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CHAPTER 24

Ganymede and Callisto

Geoffrey Collins

Wheaton College

Norton, Massachusetts

Torrence Johnson

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

1. Exploration 5. Tectonism and Volcanism


2. Interiors 6. Unanswered Questions and Future Exploration


3. Surface Materials Bibliography


4. Impact Craters

G

anymede and Callisto (Fig. 1) are the largest and outer-
most of Jupiter’s four Galilean satellites. Similar in size
to Mercury, and with surfaces dominated by dirty water ice,
they are prime examples of planet-sized icy bodies. Though
Ganymede and Callisto are neighbors and share many bulk
characteristics such as size and density, they have followed
divergent evolutionary paths. Their interior structure and
surface geology provide insight into which processes are
common and which are unique in the development of a
large icy world.

1. Exploration

1.1 Discovery

Ganymede and Callisto were discovered by Galileo Galilei
in 1610, when he first trained his telescope on Jupiter
and shortly thereafter published his results in theSiderius
Nuncius. Along with Io and Europa, they became the first
natural satellites, other than the Moon, known to science.
Galileo immediately recognized the significance of the “new
stars” traveling with Jupiter and changing their positions ev-
ery night. The orbits of what are now known as the Galilean
satellites were rapidly calculated and found to be essentially
circular and in the same plane as Jupiter’s equator. Because
Galileo made these observations centuries ago, his records

of satellite eclipses provide a long timeline to compare with

modern measurements, and they are still used to constrain

calculations of the dynamical evolution of Jupiter’s satel-

lite system under the influences of tidal dissipation and the

satellites’ mutual gravitational interactions.

1.2 Astronomical Observations

The Galilean satellites are large enough to exhibit distinct

discs (on the order of∼1 arc second in angular diameter)

when viewed through even moderate power telescopes, and

it was thus known from simple geometry that they must

be bodies comparable in size to the Moon. Precise mea-

surements of their sizes proved difficult with conventional

astronomical techniques, with published estimates from dif-

ferent observers disagreeing significantly. Even these rela-

tively uncertain size estimates were sufficient, when com-

bined with the satellites’ brightness, to indicate that their

surfaces are highly reflective compared with that of the

Moon.

In the two decades leading up to the first spacecraft ex-

ploration of the Jupiter system, astronomical techniques

advanced rapidly, particularly in the area of sensors in the

visible and near-infrared spectral range (∼0.3–2.5μm). The

pioneering planetary astronomer Gerard Kuiper used early

infrared detectors to show that Ganymede’s reflectance at 2

μm was much lower than in the visible range and suggested

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