Radiocative decay: Some of the atoms of the rocks are unstable. Their nuclei break apart
or decay, leaving behind a decay product (a daughter element).
The rate at which atoms decay is called half-life, the time it takes for half
of the parent nuclei to decay. For example, Potassium has a half-life of 1.3 billion years
and decays into Argon.
The tecnique of measuring the ratio of parent-to-daughter abundance
gives the age since the beginning of the decay. The assumptions are the following:
1: Every radioactive element will decay at a constant rate (its half-life).
2: The system is closed, ie, none of the daughter atoms has escaped from the system,
and all the daughter atoms were created by radioactive decay of the corresponding parent atom.
--> we can date rocks since they last solidified (trapping the isotopes inside).
3: The rock contained no daughter element when it formed.
You can believe that this is true if the daughter product is a gas (Potassium --> Argon)
since a gas in the solar nebula did not condense. If you find Ar trapped inside the rock
you can be sure that it came from the decay of Potassium.
To measure the age of the solar system we need samples of the condensation of the
solar nebula (fossil rocks) --> meteorites or moon rocks (not Earth's rocks).
We need to analyze different parent-daughter ratios so we can compare the resulting values
for the age of the rock.
Meteorites are good samples of the condensation of the solar nebula --> 4.6 billion years
The age of the Moon can be estimated from the decay of Uranium into Lead (half-life of 4.5 billion years).
-> The Solar system is less than half as old as the universe.