Unlike orbiters

The latest findings of the spacecraft Galileo, which has been providing valuable information about Jupiter and its satellites, relate to moon Callisto. The satellite shows no evidence of an internal magnetic field and is unlike the other satellites of Jupiter such as Io, Europa and Ganymede. In other satellites, the field is assumed to come from an internal molten core which formed when the moon was hot enough for ice, rock and metal to separate. The absence of a magnetic field on Callisto could mean that it is uniformly dense ( Nature , Vol 387, No 6630).