, and some will live for a trillion years, some for billions, and some for only millions. Now, skip forward a billion years. The only stars in the sky are either ones that lived longer than a billion years, or were born in the last billion. Go another billion....and another...and another...until you get to today. So it's no coincidence that the age distribution of the former and present stars looks the way it does.

Actually, it was (and is) about 10% helium. In the above discussion, I am ignoring the very earliest stars, the so-called Population III, which are extremely metal poor and likely quite large.

9. Mar 21, 2017

### phinds

The ATOMS are older but you can't measure the age of an atom. The compounds, the age of which can be measured, are younger than their constituent atoms.

10. Mar 21, 2017

### Staff: Mentor

I know, of course not. I just thought that some might have remained clumped throughout time. E.g. the uranium formed by the last supernova, and which we find today, shouldn't it be significantly and measurably older than 4.5? And if so, when did this nova happen? Or the ones before?

11. Mar 21, 2017

Staff Emeritus
The atoms are. The rocks are not. Essentially you are measuring the time since it was last liquid.

K-Ar dating may be simpler to see. Argon is a gas, so you expect it to bubble out of a rock when it's liquid. Now you have a rock that has potassium, but no argon. Wait, and the K-40 decays to Ar-40. Now you have argon in the rock - measure how much, and you know when the rock was last liquid. Melt the rock, the argon escapes, and the clock resets.

12. Mar 21, 2017

### Staff: Mentor

[googles] Interesting -- helium 4 is more stable than free protons and neutrons, so if they come close together they'll have a tendency to combine...if their energy is low enough. Thanks!

13. Mar 21, 2017

### Staff: Mentor

Well, supernovas require large stars, which burn faster, so yeah...
Clearly I'm not much of an expert on this, but I suppose there aren't very many candidate isotopes that are common enough and have the right half life to enable longer dating. Or perhaps more importantly, the various violences that form planets tend to distill (if that's the right word) the elements, erasing evidence of previous decay that could be used for dating.

14. Mar 21, 2017

### rootone

Helium is really annoying,
If it was not for that weird fine tuning as it is called. Helium is the end of the game.