We have samples of only a few bodies in the Solar System. We have the lunar samples returned from the Apollo and Luna missions, we have many meteorites from asteroids, and we have the rocks under our feet. We also have fifty or so Martian meteorites but we don’t know from where on the planet they originated. The remaining planets and satellites in the Solar System have only been explored remotely. Without samples we can date using radioactivity, the only way to date these surfaces is by examining the record impact craters have left throughout Solar System history.
Martian meteorite EETA79001, shergottite. Click here for original source URL
Imagine interplanetary bodies such as comets and asteroids drifting through the Solar System and occasionally crashing into other interplanetary bodies — planets and moons. The longer a surface is exposed, the more impacts will occur and the more craters will appear on it. You can think of craters as raindrops hitting a sidewalk: you can tell how long it’s been raining by how much of the surface is wet. The older the surface (the longer it rains), the more craters (wet spots) will be crowded onto it per unit area. Eventually, a surface will reach saturation – every new crater obliterates an existing one, and the distribution of craters doesn’t change. In the sidewalk analogy, the surface eventually becomes wet all over and one more raindrop doesn’t make any difference.
You would expect to see more small craters than large ones, because the crater population reflects the population of asteroids and comets hitting the surface. As the age of the surface increases, the numbers of craters of different sizes increase proportionally. Some geological processes preferentially destroy certain sized craters — for example, lava flooding on the Moon would have destroyed most of the small craters in the vicinity but larger impact basins survived. Very small craters on Mars are filled in by blowing dust. Venus is another example: its thick atmosphere burns up the smaller incoming projectiles that make small craters. All of these processes will affect the distribution of craters on a planetary surface.
Crater statistics will only tell you the relative ages of surfaces – whether one area is older or younger than another area. Once scientists were able to find the ages of lunar rocks using radioactive dating they could calibrate the ages of the same areas found with the crater counting method. The lunar cratering record can be adjusted for other planets — for instance a planet closer to the asteroid belt would probably experience more impacts. The problem is also complicated by the fact that the cratering rate was much higher early in Solar System history. Taking these factors into account this method has yielded absolute timelines for the geologic histories of other planets, including Mars.