456 Encyclopedia of the Solar System
The latest formation models attempt to address these is-
sues in a number of ways. Current models for Jupiter’s for-
mation suggest that the jovian subnebula interacts strongly
with the surrounding solar nebula as the growing giant
planet opens a “gap” in the solar nebula and material is
continually fed from the solar nebula surroundings to the
outer parts of the subnebula. This class of models can ac-
count for longer satellite accretion times and allow them to
form without being dragged into the proto-Jupiter. Another
type of formation model proposes that the inner satellites
formed in a hot dense subnebula but avoided destruction
by opening gaps themselves in the subnebula, slowing their
orbital decay. In this type of model, Callisto forms more
slowly in a thinner outer nebula environment, accounting
for some of its differences.
A final factor that may have affected the apparently dif-
ferent histories of Ganymede and Callisto is the existence
of what is known as the Laplace resonance condition. This
is a dynamical relationship between the orbital periods of
the inner three satellites, first studied by the French math-
ematician Laplace in the 19th century. Io, Europa, and
Ganymede currently exhibit a simple numerical relation-
ship (1:2:4) in their orbital periods, causing them to perturb
each other’s orbits continually, resulting in significantly non-
circular orbits. It is the existence of these noncircular orbits
that causes tidal heating in each of these satellites, resulting
most notably in the violent volcanic activity on Io, which
has the largest dose of tidal heating due to its proximity to
Jupiter [seeIo:TheVolcanicMoon]. Callisto does not
participate in this celestial dance and apparently has never
experienced tidal heating.
Despite the Laplace resonance condition, Ganymede
does not currently experience significant tidal heating be-
cause of its distance from Jupiter and the relatively small de-
gree of noncircularity of its orbit. However, calculations of
the dynamical evolution of the satellite system suggest that
Ganymede’s orbit may have been more eccentric at times in
the past, possibly resulting in a pulse of tidal heating, which
could have triggered differentiation and/or stirred up the
core and started magnetic field generation. Even though
the question of why Ganymede and Callisto have experi-
enced such different interior and geological evolution has
not been conclusively solved, it seems likely that the key
to the solution lies in some combination of differences in
formation and accretion conditions and their subsequent
orbital evolution.
3. Surface Materials
3.1 Composition of Surfaces
As noted in the discussion of astronomical discoveries, wa-
ter ice was identified as a primary surface constituent on
the surface of Ganymede and Callisto (and Europa as well)
in the 1970s by obtaining infrared spectra of these bodies.
Seen with the eye, the surfaces are darker and redder than
pure water ice, so there must be some other material mixed
with the ice, but the composition of this material has been
difficult to determine. Based on analogy to meteorite and
asteroid spectra as well as cosmochemical arguments, most
researchers have assumed that the nonwater component of
the surface is similar to the material found in primitive,
carbon-rich meteorites—a mixture of hydrated silicates
(clays) and dark, complex organic compounds (dubbed
tholins by the astronomer Carl Sagan, who studied the
production of organic material in laboratory simulations of
planetary environments). Laboratory studies of ice and min-
eral mixtures show that even small amounts of dark material
will disproportionately lower the reflectance (albedo) of the
mixture and damp out the spectral signature of water ice,
producing reflectances consistent with the observed spec-
tra of the satellites. Unfortunately, the more subtle spectral
signatures of the dark minerals are themselves obscured
in the mixed spectra by the much stronger water features,
making identification of the dark constituents difficult.
The near-infrared mapping spectrometer on Galileo pro-
vided new insights into the composition of the nonwater
constituents. This instrument not only covered the spec-
tral range accessible to Earth-based telescopes but also re-
turned spectra in the 3- to 5-μm spectral region. This part
of the infrared spectrum is inaccessible from the surface
of the Earth due to strong absorptions in the Earth’s at-
mosphere by water vapor and carbon dioxide. It is also a
key part of the spectrum for studying non-water-ice com-
ponents mixed into the satellite surfaces, since water ice is
essentially black at these wavelengths and whatever signal
is seen arises primarily from the non-water-ice component
of the surface mixture.
NIMS spectra of Ganymede and Callisto indeed proved
their value in the 3- to 5-μm range, exhibiting a number of
detectable absorption features (see Fig. 5). The strongest
feature is a relatively sharp absorption of infrared light cen-
tered at about the 4.25-μm wavelength, with weaker, but
still easily detectable, absorptions at 3.88, 4.05, and 4.57μm.
There is also a weak absorption seen centered near 3.4μm.
These absorptions are seen in the spectra from both satel-
lites but are most easily seen in the Callisto spectra, where
there is more of the dark material exposed on the surface.
The 4.25-μm feature has been identified as being caused
by the presence of CO 2 on the surface. The location of the
center of the absorption indicates that the CO 2 is not in
the form of either a solid ice or liquid, but rather occurs in
microdeposits, bonded to some other material in the soil.
The 4.57-μm absorption is believed to be due to a carbon–
nitrogen compound based on its frequency, which corre-
sponds to that expected for C≡N (a triple bond of carbon
and nitrogen). The weaker features near 3.4μm are also
believed to be due to carbon bonds with hydrogen (C–H
hydrocarbons). These features have also been identified in
space spectra of interstellar ice grains obtained by the Euro-
pean Space Agency’s Infrared Space Observatory mission.