Solar System -
Jupiter's Moons
Ganymede
Ganymede is the largest moon in the Solar System. It orbits Jupiter once every 7 days, 3 hours and 43 minutes in a low eccentricity orbit of 1,070,400 km radius. It is 5,268 km (3,270 miles) in diameter, which is about 8% more than the planet Mercury, though it has less than half the mass. It is in a 1:2 orbital resonance with Europa, and a 1:4 resonance with Io.
The moon comprises roughly equal amounts of silicate rock and water ice, and was initially thought to have a rocky core. Galileo found that Ganymede has a magnetic field, so it must have an iron-iron sulfide, liquid core. The moon is fully differentiated, with three main layers:
The moon comprises roughly equal amounts of silicate rock and water ice, and was initially thought to have a rocky core. Galileo found that Ganymede has a magnetic field, so it must have an iron-iron sulfide, liquid core. The moon is fully differentiated, with three main layers:
A sphere of metallic iron at the center (the core),
a spherical shell of rock (the mantle) surrounding the core
a spherical shell of mostly ice, mixed with some rock, surrounding the mantle. While rigid on the surface, this layer would soften further in.
a spherical shell of rock (the mantle) surrounding the core
a spherical shell of mostly ice, mixed with some rock, surrounding the mantle. While rigid on the surface, this layer would soften further in.
The ice shell is up to 800 kilometers (497 miles) thick, and continues to the surface. Like Europa and Callisto, it may have a liquid ocean some 200 km below its surface, sandwiched between ice layers. Its surface is old, dating back to early in the creation of the Solar System. One half comprises very old, dark, and highly cratered dark regions, while the other half is a lighter and little younger with extensive grooving and ridging due to tectonic activity in the distant past. Like other icy moons, the edges of craters become softened as the surface flows over geological time. Palimpsests, in the astronomical rather than literary sense, are very old craters that have almost entirely disappeared due to viscous relaxation leaving only a vague outline. This gives us clues about the past structure of the moon.
Observations using the Hubble Space Telescope found evidence of thin oxygen atmosphere, though it is far too thin to support life as we know it. It probably results from radiation breaking water down into hydrogen and oxygen, with the hydrogen escaping much more quickly due to its lower atomic weight.
Ganymede probably formed in the accretion disk that surrounded Jupiter in the early days of the Solar System. It appears to have formed fairly rapidly, in perhaps 10,000 years. Much faster than, for example, Callisto.
Observations using the Hubble Space Telescope found evidence of thin oxygen atmosphere, though it is far too thin to support life as we know it. It probably results from radiation breaking water down into hydrogen and oxygen, with the hydrogen escaping much more quickly due to its lower atomic weight.
Ganymede probably formed in the accretion disk that surrounded Jupiter in the early days of the Solar System. It appears to have formed fairly rapidly, in perhaps 10,000 years. Much faster than, for example, Callisto.
The lighter areas represent younger surface features caused by tectonics, while the darker areas are older, and show heavy cratering. The bright spots are geologically recent impact craters and their ejecta. Taken by the Galileo probe on June 26, 1996. Credit: NASA/JPL
The two large, dark areas towards the top of the image are Perrine (upper) and Nicholson (lower). The two very bright craters with "rays" emanating are Cisti on the lower left, and Tros on the upper right.
Credit: National Oceanic and
Atmospheric Administration
Credit: National Oceanic and
Atmospheric Administration
This image, from the Galileo probe, has been color-enhanced. The crater with "rays" on the upper right is Hershef, and the one at the lower right is Tashmetum.
This schematic shows Ganymede's four layers.
• a dense iron/iron-sulfide core at the center.
• a mantle of rock around the core
• a deep layer of warm soft ice
• a thin cold rigid ice crust.
• a dense iron/iron-sulfide core at the center.
• a mantle of rock around the core
• a deep layer of warm soft ice
• a thin cold rigid ice crust.
The dark area is the crater, Khensu. It is about 13 km (8 miles) in diameter. It has an unusually dark floor and a bright area around it that is material that has been thrown out. The dark area is either debris from the object that formed the crater, or the object may have passed through the icy surface to reveal a dark layer beneath. El. A much large crater 54 km (34 miles) across, can just be seen to the top-right of the image. Galileo took the image on September 6, 1996.
The larger crater is Nergal, which is about eight km (five miles) in diameter. It is in an area of bright grooved terrain called Byblus Sulcus.
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
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Jupiter's moons are grouped as follows. Select to see details of the moon or the group of moons: