Geologically rapid aqueous mineral alteration at subfreezing temperatures in icy worlds

The most active icy worlds such as Europa or Enceladus are predicted to host extensive aqueous alteration driven by water–rock interactions at elevated temperatures1–3. On the other hand, it is assumed that such alteration is kinetically inhibited at the subzero temperatures of other icy worlds, such as the mid-sized moons of Saturn and Uranus or trans-Neptunian objects1,4. Here we perform aqueous alteration experiments on a chondrite-analogue material (olivine) and find that chemical alteration processes are still efficient at temperatures as low as −20 °C, as the presence of an unfrozen water film still allows olivine to dissolve in partially frozen alkaline solutions. We infer that aqueous alteration may be enhanced by salts and ammonia present in icy worlds, and therefore remains a geologically rapid process even at subzero temperatures. Our results imply that the primary chondritic minerals in most icy bodies exceeding 400–500 km in diameter will be completely altered to hydrous secondary minerals early in their evolutionary histories. Alkaline conditions in icy worlds favour aqueous alteration processes even at subzero temperatures. As a consequence of geologically rapid alterations, the rocky cores of all icy bodies larger than 500 km in diameter should be largely composed by altered hydrated minerals formed within a few hundred million years after formation.