Cassini's salty Enceladus grains look like shards of slowly frozen ocean droplets

"The Cassini data showed us that these salt-rich grains are far more chemically diverse than an average ocean composition would suggest," Postberg said ^[9]. The grains came from Saturn's E-ring, which Enceladus supplies, and Cassini's Cosmic Dust Analyzer measured them one at a time between 2004 and 2017 ^[1]. The pattern that needs explaining is a missing pairing. If each grain were a frozen drop of the average ocean, a sodium-rich grain with chloride should usually carry carbonate too. In the Cassini spectra the two rarely shared a grain ^[3].

The experiment at the Earth-Life Science Institute in Tokyo, part of a study published in Science Advances ^[11], asked whether one body of water could produce that split. Yasuhito Sekine's group froze droplets built to match the major salts expected in the ocean, varied droplet size and cooling rate, and then mapped where the elements ended up ^[4]. In droplets about 200 micrometers across, freezing at roughly 10 K per minute or slower pulled the salts into separate regions. Faster freezing left them much more evenly mixed ^[5].

That fast-freezing run is the experiment's control. Earlier work had generally assumed the spray freezes quickly and heads straight for space ^[7]. Under fast freezing the lab droplets came out uniform, the opposite of what Cassini saw ^[5]. If this explanation holds, the grains point to slow freezing during a slower passage through the vents, perhaps along winding fractures ^[7]. The cooling rate inside Enceladus is an inference drawn backwards from the grains.

"What surprised us was that the diversity seen by Cassini could emerge from droplets originating from essentially the same ocean water," Sekine said ^[6]. In the team's proposed route, droplets tens to hundreds of micrometers across freeze and sort deep in the vents. Nearer the surface, accelerating gas drives them into the walls of narrower ice channels. They shatter into fragments that carry different salt-rich regions out to the E-ring ^[8]. The shattering stage is a proposal; the experiments as described covered the freezing ^[4].

So is the plume still a sample of the ocean? Postberg's account says yes, if you count. "The abundance of each individual component in the ocean is then reflected in the number of fragments in which a particular component is found," he said ^[10]. A single grain is a piece of a sorted droplet. The ocean's recipe sits in the frequencies across many grains. That counting rule assumes each salt-rich region is about as likely as any other to break off and reach the E-ring. The release does not say whether that was tested.

The sorting reaches beyond salts. Salts also separate from organics, and earlier analyses found many organic species appearing apart from one another at elevated concentrations ^[12]. A compound that is dilute in the ocean could turn up concentrated in a handful of grains. That makes it easier to detect, and it makes one rich grain a poor measure of how much of the compound the ocean holds.