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Why are the lunar highlands more heavily cratered than the dark lunar mare regions?

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four impact craters — Meteorite crater Four impact craters of the same size (30 km [20 miles] in diameter) imaged by spacecraft on different solid bodies of the solar system and reproduced at the same scale. They are (clockwise from upper left) Golubkhina crater on Venus, Kepler crater on the Moon, an unnamed crater on Jupiter's moon Ganymede, and an unnamed crater on Mars. The images are oriented such that the craters appear illuminated from the left; the Venusian crater is imaged in radar wavelengths, the others in visible light.

The lunar highlands are more heavily cratered than the dark lunar mare regions primarily due to differences in their geological age and surface history.12

The lunar highlands represent the older, original crust of the Moon, which has been exposed to billions of years of meteorite bombardment. In contrast, the dark mare regions are significantly younger; they were formed when basaltic volcanic flows flooded large impact basins approximately one billion years after the formation of the highlands.4213

This volcanic activity effectively resurfaced these areas, burying older craters and creating a smoother, less cratered landscape. Because the maria formed later, they have had less time to accumulate the density of impact scars found on the ancient highland terrain. By counting the number of craters per unit area, scientists can determine the relative ages of these different lunar regions.412