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Callisto is far enough distant from Jupiter to have negligible tidal
heating. Its icy crust is also thick, and is everywhere a mass of craters
that likely date to near the beginning of the solar system. It seems
never to have been heated enough for its rocky parts to differentiate
properly into core and mantle, nor has there been anything like the
surface tectonics shown by its sister moons. Nevertheless, even here
there are indications of an ocean, over 100 kilometres down, kept liq-
uid by the small amounts of radioactive heat generated by the rocky
material lying deeper still.
The Oceans of Saturn's Moons
Stretching over 280,000 kilometres of space and up to a kilometre
thick, the rings of Saturn are made mostly of water ice, some frag-
ments being as big as terrestrial icebergs. Hypotheses about the ori-
gin of Saturn's rings range from the demise of an ancient moon or
moons blown apart at the time of the Late Heavy Bombardment, 4 bil-
lion years ago, to a much more recent origin. Like those of Jupiter,
most of Saturn's 62 moons are rich in water, although in the form of
ice and not as oceans. But there are exceptions to this, even in these
distant regions. The NASA spacecraft
Cassini
has shown that active,
ocean-bearing planetary bodies can exist even 1,430,000 million kilo-
metres or so from the warmth of the Sun.
To have Giovanni Domenico Cassini's name attached to this mis-
sion was apt—unavoidable, really, given that he discovered four of
Saturn's moons, although really this was just a grace note in the life
of one of Europe's most outrageously talented and effective scientists.
His wit and social skills rivalled his scientific achievements. At the age
of 25, in 1650, Cassini was made professor of astronomy at the Univer-
sity of Bologna, Italy, by Pope Alexander VII and, to the chagrin of his
elder colleagues, was immediately paid the highest salary there. His
talents were sought to attend to delicate matters of state as well as
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