Viscosity of liquid ferric sulfate solutions and application to the formation of gullies on Mars

by Chevrier, Vincent F.; Ulrich, Richard; Altheide, Travis S.

We studied the viscosity of ferric sulfate Fe-2(SO4)(3) solutions as a model for low-temperature liquids on the surface of Mars and their implication in the formation of gullies. Viscosity varies with temperature and concentration, ranging from 7.0 x 10(-3) Pa s for 38.8 wt % at 285.15 K to 4.6 Pa s for 58.2 wt % at 260.15 K. Using the experimental results, we built a semiempirical equation of viscosity as a function of temperature and salt concentration, which was combined with a numerical model to estimate the effect of these solutions on the formation of gullies. Calculated fluid velocities ranged from 0.5 to 14 m s(-1), in accordance with estimates from image analyses. Turbulent flow occurs in the majority of the conditions and is characterized by a constant velocity (similar to 8.5 m s(-1)). At very low temperature and high concentration, the laminar regime shows reduced velocities (down to similar to 0.5 m s(-1)). In between, a transitional regime presents high velocities, up to 14 m s(-1). Using the velocities, we determined the size threshold for boulders to be moved by the liquid flow. Depending on the regime, boulders of diameter inferior to 3 m (turbulent), 4 m (transition), and down to 0.5 m (laminar) are displaced. Since laminar flow occurs only in an extremely limited range of conditions, for low temperatures (< 240 K) and supersaturated solutions, the abundance of small boulders (similar to 0.5 m) in gully channels requires lower velocities and higher viscosities than ferric sulfate solution or any other water-based liquid can reach. This suggests an important participation of debris mixed with the liquid phase.

Journal of Geophysical Research-Planets
2169-9100; 2169-9097