Io's metallic core.

Jupiter's moon Io is even more of an oddball than planetary scientists had thought. The only moon known to undergo volcanic eruptions, Io now has another distinction. Observations with the Galileo spacecraft indicate that Io has an iron core that stretches 1,800 kilometers across, or half its diameter.

Researchers have detected a metallic core in only one other solar system body-Earth, which lies far closer to the sun than Io does. "Given their relative locations in the solar system, this remarkable similarity must be accounted for in theories of solar system origin and evolution," write Galileo investigator John D. Anderson of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues in the May 3 Science.

Near Io, the craft also detected a large "hole," or less intense region, in Jupiter's magnetic field. The data suggest that something in Io's vicinity, perhaps the moon's own magnetic field, created the hole. If so, Io would be the first moon known to possess a magnetic field.

The craft recorded the data on Dec. 7, 1995, the beginning of its 2-year orbit of Jupiter and its moons. On that day, Galileo came within 899 km of Io, the closest any craft has ever gone to this moon. A decision to limit use of the craft's balky tape recorder prevented Galileo from recording close-up images of Io.

However, radio waves beamed to Earth by the craft allowed researchers to deduce that Io has an iron or iron sulfide core. Slight shifts in the frequency of the detected waves indicated that Io's gravity was causing the craft to deviate slightly from its prescribed path, which wouldn't happen if Io were a uniformly dense body.

Interior heat, resulting either from the initial collapse of material to form Io or from the tidal force continually exerted by Jupiter's gravity, may have generated the core by causing heavier material, such as iron, to sink toward the center and lighter material to float toward the surface to create the mantle of molten rock and crust.