ABSENCE OF A MAGNETIC-FIELD SIGNATURE IN PLASMA-WAVE OBSERVATIONS AT CALLISTO

The galilean moons of Jupiter are substantial bodies-three of them are larger than the Earth's Moon, and one is larger than Mercury-yet litt le has been known about them until very recently. The discovery of a m agnetosphere(1) and magnetic field(2) associated with Ganymede was sur prising, and raised the possibility that other galilean satellites,. p articularly Callisto (which is the second largest after Ganymede), als o might have an internally generated magnetic held. Here we report obs ervations of plasma waves around Callisto, detected during the recent fly-by of the Galileo spacecraft. The nature of the plasma waves indic ates that Callisto, unlike Ganymede, does not have a magnetosphere or an internal magnetic held The electron density near Callisto, however, is substantially higher than that in Jupiter's magnetosphere at this orbital radius, indicating that Callisto is a significant source of lo cally generated plasma, This plasma most probably comes from a tenuous atmosphere around Callisto, which may be similar to the hydrogen clou d around Ganymede, as the electron densities are somewhat comparable.