This is an illustration of the Chicxulub impact.
Click on image for full size
Even though the solar system was finished forming, there were still probably a lot of debris around. This debris would hit the planet and leave a crater on the planet's
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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