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R. T. Pappalardo, A. C. Barr (University of Colorado, LASP)
Europa's surface is peppered by topographic domes ~100 m tall and ~10 km wide; however, the means of forming dome topography has been elusive. It has been suggested that solid-state convection might create thermally buoyant diapirs that impinge on the surface, warping or faulting near-surface brittle ice upward into domes. However, the driving stress achievable from the thermal buoyancy in a ~20 km thick ice shell is relatively small (~0.04 MPa) so could uplift domes only ~10 m high.
Compositional diapirism can create a much larger density differences, and therefore greater driving pressures, if Europa's icy shell contains contaminants such as hydrated salts or sulfuric acid, as suggested by the Galileo NIMS data. For example, if pure warm ice rises through an ice shell contaminated with salts of density 1500 kg/m3, then an average salt volume fraction of only 2% permits a driving pressure of ~0.1 MPa, which can create a 100 m dome given an elastic overburden of effective thickness of 1 km and Young's modulus of 1 GPa.
We suggest that compositional buoyancy should be expected in a salt-rich Europan ice shell. The temperature of the upwelling ice plumes in a convecting ice shell is modeled as ~250 K, above the eutectic temperature of many plausible hydrate contaminants. Thus, hydrate contaminants will tend to drain out of rising ice plumes, while remaining in the colder overlying ice. Therefore, an upwelling process that initiates as thermal convection can trigger compositional segregation, driving diapirs buoyantly upward through a thick Europan ice shell with force sufficient to create the observed surface topography.
This work is supported by the NASA Exobiology Program.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.