HR: 12:05h
AN: P22A-08    [Abstracts]
TI: Convection in large icy satellites with self-consistent grain-size evolution
AU: * Barr, A C
EM: amyb@levee.wustl.edu
AF: Washington University of St. Louis, Department of Earth and Planetary Sciences Campus Box 1169 One Brookings Dr., St. Louis, MO 63130 United States
AU: McKinnon, W B
EM: mckinnon@levee.wustl.edu
AF: Washington University of St. Louis, Department of Earth and Planetary Sciences Campus Box 1169 One Brookings Dr., St. Louis, MO 63130 United States
AB: The viscosity of ice I is grain-size dependent for temperature and stress conditions appropriate for ice I shells of midsized and large icy satellites. Satellite thermal evolution, heat flux, critical shell thickness for convection, brittle/ductile transition depth, and potential surface deformation are grain size-dependent. We estimate grain sizes in a convecting shell using the empirical observation from polar ice sheets that d ~ A σ^-1, where A is a thermal activation term, and σ is shear stress [De La Chappelle et al., JGR 103, 1998] due to dynamic recrystallization. We use a composite volume diffusion/GBS rheology for ice I in the convection model Citcom [Barr et al., JGR, 2004] to self-consistently model strain rates and grain sizes in convecting shells. Estimates of grain size are reasonable if the grain-growth time scale is less than the convective overturn time scale (~ 10⁵ - 10⁷ yr for large icy satellites), and the shell is free of impurities that limit grain growth. For large icy satellites, the composite rheology and uniform grain size predict sluggish convection unless the grain size is small (<1 mm). Convection can only occur if d < 2 cm [Barr and Pappalardo, JGR, in press, 2005], and then only for d < 2-4 mm if initial thermal perturbations are small [McKinnon, Icarus, in press]. Dynamic recrystallization predicts, for example, d ~ 1 mm in the convecting interior and d ~ 10 cm in the stagnant lid for an ice shell 50 km thick. Compared with models with a similar but uniform grain size in the convective region, heat fluxes are larger, and shallower brittle/ductile transition depths.
DE: 6221 Europa
DE: 6222 Ganymede
DE: 6223 Callisto
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Planetary Sciences [P]
MN: Fall Meeting 2005