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Ganymede is the biggest moon of Jupiter and is also the biggest moon in the Solar system.  It has the greatest diameter as well as the greatest mass.  Ganymede even has a larger diameter than the planet Mercury, but its mass is lower than Mercury’s.

Ganymede also has a greater mass than any of the dwarf planets so far discovered.

Ganymede has a lower mass than Mercury because of its high proportion of light elements and compounds of these elements, especially water.  It is estimated that about half of Ganymede’s mass is water.


Ganymede was discovered in 1610, probably on January the 11th.  Its discovery is usually credited to Galileo Galilei (on the Right), although Simon Marius (on the left) also discovered this moon at about the same time.  Galileo had a tendency to claim all the early telescopic discoveries.


Ganymede was a young Trojan Prince.  He was the most attractive of all the mortals.  He was abducted by Zeus, the king of the Gods, in the form of an eagle, and became the cupbearer to the Gods on Mount Olympus.  The name the Romans gave to Zeus was Jupiter.  The four moons of Jupiter that Galileo discovered are all named after people that Jupiter abducted.  Simon Marius gave the name Ganymede to Jupiter's moon.

Heavy Core

Detailed measurements made by the Galileo probe suggest that Ganymede has a small, probably iron rich, core with the next layer being silicate based rock.   This is surrounded by a lighter layer consisting mainly of water ice, but with many other things in it as well.

Magnetic Field

Ganymede is the first moon to be found to have its own magnetic field.  This field is not as strong as that of the Earth, and is dwarfed by Jupiter’s magnetic field.

Liquid Ocean?

It is speculated that there is a layer of liquid salty water between a very thick icy crust and the solid core. This is still very uncertain and more measurements will be needed to see if this liquid layer exists. Data from the Galileo Probe suggests this liquid ocean does exist.

There is considerable question about whether or not there is enough heat to keep part of the water of Ganymede in a liquid form.

The tidal heat from the interactions with Jupiter and the other moons would appear to be insufficient by itself to keep some of Ganymede’s water liquid.  Other sources of heat in Ganymede are possible like the decay of radioactive elements, but the quantities of these elements are unknown.

The surface of Ganymede gets about one thirtieth of the concentration of solar energy as is received on the Earth.


Traces of Ozone (O3) have been detected and the presence of a very thin Oxygen atmosphere is inferred.

Life on Ganymede?

The only place that we are sure has life is the planet Earth.  We can only speculate about other places.  If there is liquid water in Ganymede, it would be a possible place for life.  It will probably be quite a long time before we are reasonably sure one way or the other.

Observing Ganymede

Ganymede is the only moon of our solar system, except Earth’s own moon, that can be seen from the surface of the Earth with the naked eye.  However, this is very difficult even under perfect conditions.  The main problem with seeing it is the brightness of Jupiter.

Galileo and many other astronomers have seen it through telescopes, and more recently better pictures have been taken by the Hubble Telescope.

Galileo Probe

The Galileo Probe was launched on October the 18th 1989 (my son’s birthday).  After a complicated fight path to gain velocity by fly pasts of Venus and the Earth (Twice) it arrived in the Vicinity of Jupiter; its Jupiter atmospheric probe was released on July the 13th 1995 and entered the Jovian atmosphere on December the 7th 1995.

Although the Galileo was the sixth probe to visit the vicinity of Jupiter it added enormously to our knowledge of the Jovian system as well as making many other important discoveries on its long journey.

The Magna Carta was probably not signed on Ganymede.  Historians today believe that it was actually signed by King John at Runnymede, on the flood plain of the River Thames in England in 1215.









Size comparison; The Earth on the right, Mercury upper left and Ganymede lower left.
Scientists have discovered irregular lumps beneath the icy surface of Jupiter’s largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede’s icy shell for billions of years.
This picture from NASA shows a caldera of pit.  The surface of this caldera has been interpreted as showing evidence of the flow of a thick liquid.
The Inside of Ganymede
NASA's diagram shows a metalic core surounded by rock, with a liquid ocean under an icy crust.