This is an illustration of the Chicxulub impact.
Click on image for full size
At this point in planetary history, of every planet and moon with a solid surface was bombarded with the remains of the material which formed them.
Even though the solar system was finished forming, there were still probably a lot of smaller planetesimals and debris around, too. The gravity of the large planets would attract nearby planetesimals, which would hit the planets and leave a crater on the planet's
This was the very last stage of the process whereby planetismals swept up material in their paths.
The surfaces of many planets never changed after this point in time. The surface of such a planet bears the record of the fact that it stopped changing by the unchanged appearance of the craters. The planet Mercury, the Moon, Jupiter's moon Callisto, and some portions of Mars bear such an appearance.
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