Ganymede and Callisto 451
FIGURE 2 Compilation of the best telescopic spectra of
Ganymede (green) and Callisto (blue) compared with Io (red)
and Europa (yellow). (Modified from R. Clark and T. McCord,
Icarus, v. 41, pp. 323–339, 1980).
still uncertain size estimates, the best estimates of masses
prior to 1970 suggested that the inner satellites, Io and
Europa, had rock-like densities, similar to the Moon’s, and
that Ganymede and Callisto appeared to be less dense, sug-
gesting the possible presence of large amounts of ice in their
constituent materials.
In 1972, observations of a stellar occultation by
Ganymede from two stations on the Earth provided the
first high-precision measurement of its diameter. This was
closely followed by the first spacecraft exploration of Jupiter
by thePioneer 10and 11 missions in 1973 and 1974, which
greatly improved the mass estimates of the satellites from
tracking the gravitational perturbations in the spacecraft
trajectories caused by the satellites. This led to the first
accurate determination of Ganymede’s density of about
1900 kg/m^3 , adding more evidence to the hypothesis that its
bulk composition is a mixture of rock and ice. TheVoyager 1
and 2 Jupiter encounters in 1979 provided even more data
on the satellite’s masses and accurate determination of their
shapes and volumes. These data showed Callisto is very sim-
ilar to Ganymede in its bulk properties, with a density of
about 1800 kg/m^3. Interior structure models, taking into ac-
count the high-pressure behavior of water ice, show that the
average bulk composition for both satellites is a mixture of
50–60% (by mass) anhydrous silicate “rock” with water ice.
1.3 Spacecraft Exploration
Seven spacecraft have visited the Jupiter system to date:
Pioneer 10and 11 ,Voyager 1and 2 ,Galileo,Ulysses, and
Cassini.Ulysses, a joint European Space Agency/NASA
mission to study the Sun’s environment at high latitudes,
made measurements of Jupiter’s magnetic fields, radiation
belts, and dust environment but did not study the satellites
directly.Cassini, on its way to its rendezvous with Saturn,
flew by Jupiter in 2000 and returned spectacular observa-
tions of its atmosphere andmagnetosphere,but its trajec-
tory was too far from the Galilean satellite system to provide
high-resolution views of the satellites.
The first Jupiter missions,Pioneer 10and 11 , were de-
signed to provide the first reconnaissance of the system and
to establish the intensity of the radiation belts. ThePioneer
program’s major contribution to knowledge of Ganymede
and Callisto, as mentioned earlier, was improving the mass
estimates of the satellites, leading to the first precision bulk
density measurements.
In 1979,Voyager 1and 2 , with powerful remote sensing
payloads and close targeted flybys of each Galilean satel-
lite, provided the first in-depth reconnaissance of the satel-
lites and set the stage for the geological and geophysical
exploration of these worlds.Voyager’s cameras showed that
Ganymede and Callisto, alike in many large-scale proper-
ties, have divergent geological histories (Fig. 1). Callisto’s
surface is heavily cratered at all scales, from large impact
scars over a 1000 km in diameter down to craters a few kilo-
meters in diameter, the smallest scale resolvable on Callisto
by theVoyagercameras. This battered, uniform surface
stands in stark contrast to Ganymede’s varied landscape.
Ganymede’s surface can be divided into two distinct types
of terrain, based on a sharp albedo contrast. The darker
areas (named “dark terrain”) are heavily cratered and ex-
hibitpalimpsests,much like the surface of Callisto. The
brighter parts of Ganymede’s surface (named “bright ter-
rain”) form wide lanes through the dark terrain and are
less heavily cratered, implying a younger surface.Voyager
images showed the bright terrain to have some areas that
appeared to be smooth, while other areas exhibit sets of
parallel ridges and troughs.
One of the major objectives of theGalileomission was to
perform detailed observations of the big satellites. The mis-
sion design allowed multiple close flybys at ranges 100–1000
times closer than theVoyagerencounters, enabling high-
resolution studies of their surfaces and detailed measure-
ments of their gravity fields and interactions with Jupiter’s
magnetospheric environment. High-resolution images of
the different terrains first identified byVoyagerhave il-
luminated their origins, described in detail in subsequent
sections of this chapter. The close flybys also enabled more
detailed spectroscopic observations, which identified some
of the non-water-ice components on the satellite surfaces,
including carbon dioxide embedded in the surface and ev-
idence for carbon compounds.
Repeated close flybys enabledGalileoto make preci-
sion gravity and magnetic measurements, resulting in sev-
eral major discoveries. First, Ganymede has a strongly
layered internal structure, with heavier rock and metal