The three outer Galilean satellites of Jupiter—Europa, Ganymede, and Callisto—all appear to have cores of rock. In addition to rocky material, the major constituent of these satellites seems to be ices of:

In addition to rocky material, the major constituent of Galilean satellites of Jupiter—Europa, Ganymede, and Callisto seems to be ices of water (H2O).

Explanation:

The Galilean satellites or Galilean moons are the satellites of Jupiter discovered by Galileo Galilei. They Include Io, Europa, Callisto, and Ganymede.  

Ganymede (the largest moon in the Solar System) has a liquid iron core, an internal ocean that contains a very huge quantity of water and is made up of equal measures of both silicate rock and water ice. A magnetic field is present in this moon. The atmosphere of Ganymede contains a very small amount of molecular oxygen and ozone.

Callisto has a small silicate core and a subsurface ocean of liquid water. It is made up of approximately equal measures of both rock and ice, and also contains a small amount of silicates, carbon dioxide, and organic compounds. The atmosphere of Callisto contains a very small amount of carbon dioxide and molecular oxygen and has an intense ionosphere.

Europa (smallest of the four Galilean moons) has a metallic iron-nickel core and a water-ice crust. It is made up of silicate rock and has an outer layer of water, a part of which forms the frozen water-ice crust and another part forms the liquid ocean underneath the ice. The atmosphere of Europa mainly contains oxygen.