Why are unstable isotopes useful for dating rocks
Both carbon-12 and carbon-13 are stable, but carbon-14 is unstable, which means that there are too many neutrons in the nucleus. As a result, carbon-14 decays by changing one proton into a neutron and becoming a different element, nitrogen-14 (with 7 protons and 7 neutrons in the nucleus).
The isotope originating from the decay (nitrogen-14 in the case of radiocarbon) is called the daughter, while the original radioactive isotope (like carbon-14) is called the parent.
These are the surfaces that we can get absolute ages for.
From Wikipedia, radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation.
We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic.
So, you can use the radioactive elements to measure the age of rocks and minerals. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old).
We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.
We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. We also have meteorites from asteroids and can date them, too.