The Galileo spacecraft is now another satellite in orbit around Jupiter, the largest planet in the solar system, and on June 27 came within a few hundred kilometers of Ganymede to begin a series of 11 close encounters with Jupiter’s four Galilean satellites:  Io, Europa, Ganymede, and Callisto. The four moons form a miniature solar system around the giant gaseous planet. As revealed by the Voyager spacecraft in the late 1970s, Io, the innermost, is the size and density of the Earth’s Moon and is constantly volcanically active as it is squeezed between the tidal forces of giant Jupiter and the nearest moon Europa. Europa also has a size and density similar to the Earth’s Moon, but it is covered with a layer of highly fractured water ice and possibly a liquid water ocean below.

From here outward, the Galilean satellites change character from moons that resemble the inner, silicate-rich planets to bodies that are more similar in density and composition to objects in the outer solar system. Ganymede is larger than the planet Mercury, about one-third the diameter of Earth, but its density is less than 2 grams per cubic centimeter. Its surface is composed of half ancient, heavily cratered dark terrain and half younger, bright terrain that has been heavily tectonically modified. Both these surfaces are dominated by water ice with the older terrain having more darkening agents. Callisto is similar to Ganymede in size and density, but its surface is almost completely dark terrain that is even more heavily cratered than that of Ganymede.

These basic observations raise many questions about the formation and geologic history of these satellites and the system as a whole. Among a host of important scientific objectives, Galileo is designed to obtain high-resolution images of key terrain relationships and to fill gaps in coverage from the Voyager flybys in order to address these questions. The Galileo Solid State Imaging (SSI) Team, a group that consists of geoscientists and atmospheric scientists from several institutions and is headed by Team Leader Mike Belton of the National Optical Astronomy Observatories, works to obtain these images.

A few days after the late-June encounter, in a press briefing at the Jet Propulsion Laboratory, Galileo Project Scientist Torrence Johnson, along with Jim Head and Bob Pappalardo of Brown University, described the first SSI images of Ganymede’s bright and dark terrain and the dramatic improvement in resolution over the Voyager images.

In the pair seen in Figure 1, the frame at left shows a 35 × 55 kilometer (25 × 34 mile) area of bright terrain known as Uruk Sulcus and was taken by the Voyager 2 spacecraft when it flew by in 1979, with a resolution of about 1.3 kilometers (0.8 miles) per pixel. The frame at right showing the same area was taken by Galileo and has a resolution of about 74 meters (243 feet) per pixel, more than 17 times better than that of the Voyager image. In the Voyager frame, line-like bright and dark bands can be seen, but their detailed structure and origin are not clear. In the Galileo image, each band is now seen to be composed of many smaller ridges. The structure and shape of the ridges permit planetary geologists to determine their origin and their relation to other terrains, helping to unravel the complex history of this planet-sized moon. Initial analysis suggests that extension and ductile stretching of the ice at depth is causing brittle failure in the near-surface ice and the formation of lanes of rotated fault blocks. Evidence of shear displacement can also be seen in the zone crossing the image in the upper right.

NASA/JPL Photo P-47058
Fig. 1. Comparison of Ganymede's bright terrain in Uruk Sulcus in a Voyager image [left; 1.3 kilometers (0.8 miles) per pixel] and a Galileo image [right; 74 meters (243 feet) per pixel]. In each of these frames, north is to the top, and the Sun illuminates the surface from the lower left nearly overhead (about 77° above the horizon). The area shown is at latitude 10°N, 167°W and is about 35 × 55 kilometers (25 × 34 miles). The image was taken when Galileo was 7448 kilometers (4628 miles) away from Ganymede.

In other parts of Uruk Sulcus, different relationships can be seen. In Figure 2, a mixture of terrains reveals fine details of bright areas that make up about half the surface of Ganymede. Pockmarked, ancient, heavily cratered terrain is seen at the top; it is cut by younger, line-like structures in the lower left of the image. At the lower Voyager resolution, the area in the upper right was thought to be smooth and relatively younger then the rest of the terrain in the image, perhaps having been emplaced by water ice volcanism. However, the higher crater density and the cross-cutting relationships shown in the Galileo image indicate that the previous thinking was reversed and that the material is actually older. The bright, circular feature in the lower middle is an impact crater with some dark ejecta superimposed on the linear ridges. These types of relationships revealed by Galileo are permitting scientists to work out the complex geologic history of Ganymede.

