Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 232

Galileo's Moons
Far beyond the orbit of Mars, the thinly spaced rock fragments of the
asteroids—in total amounting to only a few per cent of the Moon's
mass—circle the Sun. Long thought to be dry (they are just the wrong
side of the Sun's 'snow line'), ice has been detected even among these.
Perhaps one day such ice masses will provide a crucial water resource
for space-hopping metal miners of the future.
Far beyond the asteroid belt is the first of the gas giants, Jupiter,
some 778 million kilometres from the Sun. It is enormous, having a
mass more than twice that of the rest of the solar system (but only
one-thousandth that of the Sun). Here we are well beyond the 'snow
line'. Water has been detected in Jupiter's surface clouds, but only as
trace vapour within the hydrogen-helium atmosphere. Deep down,
there may well be more water (see Chapter 1), and this is one of the
things that the NASA spacecraft JUNO will be looking for when it has
a rendezvous with that planet in 2016. The question of water, though,
is rather clearer with Jupiter's moons.
These moons were discovered by Galileo in 1610, and the realiza-
tion that Jupiter was orbited by four worlds dealt a final blow to the
Ptolemaic view of an Earth-centred universe. It was Galileo's rival,
Simon Marius, who christened the four moons after illicit lovers of
Zeus—Io, Ganymede, Callisto, and Europa. More Jovian moons have
been discovered since: there are now 66 in total, although most are
small with distant, eccentric orbits, probably being captured pieces
of space debris. The Galilean ones, though, are substantial, with
Ganymede being a little larger than Mercury.
For three and a half centuries after Galileo, their nature remained
mysterious. From the 1950s on, analysis of their faint reflected light
led to suggestions that some had a cover of water ice. In 1971 the
astronomer John Lewis went further, and proposed that they might