Coupling of thermal evolution and despinning of early Iapetus

Research areas:
Year:
2010
Authors:
Journal:
ICARUS
Volume:
207
Number:
2
Pages:
959-971
Month:
June
ISSN:
0019-1035
Abstract:
The Cassini mission revealed two spectacular characteristics of Iapetus:
(1) a geologically old and high equatorial ridge, which is unique in the
Solar System and (2) a large flattening of 35 km consistent with the
equilibrium figure for a hydrostatic body rotating with a period of
1611, whereas the current spin period is 79.33 days. This study
describes three-dimensional simulations of solid-state convection within
an undifferentiated Iapetus. It investigates the implications for the
evolution of the interior thermal structure and its spin rate and global
shape using radially layered viscoelastic models. The role of the
concentration in the short-lived radiogenic element {[}(26)Al], just
after accretion is completed, is specifically addressed. The first
result is to show that whatever the {[}(26)Al] value, convection occurs.
As suggested by Castillo-Rogez et al. {[}Castillo-Rogez, J., Matson, D.,
Sotin, C., Johnson, T., Lunine, J., Thomas, P. {[}2007] Icarus, 190,
179-202], convection reduces the warming of the interior compared to the
conductive evolution and therefore limits the conditions for despinning.
In our calculations, two conceptual linear viscoelastic models are used.
When considering a Maxwell rheology, the interior temperature
(viscosity) never reaches a value high (low) enough to induce
despinning. In order to promote dissipation at low temperature, a
Burgers rheology, which includes an additional dissipation peak, is
introduced. For favorable parameter values, this latter rheology leads
to despinning. However, only models associated with large amounts of
short-lived radiogenic elements ({[}(26)Al] >= 25 ppb) lead to the
observed flattening. This suggests that the accretion process needs to
be completed shortly after the formation of CAIs (Calcium-Aluminum-rich
Inclusions) (