... Southwest Mercury . ... The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. Mercury: A Cratered Inferno . ... Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. ... In the Center of Spiral M77 . ...