Encyclopedia of the Solar System 2nd ed

Ganymede and Callisto 465

FIGURE 13 Furrows in Galileo Regio on Ganymede arc gently from northwest to southeast.

These are thought to be the Ganymede equivalent of the concentric rings found around impact

basins such as Valhalla on Callisto (see Fig. 8).

the satellites, making Ganymede hot enough to generate a
magnetic dynamo and keeping Callisto cold enough to not
differentiate are both challenging problems for our under-
standing of planetary geophysics. The oceans of liquid water
that exist within these bodies, sandwiched between differ-
ent phases of ice, are exotic phenomena in themselves. On
the surfaces of Ganymede and Callisto, we still don’t have a
clear idea of the composition of some of the materials that
are mixed in with the water ice; neither do we know which
of those materials come from the interiors of the satellites,
which ones come from their external environments, and

which ones are the products of chemical reactions and ra-

diation processing at the surface.

The unfortunate failure of the main antenna on the

Galileospacecraft left us without a complete global recon-

naissance of these bodies at a level of detail sufficient to re-

solve features at the scale of a kilometer or less. The small

target areas that Galileo imaged at high-resolution revo-

lutionized our understanding of these bodies, but much

of their surfaces will remain relatively unknown for the

near future. No new missions are currently being planned

to explore Ganymede and Callisto, though plans for a

FIGURE 14 Oblique view over the surface of Callisto, looking over the edge of one of the

concentric ring scarps of the Valhalla impact basin.