What are craters?
Craters are roughly circular, excavated holes made by impact events. The circular shape is due to material flying out in all directions as a result of the explosion upon impact, not a result of the impactor having a circular shape (almost no impactors are spherical). Craters are the most common surface features on many solid planets and moons — our Moon is covered with craters.
What are the major parts of a crater?
- Floor — The bottom of a crater, either bowl-shaped or flat, usually below the level of the surrounding ground.
- Central peaks — Peaks formed in the central area of the floor of a large crater. For larger craters (typically a few tens of kilometers in diameter) the excavated crater becomes so great that it collapses on itself. Collapse of the material back into the crater pushes up the mound that forms the central peak. At the same time, the rock beneath the crater rebounds, or bounces back up to add to the peak.
- Walls —- The interior sides of a crater, usually steep. They may have giant stair-like terraces that are created by slumping of the walls due to gravity.
- Rim — The edge of the crater. It is elevated above the surrounding terrain because it is composed of material pushed up at the edge during excavation.
- Ejecta — Rock material thrown out of the crater area during an impact event. It is distributed outward from the crater's rim onto the planet's surface as debris. It can be loose materials or a blanket of debris surrounding the crater, thinning at the outermost regions.
- Rays — Bright streaks extending away from the crater sometimes for great distances, composed of ejecta material.