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. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun , and because Mercury 's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon , stop again, and then set quickly over the other horizon . From Earth , Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise.