Jupiter exerts a special fascination through its sheer size: it is the largest planet, more massive than all the others lumped together, and its four giant Galilean satellites are entire worlds in themselves (one of them, Ganymede, is larger than the planet Mercury). For more than a year now, the Galileo spacecraft has swung around Jupiter in a huge, looping orbit, dodging the belts of intense radiation that surround the planet and relaying back a series of breathtaking snapshots.

The new images offer by far the most detailed views yet of the myriad worlds of the Jovian system. This journey into uncharted terrain has produced the inevitable flood of surprises and puzzles and even--in the case of the moon Europa--some new thoughts on the possibility of extraterrestrial life in our solar system.

Jupiter's Moons: Io, Europa, Ganymede and Callisto


The Great Red Spot

So far the main planet has filled mainly a supporting role in Galileo's investigations. A false-color mosaic of Jupiter's belted clouds shows, for the first time, the changing composition of the various layers in the planet's atmosphere, which is thousands of kilometers deep.

But the most remarkable images have centered on Jupiter's enigmatic Great Red Spot, a seemingly perpetual, hurricanelike disturbance whose winds reach speeds of 500 kilometers an hour. The motion of the clouds is clearly visible in a new animation. Enormous thunderheads, tens or hundreds of kilometers across, seem to dot the region around the Great Red Spot, possibly contributing to the flow of energy that maintains the fierce winds.


Cratered Callisto

The outermost of the Galilean satellites is Callisto, one of the most heavily cratered objects in the solar system. The prominent scars on this moon's face include seven chains of craters, probably produced by the impact of fragmented comets such as Shoemaker-Levy 9, which crashed into Jupiter in 1994. Planetary scientists were shocked by recent Galileo images showing that, at small scales, Callisto is oddly smooth. "We expected to see shoulder-to-shoulder craters," says Ronald Greely of Arizona State University, who is part of the Galileo imaging team. "Something strange is going on here, but we just don't know what it is."


Singing Ganymede

Ganymede, the largest of the Galilean satellites, has a battered surface much like Callisto's. The Galileo spacecraft discovered that Ganymede is not only planetlike in size but that it also shares two other attributes with Earth: it possesses a tenuous oxygen atmosphere (far too thin to breathe, however) and a magnetic field that deflects the charged particles that buzz relentlessly around Jupiter.

NASA has represented magnetic data as sounds and the rainbow-colored spectrogram illustrated. Approximately 45 minutes of observations are transformed and compressed to 60 seconds. Color is used to indicate wave intensity, red corresponding to strong waves, blue corresponding to weak waves. Time progresses to the right and frequency (pitch) increases vertically. An animation of the spectrogram and the accompanying audio (3.9 MB) can be downloaded from the Galileo web site.


Pele Erupting

Jupiter's pizza-colored moon Io is the most volcanically active body in the solar system; tidal stresses created by Io's gravitational interactions with the other Galilean satellites cook the moon's interior and power a nonstop sequence of sulfurous eruptions. Galileo is monitoring the changes on Io's surface and watching for plumes from the moon's volcanoes.

A sequence of vivid, color-enhanced images paint a suggestive portrait of Io's restless nature. The images above detail the changes around a volcano called Pele, as seen by Voyager 1 (left), Voyager 2 (middle), and Galileo (right). The Voyager frames were taken in 1979 when the two spacecraft flew past Io; the Galileo view was obtained in June, 1996. Note the changes in the shape of the deposits further from the vent while the radial dark features closer to the vent show little change.

In addition, Infrared pictures show the "hot spots" corresponding to the active regions. Other detectors have demonstrated that Io has a huge iron core below its hellish surface.


Ice Ridges

But the satellite that has generated the greatest excitement is ice-covered Europa. For years, scientists have speculated that the same mechanism that powers Io's volcanoes could warm Europa's interior, so that a global ocean could lie underneath the frozen surface. The latest Galileo views strengthen the argument, offering dramatic evidence that Europa's surface has been reworked by ice volcanoes and geysers. Remarkably high resolution images, depicting features smaller than a football field, capture complex ridges created by the tectonic deformation of Europan ice.

"I would bet that there is an ocean on Europa," says Christopher F. Chyba of the University of Arizona. In a commentary in the January 16, 1997 issue of Nature, Chyba called Europa "one of two prime candidates for a second habitable world in our own Solar System." In his new novel 3001, Arthur C. Clarke imagines life-forms that could have evolved around hydrothermal vents on Europa. Whether such a scenario is at all possible depends on whether the putative ocean really exists. Galileo scientists are looking for active ice volcanism on Europa, but definitive proof may demand a more sophisticated study. Chyba and others are already at work planning a follow-up mission that would scan Europa with ice-penetrating radar that could detect liquid water deep underground.

And if an ocean exists? Turning the X-Files slogan on its head, Chyba remarks that "it is almost impossible to predict what will be out there."