The two outer Galilean moons share in common (with Europa as well) a crust composed of water ice. Ganymede is unique among the four satellites in that it shows contrasting crustal blocks (some darker) which suggests it may have been disrupted and reassembled. It has a low number of larger craters that implies surface developments after the main early period of bombardment. Numerous grooves, cracks and ridges occur on its surface. Callisto is very heavily cratered, including some of basin size (as a lunar analog). Several of the 35 small, irregular-shaped satellites orbiting Jupiter are pictured.

### The Galilean Satellites Ganymede and Callisto

The outer Galilean moons also consist of exterior ice (probably commingled with some rock), atop an interior containing a rocky core and perhaps overlying liquid water or slush. Some scientists have speculated on the presence of a sub-ice ocean beneath Ganymede's icy crust that might harbor organic molecules and even simple life forms. Recent results from Galileo instruments that measure magnetic and electric fields appear to confirm that the ice is thick above a salty "ocean" of uncertain thickness.

We can show the surfaces of these satellites best in medium-contrast, black and white images. Ganymede, the third moon from Jupiter, at 1,070,000 km (664,898 mi), is the largest (diameter = 5,263 km [3,270 mi]) satellite in the solar system (larger than the planet Mercury). Galileo effectively imaged Ganymede, as shown in this scene:

The full view highlights the two dominant terrains, seen better in this close-up:

We judge both terrains as old (ancient) based on crater densities. The dark terrain occurs in patches, some with straight sharp boundaries, abruptly against the other type - lighter-toned terrain with ridges and grooves. The darker terrain appears older and may have been part of a crust that after breaking apart was invaded and at times surrounded by the younger material. Each terrain seems to consist of ice, with the dark areas possibly foundering in later-mobilized water-ice. Tectonic plates presumably stressed the grooved terrain to produce parallel ridges (terrestrial sea ice can develop similar but less pronounced structures). Further breakups cause smaller polygons to wedge in a sometimes jumbled patchwork.

19-59: What is particularly unusual about the patches of dark terrain? ANSWER

Below, we compare a high-resolution, Galileo image (right) of grooved terrain with a Voyager 2 image of the same area (left), proving dramatically the value of improved viewing capabilities that inevitably ensue as sensors get better and spacecraft pass closer to their targets.

The difference resulting from 75 m (246 ft) resolution (right) versus approximately 1.3 km (4264 ft) for the left image is obvious. We again depict this type of terrain, in which grooved surfaces make up a patchwork of juxtaposed individual segments, each in sharp contrast to its neighbors, in the next pair of images, taken in July of 1998, during Galileo's last, and closest, approach.

The first taken from a distance with the shadowed limb in view shows irregular wedges each internal grooving but with different orientations.

The second view show parts of three terrains that appear different in relative age. The oldest, Marius Regio, at the bottom is darker. The middle terrain, is Philus Sulcus, with one dominant set of grooves. At the top is Nippur Sulcus which appears to overlap the middle terrain.

Another view below, taken by Galileo in mid-2000, shows grooved terrain next to smooth younger terrain (Nicholson Regio) which in turn is bounded by old, more cratered terrain (Arbela Sulcus).

The craters on Ganymede, as well as those on Callisto, the last of the Galilean four, differ from those on, say, the Moon, in that nearly all of them lack raised rims and central peaks. For larger craters this should be conspicuous, but the effects of gravity almost completely obliterate these impact effects by causing these features to collapse or slump down as the ice responds by viscous flow that levels the higher sections of the structures. A notable exception are the pair of craters shown in an image collected during the July, 1998, close approach. These craters show broad raised rims (the forms resemble splash boundaries of short-lived craters produced by an object thrust into water), and dark floors, which may be material carried up from bedrock below any ocean water beneath the ice.

Occasionally, there are strings of close-spaced craters in a long row, as seen in the lower scene. Such an arrangement is likely due to the breakup of an incoming bolide or projectile that bombarded the surface in a succession of pieces, perhaps similar to the now famous 1994 sequence of collisions on Jupiter's surface by strung-out fragments of the Comet Shoemaker-Levy, about which we will comment at the end of this Section (page 19-23).

