Ganymede and Callisto 453
FIGURE 3 Cutaway diagrams showing current models for the interior structures of Ganymede
and Callisto based on Galileo gravity data. Ganymede (left) is highly differentiated, with a molten
iron core surrounded by a rocky mantle, in turn surrounded by a thick outer layer of ice. An
interior ocean of liquid water may exist sandwiched between the surface layer of Ice-I and the
higher pressure phases of ice below. Callisto (right) has an interior composed of a mixture of rock
and ice, slowly increasing in density toward the center. The outermost layer is relatively clean
water ice, with a liquid water ocean at its base. (Zareh Gorjian and Eric De Jong, NASA/Jet
Propulsion Laboratory.)
conditions the solid form remains low-density Ice-I.
Laboratory studies show that, at the high pressures reached
deep in the interiors of icy satellites the size of Ganymede
and Callisto, ice transforms to various high-density forms
over a wide range of temperatures (Fig. 4). These phases
of ice, as they are known, are denser than liquid water and
would sink in a liquid ocean. Calculations of the tempera-
ture and pressure as a function of depth within the satellites
show that if temperatures reach the required melting point,
the resulting subsurface oceans would be strange indeed—
a liquid layer sandwiched between low-density Ice-I on the
top and high-density Ice-III on the bottom (or a mixture of
high-density ice and rock in the case of Callisto).
The other major complication is whether the interior will
ever actually warm up to the ice melting point. The simple
calculations that reach the melting temperature are based
on the heat produced by radioactive decay, escaping the
interior by thermal conduction through the ice crust. How-
ever, as the temperature of ice approaches the melting point
within the satellite, another heat transfer process comes
into play, convection. Ice near its melting point is not stiff
and brittle, but can flow and deform under pressure, par-
ticularly over long periods of time. In geophysical terms, it
becomes a low-viscosity solid. Low-viscosity ice under some
FIGURE 4 Phase diagram of water ice. At low pressures near the
surface, Ice-I is less dense than liquid water. At higher pressures,
ice converts to denser phases, with higher melting points.