GANYMEDE, Moon of Jupiter

All about Ganymede

Ganymede was first discovered by Galileo in 1610, making it one of the Galilean Satellites .  Of the 17 moons it is the 7th closest to Jupiter, orbiting at a distance of 670,900 km. It is the largest moon in the solar system, much larger than the Earth's moon, with a diameter that is about 5262 km (3270 miles). If Ganymede orbited the Sun instead of Jupiter it could be classified as a planet. It is larger than Mercury and Pluto, and three-quarters the size of Mars. Ganymede's day is just over 7 Earth days long, the same time it takes to orbit Jupiter once.

Ganymede is named after Jupiter's favourite cup bearer , from Greek/Roman mythology. Ganymede contains a high amount of proportion of water ice. Its main characteristic is the grooved terrain on its cratered surface .

Facts about Ganymede

Discovered by Simon Marius & Galileo Galilei 

Date of discovery 1610 

Mass (kg) 1.48e+23 

Equatorial radius (km) 2,631 

Mean distance from Jupiter (km) 1,070,000 

Rotational period (days) 7.154553 

Orbital period (days) 7.154553 

Mean orbital velocity (km/sec) 10.88 

Orbital inclination (degrees) 0.195 

The surface of Ganymede

The surface of Ganymede has two distinctly different types of marking.  The first is that of crater zones and the second is the striations or line markings.


Ganymede seems to have had a complex geological history.  It has mountains, valleys, craters and lava flows.   It is heavily cratered especially in the dark regions which implies an ancient history. From eons of impact damage.  We modelled this in the laboratory using an impact experiment *.

  * experiment 1

  Ammonia-water lavas may have extruded from the mantle and subsequently formed some of these features. The dark regions have furrows, which are probably related to ancient impacts. In some cases, lighter material seems to have oozed up through the centre of these fractures.  Smaller fractures also cross the surface in many areas. Also, the floors of some craters appear to be partially filled with possible volcanic deposits.  The dark terrain areas was a primary target for Galileo observations in 1996 and 1997.

Galileo Image (June 1996)

Unlike the Moon, however, the craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and

Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow over geologic time and thereby wear down the ridges.  The ancient craters have since been eroded leaving only a "ghost" of a crater known as palimpsests . 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.


The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters occur over many of the grooves, indicating that the grooves are also quite ancient.  The bright regions show a different kind of landscape - one that is grooved with ridges and troughs. These features form complex patterns and some ridges have a height of a few hundred meters and run for thousands of kilometres. The grooved features were apparently formed more recently than the dark cratered areas perhaps by tension from global tectonic processes as here on Earth.  The real reason is unknown;

Galileo Image (June 1996)

however, local crust spreading does appear to have taken place causing the crust to shear and separate.  Some grooves suggest recent geological activity. The most noticeable feature is the Galileo ridge, some 4000km across.  The term "sulcus," meaning a groove or burrow, is often used to describe the grooved features. The grooved terrain is probably formed by tectonic 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 kilometres across Ganymede's surface.  Grooves are typically 5 to 10 kilometres across, and in some areas seems to be parallel blocks of crustal material separated by faults.  Similar structures can be found here on Earth in areas such as Utah , USA . Grooves are common on light terrain and were due to be investigated by the Galileo spacecraft.  We became very interested in the origin of these line marking on Ganymede and attempted to show similar markings in our laboratory experiments.*

  * experiments 2, 3 and 4

Polar Regions of Ganymede

At both the north and south polar regions relatively bright polar areas occur, probably consisting of water frosts (at top and bottom). While most of Ganymede is relatively brownish in colour, possibly due to the contamination of the icy surface by meteoritic material, these polar frosts are lighter in colour.


The interior of Ganymede

Like Callisto , Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice. Its low density of 1.94 gm/cm 3 , indicates that the core takes up about 50% of the satellite's diameter. Ganymede's mantle is most likely composed of ice and silicates, and its crust is probably a thick layer of water ice.

