Abstract
Laboratory experiments, using analog fluids displaying two-phase flow and complex rheologies, can give valuable insights into accretion processes on planets. We use Ludox, aqueous dispersions of silica nanoparticles, as planetary material analogues. These dispersions allow independent variation of spreading velocity and mechanical structure of the plate while permitting detachment faults and axial ( volcanic approximately intrusions to occur. Saline water solutions placed in contact with Ludox, cause formation of a skin through salt diffusion, whose rheology evolves from purely viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults and axial ridge structures. Lithospheric (skin) thickness at a given extension rate can be varied by changing the surface water layer salinity. The latter can also modify the pressure gradient to which the experimental lithosphere is submitted due to osmosis processes, and therefore create an equivalent to the Earth's lithospheric pressure gradient. Moreover, the mechanical properties of the skin can also be independently controlled by changing the type of colloid. The shape of the ridge axis observed in the laboratory experiments can be quite similar to the terrestrial cases. Morphologies of segmentation such as overlapping spreading centers, transform faults and rotating microplates are reproduced. Scaling laws based on fracture and fluid mechanics allow to predict the size, duration and abundances of theses features, as well as their conditions of existence on Earth. Varying the experimental fluids and conditions, we can also reach control parameters out of the range of the modern Earth. In particular, we were able to reproduce and explain two end-member cases. a) On hotter Venus, the extension morphologies encountered inside the large Artemis coronae could correspond to a combination of very high spreading rate and thin elastic axial crust. b) On icy satellites, double ridges could correspond to a very slow spreading case.