For potassium 40, the half-life is about 1.3 billion years.
At the start, let me clarify that my main concern is not the age of the earth, the moon, or the solar system, but rather the age of life, that is, how long has life existed on earth.
Many dating methods seem to give about the same ages on meteorites.
In order to use these methods, we have to start out with a system in which no daughter element is present, or else know how much daugher element was present initially so that it can be subtracted out.
We also need to know that no parent or daughter has entered or left the system in the meantime.
Back to top Radioactive elements decay gradually into other elements.
The original element is called the parent, and the result of the decay process is called the daughter element.Potassium 40 (K40) decays to argon 40, which is an inert gas, and to calcium.Potassium is present in most geological materials, making potassium-argon dating highly useful if it really works.Assuming we start out with pure parent, as time passes, more and more daughter will be produced. A ratio of infinity (that is, all daughter and no parent) means an age of essentially infinity.By measuring the ratio of daughter to parent, we can measure how old the sample is. Each radioactive element has a half-life, which tells how long it takes for half of the element to decay.For isochrons, which we will discuss later, the conditions are different.