Chloride and carbonate, two salts that scientists expected to turn up together in ice grains from the ocean of Saturn’s moon Enceladus, “rarely appeared together in the same sodium-rich particle” in data from NASA’s Cassini spacecraft. A team led by Frank Postberg of Freie Universität Berlin and Yasuhito Sekine of the Earth-Life Science Institute at the Institute of Science Tokyo reanalysed 961 mass spectra of the salt-rich grains and then froze droplets of simulated ocean water in the laboratory to explain what they saw.
Their answer is that the ocean need not be patchy at all. The grains are probably fragments of larger frozen droplets whose salts separated as they froze slowly inside the moon’s vents. The study appears in Science Advances.
Cassini’s Cosmic Dust Analyzer and 961 salt-rich spectra
The measurements come from the Cosmic Dust Analyzer on Cassini, which sampled individual ice particles in Saturn’s E-ring between 2004 and 2017. NASA’s Cassini team found in 2005 that icy particles and gas gush from the moon’s surface at roughly 800 miles per hour through warm fractures nicknamed tiger stripes, feeding the ring. Four prominent fractures, each about 84 miles (135 kilometers) long, cross the south polar terrain, and the analyzer flew through the debris and recorded what each grain was made of.
The salt-rich grains are classed as Type 3 particles, and the new study examined 961 of their spectra. If those grains were simple samples of one uniform ocean, each would carry a similar mix of salts. They do not. According to the Tokyo release carried by ScienceDaily, chloride and carbonate rarely sat together in the same sodium-rich grain, which was the puzzle the researchers set out to explain.
A Sci.News account of the paper counts at least five chemical subtypes within Type 3 grains, dominated respectively by sodium chloride, sodium carbonate or bicarbonate, sodium phosphate, sodium hydroxide and potassium salts. Grains once thought of as uniformly salty turn out to be a family of distinct recipes.
Slow freezing in the vents separates the salts
To test whether freezing alone could produce that diversity, the team froze droplets of ocean-composition water about 200 micrometers across under different conditions. Fast freezing kept the salts evenly mixed. Cooling at about 10 kelvin per minute or less caused them to separate inside the droplet, so that one region became rich in one salt and another region in a different one.
The proposed sequence has two stages. Larger droplets of ocean spray move slowly through the underground fractures, freezing gradually and segregating their salts as they go. Nearer the surface, faster-moving gas drives the frozen droplets into the walls of narrow ice channels, where they shatter into smaller fragments. Each fragment inherits the chemistry of whichever part of the parent droplet it came from, and Cassini’s analyzer later sampled those fragments in the E-ring. The paper, summarized by Astrobiology.com under the name “Cassini CDA observes compositional segregation of Enceladus’ ice grains from slow freezing and fragmentation of oceanic spray,” describes exactly this chain of events, and it links the grains’ odd chemistry to the way they formed rather than to separate patches of ocean.
Postberg put the first half of the case in terms of the observations: the Cassini data showed that the salt-rich grains are “far more chemically diverse than an average ocean composition would suggest.” Sekine described what surprised the team: “the diversity seen by Cassini could emerge from droplets originating from essentially the same ocean water.”
Separated salts and the hunt for biosignatures
The mechanism matters for the search for life because it acts as a natural concentrator, an effect the researchers consider useful rather than a nuisance. Freezing sorts dissolved material into different grains, which could make diluted organic compounds and possible biosignatures easier to detect without extensive preparation of a sample. The release says the process could greatly assist future spacecraft in doing so.
It also changes how a spectrum should be read. A grain rich in carbonate and poor in chloride does not mean that the ocean beneath it is carbonate-rich and chloride-poor; it may only mean that the droplet froze slowly and that a particular fragment of it was caught. Postberg’s second statement makes the point directly: combining the observations with the freezing experiments, he said, gives “a physical explanation: Cassini may have sampled fragments of larger frozen ocean droplets.”
Postberg has also said that future spacecraft “will have to analyze many individual ice particles in the plume” to look for signs of microbial life. The study does not report on any such search, and the underlying question it leaves open is concrete: how the average composition of the ocean itself, as distinct from the individual grains, can be recovered once freezing has scattered its salts across so many different particles. The paper is in Science Advances 12(39), DOI 10.1126/sciadv.aee7256.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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