Inhibition of Grain Boundary Sliding in Fine‐Grained Ice by Intergranular Particles: Implications for Planetary Ice Masses.

Ice in both terrestrial and planetary settings often contains rock particles. Here we present an experimental investigation of the influence of intergranular particles on the rheological behavior of ice. Experiments were performed on samples fabricated from 10‐μm ice powders +1‐μm graphite or 0.8‐μm alumina particles and subjected to elevated confining pressures. A critical particle fraction, ∼6%, was observed, below which samples behave like pure ice and deform by both grain boundary sliding (GBS) and dislocation creep, and above which GBS creep is impeded. Above this critical fraction, ice grains occur in particle‐free clusters surrounded by bands of particles mixed with fine‐grained ice, resulting in the impedance of GBS in the bands as well as sliding between the ice clusters. Our results imply that South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain >90% ice. Plain Language Summary: Ice on Mars, Ceres, and icy satellites often contains rock particles. The presence of particles in ice changes its flow behavior and thus is important for understanding the composition and evolution of planetary ice masses. Based on laboratory experiments on samples made of fine‐grained ice and intergranular particles, we determined a critical quantity of particles, about 6% by volume, below which the ice‐particle samples flow like pure ice, and above which, sliding between grains (so‐called grain boundary sliding, or GBS) is impeded. The impedance of GBS by particles has not previously been observed. At planetary conditions, GBS is often the dominant flow mechanism for pure ice. Our result thus imply that the South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain more than 90% ice. Key Points: Below the critical fraction, ∼6%, particle‐bearing ice behaves like pure ice and deforms by both grain boundary sliding and dislocation creepAbove the critical fraction, ∼6%, GBS creep is impeded in particle‐bearing ice, as bands of particles separate ice clustersIcy masses on Mars, Ceres, and outer planet satellites could contain more ice than previously expected [ABSTRACT FROM AUTHOR]