Beneath the icy crust of Enceladus, a moon of Saturn, lies an ocean. Reaching it by drilling is not yet possible, but material from the ocean still escapes: geysers of water vapor and ice particles erupt at the south pole. Scientists are looking for a way to use these emissions to understand what is happening in the depths.
A new laboratory study published in the journal Communications Earth & Environment points to a promising target - salt crystals that trap tiny droplets of liquid as they grow. The research was conducted by Liam Perera's group at the British facility Diamond Light Source.
The authors cooled a solution of sodium chloride and sodium carbonate and monitored the freezing with powerful X-ray radiation. Three-dimensional images showed how the crystals grow and what is inside them, and from the scattering of X-rays the scientists determined which minerals form. It turned out that as crystals of natron - a mineral based on sodium carbonate that can form on Enceladus - grow, droplets of the surrounding liquid become trapped inside. Such inclusions are called fluid inclusions: microscopic cavities containing liquid inside a crystal.
Large crystals trap droplets more often and hold larger samples. Among almost 3,800 individual grains, inclusions were found in only about 3% of cases, but in the largest size category they were present in all 29 grains. Each such cavity can preserve a miniature sample of the water as it was at the moment it was sealed off.
There is also an earthly reason to look closely at what else might be inside. Earlier work on ancient salt deposits reported possible microbial cells preserved in liquid cavities of crystals that grew as water evaporated. The new experiment showed only that natron is capable of trapping liquid during freezing. Whether it can likewise preserve biological material has not yet been tested. The authors suggest that the surrounding mineral may be able to protect such material from damaging radiation.
Importantly, the trapped droplet will not necessarily be an unchanged particle of the Enceladus ocean. The experiment used a solution roughly five times more concentrated than estimates based on particles collected by NASA's Cassini spacecraft: this made it possible to obtain sufficiently large crystals. In such a mixture, large natron crystals began to grow even before the main ice formation and enclosed liquid close to the original composition. In a more dilute solution resembling the ocean, ice would probably appear first: it would leave salts in the remaining water, making it saltier, and natron formed later would trap already saltier liquid. Both variants are useful: one is closer to the original water, the other shows how freezing changed it. The shape of the crystals and the surrounding material will help distinguish these histories. At temperatures on the surface of Enceladus, the trapped water itself will be frozen.
Natron has not yet been confirmed on the moon's surface. The team advises future spacecraft to carry sensitive near-infrared instruments to find this mineral and select sampling sites. Fresh deposits at geyser vents are especially promising: large droplets cool more slowly, giving crystals time and space to grow, while very rapid cooling may hinder crystallization. Large salt-rich particles are also more likely to fall back near the vents. Finding the mineral is only part of the search: where it lies and in what state also matter. In intense cold, natron should remain stable, but warmer terrain near the vents may gradually deprive it of water. The best option is recently deposited crystals: material from the interior with microscopic cavities still intact.