ABSENCE OF AN INTERNAL MAGNETIC-FIELD AT CALLISTO

Little is known about the internal properties of Callisto-the outermos t of Jupiter's four large galilean moons-other than the average densit y (about 1.8 cm(-3)). The recent unexpected discovery(1-4) that Ganyme de, and perhaps Io, has an internally generated magnetic field, combin ed with gravity results(5,6) suggesting that both Ganymede and Io are internally differentiated with metallic cores and rocky mantles, has h eightened anticipation of the results obtained by the Galileo spacecra ft in its recent fly-by of Callisto. Here we report that the spacecraf t, passing the moon at a distance of only similar to 1,100 km from the surface, detected only a small enhancement of the field strength (sim ilar to 7 nT), which may be related to changes in the jovian plasma en vironment caused by Callisto(7). Callisto does not have an internally generated magnetic field.