All terrestrial planets have a core, a mantle, and a crust. They are a bit like a boiled egg: the central yolk is the core; the white albumin is the mantle; and the shell is the crust. The crust of a terrestrial planet is thin, with the core and the mantle taking up the vast bulk, sometimes with a very large core, sometimes much smaller. Terrestrial planets have metallic cores of mostly iron, with rocky mantles and crusts. 