• 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.