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i- m resolution images, particularly those from the Galileo spacecraft, reveal craters with bright and dark ejecta in the darker regions (Fig. 6.28).

The lighter grooved terrain is found to have parallel alternating grooves and ridges running for hundreds of kilometers. The troughs and crests of these are separated by 5-10 km horizontally and several hundred meters vertically. The grooves and ridges appear to be bundled into formations referred to as 'lineations'. These lineations can be seen in some places to cut across one another, suggesting a very complex tectonic process (Fig. 6.29). This terrain can include small hills. In certain areas there is smooth terrain that might be the result of melting of the grooved terrain from below, or the result of cryovolcanism. Upon close examination it is easy to see that the grooved terrain cuts into the dark cratered terrain, indicating that the dark terrain is older. A good example of Ganymede's dark terrain is a region named Galileo Regio (Fig. 6.30). Galileo Regio is a large oval area ~2,800-3,200 km in diameter [418]. This feature had been seen by early telescopic observers; Dollfus and others making sketches of albedo markings. More recently even amateur astronomers, with modern 10-in. telescopes utilizing CCD cameras and webcams, are capturing these albedo features!

While the light, grooved terrain certainly presents evidence of tectonism, Galileo discovered the affects of tectonic activity in the darker regions as well. The dark terrain of Nicholson Regio is intensely fractured by tectonic forces, and one large crater was seen to have been torn apart [419].

Fig. 6.28. A Galileo spacecraft view of the bright, rayed crater Osiris on Ganymede. North is to the bottom of the image. (Credit: NASA/JPL/Caltech)

Another dark region is Marius Regio. Marius Regio is a large area that lies near Galileo Regio. This combined area is of great interest because the two regios are separated by a sulci named Uruk Sulcus (Fig. 6.31). The separation of the two regios appears to have been the result of some form of plate tectonics [420]. Crustal spreading may be responsible for the formation of Uruk Sulcus [421].

About 65% of the surface area of Ganymede consists of bright terrain with relatively low crater populations. Most of this bright terrain is heavily grooved. These tectonically resurfaced areas generally show numerous triangular ridges and troughs, probably resulting from tilt-block normal faulting, resulting in the destruction of pre-existing surface forms. The eccentricity of Ganymede's early orbit may have been high enough in the past that tidal heating and flexing drove internal activity such that the internal heating aided the formation of bright terrain [422]. Many of the grooves probably formed at different times, during different deformational episodes, responding to different stress fields [423]. Pappalardo et al., made an initial analysis of Galileo images taken of the Uruk Sulcus area. Ridges and grooves are widespread in this region, with the terrain described as 'parallel ridged terrain'. The tectonic activity in the region is multifaceted. In addition to tilt block normal faulting, evidence is also found for horst and graben faulting, strike-slip deformation, high extensional strain, domino-style normal faulting, and horizontal shear and transtension. In Uruk Sulcus, spacing of ridges and troughs is ~8 km, with crest-to-trough height differences usually 300-400 m and as great as 700 m, with terracing apparent on some of the ridge walls [424]. Pappalardo et al., conclude there is abundant evidence that tilt-block-style normal faulting, horst-and-graben style normal faulting, and strike-slip deformation has modified pre-existing terrain through destruction of the older surfaces [425].

Fig. 6.29. Ridges, grooves, craters, and the relatively smooth areas in Uruk Sulcus on Ganymede were captured in this image by the Galileo spacecraft. The patterns of ridges and grooves indicate that extension (pulling apart) and shear (horizontal sliding) have both shaped the icy landscape. (Credit: NASA/JPL/Caltech)

Fig. 6.29. Ridges, grooves, craters, and the relatively smooth areas in Uruk Sulcus on Ganymede were captured in this image by the Galileo spacecraft. The patterns of ridges and grooves indicate that extension (pulling apart) and shear (horizontal sliding) have both shaped the icy landscape. (Credit: NASA/JPL/Caltech)

The dark regions make up ~35% of the surface of Ganymede. Its geologic history is different from that of the bright areas. As alluded to, the dark terrain is covered with widespread, heavy cratering (Fig. 6.32). It is also faulted, to some extent. In the dark regions we see cratering, hummocky-hilly terrain, palimpsests, furrows, faults and fractures, knobs and massifs, furrow rims, and low-albedo plains. The geologic processes thought to have occurred in Galileo Regio in particular include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possible cryovolcanism and isostatic adjustment. However, scientists have not found unequivocal evidence for cryovolcanism within the dark regions; no features that may be source vents have been found. Impact cratering is a significant process within the dark regions of Ganymede. Galileo Regio is the largest contiguous region of dark terrain on Ganymede [426]. It is thought that the darker coloration of these regions consists of a thin, low albedo veneer overlying a

Images Ganymede From Galileo Probe
Fig. 6.31. A high-resolution mosaic of the Uruk Sulcus region on Ganymede, taken by the Galileo spacecraft. Note the parallel ridges and troughs. (Credit: NASA/JPL/Caltech)

cleaner substrate that contains small amounts of admixed dark meteoritic material [427]. By contrast, the low-lying ice of sulci is relatively clean.

