Also radar maps, showing mountains and valleys, have been made of all the inner planets. Some meteors are fragments of other planets, primarily Martian meteors. From what we observe we can piece together the geologic history of the planets. The surface is a reflection of what's going on inside the planet. The rocky planets are spherical because of gravity.
Gravity also is responsible for the layered structure of planet interiors.
This is called differentiation. 1.Gravitational energy left from the formation of the planet, 2.Meteor bombardment 3.Decay of radioactive material trapped in the body of the planet. Therefore at this time gravity concentrated the densest material in the center and the lighter material rose to the top. The lithosphere is the outer layer of solid rock, containing the crust and the top part of the mantle. When the Earth cooled, the lithosphere cracked into plates which now move around forming continents, etc.