Final answer:
The primary source of erosion on the Moon is impacts from interplanetary debris such as asteroids and comets. Change on the Moon takes a long time due to the slow process of erosion and modification through impacts over a very long period of time.
Step-by-step explanation:
The primary source of erosion on the Moon is impacts from interplanetary debris such as asteroids and comets. These impacts break up the lunar surface and scatter the fragments, leading to the formation of craters. Over billions of years, these impacts have reduced much of the Moon's surface layer to particles about the size of dust or sand.
Change on the Moon takes a long time because the rate of cratering has been roughly constant for the past 3 billion years, but was much greater in earlier times. As a result, the Moon's surface has undergone a slow process of erosion and modification through impacts over a very long period of time.