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.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.
These differing atoms are called isotopes and they are represented by the sum of protons and neutrons in the nucleus. Carbon has 6 protons in its nucleus, but the number of neutrons its nucleus can host range from 6 to 8.For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter").It is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate on average is predictable.When we age date a planet, we are actually just dating the age of the surface, not the whole planet.We can get absolute ages only if we have rocks from that surface.When there is a scientific discussion about the age of, say a meteorite or the Earth, the media just talks about the large numbers and not about the dating technique (e.g. On the other hand, when the media talk about "more recent events," ages that are more comprehendible, such as when early Man built a fire or even how old a painting is (or some ancient parchment), then we bring up the dating technique in order to better validate the findings.