Ganymede: the Giant Moon


Ganymede, larger than the planet Mercury, is the largest Jovian satellite and the largest moon in the solar system. Its distinctive surface is characterized by patches of dark and light terrain. Bright frost is visible at the north and south poles. The very bright icy impact crater, Tros, is near the center of the image in a region known as Phrygia Sulcus. The dark area to the northwest of Tros is Perrine Regio; the dark terrain to the south and southeast is Nicholson Regio.

Ganymede's surface is characterized by a high degree of crustal deformation. Much of the surface is covered by water ice, with a higher amount of rocky material in the darker areas. This global view was taken when Galileo was 1.68 million kilometers from Ganymede; the finest details that can be discerned are about 67 kilometers across.

Ganymede is the largest satellite in the solar system with a diameter of 5,268 km (3270 miles). It is larger than Mercury and Pluto, and three-quarters the size of Mars. If Ganymede orbited the Sun instead of orbiting Jupiter, it would easily be classified as a planet.

Model of Ganymede's Interior

Since Ganymede has a low density of 1.94 grams/cubic centimeter (water's density = 1.00), it was originally estimated that the satellite is half water ice with a rocky core extending to half of the satellite's radius. However, the first two flybys by Galileo had detected a magnetic field around Ganymede, which strongly indicates that the satellite has metallic core about 250 to 800 miles in. The mantle is composed of ice and silicates and a crust which is probably a thick layer of water ice.

Voyager images of Ganymede have shown that the satellite has a complex geological history. Ganymede's surface is a mixture of two types of terrain. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain which forms intricate patterns across Ganymede. The term "sulcus," meaning a groove or burrow, is often used to describe the grooved features. This grooved terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 meters (2000 feet) have been observed in the Voyager imagery and the grooves run for thousands of kilometers across Ganymede's surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark, cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike the Moon). The largest area on Ganymede is called Galileo Regio, and is one of the areas targeted by the Galileo spacecraft.

The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to slow and gradual adjustment to the soft icy surface. These large "phantom craters" are called palimpsests, a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing. Palimpsests range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede's craters - rays tend to be bright from craters in the grooved terrain and dark from the dark cratered terrain.

Recently, the Hubble Space Telescope detected a thin oxygen atmosphere on Ganymede.

Ganymede Quick-Look Statistics

Discovery: Jan 11, 1610 by Galileo Galilei
Diameter (km): 5,268
Mass (kg): 1.48e23 kg
Mass (Earth = 1) 0.0247
Surface Gravity (Earth = 1): 0.145
Mean Distance from Jupiter (km): 1,070,000
Mean Distance From Jupiter (Rj): 15.1
Mean Distance from Sun (AU): 5.203
Orbital period (days): 7.154553
Rotational period (days): 7.154553
Density (gm/cm?3) 1.94
Orbit Eccentricity: 0.002
Orbit Inclination (degrees): 0.183
Orbit Speed (km/sec): 10.9
Escape velocity (km/sec): 2.74
Visual Albedo: 0.43
Subsolar Temperature (K): 156
Equatorial Subsurface Temperature (K): 117
Surface Composition: Dirty Ice

Crater & spectr

The top figure is an image of the crater Melkar on Ganymede, at a wavelength of 0.85 microns, taken by the Near Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft, The crater is illuminated by the Sun from the left. The finest detail that can be seen is approximately 30 km in size. What is most obvious, and of great interest, are the two concentric ring structures and the central dome. The walls of these rings are in shadow on the left, and are in sunlight on the right. To understand how these rings and central dome are thought to form, consider a pebble dropped into a pond. Ripples spread out from the center, oscillating up and down. The rings and dome forming Melkar are a snapshot of these ripples in the ice of Ganymede, possibly caused by the impact of a comet or asteroid. Similar features on the Moon are only associated with much larger craters as the stronger Moon rock behaves this way only with large impacts. NIMS can obtain images at many different wavelengths from 0.7 to 5.2 microns.

The spectrum shows the amount of reflected light as a function of wavelength from the crater floor of Melkar. Several distinct absorption features, caused by water ice, are evident at 1.5 and 2.0 microns. Beyond 3.0 microns the intensity increases again as the longer wavelengths are more sensitive to Ganymede's thermal radiation. The shape of the absorption features suggest that the ice is mixed with hydrated minerals. These relatively dark minerals probably cause the variations in ice brightness seen at visible wavelengths.

Impact craters

The number of impact craters seen in this image of Ganymede testify to the terrain'sgreat age, dating back several billion years. The image's left edge slices through a 19-kilometer-diameter (12-mile) crater. The dark and bright lines running from lower right to upper left and from top to bottom are deep furrows in the ancient crustof dirty water ice.

New over old

New terrain overlays older terrain, which overlays still older surface, in this view of part of the surface of Jupiter's moon Ganymede. The key characteristics and relationships of the major terrain types on tectonically activeGanymede are seen at a resolution 16 times better than images taken by the Voyager spacecraft in 1979.

