Ganymede Sings, Europa Dazzles, Callisto Confounds

The dark, dusty landscape confused and puzzled the crowd of press and JPLers at von Kármán as they examined the closest images yet of Callisto, the outermost Galilean satellite, long thought to bear the most ancient, most heavily cratered, and least modified surface in the Jovian system. Here was a surprise even for those who follow the Galileo Mission and have come to expect surprises.

The press conference of December 12, less than a week after Galileo's anniversary of its first year at Jupiter, presented some newly released images of Europa and Callisto from the first Callisto flyby (C3) and focused on the newly confirmed magnetic field and recently discovered iron core of Ganymede. The panel, introduced by Jane Platt (JPL Public Information) and moderated by Dave Seidel (JPL Public Education), included Don Gurnett (Iowa), Margaret Kivelson (UCLA), Gerald Schubert (UCLA), Kelly Bender (Arizona State), and John Anderson and Project Scientist Torrence Johnson (JPL).

Images from the Callisto Flyby

Kelly Bender presented new, SSI (solid-state imaging) images from the November 4 C3 flyby. The 1136-km pass by Callisto was the closest ever for that moon, and the 34,000-km pass by Europa was four times closer than any pass since the tour started last June. The images prepared by the SSI Team did not disappoint.

The close-up of Callisto on the front page focuses on a portion of a chain of about 25 small impact craters. Based on low-resolution Voyager images, investigators expected that tiny craters would be found peppering Callisto's ancient crust. But look at the picture. The surface seems to be covered with a smooth, dark material. No little craters, and the rims of the bigger craters have slumped to expose the bright, icy subsoil, which seems to poke out of a sea of soft soot. Seemingly, some unexpected process has caused an unknown material to fill in the lower elevations.

Europa continues to amaze. This surface shows practically no craters at all, and a spidery network of wandering and arcing ridges, with new ones overlaying older ones, seems to fill the field. These thin ridges generally carry a dark groove down their centers, and their raised edges are often knotted and bumpy. Some sections show the smudgy, dark deposits characteristic of triple bands, while other sections are clean. The extreme youth of the surface suggests very recent, even current icy volcanism. The bands of symmetrical, parallel ridges suggest spreading centers, like the Earth's mid-oceanic ridges and zones of sea-floor spreading.

The Ganymede Discoveries

The Ganymede discoveries represented the careful analysis of data from the first two close flybys, G1 (June 27) and G2 (Sept. 6).

Don Gurnett's plasma wave instrument (PWS) is basically a radio receiver that detects waves in the charged plasma flowing through planetary magnetic fields. The G1 data were first processed as a brightly colored spectrogram, which bore the unmistakable visual signature of a discrete magnetosphere nestled inside the larger Jovian field. To present these data as an audible record to the press, Gurnett's Plasma Wave Science Team compressed about 50 minutes of data into one minute and lowered the pitch ninefold to within the range of human hearing. The result, played with an animation of the spacecraft swooping past the moon, was the Ganymede Symphony: about 10 seconds of Jupiter's murmuring, sizzling magnetosphere; then a crescendo of bangs and pops as Galileo entered Ganymede's magnetosphere (characterized by a 500-nanotesla field); a rising, then falling tone with a background of low, growling rumbles (called a chorus) as it sailed past the moon; then, finally, another crash followed by relative calm as it reentered the Jovian domain.

Margaret Kivelson's magnetometer provided the data from G1 that her Magnetometer Science Team used to model the suspected magnetic field as a bar magnet, tilted about 10 degrees off the moon's spin axis and strong enough to warp the Jovian field. On the G2 pass, the same onboard magnetometer yielded confirmation of their model.

John Anderson and his Celestial Mechanics Team reprised their discovery of Io's iron core (see "Meet the Radio Science Teams," The Galileo Messenger, issue 39) with a similar treatment of Ganymede. Tracking the spacecraft trajectory via radio frequency shifts known as Doppler data can distinguish a perfect sphere from an oblate one. Comparing the deduced shape of the satellite with its known density, Anderson suggests that under Ganymede's warped and faulted icy crust lies an 800-km thick layer of warm, plastic ice, an equally thick mantle of rock, and a dense iron/iron sulfide core. Actually, without its ice, Ganymede's interior looks a lot like Io's, with perhaps a little more rock and a little less core.

Gerald Schubert put these newly revealed structures and properties into evolutionary, geodynamic perspective. He explained that a planetary magnetic field like the Earth's is understood to be generated by fluid motions in the liquid outer portion of its metallic iron core that generate electric currents--a geodynamo, actually. This seems to be the case for Mercury, and may be for Io as well. Schubert assumes an early, homogeneous accretion of materials, then a settling of the heaviest to the core, with the generation of lots of heat. But Ganymede has cooled through its history. While he was willing to concede that the moon may develop some heat from the kind of tidal flexing that heats Io, he thought it not sufficient to account for the crustal movements that have scarred its surface. He speculated that Ganymede may have enjoyed an orbit closer to Jupiter in some past epoch.

In the question-and-answer session that followed, some curious comparisons between the three icy surfaces were made. Both Europa and Ganymede show zones of parallel ridges, but only Europa's seem to reflect surface spreading--Ganymede's seem to come from faulting after pulling stresses. Also, contrary to pre-Galileo expectations, Ganymede shows many more tiny craters than Callisto, whose surface is strangely softened by surface slumping.

While our understanding of the internal structures and surface features of the Galilean moons is clearly ahead of even 3 months ago, every new image continues to baffle and confound even the most experienced investigators. It's exhilarating and frustrating at the same time--but then, I think we always expected it would be.

• From the Project Manager
• Project Galileo's Long Reach
• What a Year This Has Been!
• The Galilean Moons...Close Up
• Educational Outreach Corner
• Up to Date
• Carl Sagan, Project Galileo's Friend