Recent observations from NASA's Galileo spacecraft reveal that Callisto, one of Jupiter's sixteen moons, is like an undifferentiated “dirty snowball” that has no internal magnetic field or magnetosphere. The outermost of the four large moons discovered by Galileo may be enshrouded by a tenuous atmosphere, however. The new findings, reported in the latest issue of Nature, are surprising because earlier data from Galileo indicates that two of Jupiter's other Galilean moons—Ganymede and Io—have magnetic fields. John D. Anderson of the Jet Propulsion Laboratory in Pasadena, Calif., and colleagues presented gravimetric data showing that Callisto is composed of 40% compressed ice and 60% rock mixed with metal. The undifferentiated composition is consistent with the absence of a magnetic field, which was confirmed by Krishan Khurana of the University of California and his colleagues.