Page 18 - Curriculum Visions Dynamic Book

would be the same for Saturn.
Jupiter’s moons
When a probe was sent out from Earth to look at Jupiter and its moons in the mid and late 1990s, it was appropriately named Galileo (see page 9). In 1610 Galileo Galilei was
the first person to see the moons of Jupiter, using the newly developed telescope. The moons together are called the Galilean satellites in his honor. Indeed, Galileo used the moons as another piece of evidence that the planets orbit the Sun—something that, at the time, was widely disbelieved.
Galileo saw four moons. We now call them Callisto (pages 33–34), Europa (page 29), Ganymede (pages 30–32), and
Io (pages 21–26)—names associated with Jupiter in Greek mythology. We now also know there are more than these four moons. There are 16 that can be called distinct satellites, 13 that were found by Earth observation, and three more that were discovered by Voyager.
The inner and outer moons vary in some respects. The inner eight have orbits that are nearly circular, while the outer eight have very oval orbits.
The moons of Jupiter could have developed from dust in the solar system at the same time as Jupiter did. But Jupiter had an influence on all of them. The high temperature of the growing early planet may have stopped the condensation of gases on the inner moons (including Io and Europa), and so they remained rocky. The outer moons (including Ganymede and Callisto), on the other hand, were in a cooler region of space; so water, for example, did not boil off into space.
The outermost eight satellites probably did not form at the
same time as the inner ones, but were captured by Jupiter at a
much later date.
 In this view the four Galilean satellites (moons) are shown to scale, although not in their correct orbital positions.
Io is closest to Jupiter, then come Europa, Ganymede, and Callisto.
Each satellite is amazingly different, even though they are relatively close to Jupiter (350,000 kilometres for Io; 1.8 million kilometres for Callisto). North is at the top of these images taken by NASA’s Galileo spacecraft.
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