[1] Ice resistance to tensile fracture influences surface morphodynamics on outer planetary satellites such as Titan, yet measurements of tensile strength and fracture toughness of polycrystalline water ice at temperatures below terrestrial conditions (<220 K) have not been previously reported. We investigated these parameters from 260 K to 110 K using a walk-in freezer, and chilling by dry ice and liquid nitrogen. We also investigated the influence of solid impurity concentration and the spread in crystal grain size distribution. Although fracture toughness showed no sensitivity to temperature, we find that tensile strength increases with decreasing temperature at 7 kPa KÀ1 for all ice types tested. For pure water ice, samples made from uniform-sized seed crystals were stronger than mixed-grain-size ice, suggesting that strength is limited by the coarse tail of the size distribution. Samples tested submerged in liquid ethanol were 0.45 MPa weaker than in air; increasing porosity reduced tensile strength. Tensile strength increased linearly with concentration of urea, basalt and ammonium sulfate. These results suggest that on Titan and other icy satellites, the tensile strength of fine-grained polycrystalline water ice containing solid impurities may be several times greater than the 1 MPa value commonly used in modeling. For low strain rate processes where fracture propagation rather than fracture initiation limits strength, a temperature invariant fracture toughness of 0.15 MPa m 1/2 is appropriate. Understanding ice diagenesis on Titan, and the resulting composition, grain size distribution, and porosity, is needed to accurately model surface processes that are limited by ice resistance to fracture.

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Methane hydrate formation and dissociation are buffered by salinity in a closed system. During hydrate formation, salt excluded from hydrate increases salinity, drives the system to three-phase (gas, water, and hydrate phases) equilibrium, and limits further hydrate formation and dissociation. We developed a zero-dimensional local thermodynamic equilibrium-based model to explain this concept. We demonstrated this concept by forming and melting methane hydrate from a partially brine-saturated sand sample in a controlled laboratory experiment by holding pressure constant (6.94 MPa) and changing temperature stepwise. The modeled methane gas consumptions and hydrate saturations agreed well with the experimental measurements after hydrate nucleation. Hydrate dissociation occurred synchronously with temperature increase. The exception to this behavior is that substantial subcooling (6.4°C in this study) was observed for hydrate nucleation. X-ray computed tomography scanning images showed that core-scale hydrate distribution was heterogeneous. This implied core-scale water and salt transport induced by hydrate formation. Bulk resistivity increased sharply with initial hydrate formation and then decreased as the hydrate ripened. This study reproduced the salinity-buffered hydrate behavior interpreted for natural gas-rich hydrate systems by allowing methane gas to freely enter/leave the sample in response to volume changes associated with hydrate formation and dissociation. It provides insights into observations made at the core scale and log scale of salinity elevation to three-phase equilibrium in natural hydrate systems.

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