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Flow strength of highly hydrated Mg- and Na-sulfate hydrate salts, pure and in mixtures with water ice, with application to Europa

Journal of Geophysical Research E: Planets

By:
, , , and
DOI: 10.1029/2005JE002475

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Abstract

We selected two Europan-ice-shell candidate highly hydrated sulfate salts for a laboratory survey of ductile flow properties: MgSO4 ?? 7H2O (epsomite) and Na2SO4 ?? 10H2O (mirabilite), called MS7 and NS10, respectively. Polycrystalline samples in pure form and in mixtures with water ice I were tested using our cryogenic high-pressure creep apparatus at temperatures 232 ??? T ??? 294 K, confining pressures P = 50 and 100 MPa, and strain rates 4 ?? 10-8 ??? ???dot;e ??? 7 ?? 10-5 s-1. Grain size of NS10 samples was > 100 ??m. The flow strength ?? of pure MS7 was over 100 times that of polycrystalline ice I at comparable conditions; that of pure NS10 over 20 times that of ice. In terms of the creep law ???dot;e = A??n e-Q/RT, where R is the gas constant, we determine parameter values of A = 1012.1 MPa-ns-1, n = 5.4, and Q = 128 kJ/mol for pure NS10. Composites of ice I and NS10 of volume fraction ?? NS10 have flow strength ??c = [??NS10??NS10J + (1 - ?? NS10)??iceIJ]1/J where J ??? -0.5, making the effect on the flow of ice with low volume fractions of NS10 much like that of virtually undeformable hard rock inclusions. Being much stronger and denser than ice, massive sulfate inclusions in the warmer, ductile layer of the Europan ice shell are less likely to be entrained in convective ice flow and more likely to be drawn to the base of the ice shell by gravitational forces and eventually expelled. With only smaller, dispersed sulfate inclusions, at probable sulfate ?? < 0.2, the shell may be treated rheologically as pure, polycrystalline ice, with boundary conditions perhaps influenced by the high density and low thermal conductivity of the hydrated salts. Copyright 2005 by the American Geophysical Union.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Flow strength of highly hydrated Mg- and Na-sulfate hydrate salts, pure and in mixtures with water ice, with application to Europa
Series title:
Journal of Geophysical Research E: Planets
DOI:
10.1029/2005JE002475
Volume
110
Issue:
12
Year Published:
2005
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Geophysical Research E: Planets
First page:
1
Last page:
10
Number of Pages:
10