The giant planet Saturn, with its flat rings, is probably the most widely recognized astronomical image. Early astronomers noted its 29-year orbit and assumed that it moves sluggishly. Composed mostly of hydrogen, its atmosphere and structure are similar to Jupiter's, but its density is much lower. Saturn is so light that it could float on water. Like Jupiter, Saturn rotates at great speed causing its equator to bulge outward. Saturn also has an appreciable magnetic field. Winds in its upper atmosphere can travel at l100 mph but major storms are rare. White spots tend to develop during Saturn's northern-hemisphere summer, which happens every 30 years or so, the last being in 1990.