In geology, a crust is the outermost soli shell of a planet or moon. Crust is chemically and mechanically different from underlying material. Crusts of Earth, our Moon, Mercury, Venus, and Mars have been generated largely by igneous processes, and these crusts are richer in incompatible elements than the underlying mantles. Crusts are also present on moons of outer planets and have formed by similar or analogous processes: for instance, Io, a moon of Jupiter, also has a crust formed by igneous processes.