Experimental and theoretical deformation of ice-rock mixtures: Implications on rheology and ice content of Martian permafrost
The distribution and amount of ground ice on Mars is an important issue for the future exploration of this planet. The presence of ground ice is demonstrated by landforms due to the viscous deformation, such as softened terrain and lobate debris aprons. The proportion of ice needed to produce the viscous deformation is still unknown. Constant load triaxial tests are conducted at differential stresses of 1.9- 8.5 MPa, confining pressure of 12 MPa and temperature of 263 K in order to determine experimental limits on the proportion of ice in the Martian subsurface. Samples are ice-rock mixtures made of quartz grains of different size distributions with ice content from 25% to 48%. Creep is observed in most tests with viscosity from 10 to 50 times higher than pure ice (same conditions) that fits the range of solid/liquid suspensions. Tests also conclude to the occurrence of a brittle-ductile transition, at ice content approximately lower than 28%. This transition is not due to the external conditions but to the composition of the material in agreement with data about terrestrial rock glaciers. This transition is likely due to the granular effect of the solid grains which becomes dominant in the deformation at high solid content. Martian's softened terrains suggest a viscous deformation over the first kilometer of the Martian permafrost. The ice fraction at this depth should therefore be of 28% minimum to produce a viscous deformation. Such ice proportion allows to estimate the total amount of water in the ground to correspond to a 200 m thick ocean spread over the whole planet. Calculated strength envelopes of the Martian subsurface highlight the possibility to produce rheological discontinuities by ice-rich layers in the subsurface.