Viscosity measurements of aqueous magnesium sulfate solutions under high pressure: Implications for subsurface fluids in large icy satellites

Subsurface oceans and brines beneath the thick icy crust of large icy satellites such as Europa, Ganymede, Callisto, and Titan are among the most promising targets for exploring potential habitability. The physical properties of these liquids, particularly viscosity, play a fundamental role in governing fluid dynamics, as well as material and heat transport occurring within high-pressure environments. Although magnesium sulfate (MgSO₄) is likely one of the primary dissolved salts in these extra-terrestrial oceans, its viscosity under high-pressure conditions remains poorly understood. In this study, a falling-sphere viscometer was developed with a diamond anvil cell (DAC) to measure the viscosity of 10 wt% MgSO₄ solutions at pressures up to 1100 MPa and temperatures ranging from 263 to 313 K. Our results showed that MgSO₄ solutions exhibited viscosities more than 1.5 times as high as that of pure water at the same pressure and temperature conditions. At low temperature, the viscosity of MgSO₄ solutions increased monotonically with pressure, whereas pure water exhibited a minimum viscosity at ∼200 MPa. This difference reflects the strong ionic effects on the disruption of water structure and construction of hydration shell by Mg²⁺ and SO₄²⁻ ions. By extrapolating our findings to subsurface ocean conditions, we estimated that 10 wt% aqueous MgSO₄ oceans/brines in icy satellites would have viscosities between 1 and 13 mPa·s at pressures below 700 MPa. This finding suggests that aqueous MgSO₄ fluids potentially present in icy satellites can exhibit higher viscosities compared with pure water, whose viscosities are typically limited to the narrow range of 1–2 mPa·s.