There is significant impact cratering on Ganymede, indicating that Ganymede suffered its fair share of the effects of the 'Great Bombardment' period (Fig. 6.33). Most of the impacts appear to have occurred early in Ganymede's history. This is evident from the fact that most of the cratering evidence is contained in the dark

Fig. 6.32. A portion of the Galileo Regio region on Ganymede, revealing the heavily cratered dark terrain that makes up about half of the surface of this moon. Some craters are cut by numerous fractures, showing that the ancient crust was highly deformed early in Ganymede's history. A Galileo spacecraft image. (Credit: NASA/JPL/Caltech)

Fig. 6.32. A portion of the Galileo Regio region on Ganymede, revealing the heavily cratered dark terrain that makes up about half of the surface of this moon. Some craters are cut by numerous fractures, showing that the ancient crust was highly deformed early in Ganymede's history. A Galileo spacecraft image. (Credit: NASA/JPL/Caltech)

Fig. 6.33. The Galileo spacecraft reveals that the grooved terrain of Ganymede's Nippur Sulcus is composed of ridges and troughs spaced 1-2 km apart. A few broad ridges have smaller ridges on top of them. A 12 km crater is superimposed on these ridges. (Credit: NASA/JPL/Brown University)

regions of the surface, with the grooved terrain having resurfaced the moon in a later period. Yet, there are a number of impact crater features in these grooved areas, indicating that some of the impacts occurred after the grooves appeared. Relatively speaking, Ganymede is much more heavily cratered than Europa, but not nearly so as Callisto.

Craters on Ganymede do not behave the same as craters on our Moon or on Mars. For example, many of the craters on the Moon, large and small, show deep excavations with crater rims and central peaks. On Ganymede, as craters increase in diameter, the floors become shallower, and the crater rims less prominent; in other words, they become flatter with size. There are many examples of craters on Ganymede that are so shallow as to almost disappear. These features are referred to as 'palimpsests' and some of them are 400 km in diameter (Fig. 6.34). Palimpsests consist of four surface units: central plains, an unoriented massif facie, a concentric massif facie, and outer deposits. Deposits represent fluidized impact ejecta [428]. 'Palimpsests' account for a large portion of the surface and thus must have originated in Ganymede's early history. During this time Ganymede's surface was probably not as firm as it is now, and would have been able to flow over time and soften the relief of the crater. Or, alternately, the larger impacts were able to pen-£ etrate to the slush below at the time, allowing fluid to flood the crater floor.

£ There are young craters with brilliant ejecta ray systems that occur in both c the dark regions and the light, grooved terrain (Fig. 6.35). The most conspicuous

O ^ of these is named Osiris. This crater is 150 km in diameter with rays extending

•> V 1,000+ km [429]. Like our Moon, there is also evidence of some large impacts on

,0 Ganymede. There are several relics of multiple-ringed basins on Ganymede, with the q © largest of these being Gilgamesh in the southern hemisphere. Gilgamesh has a 150 km

O wide central depression, and an outermost ring with a 225 km radius [430].

The Galileo spacecraft detected a thin ionosphere, suggesting that Ganymede had a tenuous atmosphere. The Hubble Space Telescope had detected a tenuous oxygen atmosphere. Galileo also detected the presence of a magnetic field around Ganymede. This was also evidence for conditions allowing polar-aurorae. As it turns out, the prerequisites for an auroral display are a magnetic field, circulating charged particles, and a tenuous atmosphere. Ganymede has about 50% water ice on its surface. Apparently, both Europa and Ganymede release oxygen by

Fig. 6.34. A small portion of Galileo Regio showing the light-colored palimpsest of Memphis Facula. Although smooth in appearance, surprisingly the 340-km wide feature originated as the site of a massive impact. This Galileo spacecraft image reveals that the crater walls have slumped, the floor has risen isostatically, and the remaining topography has been smoothed out by slush. (Credit: NASA/JPL/Caltech)

Fig. 6.35. The Galileo spacecraft cau ght th is image of the dark-floored crater Khensu on Ganymede. Khensu possesses an unusually dark floor and a bright ejecta blanket. The dark component may be residual material from the impactor, or it may be that the impactor punched through the bright surface to reveal a dark layer beneath. (Credit: NASA/JPL/Brown University)

Fig. 6.35. The Galileo spacecraft cau ght th is image of the dark-floored crater Khensu on Ganymede. Khensu possesses an unusually dark floor and a bright ejecta blanket. The dark component may be residual material from the impactor, or it may be that the impactor punched through the bright surface to reveal a dark layer beneath. (Credit: NASA/JPL/Brown University)

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