Physikalische Planetologie
Stagnant lid convection in the medium-sized icy satelites of Saturn
The multi-faceted Saturnian system includes a group of medium-sized, regular satellites in a size range from
bareley 200 km to well over 700 km. They comprise Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus. Their
low densities indicate that ice is a major constituent. We construct models of thermal evolution for all of these
bodies except the tidally heated Enceladus, as we do not take into account heat sources other than accretional
and radiogenic heating in the silicate fraction of the satellites. Previous models of terrestrial planetary bodies
have assumed the boundary between the lithosphere and the convecting mantle to be defined by an isotherm at
the minimum temperature enabling solid state creep on geological time scales. As was demonstrated in
laboratory experiments and verified in theoretical studies, convection in a volumetrically heated fluid with
strongly temperature-dependent viscosity occurs in a sublayer located underneath a stagnant lid. The viscosity
contrast across the sublayer remains constant, while the temperature at the bottom of the stagnant lid varies
with time. In our model calculations, we apply the concept of stagnant lid convection to the icy satellites stated
above. The most significant model parameters are satellite radius, mean density, silicate volume fraction, and
ambient temperature at time of accretion. Furthermore, we assume homogeneous accretion.
Initially, heat generated in the satellites
is transported by conduction. If internal heating is sufficient, a stagnant lid covering a convecting sublayer may
form during thermal history. At first, the convecting sublayer may enclose a conducting core, which quickly
dissolves. As the satellite cools, its stagnant lid grows until convection ceases and conduction becomes the
sole mode of heat transfer again.
The temperatures at the upper boundary of the convecting
region are higher than in previous models, as is the quasi-isothermal temperature characterizing this layer.
Results of thermal history model calculations are reviewed with respect to observed present-day surface
features on the Saturnian satellites.
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