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Ganymede Fact Sheet
What Is Known About Ganymede SO FAR
Galileo Image of Ganymede (June 1996)
Table of Contents
Ganymede Summary
The Galilean Moons
Ganymede, Callisto, Io and Europa
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 1 Image (1979)
Galileo Image (June 1996)
Voyager 2 Image (1979)
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.
Galileo Image (June 1996)
Voyager 2 Image (1979)
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
The Discovery of the Galilean Satellites
Find out how Galileo Galilei discovered the moons around Jupiter.
Ganymede Images
Ganymede Animations
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Galileo Flybys
Galileo has made four close flybys of Ganymede during its
orbital tour of Jupiter.
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