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By studying other planets, we are learning more about our own planet.
The effects of impacts and how they might affect us here on Earth, global climate change (Venus vs.
For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface.
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.
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).