Numerical simulations of convection in Europa's ice shell: Implications for surface features

Abstract

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.

Europa Lander Mission: A Challenge to Find Traces of Alien Life

“…The extreme position in this debate, a thick shell of a few tens of kilometers likely convective in its lower part (e.g., Pappalardo et al, 1999) is supported by thermodynamic models (Hussmann et al, 2002;Tobie et al, 2003). The uncertainty in thickness translates directly into an uncertainty in the heat transfer mechanism: if the shell is thick, the rigid surface could be underlain by a layer of convecting water ice (e.g., Tobie et al, 2003;Showman and Han, 2004), whereas a thin shell would instead transport the heat by conduction, and can account for melting of water through the surface .…”

“…In spite of numerous observational evidences the existence of the global ocean on Europa remains a subject of debate (Carr et al, 1998;Pappalardo et al, 1999;Stevenson 2000;Schenk et al, 2008). Galileo observations and measurements cannot give a definitive constraint about the thickness of the ice shell, provided the ocean is there.…”

Tectonic history of Europa: Coupling between internal evolution and surface stresses

“…We can evaluate the volumetric tidal heating rate using a Maxwell viscoelastic model (e.g., Segatz et al, 1988;Ojakangas and Stevenson, 1989). The average volumetric dissipation rate strongly depends on the temperature, and it can be calculated by (Showman and Han, 2004)…”

Possibility of Convection for Diffusion (Newtonian) Viscosity in the Ice Shell of Europa?

“…Nevertheless, other recent studies of Europa (Hussmann et al, 2002;Nimmo and Manga, 2002;Sotin et al, 2002;Spohn and Schubert, 2003), including numerical investigations (Tobie et al, 2003;Showman and Han, 2004), have used Newtonian (stress-independent) viscosity for water ice. However, the stability against convection of the shell has been not previously analyzed for realistic Newtonian viscosities, and diffusion (Newtonian) creep has not been experimentally observed in water ice (although a flow law for diffusion creep has been proposed from the values of relevant constants; Goldsby and Kohlstedt, 2001).…”