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Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics. Links to external sites will be displayed in another window.
See the "Not Round" page from the THEMIS site for what can make an impact crater not round (links will appear in a new window).
Volcanism is any eruption of molten lava onto the surface.
The molten rock has a lower density than solid rock so it rises.
The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts.
The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact.