Picture a spacecraft flying through a cloud of frozen spray, thrown up from cracks near the south pole of a moon smaller than England. That is roughly what NASA's Cassini did between 2004 and 2017, sampling icy grains flung from Enceladus into Saturn's E-ring. Beneath that moon's crust sits a global ocean, and these grains were the closest thing scientists had to a bucket dropped straight into alien seawater. But when a team led by Frank Postberg at Freie Universität Berlin examined 961 salt-rich particles, something odd emerged.

If the grains all came from the same ocean, shouldn't they contain roughly the same mix of salts? Instead, some were dominated by sodium chloride, others by carbonates, phosphates, or potassium chloride. Chloride and carbonate almost never appeared together in the same sodium-rich grain. One ocean, seemingly, was producing wildly inconsistent souvenirs.

The answer, it turns out, was not about different oceans but about different weather, of a sort. Researchers at the Earth-Life Science Institute in Tokyo, led by Yasuhito Sekine, recreated Enceladus-like droplets in the laboratory and froze them under varying conditions. When droplets around 200 micrometres wide froze slowly, at roughly 10 kelvin per minute or less, their salts separated into distinct pockets. Frozen quickly, the same ingredients stayed evenly blended.