The Galileo spacecraft returned these close-ups of Ganymede revealing that its surface has been extensively bombed by comets and asteroids and dramatically wrinkled and torn by the same forces that make mountains and move continents on Earth.

In addition, the physics instruments on Galileo made the major discovery that Ganymede possesses its own magnetosphere, a bubble-shaped region of charged particles that surrounds many of the planets but has never been found to exist around a moon. The finding indicates that Ganymede very likely creates its own magnetic field. Possible sources of a magnetic field include a molten iron core or even a thin layer of conducting salty water underneath its icy crust. The newly found magnetosphere is contained within Jupiter's magnetosphere.

The plasma wave spectrometer showed that the densities of charged particles around Ganymede increased by a factor of more than 100 near Galileo's closest approach. This indicates that Ganymede is surrounded by a thin ionosphere, which in turn suggests the existence of a tenuous atmosphere.

Stereo View of Ganymede's Galileo Region
Topographic detail is seen in this stereoscopic view of the Galileo Regio region Ganymede. The picture is a computer reconstruction from two images taken by NASA's Galileo spacecraft this summer. One image of the Galileo Regio region was taken June 27, 1996, at a range of 9,515 kilometers (about 5,685 miles) with a resolution of 76 meters. The other was taken September 6, 1996 at a range of 10,220 kilometers (about 6,350 miles) with a resolution of 86 meters. The topographic nature of the deep furrows and impact craters that cover this portion of Ganymede is apparent. The blue-sky horizon is artificial.

Stereo Image of Galileo Regio on Ganymede
New topographic detail is seen in a stereoscopic view of this part of Ganymede. The newly processed picture is a computer reconstruction from two images taken by NASA's Galileo spacecraft. One image of the Galileo Regio region was taken June 27, 1996, at a range of 9,515 kilometers (about 5,685 miles) and the other was taken at a range of 10,220 kilometers (about 6,350 miles) on September 6, 1996. The topographic nature of the deep furrows and impact craters that cover this portion of Ganymede is apparent. The blue-sky horizon is artificial.

NIMS Ganymede Surface Map
Galileo has eyes that can see more than ours can. By looking at the infrared wavelengths, the NIMS (Near Infrared Mapping Spectrometer) instrument can determine what type and size of material is on the surface of a moon. Here, 3 images of Ganymede are shown. Left: Voyager's camera. Middle: NIMS, showing water ice on the surface. Dark is less water, bright is more. Right: NIMS, showing the locations of minerals in read, and the size of ice grains in shades of blue.

Uruk Sulcus Region
Ridges, grooves, craters and relatively smooth areas in the Uruk Sulcus region of Jupiter's moon Ganymede are shown in this high-resolution image captured by NASA's Galileo spacecraft during its first flyby of Ganymede on June 27, 1996. This image was taken when Galileo was 7,448 kilometers (4.628 miles) away from Ganymede; north is to the top of the picture, and sunlight illuminates the surface from the lower left nearly overhead (77 degrees above the horizon). The area shown, at latitude 10 degrees north, longitude 168 degrees west, is about 55 by 35 kilometers (34 by 25 miles); the smallest features that can be discerned are 74 meters (243 feet). The line-like features are sunlit ridges, often arranged in parallel sets. The patterns of ridges and grooves indicate that extension (pulling apart) and shear (horizontal sliding) have both shaped the icy landscape.

Voyager/Galileo Comparison
These images demonstrate the dramatic improvement in the resolution of pictures that NASA's Galileo spacecraft is returning compared to previous images of the Jupiter system. The frame at left was taken by the Voyager 2 spacecraft when it flew by in 1979, with a resolution of about 1.3 kilometers (0.8 mile) per pixel. The frame at right showing the same area was captured by Galileo during its first flyby of Ganymede on June 27, 1996; it 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 scientists to determine their origin and their relation to other terrains, helping to unravel the complex history of the planet-sized moon. In each of these frames, north is to the top, and the sun illuminates the surface from the lower left nearly overhead (about 77 degrees above the horizon). The area shown, at latitude 10 degrees north, 167 degrees west, is about 35 by 55 kilometers (25 by 34 miles). The image was taken June 27 when Galileo was 7,448 kilometers (4.628 miles) away from Ganymede.

Ridges and Troughs
A mosaic of four Galileo high-resolution images of the Uruk Sulcus region of Jupiter's moon Ganymede is shown within the context of an image of the region taken by Voyager 2 in 1979. The image shows details of parallel ridges and troughs that are the principal features in the brighter regions of Ganymede. The Galileo frames unveil the fine-scale topography of Ganymede's ice-rich surface, permitting scientists to develop a detailed understanding of the processes that have shaped Ganymede. Resolution of the Galileo images is 74 meters (243 feet) per pixel, while resolution of the Voyager image is 1.3 kilometers (0.8 mile) per pixel. In this view, north is to the top, and the sun illuminates the surface from the lower left nearly overhead. The area shown, at latitude 10 degrees north, longitude 168 degrees west, is about 120 by 110 kilometers (75 by 68 miles) in extent. The image was taken June 27 at a range of 7,448 kilometers (4,628 miles).

