Elasticity and dislocations in ice X under pressure

We determine the compressibility, the elasticity and the slip systems for ice X in the 100-350 GPa pressure range from first-principles calculations. Our results show that ice X has a high elastic anisotropy, which increases by more than two orders of magnitude over this pressure range. We use the Peierls-Nabarro model to compute the dislocation structure for the most likely glide systems. We obtain that the easiest slip systems are 〈1 1 1〉{1 1 0} at low pressures, and 〈1 0 0〉{1 1 0} at higher pressure. This suggests that the mantle of large icy planets may have different rheological properties at shallow and at deep conditions. In contrast the mantle of smaller icy planets would be more homogeneous from a rheological point of view.