458 Encyclopedia of the Solar System
identified in NIMS spectra. A combination of thermal seg-
regation and sputtering has also been invoked to explain the
distribution of water frost on Ganymede, where the equato-
rial zone most impacted by magnetospheric plasma has less
visible surface frost, and the high-latitude regions appear
to be coated with frost that may have migrated there from
the equatorial areas and been retained there due to lower
temperatures. Ganymede’s frosty polar caps (the northern
cap is visible in Fig. 1) closely follow the region where
Ganymede’s magnetosphere becomes connected with the
external Jupiter magnetosphere, indicating that charged
particles play an important role in creating this feature.
Another interaction with the external environment of the
satellites is micrometeoroid bombardment. All the satellites
are exposed to the flux of tiny grains from interplanetary
space striking their surfaces. In the cases of Ganymede and
Callisto, the interplanetary particle fluxes are enhanced due
to Jupiter’s gravity.Galileocarried a sensitive detector that
measured the surrounding dust environment as the space-
craft orbited Jupiter. It also made measurements on the
close passes by the satellites to sample the population or dust
particles near the satellites. The dust investigators found
that both satellites have a population of small (micrometer-
sized) particles loosely bound by gravity in the space sur-
rounding the satellites. These measurements are consistent
with icy dust grains that have been blasted off the satellite
surfaces by the impact of interplanetary micrometeorites.
The ices on the surfaces of Ganymede and Callisto are
weakly warmed by the Sun and are exposed to near-vacuum
conditions. Even at the cold temperatures in the Jupiter sys-
tem, ice will slowly sublime and escape as a gas. Water ice
could sublimate at a rate of meters per million years, but on
Ganymede and Callisto it is soon choked off by a blanket
of non-ice dust, since the surface ice is not pure and the
dust does not sublimate. Sublimation will occur millions of
times faster for SO 2 ice, and CO 2 ice will sublimate thou-
sands of times faster than that, so incorporation of these
compounds into the ice bedrock will drive the sublimation
erosion process much faster.3.3 Regolith
Bright ice crystals and dark non-ice dust both exist on the
surfaces of Ganymede and Callisto, but they are largely seg-
regated from each other. If one were to pick up a sample of
the loose surface material (the regolith), it would probably
be composed of mostly ice or mostly dust, and not a mixture
of the two. High-resolution images show very high albedo
contrasts over small spatial scales, with relatively pure icy
material outcropping in patches surrounded by blankets of
dark non-ice material. This effect is most pronounced on
Callisto (Fig. 6). It appears that the ice bedrock is composed
of a mixture of ice and non-ice dust. When a fresh outcrop
of bedrock is exposed at the surface, the ice will begin toFIGURE 6 Callisto’s surface is characterized by bright icy hills and impact crater rims
surrounded by blankets of dark dust. This surface is thought to result from sublimation of an
ice/dust mixture, leaving a lag deposit of loose dust in the low areas and depositing bright frost
on steep slopes and hilltops. Note the raised tongue of a landslide deposit consisting of loose
dark material emanating from the shadowed wall in the prominent crater in the northeastern
section of the image. This area is located within the Asgard impact basin.