Crater counting
A way of estimating how old a planetary surface is by counting how many craters it has: more craters means an older surface.
Impacts happen across the Solar System all the time, so a surface collects craters the longer it is exposed, rather like a pavement collecting chewing-gum spots. Crater counting turns this into a clock. Scientists measure every crater above a chosen size in a mapped area and compare the density with a model of how the impact rate has changed through time.
The model is calibrated using the Moon, because Apollo and Luna missions returned rocks whose ages could be measured precisely by radiometric dating, from areas whose craters had also been counted. That link lets researchers date surfaces on Mars, Mercury and asteroids that no one has sampled. For instance, the floor of the enormous Rheasilvia basin on Vesta, the probable source of the HED meteorites, was dated this way using images from the Dawn spacecraft, though different models gave rather different ages.
The method has limits. Small craters can be wiped out by erosion or lava, and secondary craters made by debris from a larger impact can be mistaken for primary ones. On a lunar map you can see the principle for yourself: the bright, smooth maria have far fewer craters than the older, battered highlands.
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