Parts of Ganymede's surface have jumbled icy blocks that look like small mountains; thus:

19-60: How do planetary scientists really know that Ganymede, and Callisto as well, are made up of water ice? If liquid water were to reach the surface, could it flow any distance? ANSWER

The Near Infrared Mapping Spectrometer on Galileo is capable of producing maps like the one below of Ganymede that show the broad distribution of ice (green or blue) versus what may be a mix of ice and rock (black or red).

Gallileo has measured an internal magnetic field and surrounding magnetosphere associated with Ganymede, making it the fourth solid planet/satellite to have its own field (others: Earth; Mercury; Io). The origin of this field is still being investigated but the implication is that there is considerable iron in its core.

Callisto (4800 km [2981 miles] diameter; 1,883,000 km [1,169, 343 miles] from Jupiter's center) is dominated by a single terrain made up of darker materials (possibly like the dark terrain of Ganymede). Seen in full as a photomosaic made from nine Voyager 1 images, this satellite appears to be pockmarked by thousands of lighter spots, which are impact scars in the icy surface:

Note the details in the view below.

Later, in 2001, the Galileo space probe produced this full color image of Callisto.

19-61: If the surface of Callisto is dark where uncratered, why do the craters have white tones? ANSWER

Seen close-up by Galileo, this is a typical impact crater on Callisto. Note the jumbled nature of the rim.

The most prominent mega-feature on Callisto is the great series of concentric ice rings that define Valhalla Basin:

This multiringed structure has counterparts in the Caloris Basin of Mercury and Mare Orientale on the Moon. The rings formed during or shortly after a collision that scooped out the central basin. They are crudely analogous to multiple rings in water caused by a stone. The rings in ice tended to stay "frozen" in place as they formed. Over time, these topographic rims, and those of larger craters, tended to diminish in height due to the flow of ice outward from the rise.

19-62: Consider this next image of the ringed crater Asgard, west of Valhalla and smaller. How many rings can you count? ANSWER

Callisto has some structural features. One example is this graben with fault scarp.

During a very close approach flyby, Galileo obtained these views of the surface of Callisto (resolution better than 3 meters) which showed ice peaks several hundred meters high, and small impact craters.

Even as this section was undergoing review and revision, JPL scientists have announced that the Galileo probe detected organic molecules on Ganymede and Callisto. Details as to types or species are not available.

In addition to the large Galilean satellites, Jupiter has 35 irregular-shaped satellites (several discovered by Voyager and Galileo; others since then have been found by telescope; the number of small satellites will likely increase with future observations). Typical of the smaller satellites is Amalthea (150 x 270 km [93 x 167 miles], whose irregular shape and reddish color (sulphur coating?} is shown here:

Amalthea is the largest of the four inner small satellites (in orbits between Jupiter and Io), shown in this next image in a composite photo constructed from different observations:

From left to right (with lengths as shown): Melis (60 km; 37 miles); Adrastea (20 km; 12 miles) (discovered by the Galileo spacecraft); Amalthea (247 km; 154 miles); Thebes (116 km; 72 miles). Of the 8 beyond the orbit of Callisto, Elara is the largest (80 km; 50 miles); they occur in four pairs.

The smallest satellites (only a few miles [kilometers] in size) have been discovered by watching Jupiter over short time periods for any light spots (reflecting minor moons) that shift in position, as illustrated by this sequence of telescope observations:

To summarize our present knowledge of what the space program has learned about the largest planet in the Solar System, extracting from a December 1997 JPL Press release:

The key findings of Galileo's primary mission include:

> The existence of a magnetic field from Jupiter's largest moon, Ganymede.

> The discovery of volcanic ice flows and melting or "rafting" of ice on the surface that support the presence of liquid oceans underneath at some point in Europa's history.

> The observation of water vapor, lightning, and auroras on Jupiter in the outer jovian atmosphere.

> The discovery of an atmosphere of hydrogen and carbon dioxide on the moon, Callisto.

> The presence of metallic cores in Io, Europa, and Ganymede (but not Callisto).

> Evidence of very hot volcanic activity on Io and observations of dramatic changes compared to previous observations.

Galileo, one of the triumphs of the planetary exploration program, finally had its orbit lowered, as propulsion gas ran out, and was slowed so as to plunge into the jovian atmosphere on September 21, 2003. But the tremendous achievements of this satellite, especially in the wealth of information about its major moons, has encouraged scientists to ask NASA for a follow-up mission, named JIMO (Jupiter Icy Moons Orbiter), which if approved could reach the planet as early as 2011.

Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net