The Brittle-Ductile Transition in Mixtures of Rock and Ice: Experiments at Planetary Conditions

Introduction: Mars exhibits a wide variety of landforms that are indicative of ductile flow, ranging from the viscous creep of ice-rich permafrost [1] to the glaciation of thick Martian ice sheets [2] to the surface mobility of thin debris flows [3]. All of these flow features probably contain significant amounts of dust, which for example will likely be present throughout the impact-disrupted subsurface megare-golith [4], within dark bands in the Polar Layered Deposits [2], and inside sublimation lag layers atop ablating near-surface ice [5]. Due to the unknown rheological effects of dust upon water ice undergoing creep, the proportion of ice required to produce such deformation is uncertain. Mangold et al. [6] attempted to constrain the volumetric ice fraction by conducting constant load triaxial tests at differential stresses of 1.9-8.5 MPa, confining pressures of 12 MPa, and temperatures of 263 K. For ice contents ranging from 25% to 48%, they obtained relative viscosities 10-50 times higher than that of pure ice under the same conditions [6]. Moreover, Mangold et al. [6] concluded that the brittle-ductile transition in ice-rock mixtures occurs at ice fractions lower than 28%; hence, if the upper kilometer of the Martian megaregolith is comprised of ice-rich permafrost undergoing ductile deformation, then this 28% minimum value implies a global equivalent subsurface layer of at least 200 m [6]. We have recently begun an ongoing investigation into the rheology of ice-rock mixtures at planetary conditions, the main objective of which is to update the 1992 work of Dur-ham et al. [7] in order to assess the effects of particulates upon grain-size sensitive creep of water ice at low temperatures ranging from 77 K to 223 K. As part of this research program, our initial experiments on ice-rock mixtures should provide more detailed constraints on the nature of the brittle-ductile transition at low temperatures—and, by extension, the minimum ice content required for ductile flow on Mars. The Brittle-Ductile Transition: The maximum particle loading tested in our earlier 1992 work was 56% by volume [7]. At higher loading, conventional testing becomes problematic as the higher differential stresses that must be applied move the deformation regime closer to the brittle-ductile transition, a rather fundamental change in deformation mechanism, with a strong shift in the dependence of viscosity on temperature, pressure, and strain rate. The brittle-ductile transition in all rocks, including ice + rock mixtures, is gradual , with cataclastic fracturing and crushing …