A document by Cassandra Seltzer. Click on the document to view its contents.

Icy moons in the outer Solar System contain rocky, chondritic interiors, but this material is rarely studied under confining pressure. The contribution of rocky interiors to deformation and heat generation is therefore poorly constrained. We deformed LL6 chondrites at confining pressures ≤ 100 MPa and quasistatic strain rates, and recorded acoustic emissions (AEs) using ultrasound probes. We defined a failure envelope, measured ultrasonic velocities, and retrieved elastic moduli for the experimental conditions. Chondritic material stiffened with increasing confining pressure, and reached its peak strength at 50 MPa confining pressure. Microcracking events occurred at low stresses, during nominally “elastic” deformation, indicating that dissipative processes are possible in rocky interiors. These events were most energetic at lower differential stresses, and occurred more frequently at lower confining pressures. We suggest that chondritic interiors of icy moons are therefore stronger, less compliant, and less dissipative with increasing pressure and size.

Titan is unique among icy moons for its active surface processes and extensive erosional features. The presence of coarse sediment suggests that mechanical weathering breaks down Titan’s surface material, but the exact processes of mechanical weathering are unknown. We tested the idea that topographic features perturb ambient crustal stresses enough to generate or enhance fractures. We used a boundary element model to predict the likely stress state within hypothetical Titan landforms, including river valleys and isolated ridges, and to model the locations and types of resulting fractures. Our results suggest that topographic stress perturbations are indeed sufficient to generate fractures and drive mechanical weathering, with little dependence on the density of the material making up Titan’s crust and landforms and no dependence on its elastic moduli. For material density of 800 to 1200 kg/m3, opening-mode failure is predicted to occur within hypothetical Titan landforms with relief exceeding 20 m at ambient horizontal stresses up to 1 MPa of compression, which encompasses typical predicted tidal stresses ranging between 10 kPa of compression and 10 kPa of tension. Under the same conditions, shear fracture is predicted to occur if cohesion of the material is less than 100 kPa or if hydrocarbon fluids reduce local effective normal stresses. We therefore suggest that Titan’s crust may be highly fractured and permeable, and that the predicted fractures could create pathways for sediment generation and subsurface transport of fluids.

As partially molten rocks deform, they develop melt preferred orientations, shape preferred orientations, and crystallographic preferred orientations (MPOs, SPOs and CPOs). We investigated the co-evolution of these preferred orientations in experimentally deformed partially molten rocks, then calculated the influence of MPO and CPO on seismic anisotropy. Olivine-basalt aggregates containing 2 to 4 wt% melt were deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa at shear stresses of τ = 0 to 175 MPa to shear strains of γ = 0 to 2.3. Grain-scale melt pockets developed a MPO parallel to the maximum principal stress, s1, at γ < 0.4. At higher strains, the grain-scale MPO remained parallel to s1, but incipient, sample-scale melt bands formed at ~20° to s1. An initial SPO and CPO were induced during sample preparation, with [100] and [001] axes girdled perpendicular to the long axis of the sample. At the highest explored strain, a strong SPO was established, and the [100] axes of the CPO clustered nearly parallel to the shear plane. Our results demonstrate that grain-scale and sample-scale alignments of melt pockets are distinct. Furthermore, the melt and the solid microstructures evolve on different timescales: in planetary bodies, changes in the stress field will first drive a relatively rapid reorientation of the melt network, followed by a relatively slow realignment of the crystallographic axes. Rapid changes to seismic anisotropy in a deforming partially molten aggregate are thus caused by MPO rather than CPO.