Ransford, G. A., A. A. Finnerty, K. D. Collerson (1981) Europa's petrological thermal history. Nature 289: 21-24.
principle objective: demonstrate that water/ice condensed in Europa is mostly contained in hydrous silicates
- Europa accreted from ice and silicates; was heated by radiogenic sources,
heat distributed by conduction;
previous models: high T reached soon after accretion, differentiation /
segregation of silicate core over liquid water mantle, low relief indicates
thick ice crust on surface; water mantle may remain liquid indefinitely due to tidal heating, or may be cooled by convection in the ice crust, solidified to form thick ice mantle
- assertion: accretional heating would not dehydrate any nominally hydrous minerals because accretional heating insufficient for small body in jovian nebula (cf. solar nebula); even if Europa accreted as anhydrous silicates + water ice, later thermal evolution would melt ice and hydrate silicates
- assume chondritic bulk composition (mostly olivine, assume forsterite) + 5 weight % H2O; olivine hydrates to serpentine, chlorite, brucite (fig. 1: P-T diagram); role of Fe uncertain
- 5 wt. % water would be consumed by 1/3 of planet's volume (assume planet is all olivine), yielding interior of 2/3 olivine + 1/3 serpentine, etc., or outer hydrous silicate layer ~270 km thick
- what about convection? interior heated (radiogenic, maybe tidal?);
convection of anhydrous silicates occurs, but hydrous silicate layer need not convect, based on Moon evolution; "additional information on the rheology of 'wet' serpentine and chloritite will be needed before the possibility of convection within hydrated mineral assemblages can be properly evaluated" - see next paper, below, in which assertion is made that hydrous silicates are brittle, especially when oversaturated
Finnerty, A. A., G. A. Ransford, D. C. Pieri, K. D. Collerson. Is Europa
surface cracking due to thermal evolution? Nature 289: 24-27.
thesis: "some of the surface features of Europa may have originated by
processes within a model planet in which hydrated silicates are stable"
- below dehydration T, serpentinites and chloritites are strong and brittle, they become ductile with incr. T, P; at and above dehydration T, strength weakens greatly, rocks become brittle; free water fills pores
- water ice behaves in brittle manner until close to melting T, so thick ice crust should behave differently than thin; brittle ductile transition should occur below ~24 km on Europa; thus large-scale cracks in Europa's ice crust indicate anisotropic stresses propagated through ice thickness < 24 km over deforming interior (unclear)
- possible cause of large scale linear features (triple bands): dehydration of hydrous silicates at low pressures by reactions such as: serpentine (hydr) + brucite (hydr) = forsterite (anhydr) + water
positive volume change on dehydration is small at high P but pronounced at low P
- if Europa accreted at low T, hydrous silicates may be distributed throughout planet; radiogenic heating would eventually increase T to dehydration T of hydrous silicates (500-700 C =~800-1000 K); H2O must either increase local rock volume,. or escape; in latter case, may hydrate overlying rock (but overlying rock cooler)
- dehydration "front" would coincide with upward movement of this isotherm; dehydrated lower region decreases in volume, upper region being hydrated expands; volume change greater at lower P, overall V increase;
eventually, water saturates hydrous silicate layer (~270 km deep, below thin ice crust); any additional water will embrittle hydrous layer, and convection in underlying hot anhydrous silicate interior may crack hydrous layer
- triple bands manifest liquid water-ice breccia-hydrous silicate eruption, followed by clean. "xenolith-free" water;
concentrations of fractures near the anti-jovian point due to tensile stress by expansion of hydrous silicate layer; distribution of surface features may be related to upwelling and downwelling in convecting interior
- my comments: convection needs to be addressed using information on P-T-dependent rheology of hydrous silicates; if hydrous silicate layer convects, would it mix with anhydrous interior? what happens to heat brought up to hydrous-anhydrous boundary from hot, convecting interior? would increased T in hydrous layer cause it to convect?