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 K À1 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.