The strength of clathrate-rich mixtures of ice I and methane clathrate hydrate

The strengths of water ice I and of methane clathrate hydrate at planetary conditions are known individually, but that of ice-clathrate mixtures is largely unconstrained by experiment. Subsurface materials on icy satellites of the outer solar system may well include such mixtures, so an understanding of their mechanical properties is demanded. The question is especially important because methane clathrate and ice I differ hugely in strength, the clathrate being several orders of magnitude more viscous than ice at the same stress and temperature. We present the strengths of various methane clathrate-rich ice + clathrate mixtures (5%, 15%, 25% ice by volume) as measured in triaxial deformation experiments. Our preliminary results show that mixtures with as little as 15% ice exhibit weak, ice-like behavior. A general constitutive law for the strength of two-phase mixtures is elusive, but in mixtures of phases with extreme viscosity contrast substantially more than 25% of the weaker phase is usually required before the strength of the aggregate starts resembling that of the weaker phase. Thus the current results are surprising. Methane clathrate has been suggested as a reservoir of atmospheric methane on Titan, and has been implicated in the origin of geysers on Enceladus, but its presence in massive formations or thick layers has been problematic for tectonic models because of its near rock-like strength. If relatively small amounts of ice can weaken the aggregate, the existence of massive clathrate may become less objectionable, at least on mechanical grounds.