  Existence of subsurface oceans on the satellites of the giant planets and
Trans-Neptunian objects has been predicted for some time. Oceans on icy worlds
exert a considerable influence on the dynamics of the ice-ocean system and,
because of the astrobiological potential, represent an important objective for
future missions. The Pluto-Charon system is representative of an icy moon
orbiting a dwarf planet formed from the remnants of a giant impact. Evolution
of icy moons is primarily controlled by the mode and efficiency of heat
transfer through the outer ice shell, which is influenced by the presence of
impurities, by tidal dissipation in the ice shell, and the radioactive element
budget in the core. Previous studies on the evolution of the Pluto-Charon
system considered either only the thermal or the tidal evolution, and in the
cases where both were considered, the important effect of the presence of
impurities in the liquid oceans was not addressed. We consider the joint
tidal-thermal evolution of the system by combining a comprehensive tidal model
that incorporates a viscoelastic tidal response with a parameterized convection
model developed for icy worlds. This approach enables an extensive analysis of
the conditions required for formation and maintenance of subsurface liquid
oceans to the present. Our results show that because of fast circularization
and synchronization of the orbits, tidal heating is only important during the
early stages of evolution (<1 Myr). We test the sensitivity of our results to
the initial orbital and thermal parameters. In all the cases, oceans on Pluto
are always predicted to remain liquid to the present, ranging from 40 km to
150-km thick, whereas oceans on Charon have solidified. This is supported by
New Horizons observations of extensional faults on Pluto and both extensional
and compressional faults on Charon.
