Ice/hydrate Eutectics: the Implications of Microstructure and Rheology on a Multi-phase Europan

Introduction: Galileo’s Near-Infrared Mapping Spectrometer observed distorted and asymmetric water absorption bands suggestive of H2O in a physical state other than crystalline ice on the surface of Europa.[1,2] The microstructure and related mechanical response of the crust, then, is dependent not on just pure ice, but more likely on a polyphase aggregate of ice and (a) non-ice phase(s). Laboratory spectroscopy studies and thermochemical models based on chondritic abundances point to several possible candidates for the non-ice component [1,2,3,4]. We present part of our ongoing investigation into the phase morphology and deformation behavior on two binary ice-salt hydrate systems that are likely volumetrically important on Europa: H2O-Na2SO4 and H2O-MgSO4. In both cases, the aggregates are characterized by a eutectic solidification/melting reaction. Methods: The samples in this study were solidified from a homogeneous liquid solution. Compositions corresponding to a system’s stable or metastable eutectic (Fig. 1) were immersed in a cryobath that was calibrated so as to monitor heat release upon crystallization; the method allowed quantification of phase growth rate during solidification. Temperature cycling about the eutectic was implemented to promote growth of stable phases [5]. Microstructural analysis was performed using images obtained from Cryogenic Scanning Electron Microscopy (CSEM). Compressional creep tests (240 ≤ T(K) ≤250; 2.7 ≤ σ(MPa) ≤ 12.7) were carried out in an apparatus employing gaseous N2 (P = 50MPa) as the confining medium; specimens (fully-dense cylinders 25-mm dia., 66-mm long) were jacketed in In (250-μm thick) to separate them from the pressurized gas. Two samples in the system H2ONa2SO4 and three in the system H2O-MgSO4 were tested. Eutectic microstructure: All samples grown from solution exhibited classical eutectic microstructures, that is, the solids consist of “colonies” of a fine, ordered intergrowth of two phases; colonies impinge upon one another as they grow, forming colony boundaries. We found that the pattern, or morphology, the intergrowth takes within a colony is unique to each ice/sulfate system, regardless of its bulk composition. As nominally identical structures have been long identified in metallurgy, we borrow their terminology[6] in stating that (i) the H2O-Na2SO4 system forms a “broken lamellar” structure and (ii) H2O-MgSO4 system forms a “complex regular lamellar” structure. The broken lamellar morphology is characterized by uniform blade-like mirabilite (Na2SO4•10H2O) arranged in roughly parallel columns within a water ice matrix (Fig. 2a). The complex regular structure seen in the system H2O-MgSO4 involves an interconnected maze-like structure with dispersed regions of aligned plates, or lamellae (Fig. 2b); both the stable eutectic phase, MgSO4•12H2O (“MS12”) and the metastable eutectic phase MgSO4•7H2O (“MS7”; epsomite) exhibited this microstructure, differing only in scale. Mechanical response: The mechanical response of the polyphase aggregates can be characterized by the Dorn or “power-law” model, in which the steadystate strain rate is thermally activated and a nonlinear function of stress, i.e., n a exp( E / RT) ε ∝ σ −