450 Encyclopedia of the Solar System
FIGURE 1 Global views of Ganymede (left) and Callisto (right), obtained by the camera on theGalileospacecraft.that water ice might be responsible. Vassily Moroz, a plan-
etary scientist working at the Crimea Observatory in the
Soviet Union, made even more detailed infrared color mea-
surements and concluded that water ice was the best expla-
nation for Ganymede’s spectrum.
At the spectral resolution and signal-to-noise ratio of
these pioneering measurements, however, a conclusive
identification of the surface composition could not be made
because several other candidate materials, including ices
of carbon dioxide and ammonia, were known to have ab-
sorptions in the same part of the infrared spectrum. The
issue was settled conclusively for Ganymede and Europa
by a team led by Carl Pilcher, then at MIT, who published
the first high-resolution infrared reflection spectra for these
satellites in 1972 and compared them in detail with labora-
tory spectra of ices at low temperature. All the significant
absorption features in the 1- to 2.5-μm region matched
spectra of water ice and ruled out any major contribution
from other ices. Callisto’s spectrum also displays water ice
and hydrated silicate features, although the water signature
is subdued compared with Ganymede’s strong water ice
spectral absorptions,due to the larger amount of dark
material mixed with the ice on Callisto’s surface.
Figure 2 shows a compilation of the best telescopic spec-
tra of Ganymede and Callisto compared with Io and Europa.
The dominant features in all the spectra except Io’s are the
deep absorptions at wavelengths longer than 1μm due to
the presence of hydrated materials and water ice. Labora-
tory studies of water ice reflectance and theoretical simu-
lations of spectra from mixtures of material have demon-
strated that the observed spectra can be explained by water
ice/frost, mixed with varying amounts of a spectrally neutral
darker component with a reddish color in the visible portion
of the spectrum (i.e., one having absorption at ultraviolet
and blue wavelengths). The nature of the non-water-ice
component in the satellites’ surfaces is still under inves-
tigation, but spectra of different regions on both satellites
taken by the Near-Infrared Mapping Spectrometer (NIMS)
instrument on theGalileomission are providing clues to the
identification of this material (see Section 3).1.2.1 MASSES AND DENSITIES
The mass of a distant planetary object is normally impos-
sible to determine from remote astronomical observations
alone, unless it happens to have a companion whose orbit
can be determined, as is the case for the giant planets with
their satellite systems, the Pluto/Charon system, and more
recently numerous asteroids and several trans-Neptunian
objects. The Galilean satellites represent a more difficult
case. They do not themselves have satellites, but the mu-
tual gravitational attraction among these large satellites pro-
duces significant and measurable changes in their orbits
about Jupiter. The mathematician Pierre Laplace studied
these interactions in the late 19th century, and subsequent
developments in this new branch of dynamical astronomy
permitted reasonable estimates of the satellites’ masses to
be made in the early 20th century. When combined with the