Mixture of Terrains in Uruk Sulcus
A mixture of terrains studded with a large impact crater is shown in this view of the Uruk Sulcus region of Jupiter's moon Ganymede taken by NASA's Galileo spacecraft during its first flyby of the planet-sized moon on June 27, 1996. The image shows fine details of bright areas that make up about half of the surface of Ganymede. Pock-marked, ancient, heavily cratered terrain is seen at the top; it is cut by younger, line-like structures in the lower left of the image. 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 allow scientists to work out the complex geologic history of Ganymede. In this view, north is to the top and the sun illuminates the surface from the lower left nearly overhead. The area shown, at latitude 10 degrees north, longitude 168 degrees west, is about 55 by 35 kilometers (34 by 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 7,448 kilometers (4.628 miles).

Galileo Regio
This view of a part of the Galileo Regio region on Jupiter's moon Ganymede shows fine details of the dark terrain that makes up about half of the surface of the planet-sized moon. One of many ancient impact craters in the region is visible at the middle left. The crater is cut by numerous fractures, showing that the ancient crust was highly deformed early in Ganymede's history. Dark areas may have originated from dark material thrown off by dark meteorites hitting the surface in thousands of impact events. In this view, north is to the top and the sun illuminates the surface from the lower left about 58 degrees above the horizon. The area shown, at latitude 19 degrees north, longitude 149 degrees west, is about 19 by 26 kilometers (12 by 16 miles); resolution is about 80 meters (262 feet) per pixel. The image was taken June 27 at a range of 7.652 kilometers (4,755 miles).

Ancient Terrain in Galileo Regio
Ancient impact craters shown in this image of Jupiter's moon Ganymede taken by NASA's Galileo spacecraft testify to the great age of the terrain, dating back several billion years. At the margin at the left, half of a 19-kilometer-diameter (12-mile) crater is visible. The dark and bright lines running from lower right to upper left and from top to bottom are deep furrows in the ancient crust of dirty water ice. The origin of the dark material is unknown, but it may be accumulated dark fragments from many meteorites that hit Ganymede. In this view, north is to the top, and the sun illuminates the surface from the lower left about 58 degrees above the horizon. The area shown is part of Ganymede's Galileo Regio region at latitude 18 degrees north, longitude 147 degrees west; it is about 46 by 64 kilometers (29 by 38 miles) in extent. Resolution is about 80 meters (262 feet) per pixel. The image was taken June 27 at a range of 7.563 kilometers (4,700 miles).

Detail of the furrowed region of Galileo Regio
View of the Galileo Regio region on Ganymede showing fine details of the Galileo image fit into the larger scale, but much lower resolution view of the region taken 17 years earlier by Voyager. The broad curved furrow patterns are characteristic of the darker regions of this moon. North is to the top of the picture and the sun illuminates the surface from almost overhead in the Galileo picture.

Detail of Uruk Sulcus
View of a portion of the Uruk Sulcus region on Ganymede showing how the fine details of the grooved terrain that are the principal features in the brighter regions of this satellite relate to the global view of the moon. North is to the top of the inset picture and the sun illumination is almost overhead. The global view is a Voyager picture taken in 1979.

Detail and context of Galileo Uruk Sulcus pictures
View of the region of Ganymede's Uruk Sulcus placed on a lower resolution Voyager view taken 17 years earlier. North is to the top of the picture and the sun illuminates the surface from almost overhead in the Galileo view. The finest details that can discerned in the Galileo picture are about 80 meters across. The four boxes outlined in white show the extent of Galileo's initial look at this area.

High Resolution detail of Galileo Regio
This picture shows a segment of a Galileo image of the Galileo Regio region on Jupiter's moon Ganymede. Features in the curved furrows that characterize this region can be seen even with very high sun illumination. North is to the top of the picture. The finest details that can discerned are about 80 meters across.

Nippur Sulcus Region on Ganymede
New terrain overlays older terrain, which overlays still older surface, in this view of part of the surface of Jupiter's moon Ganymede, taken by the camera onboard NASA's Galileo spacecraft. Galileo obtained the images that make up this mosaic when it flew past Jupiter's moon Ganymede for the second time on September 6, 1996. An area about 54 kilometers (33 miles) wide and 90 kilometers (55 miles) high is shown. Northern Marius Regio (the dark terrain at bottom), Philus Sulcus (bright terrain at center), and Nippur Sulcus (bright terrain at top) are seen illuminated by the Sun from the southeast (north is at the top).

The key characteristics and relationships of the major terrain types on tectonically active Ganymede are seen at a resolution 16 times better than images taken by the Voyager spacecraft in 1979. At the bottom, the ancient dark terrain is seen to be very deformed by tectonic fractures and faults. An impact crater about 18 kilometers (about 11 miles) in diameter has been highly modified by faulting. More recent cross-cutting fractures and faults at center illustrate to scientists the sequence of events that have created the younger bright terrain. The lines in the middle left of the image are faults that are cross-cut by younger faults in the upper part of the image. The smooth band in the upper middle of the image may represent water-ice volcanic deposits flooding a fault valley. Clusters of small craters, representing ejecta transported from distant craters and re-impacting here, are seen in the middle of the photo. The images that make up this mosaic were taken at a range of about 11,620 kilometers (about 7,200 miles).

Jupiter Ganymede

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