Ganymede and Callisto 457
FIGURE 5 GalileoNIMS spectra of Ganymede and
Callisto, showing absorption of infrared light by
various surface materials. See text for details.
(Modified from T. McCord et al., J. Geophys. Res., v.
103, pp. 8603–8626, 1998.)Recent observations of the satellites of Saturn by a similar
instrument on theCassinispacecraft show nearly identical
CO 2 absorption and similar C≡N and C–H features [see
PlanetarySatellites]. The other two absorption fea-
tures (3.88 and 4.05μm) seen on Ganymede and Callisto
appear to be unique to the Jupiter system and are thought
to be due to S–H bonds and sulfur dioxide, respectively.
3.2 Surface and Atmosphere Interactions
with Local Environment
The environment around Jupiter is awash in radiation from
charged particles trapped in Jupiter’s intense magnetic field.
Since they are trapped in the field, which rotates rapidly
with Jupiter’s spin, the particles sweep past the satellites in
their comparatively slow orbits. Thus, the side of a satellite
facing away from the direction of orbital motion (the trailing
hemisphere) is exposed to a much higher dose of radiation
than the leading hemisphere. When the charged particles
strike the surfaces of the satellites, they can send surface
molecules flying (a process called sputtering), and they can
break molecular bonds in the surface material, causing new
chemical reactions to occur and creating new compounds.
Laboratory studies suggest that the CO 2 ,C≡N, and C–H
features seen in the spectra of Ganymede, Callisto, and
other icy satellites in the solar system may have a com-
mon origin due to charged particle irradiation of minerals
containing potassium cyanide and possibly other cyanogens
(carbon–nitrogen bearing compounds). Irradiation of these
compounds by energetic particles in the presence of wa-
ter ice is believed to be an important source of the CO 2
found embedded in the mineral/ice matrix on the surfaces
of Ganymede and Callisto. On Callisto, the distribution of
CO 2 mapped by NIMS shows a marked concentration on
the trailing hemisphere of the satellite, as would be expected
from radiation-induced CO 2 production.
Sulfur is an important ion in the Jupiter system, contin-
ually supplied to space by the escape of sulfur and sulfur-
dioxide gases from volcanic Io. The sulfur becomes ionized
and joins the low-energy plasma streaming through Jupiter’s
magnetosphere, which then washes up on the other satel-
lite surfaces. Implantation of sulfur into the surfaces of the
icy Galilean satellites has been suggested in the past as the
likely reason for the low reflectance of the satellites in the ul-
traviolet part of the spectrum. The sulfur-induced infrared
absorption features are also plausible results of bombarding
the icy surfaces with sulfur-rich plasma. However, the dis-
tribution of SO 2 on the surfaces of Ganymede and Callisto
do not show a strong leading–trailing hemisphere asymme-
try as one would expect, indicating that perhaps there is also
sulfur in the ice bedrock.
Both Ganymede and Callisto exhibit other effects from
their continual bombardment by charged particles in
Jupiter’s magnetosphere. Both molecular oxygen, O 2 , and
ozone, O 3 , have been detected in telescopic spectra of
their surfaces. These oxygen compounds appear to exist
as microscopic bubbles trapped in the matrix of the icy sur-
face material and have also been attributed to irradiation
by charged particles.
In addition to the frozen and trapped gases in their
surfaces, Ganymede and Callisto have very tenuous atmo-
spheres. On Ganymede, Hubble Space Telescope spec-
tra have identified molecular oxygen, and the Galileo
ultraviolet instrument detected a thin veil of hydrogen in the
surrounding space. These gases are apparently produced
by a combination ofsublimationand sputtering from the
icy surface. Callisto’s atmosphere is similarly of very low
density, and the only detectable gas so far has been CO 2