2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

42
Symposium 1: Structural Geology, Tectonics and Geodynamics
1.A.4
Thermal convection in the outer layer of icy satellites
Yao Chloé 1 , Deschamps Frédéric 1 , Tackley Paul 1 & Sanchez-Valle Carmen 2
1 ETH Zurich, Institute of Geophysics, Sonneggstrasse 5, CH-8092 Zurich (chloe.yao@erdw.ethz.ch)
2 ETH Zurich, Institute of Geochemisty and Petrology, Clausiusstrasse 25, CH-8092 Zurich
Recent spacecraft missions exploring the outer planets of the Solar System provided new data that increased our knowledge
about the internal structure of icy satellites. The existence of a global sub-surface ocean located between an outer
layer of ice I and a basal layer of high pressure ices have been proposed forty years ago (Lewis, 1971). As the primordial
ocean cools down, ice crystallizes both at its top and its bottom, and if the heat transfer in the outer ice I layer is not efficient
enough, a liquid ocean can be maintained in between the two layers of ice. The presence of an ocean is thus controlled
by the heat transfer in the outer ice layer. Convection is likely the most efficient way to transfer heat through this
ice layer (McKinnon, 2006; Deschamps & Sotin, 2001), but the regime of convection (and therefore the heat transfer) depends
on the rheology of the fluid. In the case of ice, viscosity is strongly temperature dependent and thermal convection
in the outer ice shell follows a stagnant lid regime. A rigid stagnant lid forms at the top of the system, and convection is
confined in a sublayer (Davaille & Jaupart, 1993).
Figure 1. Thermal structure of the convective layer for Ra=5.7.105 and Δμ=105
Many numerical studies including strongly temperature-dependent viscosities have been performed in 2D Cartesian geometry
allowing the determination of scaling laws between the temperature of the well-mixed interior and the ratio of the
top viscosity to the bottom viscosity.
In this work, we present new models of thermal convection in spherical geometry, which we use to model the heat transfer
through the outer layer of icy moons. We use STAGYY (Tackley, 2008) to run simulations in 3D spherical geometry (fig.
1) with a ratio of the core radius to the total radius of 0.80 (meaning that the ice shell represents 20% of the satellite’s
radius). We consider the only source of heat to be from the bottom of the ice layer (internal heating is neglected). Under
these conditions, we study the dependence of the temperature of the well-mixed interior (Tm) on the effective Rayleigh
number (Ra) (describing the vigor of convection) and the ratio of viscosity (Δμ) (fig. 2). For a range of Ra between 105 and
107 and a range of Δμ between 104 and 106, we obtained the scaling law:
Swiss Geoscience Meeting 2011
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