Here's why:
* Cratering Rate: The Moon is constantly bombarded by asteroids and meteoroids. Over time, the surface accumulates impact craters.
* Maria Formation: The maria are vast, dark plains of volcanic basalt that filled in low-lying areas on the Moon's surface. This volcanic activity occurred relatively late in the Moon's history.
* Relative Ages: Since the maria formed after the highlands, they have had less time to accumulate craters. The highlands, having existed for a longer period, have been subjected to a greater amount of bombardment.
Therefore, the higher density of craters on the highlands indicates that they are much older than the maria.
