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[9.07] Onset of Convection in Ice I with a Composite Newtonian/Non-Newtonian Rheology

A. C. Barr, R. T. Pappalardo (Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder)

Efforts to judge whether the ice I shells of Europa, Ganymede, and Callisto convect, and under what conditions convection can drive endogenic resurfacing have been hampered by uncertainties in ice rheology, shell thickness, and physics of tidal dissipation. Recent laboratory experiments suggest that ice I exhibits a stress- and temperature-dependent rheology at conditions appropriate for the shells.

We use numerical methods to determine the conditions required to trigger convection in an initially conductive, basally heated ice I shell with a composite Newtonian/non-Newtonian rheology and uniform grain size. The critical Rayleigh number varies as a power of the amplitude of initial temperature perturbation, ice grain size, and wavelength of the initial temperature perturbation. Regardless of the controlling rheology, a finite amplitude temperature perturbation is required to permit convection, which may require tidal dissipation.

If tidal dissipation is distributed in the shells over length scales similar to the shell thickness, the critical thickness for convection is <30 km if the ice grain size is less than 1 mm, and plume growth is controlled by Newtonian volume diffusion. The critical shell thickness is <30 km for grain sizes greater than 1 cm, when thermal stresses activate dislocation creep. For intermediate grain sizes, GSS creep controls plume growth, and critical shell thicknesses are close to the maximum permitted on the satellites. If tidal dissipation is concentrated on horizontal length scales longer than the shell thickness, the critical shell thickness for convection is larger, and requirements on the ice grain size more stringent. The convective stability of the ice shells may therefore depend on the evolution of grain size and distribution of tidal dissipation in the shells, in addition to rheological, thermal, and physical parameters.

This work is supported by the NASA Exobiology and GSRP Programs.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.