Final answer:
The lunar maria are smoother than the surrounding highlands because they were formed by lava flows that filled in large basins, and they have endured fewer impacts due to their younger age. The highlands are older, more elevated, and heavily cratered. Differences in crater density between the maria and highlands signify different geological ages and histories of volcanic activity and cratering.
Step-by-step explanation:
Most astronomers believe that the lunar maria are smoother than the surrounding heavily cratered areas due to their younger geological age and the process of maria formation. The surface of the lunar maria consists of dark basaltic dust, produced by the solidification of lava after ancient volcanic activity. This smooth basaltic landscape was created by lava flows that filled in large basins on the Moon's surface, making them less cratered than the highlands.
Moreover, the maria cover about 17% of the lunar surface and are mostly located on the side of the Moon that faces Earth. They are relatively crater-free compared to the highlands because the highlands have been exposed to space environment for a longer period, accumulating more impacts over time.
The lunar highlands, on the other hand, are the lighter, more heavily cratered regions that are generally several kilometers higher compared to the maria. They are older and have experienced more impacts, creating a rougher landscape with more craters. The difference in crater density does not necessarily imply that the highlands are 10 times older; it indicates different periods of volcanic activity and impact events.