Voyager images were used to create a global view of Ganymede. 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 activity of the surface in the past. Based on geochemical and geophysical models, scientists expect Ganymede's interior to either consist of: a) a mixture of rock and ice or b) a 'core' of rock, possibly iron under a deep layer of warm soft ice capped by a thin cold rigid ice crust.   More recent information from the first two flybys by Galileo shows detection of 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.

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 centre 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. (Copyright 1999 by Calvin J. Hamilton)

The atmosphere of Ganymede

Ganymede has no known atmosphere, but recently the Hubble Space Telescope detected ozone at its surface. The amount of ozone is small compared to Earth. It is produced as charged particles trapped in Jupiter's magnetic field rain down onto the surface of Ganymede. As the charged particles penetrate the icy surface, particles of water are disrupted leading to ozone production. This chemical process hints that Ganymede probably has a thin oxygen atmosphere like that detected on Europa .  Hubble found ozone's spectral fingerprint during observations of Ganymede made by Keith Noll and colleagues at the Space Telescope Science Institute, Baltimore , Maryland . Unlike ozone production in Earth's atmosphere, Ganymede's ozone is produced by charged particles trapped in Jupiter's powerful magnetic field (much like the Earth's Van Allen radiation belts). Jupiter's 9-hour, 59-minute rotation sweeps these particles along at tremendous speed, where they overtake the slower moving Ganymede and rain down onto the surface. The charged particles penetrate the ice surface where they disrupt water molecules, but the exact steps leading to ozone production are not yet fully understood, according to Noll.

Ganymede is much colder than Earth, with these daytime temperatures ranging across the surface from 90 to 160 Kelvin (or -297 to -171 degrees Fahrenheit). Jupiter and its moons receive less than 1/30th the amount of sunlight that the Earth does, and Ganymede has not enough atmosphere to trap heat.

Comparing Ganymede with another Jovian Moon, Callisto


Before the Galileo spacecraft enlightened us with information and new images of Ganymede, it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice surface. The dark regions of Ganymede are similar to the surface of Callisto.  Data now indicates that Callisto has a more uniform composition while Ganymede comprises a three-layer structure: a small molten iron or iron/sulphur core surrounded by a rocky silicate mantle with an icy shell on top.   Ganymede may be more akin to Io but with an additional outer layer of ice.  Unlike Europa and Io, Ganymede is not undergoing tidal heating today. Whereas Ganymede seems to have differentiated layers reminiscent of Earth, there seems to be little internal structure to Callisto, although Galileo data indicates an increase in rock density towards the centre.  Unlike Ganymede, with its complex ridges and valleys, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history.  The history of tectonic activity on Ganymede may be related to its orbital and tidal evolution. Callisto may represent what the other Galilean moons were like early in their history.


Surface features

Although Ganymede has its fair share of craters, it most unusual feature is the myriad of lines crossing its surface.  Callisto's surface is covered entirely with craters. The surface is very old. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.

Huge areas of relatively dark, heavily cratered landscape cover both Callisto and Ganymede.  Numerous parallel troughs (furrows) could be part of a vast impact structure.   Like Ganymede, Callisto's ancient craters have collapsed. They no longer show the high ring mountains, radial rays and central hollows common to craters on the Moon .


Whereas Ganymede may have a very thin atmosphere of ozone, Callisto has a very thin atmosphere composed of carbon dioxide.

Our laboratory experiments.

Experiment 1  Forming impact craters

Having studied the crater-like markings on photographs of Ganymede we decided to conduct our own experiment to see if impact markings from objects such as ‘meteorites’ would leave these types of mark.  We collected limestone nodules from Salisbury Plain to use for this experiment.

Our first findings were that the impact zones were similar to those seen on Ganymede, with the crater and the outer rim ridge.  As we increased the height from which the impact nodules were dropped we found that the debris ejected from the impact zone went further and further.  We took measurements of the nodules, the craters formed and the distance of ejected material from the impact zone.  These results showed us that:

a)      crater width was proportional to the nodule size

b)     crater depth increased with height from which the nodule was dropped

c)      distance of ejected material increased with both height of drop and size of nodule. 

