Jupiter: Moons: Ganymede
Ganymede [GAN-ee-meed] is the largest moon of Jupiter and is the largest in our solar
system with a diameter of 5,262 km (3,280 miles). If Ganymede orbited the Sun instead of
Jupiter it could be classified as a planet. 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
, 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.
Ganymede has no known atmosphere, but recently the Hubble Space Telescope detected
ozone at its surface. The amount of ozone is small as 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 tenuous oxygen atmosphere like that detected on Europa.
Ganymede has had a complex geological histroy. It has mountains, valleys, craters and
lava flows. Ganymede is mottled by both light and dark regions. It is heavily cratered,
especially in the dark regions, implying ancient origin. The bright regions show a different
kind of terrain - one which is grooved with ridges and troughs. These features form complex
patterns, have a vertical relief of a few hundred meters, and run for thousands of kilometers.
The grooved features were apparently formed more recently than the dark cratered area,
perhaps by tension from global tectonic processes. The real reason is unknown; however,
local crust spreading does appear to have taken place, causing the crust to shear and
Copyright © 1997-1999 by Calvin J. Hamilton.