Melting Point

Impact crater floors are commonly flat and relatively smooth, the result of the cooling and solidification of impact melt generated by the impact event itself. Often, the pool of impact melt cracks as it cools, a process well illustrated by the striking Abedin crater. Although not visible in the frame above, this crater also hosts cooling cracks on its floor. It also boasts numerous terraces along its inner wall, which likely formed after the impact melt solidified. Note how the fine-grained texture of the inner walls contrasts with the crater's floor.

http://photojournal.jpl.nasa.gov/catalog/PIA19231