Chemical reactions known as “aqueous alteration” can occur when rocky materials react with liquid water. Because such reactions release heat, they may foster conditions conducive to planetary habitability. However, it has been assumed that subsurface temperatures on mid-sized satellites such as the moons of Saturn and Uranus are too low for aqueous alteration to occur at the interface between their outer H2O-rich outer shells and inner rocky cores.
A study by Amber Zandanel (Université Grenoble Alpes, France) and colleagues aimed to determine whether aqueous alteration reactions are possible at the water-rock interfaces of icy satellites. To do so, they studied the evolution of olivine grains (a common analog for primitive planetary compositions) in solutions containing 0%, 0.8%, and 8% ammonia at temperatures of -20°, 4°, and 20°C over the course of 442 days. Because dissolved salts suppress water’s freezing point, higher concentrations of ammonia result in more fluid existing even at temperatures below the freezing point of water. They then determined the presence and amount of liquid remaining near the crystal grains, using cross-polarized light microscopy and Raman spectroscopy, to identify the composition of the remaining liquid. Results showed that even at temperatures as low as -20°C, liquid water persists and facilitates chemical alteration of olivine grains due to high concentrations of ammonia in the remaining liquid.
This study shows that the interiors of the mid-sized moons around Saturn and Uranus may have been much more geologically active than previously thought, since their H2O-rich layers are likely to have a dissolved salt component. Punctuated heating events, such as passage through tidal resonances and large impact events, will accelerate aqueous alteration processes, suggesting that a core of originally chondritic composition could be significantly altered to a secondary mineral assemblage early in a moon’s history, while possibly fostering habitable conditions favorable to early life. READ MORE