Ice is commonly found mixed with rock particles in a variety of planetary and terrestrial settings, including on the surfaces of icy satellites, in Mars’s polar ice caps, and at the base of Earth’s glaciers and ice sheets. Previous laboratory experiments have shown that below a particle fraction of about 15%, the strength of these mixtures is comparable to that of pure ice, whereas above this threshold, the sample strength increases proportionately to the fraction of particles. But because these studies were conducted under widely ranging pressure and temperature conditions, researchers have yet to determine which physical mechanisms are responsible for these effects.
Now Qi et al. report the results of a series of laboratory experiments that offer fresh insight into how intergranular rock particles influence the rheology of ice. After fabricating samples of fine-grained ice powder blended with either 1 micrometer of graphite or 0.8 micrometer of alumina particles, the team subjected these materials to sequentially higher stresses to observe how the samples deformed.
The results indicate that at particle fractions below about 6%, the samples behaved like pure ice, deforming both by grain boundary sliding and by dislocation creep, a process whereby ice crystals deform plastically. Above this critical 6% threshold, however, the authors found that bands of rock particles surrounding colonies of ice crystals impeded grain boundary (or colony boundary) sliding, the type of viscous flow believed to dominate the deformation of ice in numerous planetary and terrestrial settings.
This inhibition of grain boundary sliding at relatively low fractions of intergranular particles has important implications for determining the compositions and histories of ice masses throughout the solar system. If, for example, grain boundary sliding is inhibited in Mars’s south polar layered deposits, their viscosity would be 3 orders of magnitude higher than previously calculated. The authors therefore argue that these deposits could have experienced the extensive, long-term, viscous relaxation reported by other researchers only if they consist of more than 94% ice.
These groundbreaking experiments may also help researchers constrain the composition of Ceres, the largest object in the asteroid belt, as well as outer planet satellites like Callisto and Ganymede, which have experienced significant crater relaxation. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080228, 2018)
—Terri Cook, Freelance Writer