  Geophysical measurements can reveal the structure of icy ocean worlds and
cycling of volatiles. The associated density, temperature, sound speed, and
electrical conductivity of such worlds thus characterizes their habitability.
To explore the variability and correlation of these parameters, and to provide
tools for planning and data analyses, we develop 1-D calculations of internal
structure, which use available constraints on the thermodynamics of aqueous
MgSO$_4$, NaCl (as seawater), and NH$_3$, water ices, and silicate content.
Limits in available thermodynamic data narrow the parameter space that can be
explored: insufficient coverage in pressure, temperature, and composition for
end-member salinities of MgSO$_4$ and NaCl, and for relevant water ices; and a
dearth of suitable data for aqueous mixtures of Na-Mg-Cl-SO$_4$-NH$_3$. For
Europa, ocean compositions that are oxidized and dominated by MgSO$_4$, vs
reduced (NaCl), illustrate these gaps, but also show the potential for
diagnostic and measurable combinations of geophysical parameters. The
low-density rocky core of Enceladus may comprise hydrated minerals, or anydrous
minerals with high porosity comparable to Earth's upper mantle. Titan's ocean
must be dense, but not necessarily saline, as previously noted, and may have
little or no high-pressure ice at its base. Ganymede's silicious interior is
deepest among all known ocean worlds, and may contain multiple phases of
high-pressure ice, which will become buoyant if the ocean is sufficiently
salty. Callisto's likely near-eutectic ocean cannot be adequately modeled using
available data. Callisto may also lack high-pressure ices, but this cannot be
confirmed due to uncertainty in its moment of inertia.
