crust:  The solid outermost layer of a planet or moon is called the crust.  The crust is usually a thin layer, taking up about 1-2 percent of the planet's total radius.  If you think of a planet as being like a peach, the crust would be like the skin of the fruit.     











current:  Protons are positively charged particles in an atom.  Electrons are negatively charged particles in an atom.  The steady flow of either of these charged particles is called an electric