NASA/JPL Photo P-47065
Fig. 2. In this view of Uruk Sulcus, part of Ganymede’s bright terrain, north is to the top and the Sun illuminates the surface from the lower left nearly overhead. The area shown, at latitude 10 N, longitude 168°W, is about 55 × 35 kilometers (34 × 25 miles), and the smallest features that can be seen are 74 meters in size. The image was taken on June 27 at a range of 7448 kilometers (4628 miles).

Images of the dark terrain provided similar increases in resolution and show important details of this ancient icy landscape (Figure 3). Abundant impact craters seen in this image of Galileo Regio [resolution about 80 meters (262 feet) per pixel] testify to the great age of the terrain, dating back several billion years. At the bottom margin, half of a 19-kilometer (12 mile) -diameter crater is visible. The dark and bright lines running from lower left to upper right and from left to right across the middle are deep furrows in the ancient crust of dirty water ice. These furrows may be ancient impact basin rings or tectonic structures resulting from very early convection in a liquid water mantle. The origin of the dark material is unknown, but it may be accumulated dark fragments from many meteorite falls that accumulated on Ganymede. Analysis of this and similar Galileo images will allow planetary geologists to assess these theories and to compare these results with images taken during encounters with other moons of Jupiter in the coming months.

NASA/JPL Photo P-47067
Fig. 3. Part of Ganymede’s Galileo Regio dark terrain (latitude 18°N, longitude 147°W) about 46 × 64 kilometers (29 × 38 miles) in extent. Resolution is about 80 meters (262 feet) per pixel. The image was taken at a range of 7563 kilometers (4700 miles). North is to the top, and the Sun illuminates the surface from the lower left at about 58° above the horizon.

NASA/JPL Photo P-47170     

Fig. 4. Jupiter’s moon Europa, as seen in this image taken June 27, 1996, by Galileo, displays features in some areas resembling ice floes seen in Earth's polar seas. Europa’s icy crust has been severely fractured, as indicated by the dark linear, curved, and wedged-shaped bands seen here. These fractures have broken the crust into plates as large as 30 kilometers (18.5 miles) across. Areas between the plates are filled with material that was probably icy slush contaminated with rocky debris. Some individual plates were separated and rotated into new positions. Europa's density suggests that it has a shell of water ice as thick as 100 kilometers (about 60 miles), parts of which could be liquid. Currently, water ice could extend from the surface down to the rocky interior, but the features seen in this image suggest that motion of the disrupted icy plates was lubricated by soft ice or liquid water below the surface at the time of disruption. This image covers part of the equatorial zone of Europa and was taken from a distance of 156,000 kilometers (about 96,300 miles). North is to the right and the Sun is nearly directly overhead. The area shown is about 360 × 770 kilometers (220 × 475 miles or about the size of the state of Nebraska), and the smallest visible feature is about 1.6 kilometers (1 mile) across.

Recently an amazing finding led NASA scientists to report the possible detection of signs of fossil life on Mars. This has raised interest in another exciting aspect of the Galileo mission, the nature of the interior of the satellite Europa. During the Ganymede-1 encounter, SSI team members were able to obtain a few images of the surface of Europa. In a press briefing on August 13, SSI Team Member Ron Greeley of Arizona State University described Europa images that showed a cracked and previously mobile surface, which appeared as if thin plates of “sea-ice” had separated and drifted apart, causing liquid or slushy water to well up and freeze, creating dark patches in between (Figure 4). Of course, if liquid water does (or did) exist in the shallow interior of Europa, this could be a natural site for the development of life. Further encounters will provide much better resolution imaging of Europa to reveal surface features in more detail.

Information for this report was contributed by Jim Head, SSI Team member from Brown University. These images and information on their interpretation and the Galileo mission in general can be seen on the World-Wide Web at and