Jupiter is primarily composed ofhydrogenwith a quarter of its mass beinghelium, although helium only comprises about a tenth of the number of molecules. It may also have a rocky core of heavier elements,but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planet's shape is that of anoblate spheroid(it has a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is theGreat Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen bytelescope. Surrounding Jupiter is a faintplanetary ringsystem and a powerfulmagnetosphere. Jupiter has at least 67 moons, including the four largeGalilean moonsdiscovered byGalileo Galileiin 1610.Ganymede, the largest of these, has a diameter greater than that of the planetMercury.