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

Álvarez‐Gómez

Tejero

et al. 2007

Icarus

Self Cite

Although it is mostly accepted that the lower part of the ice shell of Europa is actively convective, there is still much uncertainty about the flow mechanism dominating the rheology of this convective layer, which largely depends on the grain size of the ice. In this work, we examined thermal equilibrium states in a tidally heated and strained convective shell, for two rheologies sensitive to grain size, grain boundary sliding and diffusion creep. If we take a lower limit of 70 mW m −2 for the surface heat flow, according to some geological features observed, the ice grain size should be less than 2 or 0.2 mm for grain boundary sliding or diffusion creep respectively. If in addition the thickness of the ice shell is constrained to a few tens of kilometers and it is assumed that the thickness of the convective layer is related to lenticulae spacing, then grain sizes between 0.2 and 2 mm for grain boundary sliding, and between 0.1 and 0.2 mm for diffusion creep are obtained. Also, local convective layer thicknesses deduced from lenticulae spacing are more similar to those here derived for grain boundary sliding. Our results thus favor grain boundary sliding as the dominant rheology for the water ice in Europa's convective layer, since this flow mechanism is able to satisfy the imposed constraints for a wider range of grain sizes.

Section: Introductionmentioning

confidence: 99%

Heat flow and thickness of a convective ice shell on Europa for grain size-dependent rheologies

Álvarez‐Gómez

Tejero

et al. 2007

Icarus

Self Cite

Equilibrium Convection on a Tidally Heated and Stressed Icy Shell of Europa for a Composite Water Ice Rheology

2010

Earth Moon Planets

Self Cite

Water ice I rheology is akey factorfor llllderstanding the thermal and mechanical state of the outer shell of the icy satellites. Ice flow involves several deformation mechanisms (both Newtonian and non-Newtonian), which contribute to different extents depending on the temperature, grain size, and applied stress. In this work I analyze tidally heated and stressed equilibrium convection in the ice shell of Europa by considering a composite viscosity law which includes diffusion creep, basal slip, grain bOlmdary sliding and dislo cation creep, and. The calculations take into aCcOllllt the effect of tidal stresses on ice flow and use grain sizes between 0.1 and 100 mm. An Arrhenius-type relation (useful for parameterized convective models) is fOlllld then by fi tting the calculated viscosity between 170 and 273 K to an exponential regression, which can be expressed in terms of pre exponential constant and effective activation energy.

Heat Flow and Thermal State of the Crust of the Icy Galilean Satellites

2012

Earth Moon Planets

Self Cite

The use of lithosphere strength indicators (as for example the effective elastic thickness of the lithosphere or the depth to the brittle-ductile transition) can give clues on the surface heat flow and the thermal state of the ice crust of icy satellites, which in turn has profound implications for our understanding of the internal evolution, and the possible existence of internal oceans, in these planetary bodies. As shown in this review, this kind of approximations has found very high (and possibly recent) surface heat flows for Europa, and ancient high heat flows for Ganymede. On the other hand, the heavily cratered of Callisto suggests a cold and inactive outer ice shell throughout the entire history of this body.However, irrespective of the greatly different geology recorded on the surface of the icy Galilean satellites, there are evidences from orbital magnetic observations internal oceans in all them. By combining heat flow estimations and the existence of internal oceans, a more complete description of the thermal history of the icy Galilean satellites should emerge.