Figure 3. Required minimum temperatures of hydrothermal fluids on Enceladus.

Results are obtained from the cross-sections between the solubility of silica at 0 °C and the ΣSiO₂ values determined by the serpentine–talc buffer as a function of seawater pH at 0 °C for different values of hydrothermal fluid pH. In other words, the required minimum temperatures are determined when the ΣSiO₂ value for a given hydrothermal fluid exceeds the solubility of silica at 0 °C. The silica solubility is obtained for Na⁺ concentration of 0.1 mol kg⁻¹ and pressure 30 bar. The pH of Enceladus' seawater has been suggested to be in the range of 8.5–10.5 (refs 1, 5) (the shaded area). The solid lines show the results when hydrothermal fluid pH values change to seawater pH values by cooling and mixing with oceanic water (see text).