The swirly "surface" of Jupiter that we view from Earth is really the ammonia and methane clouds that make up the planet's very top layer. Because the planet is entirely formed of gases, standing on the surface would be impossible, yet there is no surface to stand on. The average distance between Jupiter's clouds is about 5500 miles. Jupiter has three major geographic features: oceans, ice caps and volcanoes. Beneath its cloud cover are a thick crust and an interior made of hydrogen and helium with small amounts of zinc, iron, magnesium and calcium. The ocean that covers most of Jupiter is called a "magma ocean". It lies deep under the gas giant's thin atmosphere and is composed mainly of molten rock. Although it is deep, the ocean probably doesn't reach down more than 10 miles because above that depth the pressure would be too great. However, due to the intense gravity of Jupiter, the ocean may be heated to 400 degrees Fahrenheit (200 degrees Celsius). Ice caps cover much of Jupiter's southern hemisphere. They were once thought to be permanent places where water was locked up in ice, but new research suggests that they may change periodically or even sporadically. In fact, several planets have been discovered by scientists using radar technology that have changing ice caps! Jupiter has four large moons: Io, Europa, Ganymede and Callisto.