[1] Numerical mantle convection models indicate that subducting slabs can reach the core-mantle boundary (CMB) for a wide range of assumed material properties and plate tectonic histories. An increase in lower mantle viscosity, a phase transition at 660 km depth, depth-dependent thermal expansivity, and depth-dependent thermal diffusivity do not preclude model slabs from reaching the CMB. We find that ancient slabs could be associated with lateral temperature anomalies $500°C cooler than ambient mantle. Plausible increases of thermal conductivity with depth will not cause slabs to diffuse away. Regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the CMB, even for radially simple mantle structures. The observation from tomography showing only a few continuous slab-like features from the surface to the CMB may be a result of complex plate kinematics, not mantle layering. There are important consequences of deeply penetrating slabs. Our models show that plumes preferentially develop on the edge of slabs. In areas on the CMB free of slabs, plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin. However, in areas beneath slabs, the basal thermal boundary layer would be thicker and plume formation infrequent. Beneath slabs, a substantial amount of hot mantle can be trapped over long periods of time, leading to ''mega-plume'' formation. We predict that patches of low seismic velocity may be found beneath large-scale high seismic velocity structures at the core-mantle boundary. We find that the location, buoyancy, and geochemistry of mega-plumes will differ from those plumes forming at the edge of slabs. Various geophysical and geochemical implications of this finding are discussed.

Europa's icy surface displays numerous small (5‐ to 30‐km‐diameter) pits, spots, and uplifts that have been suggested to result from convection in the ice shell. To test this hypothesis, we present numerical simulations of convection in Europa's ice shell, including temperature‐dependent viscosity and tidal heating. Ice shells 15 and 50 km thick are considered, consistent with several estimates of the shell thickness on Europa. The convection produces deep pits (consistent with some of the observed features) when the lithospheric viscosity is 103–105 times greater than that of the underlying ice, but greater viscosity contrasts lead to topography insufficient to explain the observed pits. If ductile creep is the only deformation mechanism, these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 1010. However, the strength of Europa's surface may be low enough for plastic deformation to play a role in the convection, opening the possibility that convection could produce some of Europa's pits. For plausible viscosities (1013 Pa s at the melting temperature), the pits are 100–300 m deep and 10–20 km in diameter; greater or lesser viscosities lead to wider or narrower pits, respectively. None of our simulations produced isolated uplifts of any diameter, however, so these probably formed by another mechanism. The convection can induce surface stresses >1 bar, which exceeds the inferred strength of Europa's crust and indicates the likelihood of surface disruption. The maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0.05–0.07 W m−2.

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We present two‐dimensional numerical simulations of thermo‐compositional convection to test the hypothesis that the combined buoyancy from both thermal and salinity contrasts in Europa's ice shell can produce the numerous uplifts and pits on Europa's surface. Our simulations show that uplifts and pits with amplitude of 100–500 m and diameters of 10–30 km (similar to some of the observed features) can be produced in a 10–30 km‐thick ice shell with 2–10% compositional density variations if the viscosity contrast due to temperature variation does not exceed 106. The pit and uplift formation time and lifetime are approximately proportional to the surface viscosity, ranging from 104 years to 107 years for viscosity contrasts of 104–106. Convection cannot produce substantial surface topography if the viscosity contrast exceeds 107–108. These results imply that thermo‐compositional convection can only produce Europa's pits and uplifts if Europa's surface is weak.

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[1] We present 2D and 3D numerical simulations of convection to test the role of shear heating and fracture zones on Europan ridge formation. Our simulations show that a pre-existing fracture zone promotes upwelling and lithospheric thinning, leading to topographic uplift of 50 m. Shear heating also promotes lithospheric thinning and buoyant ascent, producing a ridge-like feature with topography up to 120 m. Topography remains linear along strike even under the influence of heterogeneous 3D convection within the ice shell. Although the central trough is not reproduced in the simulations, our results support the idea that shear heating can produce ridge-like structures on Europa. Citation: Han, L., and A. P. Showman (2008), Implications of shear heating and fracture zones for ridge formation on Europa, Geophys. Res. Lett., 35, L03202,