Jupiter is thought to have a small, hot, rocky core. This is surrounded by concentric layers of hydrogen , first in a liquid-metallic state (which means that its atoms, although not bonded to each other, are so tightly packed that the electrons can move easily from atom), then non metallic liquid, and finally gaseous. The planet's atmosphere consists of approximately 86% hydrogen and 14% helium, with traces of methane, water and ammonia. The liguid -metallic hydrogen layer is a good conductor of electrical currents. These are the source of a powerful magnetic field that is many times stronger than the Earth's and enormous in extent. It stretches for several million kilometers toward the Sun and for several hundred million kilometers away from it. The magnetic field traps charged particles from the Sun, forming a zone of intense radiation outside Jupiter's atmosphere that would be fatal to a human being exposed to it. The motions of the electric charges cause radio emissions. These are modulated by the rotation of the planet and used to estimate the period of rotation, which is about 9.9 hr.