Ridges and troughs

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, which in turn is shown within the context of a full-disk image of Ganymede.The image shows details of parallel ridges and troughs that are the principal features in the brighter regions of Ganymede.

Science objectives

Ganymede Objectives

1. Characterize any volcanism
2. Determine the nature and timing
of any tectonic activity
3. Determine the history of formation and degradation
of impact craters
4. Determine the nature of the surface materials

Stereo view

New topographic detail is seen in a stereoscopic view of this part of Ganymede. The picture is a computer reconstruction from two Galileo images. The topographic nature of the deep furrows (the trough depth is one kilometer) and impact craters that cover this portion of Ganymede is apparent. The blue-sky above the horizon is artificial.

Water and minerals

Galileo infrared observations are used to examine Ganymede's surface composition.The central false-colorimage shows the distribution of water ice on Ganymede's surface (brighter areas have more ice). This image shows that water ice is relatively depleted in the regions that appear visually dark in the corresponding Voyager mosaic on the left..The texture differences implied by this false color map in three wavelengths may suggest that water migrates from the equator to the poles.

  • 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 red, and the size of ice grains in shades of blue.

Electric field spectrogram

This electric field spectrogram shows the very strong interaction between Ganymede and the Jovian magnetosphere. The wealth and diversity of the wave signatures shown here provide evidence of a small magnetosphere surrounding Ganymede. The band of noise labeled fUH is at the upper hybrid resonance frequency and can be used to determine a plasma density of approximately 100 particles per cubic centimeter. The broadband bursts at the beginning and end of the interaction period are typical of the plasma wave signature for a magnetopause, or boundary of a magnetosphere. The banded emissions after closest approach are electron cyclotron harmonic emissions which are known at Earth to contribute to the generation of the aurora. The bright, broadband emission centered on closest approach and the emissions identified as "chorus" in the spectrogram are called whistler-mode emissions. The maximum frequency of these emissions enable the determination of a maximum in the Ganymede magnetic field traversed by Galileo of about 400 nanoTesla. The narrowband radio emissions extending primary to the right of the Ganymede interaction in the spectrogram are the first known radio emissions from a planetary satellite; these are similar to radio emissions studied at Earth and the outer planets, including Jupiter.

Geological mystery

This image is centered on an unusual semicircular structure about 33 kilometers (20 miles) across. A 38 kilometer (24 mile) long, remarkably linear feature cuts across its northern extent, and a wide east-west fault system marks its southern boundary. The origin of these features is the subject of much debate among scientists analyzing the data. Was the arcuate structure part of a larger feature? Is the straight lineament the result of internal or external processes? Scientists continue to study this data in order to understand the surface processes occuring on this complex satellite.


The cut-out reveals the interior structure of this icy moon. This structure consists of four layers based on measurements of Ganymede's gravity field and theoretical analyses using Ganymede's known mass, size and density. Ganymede's surface is rich in water ice and Voyager and Galileo images show features which are evidence of geological and tectonic disruption of the surface in the past. As with the Earth, these geological features reflect forces and processes deep within Ganymede's interior. Based on geochemical and geophysical models, scientists expected Ganymede's interior to either consist of: a) an undifferentiated mixture of rock and ice or b) a differentiated structure with a large lunar sized 'core' of rock and possibly iron overlain by a deep layer of warm soft ice capped by a thin cold rigid ice crust. Galileo's measurement of Ganymede's gravity field during its first and second encounters with the huge moon have basically confirmed the differentiated model and allowed scientists to estimate the size of these layers more accurately. In addition the data strongly suggest that a dense metallic core exists at the center of the rock core. This metallic core suggests a greater degree of heating at sometime in Ganymede's past than had been proposed before and may be the source of Ganymede's magnetic field discovered by Galileo's space physics experiments.

Dark floor craters

The dark-floored crater, Khensu, is the target of this image of Ganymede. The solid state imaging camera on NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. Khensu is located at 2 degrees latitude and 153 degrees longitude in a region of bright terrain known as Uruk Sulcus, and is about 13 kilometers (8 miles) in diameter. Like some other craters on Ganymede, it possesses an unusually dark floor and a bright ejecta blanket. The dark component may be residual material from the im pactor that formed the crater.Another possibility is that the impactor may have punched through the bright surface to reveal a dark layer beneath.

Another large crater named El is partly visible in the top- right corner of the image. This crater is 54 kilometers (34 miles) in diameter and has a small 'pit' in its center. Craters with such a 'central pit' are common across Ganymede and are especially intriguing since they may reveal secrets about the structure of the satellite's shallow subsurface.

Science objectives for Uruk Sulcus and Galileo Regio

Uruk Sulcus

Western Uruk Sulcus contains a confined band of light material and grooves having a moderately complex stratigraphy, bounded by Galileo Regio to the north and Marius Regio to the south. The margin of Galileo Regio is partly broken up into small deformed blocks, on which light mantling deposits have been superimposed. The lines along which breakup occurred, which have been occupied by groove lanes having a regionally dominant NE-SW trend, appear to be defined by furrows in the dark terrain. This observation will sample the bright grooved terrain of Uruk Sulcus at high resolution.

