RHEOLOGICAL INVESTIGATION OF CRYOVOLCANIC BRINES: VISCOSITY OF CHLORIDE AND SULFATE BRINES

Rheological properties of potential cryovolcanic products are fundamental in determining how flows are emplaced and what morphologies result. Evolving cryogenic compositions span a similar viscosity range as silicate lavas allowing similar morphologies to occur (e.g. domes or flows). Few experimental studies have been conducted to provide rheological data relevant to cryovolcanism. A broad compositional range of liquids need to be fully characterized to investigate the effect of crystallization. We conduct measurements of briny liquids to model viscosity as a function of temperature and composition.

Binary aqueous salt systems [NaCl, KCl, NH4Cl, MgSO4, K2SO4, (NH4)2SO4] were chosen for their simplicity, well characterized nature, and relevance to cryovolcanism on Europa. Three concentrations on the water-rich side of the eutectic were synthesized for each composition. Isothermal liquid viscosity measurements were made at intervals between 30°C and -20°C. Cooling experiments (2, 1, and 0.5 K/min) were also run to provide information about viscosity evolution during undercooling and the kinetics of crystallization onset.

MgSO4 shows the most variation in viscosity for both temperature and concentration. At 0°C the viscosity of pure water is ~1.5 mPas, while the viscosity of 5 and 15 wt% solutions are 2.0 and 5.5 mPas. Viscosity increases to 5 and 14 mPas respectively at -20°C. For the other compositions, the same concentrations have less effect on viscosity, which range from 1.3 – 2.3 mPas at 0°C and from 2.5 – 4.5 mPas at -20°C. The viscosity of Europa’s ocean at the base of the ice crust is likely to be of the order of 2 mPas at an approximate liquidus temperature of 0°C.

Continued work will include measurements of methanol, ammonia, and carbonate systems. Viscosity measurements will also be conducted for higher concentrations on the solute-rich side of the eutectic. This will provide better constraints on physical and fluid dynamic evolution of flows on icy worlds.