Early Iapetus and the origin of its odd shape

Details Early Iapetus and the origin of its odd shape Early Iapetus and the origin of its odd shape

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..854r&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

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The Cassini mission revealed two spectacular characteristics of Iapetus: (i) a high, equatorial ridge, which is unique in the Solar System and (ii) a large flattening (a-c = 35 km) inconsistent with its current spin rate. In order to explain these two striking observations, Castillo-Rogez et al. [1] proposed that Iapetus froze its shape as it despan from a rapid spin rate of a few hours to the present synchronous rotation (~79 days). Such a despinning is possible if an additional heat component was present during its early history, including short-lived radiogenic elements such as 26Al, and if heat transfer is inefficient to cool down the interior. The efficiency of heat removal is mainly controlled by the occurrence of thermal convective instabilities, which is determined by the rheological structure of the interior. The rheological profile also controls the despinning rate and the shape relaxation. In this study, we investigate the onset of convection during the early evolution of Iapetus and its consequences on the scenario proposed by Castillo- Rogez et al. [1] convection for fluids with large viscosity contrasts in 3-D spherical geometry, using the numerical tool OEDIPUS ([2], [3]). Our goals are to evaluate for a large range of plausible initial conditions (1) the onset time of convection, (2) the evolution of the viscosity structure, (3) the resulting despinning rate and (4) the relaxation of Iapetus' flattening. The despinning rate due to dissipation of the rotational energy in the interior is computed using the method of Tobie et al. [4] and the relaxation rate are calculated using the spectral approach developed by Čadek [5] and Tobie et al. [6]. Two different viscoelastic linear rheologies are considered: (i) a Maxwell model which is described by an elastic shear modulus, μ, and longterm viscosity, η, and (ii) a Burger model, which includes a transient shear modulus, μB, and a shortterm viscosity, ηB, in addition to the two Maxwell parameters. Our simulations indicate that thermal convection starts when the mean viscosity gets lower than ~1017 Pa.s, which corresponds to a temperature of 210 K for pure H2O ice. When convection initiates, a large number of cold instabilities develop at the base of a stagnant lid due to the large value of the Rayleigh number (Rai=108) (Figure 1). The time required to reach an internal temperature of ~210 K is mainly determined by the time tacc at which the satellite accretion is completed. Heating due to short-lived radiogenic elements contributes to the internal budget only for short time of accretion, tacc. Therefore, for tacc=1.5 Myr, the onset time of convection, tonset , is ~5.8 Myr, whilst for tacc=10 Myr, the onset time for convection can be as long as ~1 billion years. After the onset of convection, the convective motions are confined within a quasi-isoviscous warm internal core beneath a cold viscoelastic lithosphere, where most of the viscosity variations occur. As the satellite cools down, the internal viscosity progressively increases and the lithosphere thickens (Figure 2a). The vigour of convection progressively decays, but for most of the simulations convective motions still operate after 2-3 billion years (the typical duration of our numerical experiments). The evolution of the viscosity profile obtained for different values of tacc have been used to compute the despinning rate and the subsequent slowdown of the satellite rotation. When a Maxwell rheology is assumed, despinning is not obtained in any of our models. Despinning is only observed for models with tacc< 2 Myr if a Burger rheology is considered with μ=μB and ηB=η/17 [7] (Figure 2b). For the few models that despan, we compute the evolution of the satellite flattening for different initial rotation periods (Figure 2c). Only calculations performed with an initial period of 8 hours lead to the present-day value of flattening (~ 35 ± 3 km, [8]). on the early evolution of Iapetus. We show that whatever the time of completion of accretion considered here, thermal convection occurs, which limits the warming of the interior compared to the conductive evolution models of Castillo-Rogez et al [1]. Consequently, the interior is faintly dissipative, so that only few models lead to despinning and a very limited number of models can explain the current shape of Iapetus. References [1] Castillo-Rogez, J. et al (2007) Icarus, 190, 179- 202 [2] Choblet, G. (2005) Journal of Computational Physics, 205, 269-291 [3] Choblet, G et al (2007) Geophysical Journal International, 170, 9-30 [4] Tobie, G et al. (2005). Icarus, 177, 534-549 [5] Cadek, O. (2003) EGS-AGU-EUG Joint Assembly, 5279 [6] Tobie, G et al (2008) Icarus, doi:10.1016/j.icarus.2008.03.008. [7] Reeh, N. et al. (2003) Ann. Glaciol., 37 [8] Thomas, P. et al. (2007) Icarus, 190, 573-584

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