Final answer:
There are two theories explaining crater densities on the Moon between 3.9 - 3.2 Ga: Collision and Evaporation, and Impact and Erosion.
Step-by-step explanation:
There are two theories explaining crater densities on the Moon between 3.9 - 3.2 billion years ago: Collision and Evaporation, and Impact and Erosion. The Collision and Evaporation theory suggests that the craters were formed when objects collided with the Moon and then evaporated, leaving behind a crater. This theory explains the circular shape of the craters and the lack of oval-shaped craters. The Impact and Erosion theory proposes that the craters were caused by high-speed impacts from interplanetary debris like asteroids and comets. These impacts excavated craters of larger size compared to the impactor and created raised rims, ejecta blankets, and central peaks.