Questions we hope to answer:

  • What is the light terrain and where did it come from?
  • In Uruk Sulcus, the light material seems only to be a very thin coating on older rough dark terrain. This could be evidence of explosive eruptions predicted to accompany extrusion of ice-volcanic melts. Higher resolution images of this area will help to answer this.
  • A better understanding of the complex stratigraphy of the light terrain at high resolution, will give us insight into the history of resurfacing and deformational processes. This will help us to understand the basic geologic processes that formed the light terrain.
  • What is the origin of grooves in Ganymede's bright terrain?
  • How does groove morphology change in different terrain types?
  • Why do the groove lanes have a common orientation? On a global scale? On a regional scale?
  • What is the detailed morphology of the deformed dark blocks along the margin of Galileo Regio?
  • What are the crater morphologies like in this region?
  • How do they relate to those in other regions, and what do they tell us about craters on Ganymede?
  • What are the albedo patterns in the light terrain and what can they tell us about the geology and physical characteristics of those regions?

Galileo Regio

This area contains furrows of several orientations. In some cases the furrows cross-cut one another. There are also intervening patches of dark smooth material, which might be volcanic in origin. In the center where all the frames overlap, there is a feature that could either be a small palimpsest (an ancient impact scar) or a crater which is partly buried by material that emanated from a furrow or other source.

Questions we hope to answer:

  • What is the morphology of the furrows and what are the relative ages of features in Galileo Regio?
  • What is the geological history of the dark terrain in this location?
  • What are the albedo patterns in the dark terrain and what can they tell us about the geology and physical details of those regions?
  • What are the crater morphologies like in this region?
  • How are Ganymede craters similar to or different from craters on other planetary bodies?

High resolution pictures of Galileo Regio terrain will be analyzed with respect to relations between furrows, the region's dark materials, and the partially filled-in crater.


Fine Details

Dramatic view of fine details in ice hills and valleys in an unnamed region on Jupiter's moon Ganymede. North is to the top of the picture and the sun illuminates the surface from the left. The finest details that can discerned in this picture are only 11 meters across (similar to the size of an average house) some 2000 times better than previous images of this region. The bright areas in the left hand version are the sides of hills facing the sun; the dark areas are shadows. In the right hand version the processing has been changed to bring out details in the shadowed regions that are illuminated by the bright hillsides. The brightness of some of the hillsides is so high that the picture elements 'spill over' down the columns of the picture.


Two highly fractured craters are visible in this high resolution image of Jupiter's moon, Ganymede. NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. North is to the top of the picture and the sun illuminates the surface from the southeast. The two craters in the center of the image lie in the ancient dark terrain of Marius Regio, at 40 degrees latitude and 201 degrees longitude, at the border of a region of bright grooved terrain known as Byblus Sulcus (the eastern portion of which is visible on the left of this image). Pervasive fracturing has occurred in this area that has completely disrupted these craters and destroyed their southern and western walls. Such intense fracturing has occurred over much of Ganymede's surface and has commonly destroyed older features.


Complex tectonism is evident in these images of Ganymede's surface. The solid state imaging camera on NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. The 80 kilometer (50 mile) wide lens- shaped feature in the center of the image is located at 32 degrees latitude and 188 degrees longitude along the border of a region of ancient dark terrain known as Marius Regio, and is near an area of younger bright terrain
named Nippur Sulcus. The tectonism that created the structures in the bright terrain nearby has strongly affected the local dark terrain to form unusual structures such as the one shown here. The lens-like appearance of this feature is probably due to shearing of the surface, where areas have slid past each other and also rotated slightly. Note that in several places in these images, especially around the border of the lens-shaped feature, bright ridges appear to turn into dark grooves. Analysis of the geologic structures in areas like this are helping scientists to understand the complex tectonic history of Ganymede.


Two impact craters surrounded by an unusual ejecta blanket dominate this high resolution image of the surface of Jupiter's moon, Ganymede. NASA's Galileo spacecraft imaged this region as it passed Ganymede during its second orbit through the Jovian system. North is to the top of the picture and the sun illuminates the surface from the southeast. Nergal, the larger crater, is about eight kilometers (five miles) in diameter, while the smaller (unnamed) crater to its west is three kilometers (1.8 miles)
across. The craters are situated in a region of bright grooved terrain named Byblus Sulcus, located in the northern part of Marius Regio at 39 degrees latitude and 201 degrees longitude. The distinctive ejecta blanket that surrounds them is darker nearer the craters and brighter further away. The inner region of the ejecta is characterized by a lobate appearance indicative of the flow of a liquid (or slushy) substance over the surface. The flow was probably icy surface material melted by the energy released during the impact that formed the crater.