The Galileo spacecraft, bruised but unbowed, continued its winning streak as it returned dazzling images of Jupiter's planet-size satellite, Ganymede. A series of snapshots released by NASA on July 10 unveil a alien-looking world, studded with craters and etched with deep grooves. Even more surprisingly, something seems to be active within this seemingly frozen wasteland: Galileo's instruments reveal that Ganymede has a magnetic field, possibly generated in a molten core or in a vast subterranean layer of saltwater.
Galileo, which began orbiting Jupiter last December 7, has had its share of technical difficulties. A faulty main antenna that never deployed forced engineers to reprogram the space probe so that it could radio highly compressed data to Earth through its tiny backup antenna. The detailed resolution of the current images is a testament to the success of the program. Galileo flew within 835 kilometers of Ganymede during its recent encounter. The just released images, which were mostly taken from a distance of about 7,500 kilometers, show details a mere 80 meters across.
By far the most bizarre aspect of Ganymede's face is its strange network of ridges and furrows. Unlike terrestrial topography, however, the ups and downs of Ganymede are carved in ice, not rock. Large parts of the satellite seem to have been reshaped by ice volcanoes and tectonic forces that have thrust up icy mountains. The craters that dot the surface also have an unusual, flattened appearance.
Galileo also captured a snapshot of Io, another of Jupiter's giant satellites. Io is the most volcanically active body in the solar system; its surface is constantly reshaped by sulfurous eruptions. The new image from Galileo shows some remarkable changes since the Voyager spacecraft visited Io 17 years ago; fallout of sulfur and sulfur dioxide frost has created a huge white area in the satellite's southern hemisphere.
Planetary scientists are just beginning to digest the meaning of the Galileo results. Over the next 17 months, the probe will make three more passes by Ganymede, during which it will map other regions and further study the local magnetic field. Io too will come under closer scrutiny. And Galileo will relay images of Jupiter (NASA has just released a preliminary view of the planet's cyclonic Great Red Spot) and the planet's other giant satellites: battered Callisto and Europa, whose billiard-ball smooth surface may conceal a worldwide ocean.