<P> Charged particles have a considerable influence on the surface properties of Galilean moons . Plasma originating from Io carries sulfur and sodium ions farther from the planet, where they are implanted preferentially on the trailing hemispheres of Europa and Ganymede . On Callisto however, for unknown reasons, sulfur is concentrated on the leading hemisphere . Plasma may also be responsible for darkening the moons' trailing hemispheres (again, except Callisto's). Energetic electrons and ions, with the flux of the latter being more isotropic, bombard surface ice, sputtering atoms and molecules off and causing radiolysis of water and other chemical compounds . The energetic particles break water into oxygen and hydrogen, maintaining the thin oxygen atmospheres of the icy moons (since the hydrogen escapes more rapidly). The compounds produced radiolytically on the surfaces of Galilean moons also include ozone and hydrogen peroxide . If organics or carbonates are present, carbon dioxide, methanol and carbonic acid can be produced as well . In the presence of sulfur, likely products include sulfur dioxide, hydrogen disulfide and sulfuric acid . Oxidants produced by radiolysis, like oxygen and ozone, may be trapped inside the ice and carried downward to the oceans over geologic time intervals, thus serving as a possible energy source for life . </P> <P> The first evidence for the existence of Jupiter's magnetic field came in 1955, with the discovery of the decametric radio emission or DAM . As the DAM's spectrum extended up to 40 MHz, astronomers concluded that Jupiter must possess a magnetic field with a strength of about 1 milli teslas (10 gauss). </P> <P> In 1959, observations in the microwave part of the electromagnetic (EM) spectrum (0.1--10 GHz) led to the discovery of the Jovian decimetric radiation (DIM) and the realization that it was synchrotron radiation emitted by relativistic electrons trapped in the planet's radiation belts . These synchrotron emissions were used to estimate the number and energy of the electrons around Jupiter and led to improved estimates of the magnetic moment and its tilt . </P> <P> By 1973 the magnetic moment was known within a factor of two, whereas the tilt was correctly estimated at about 10 ° . The modulation of Jupiter's DAM by Io (the so - called Io - DAM) was discovered in 1964, and allowed Jupiter's rotation period to be precisely determined . The definitive discovery of the Jovian magnetic field occurred in December 1973, when the Pioneer 10 spacecraft flew near the planet . </P>

What is the origin of the magnetic fields of jupiter and saturn