On the internal structure and dynamics of Titan

Research areas:
Symposium on the Jovian System after Galileo/The Saturnian System before Cassini-Huygens, NANTES, FRANCE, MAY 11-15, 1998
The purpose of this paper is to study the evolution of Titan from the
primordial core overturn to the present, and to investigate the possible
existence of both a deep liquid layer and an iron core, depending on the
composition of chondrites and the primordial amount of volatiles
included in ices. Models of Titan's interior are constructed using
theoretical models based on thermal and mechanical properties of ices
and silicates. Depending on both the heat transfer efficiency in
chondrites and ices, and the amount of heat present in the interior,
many properties of the deep structure of the satellite are deduced. Heat
transfer through the convecting shell in planets is commonly estimated
using parameterized laws which relate the vigor of convection to the
heat flux at the top of the convecting shell. These laws, established
from studies with constant viscosity fluids have been changed in order
to take into account the very large viscosity contrasts in the different
layers. Models also require a good knowledge of both thermodynamic and
rheological parameters of materials at high pressure. In Titan, many
volatiles were probably present in the primordial liquid layer. These
volatiles must decrease the freezing temperature of the liquid which is
of fundamental importance for the evolution of the satellite. Recent
experimental results on the system NH(3)-H(2)O are included in the
present models.
Evolution of the core - Using numerical models incorporating recent
results on thermal convection for fluids with strongly temperature
dependent viscosity, the thermal evolution of Titan's core is presented
for two possible compositions of the planetoids. In the first case, the
chondritic part of the planetoids was possibly composed of CI chondrites
and the core is simply composed of silicates, whereas in the second
case, chondrites with a large amount of metallic iron (EH enstatite
chondrites) were accreted during Titan's formation. Diffusive heating
increases the averaged temperature of the homogeneous chondritic core up
to a critical value where marginal convection may occur about 1 Ga after
the core overturn. In case 1, the onset of convection is accompanied by
partial melting of the silicate core. Then, the vigor of convection
keeps increasing and would still be vigorous at the present time.
Partial melting of silicates below a thick thermal boundary layer at the
top is very likely at present. In the other model (case 2), metallic
iron starts melting before the onset of convection and implies a rapid
overturn of the chondritic core into a layered structure with a dense
liquid iron core surrounded by a silicate layer. In this layered core,
convection in the silicate layer is not very vigorous, but probably
still exists.
Evolution of the icy layers - Radiogenic heating of silicates is
transferred by convection through the ice shell. If convection is
vigorous, the heat flux through the ice shell is larger than the heat
flux from the core and crystallization in the liquid shell occurs both
at the top and at the bottom. Then, two different evolutions can be
expected: (i) the decrease of temperature due to thickening is small and
the Rayleigh number of the ice I shell increases when the layer thickens
(pure H(2)O case). Freezing of the liquid layer is very rapid and Titan
is presently composed of a thick icy mantle, which convects vigorously;
and (ii) the freezing temperature decreases strongly when pressure
increases so that the Rayleigh number does not increase when ice I
thickens because viscosity increases rapidly (NH(3)-H(2)O case). As a
consequence, the thickening of ice I is very slow. The present structure
of Titan depends on the primordial composition of the liquid layer, but
it is probable that a liquid layer, which could be more than 350 km
thick, still exists in the interior of the satellite. Such a layer may
be determined by the Cassini measurements. (C) 2000 Elsevier Science
Ltd. All rights reserved.