Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 233

have sufficient inner heat sources (from radioactive decay in their
rocky cores) to produce a liquid ocean beneath icy carapaces.
A couple of years later, in 1973, the first of the exploring spacecraft,
Pioneer 10 , flew past Jupiter and began to send images of that planet
and its moons. More spacecraft followed—the Voyager , Ulysses , Cassini ,
and Galileo missions. They ignited a revolution in understanding that
continues apace today. The sheer diversity of the moons of Jupiter,
and of those of more distant planets, came as a tremendous—and
exhilarating—surprise. Each one seems to have its own, quite distinct,
identity. And among them there are true oceans.
Appropriately enough, it was NASA's Galileo spacecraft, which
took off into space on 18 October 1989, that was to gather some of the
key data. It was to fly over 4.5 billion kilometres before plunging into
the atmosphere of Jupiter on 21 September 2003. On its journey it was
faithful to its namesake, discovering new oceans beneath the icy shells
of Ganymede, Callisto, and Europa. 144 Only Io, the innermost of the
Galilean moons, has no water; any water would long ago have evapo-
rated into space due to the moon's proximity to a hot young Jupiter.
Instead, it is the most volcanically active body in the solar system, its
interior being continually churned and melted by the powerful tidal
forces exerted by its parent planet.
Of the watery Jovian moons, Europa is in many ways the most
curious—and the most promising for those seeking life in alien
oceans. Looked at as a whole, its surface is smooth, bright, icy—and
geologically new, for few meteorite or comet craters mar its overall
evenness. It formed—or was renewed—only some 50 to 100 million
years ago, and therefore dates from the time of Earth's dinosaurs.
Looked at more closely, it is criss-crossed by networks of dark lines
with, here and there, patches of jumbled 'chaotic terrain' that look
like masses of angular icebergs encased within sea ice (see Plate 7).
The surface is clearly a dynamic one 145 which has undergone a long