36th DPS Meeting, 8-12 November 2004
Session 16 Jovian System
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

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[16.12] Effects of plasticity on convection in an ice shell: Implications for Europa

L. Han, A.P. Showman (Univ. Arizona)

Europa's surface exhibits numerous pits, uplifts, and disrupted chaos terrains that have been suggested to result from convection in the ice shell. To test this hypothesis, we present numerical simulations of convection in an ice shell including the effects of plasticity, which provides a simple continuum representation for brittle or semibrittle deformation along discrete fractures. Plastic deformation occurs when stresses reach a specified yield stress; at lower stresses, the fluid flow follows a Newtonian, temperature-dependent viscosity. Three distinct modes of behavior can occur. For yield stresses exceeding 0.2-1 bars, depending on the melting-temperature viscosity and ice-shell thickness, plastic effects are minimal and stagnant-lid convection, with no surface motion and minimal topography, results. Intermediate yield stresses of 0.2-1 bars allow quasiperiodic, catastrophic overturns of the upper conductive lid, with (transient) stagnant lids forming in between overturn events. Small yield stresses of 0.1-0.2 bars allow continual recycling of the upper lid, with simultaneous, gradual ascent of warm ice to the surface and descent of cold, near-surface ice into the interior. Between the stagnant-lid regime and the episodic-overturn regime, we find a transitional regime characterized by the existence of a cold, quasi-stagnant upper lid that deforms plastically, leading to surface velocities of several mm/year. These simulations imply that extensive surface disruption would occur on Europa, and they suggest that, if yield stresses of 0.2-1 bar are relevant to Europa, then convection in Europa's ice shell can produce chaos-like structures at the surface. However, our simulations have difficulty explaining Europa's numerous pits and uplifts. When plasticity forces the upper lid to participate in the convection, dynamic topography of 100 to 150-m amplitude results, but the topographic structures generally have diameters of 30--100 km, an order of magnitude wider than typical pits and domes. Horizontal discontinuities in yield stress sometimes allow isolated narrow depressions to form, but none of our simulations produced isolated uplifts of any diameter. This work was supported by NASA PG&G and NSF Planetary Astronomy.

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