We compared pictures of impact zones on Ganymede with those found on out own Moon., both being approximately  3 – 3.5 billion years old.  We noticed that, unlike out Moon, many of the impact craters have faded or been eroded.  We think that this erosion could partly be due to surface tension and tectonic activity.  We also think that, as Ganymede is much larger than our own Moon it has a large enough gravitational field to attract and hold particles in some type of atmosphere.  Such an  atmosphere could be responsible for some of the erosion seen on Ganymede.

Experiment 2

Mantle movement and surface cracking

Having looked at that crater zones we also tried to discover why Ganymede has striations or line markings.  We wondered if this could be due to tectonic movement, with a moving, swelling mantle causing the crust to crack, as had been suggested by our research.  We used tomatoes to test this theory.  By heating the tomatoes in boiling water the soft liquid centre expanded, cracking the thin, rigid skin.  We found that this produced quite straight cracks, starting at a tiny point and rapidly running in a straight line as the softer, more liquid material pushed outwards. 

Experiment 3

Molten cooling to form troughs and ridges

In this experiment we tried to find out the mechanism behind the formation of lines on Ganymede.  We began looking at ‘magma’ cooling by melting wax and pouring it onto an iced tray of aluminium foil.  This experiment showed two things.  Firstly, that as the wax cooled it formed rivulets and tiny ridges due to cracking.  We stained some of these cracked areas blue to show them more easily.  Secondly we found that, as the wax cooled and contracted so the aluminium foil was distorted and put under severe strain, almost to the point of tearing.   Although this experiment resulted in both tension in the existing surface material (the aluminium foil) and cracking of the new surface material (the wax) we did not yet have the straight lines we were looking for.

Experiment 4

In our final experiment we looked at the lines on Ganymede which interlocked or crossed over and wondered how they might have been formed.  Rather than just crustal cracking due to mantle activity, we tried to look at the results following tectonic movement – the flowing and subsequent cooling of magma.  We had watched a video of pillow lava here on Earth, showing magma cooling but this did not produce the straight-line effects we were looking for.  We concluded that it could be because the water surrounding the active vents was subject to considerable heating giving the magma time to ‘ooze’ before setting into rounded shapes.   We decided to use boiling sugar, which we previously melted in a pan.  This was quickly transferred to icy water.  We used very cold water to simulate the cold surface temperatures on Ganymede.  As the boiling sugar cooled quickly under the water it cracked and almost instantly formed long, interwoven,  crystal like structures,  showing the long, straight lines we had hoped to see.



Our conclusions

We have researched Ganymede and carried out several experiments.  We have formed some ideas about Ganymede but we await further information from the spacecraft of the future to extend and develop out ideas.

We think that, as Ganymede’s unusual surface features comprise craters and furrows or striations:

·         That these craters are due to impacts of objects from space (particularly those drawn in by the huge gravitational field of Jupiter).

·         That the furrows could be the result of several things:

a)      tectonic movement due to magma movement and crustal tension

b)     Glacial movement such as has been carved out on Earth

c)      Rapid magma cooling in the surface temperatures later eroded by the thin atmosphere and ‘winds’ of Jupiter.

·         That the furrow features could have been a result of countless comet impacts which have been torn apart by the gravitational forces of Jupiter, such as observed in the Shoemaker-Levy 9 comet.

·         That the mechanism, which drives the tectonic movement, could be related to gravitational forces, as is believed to exist on Io.  The orbit of Ganymede is further away from Jupiter than is Io, it is also much larger.  This may reduce the gravitational-stress effects probable on Io resulting in active volcanoes.  Ganymede is much closer to Jupiter than Callisto, its sister moon, where little tectonic activity is evident.  Perhaps this ‘halfway house’ position has given Ganymede its own identity.

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