The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.

The basic theory of radiometric dating is briefly reviewed.

Radiometric dating--the process of determining the age of rocks from the decay of their radioactive elements--has been in widespread use for over half a century.

There are over forty such techniques, each using a different radioactive element or a different way of measuring them.

The biggest assumption is that, to first order, the number of asteroids and comets hitting the Earth and the Moon was the same as for Mercury, Venus, and Mars. The bottom line is that the more craters one sees, the older the surface is.

This can be interpreted in two ways: why it is important to know the age of a planet or how is age dating important in determining the age of a planet?