What slow freezing separates
Cassini flew through the ice plume at Enceladus's south pole and found salts and organic compounds in the grains. Frank Postberg, a planetary scientist at Freie Universität Berlin, and his team rebuilt that path with Cassini measurements, laboratory experiments and models. A droplet forms when a gas bubble bursts at the ocean surface, and vapour carries it out through a fracture. The freeze is not instant. Slow freezing separates sodium chloride from sodium carbonate inside the same droplet, and organic material gathers in another part of it.[1]
On the way up, a frozen droplet reaches about 1000 km/h. A strike on the fracture wall breaks it into fragments a few micrometers across. Many grains that escape are then one concentrated substance, already split from the rest. Postberg says Enceladus takes on much of the sample preparation a chemistry laboratory would otherwise do.[1]
The search drops to one grain
If microbial material were in one of those droplets, the same slow freeze would separate it from the salts. After the break, that material would sit in a small share of the ice grains, at high concentration. A measurement that folds the whole plume into one number would dilute that share. The place to read is the single grain. Postberg's laboratory has already tested whether specialised instruments can pick out cell material in one plume grain.[1]
The separation may come from freezing against the fracture wall rather than from the makeup of the ocean as a whole. A grain that is nearly pure salt, or nearly pure organic material, still has to be tied back to the water it left. ESA's L4 mission, now in planning, is aimed at signs of life on Enceladus. The observable sign is salt gathered in one grain and organic material in another, at the concentrations the slow-freeze path produces.[1]