The viscosity of aqueous solutions as analogs to cryovolcanic liquids

Abstract

Effusive cryovolcanism may have occurred now or in the past on several icy bodies of the outer solar system. Our understanding of the physical properties of potential cryolavas is hindered by the limited range over which experimental data exist at appropriate conditions. The viscosity of the purely liquid phase is poorly constrained, yet, is a required component for understanding subsurface oceans, cryomagma ascent/transport, and subsequent evolution during emplacement. However, limited data exist for the viscosity of brines extending down temperature (<0°C). We have thus measured the viscosities of a broad range of binary aqueous systems at a range of temperatures and concentrations. These include the water-rich side of the eutectic for chloride (Na, K, NH4), sulfate (Mg, K, NH4), ammonia, and methanol systems between −35 and 30 °C. For geological applications, the effect of temperature on viscosity is often a larger control than the effect of concentration. We therefore constructed a model of viscosity based on the Vogel-Fulcher-Tammann equation, commonly used for silicate melt viscosities. This semi-empirical, temperature dependent model was parameterized for a given composition to account for the concentration dependence. Our model reproduced the experimental dataset to within 10 % relative uncertainty (typically within 5 %) and reproduces exactly the curve defined for pure water. The model is capable of extrapolating down to sub-ambient temperatures relevant to cryovolcanism without any singularities. Preliminary investigation of ternary systems (H2O-NaCl-MgSO4 & H2O-NH3-CH3OH) suggests this model can be successfully extended to more complicated multi-component systems. This relatively straightforward scalability and the coupled temperature and concentration dependence are key improvements over previous, more complicated models. The viscosity data produced in this study covers a poorly investigated temperature range and can be applied to cryovolcanic processes on icy bodies.