The four moons that Galileo saw when he pointed his telescope at Jupiter
are now known as "the Galilean satellites." They are Jupiter's four largest
moons, all easily seen with even a small telescope. These moons have now
been visited by several NASA space probes which have revealed them to
be fascinating worlds. In order of distance from Jupiter, they are:
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Io:
the closest to Jupiter, Io is a moon caught in a constant tug-of-war
between the giant planet and the other Galilean satellites. As a result,
Jupiter's gravity produces a constant squeezing of Io, heating its
interior. Io is the only body in the solar system other than the Earth
on which active volcanos exist. These volcanos spew sulfur and sulfur
dioxide, giving the moon a colorful surface of reds, oranges, and white.
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Europa:
Next furthest out is Europa. Being further from Jupiter, Europa
is not as heated by Jupiter's pull as Io, and its surface is covered in
ice layer a few kilometers thick. However, below that ice is thought to
be a water ocean a few hundred kilometers deep. The motion of the ice layer
on top of this ocean results in deep fractures in the surface ice which
show up as lines crossing the surface.
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Ganymede:
Ganymede is the largest of the Galilean satellites, larger
than our own Moon and even larger than the planet Mercury. Many impact
craters can be seen on Ganymede, a sign that, unlike Io and Europa,
Ganymede's surface is not changing much. However, the surface does show
grooves and cracks, suggesting that at one point in its history it
was more active.
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Callisto:
The most distant Galilean moon, Callisto is covered with
impact craters. Because it is so far from Jupiter, Jupiter's gravitational
pull does very little to heat Callisto, making it essentially a solid,
unchanging ball of ice. The many old craters and lack of cracks or grooves
on its surface indicate that very little has changed on the surface of
Callisto since it formed.
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