Salt crystals could preserve Enceladus ocean water

Saturn's moon Enceladus hides a salty ocean under kilometres of ice, but it does not keep everything to itself. Jets near its south pole fire water vapour and ice grains into space, which NASA's Cassini spacecraft flew through and sampled. A new laboratory study suggests where a future mission should look on the surface for the best-preserved traces of that ocean: inside salt crystals.

The work, led by Liam Perera of Diamond Light Source in the UK and published in Communications Earth & Environment, is summarised by Phys.org.

Watching salt freeze

The team cooled a solution of sodium chloride and sodium carbonate while scanning it with powerful X-rays. 3D images showed crystals forming in real time, and X-ray scattering identified which minerals grew. The key one was natron, a sodium carbonate mineral expected to form on Enceladus.

As natron crystals grew, they wrapped around tiny pockets of the surrounding liquid, known as fluid inclusions. Only about 3% of nearly 3,800 separated grains contained them, but bigger crystals were much more likely to: all 29 grains in the largest size class did. Each pocket is effectively a sealed sample of the water present at the moment it closed.

Why it matters for finding life

On Earth, studies of ancient salt deposits have reported possible microbial cells preserved in fluid pockets inside crystals. The new experiment shows that natron can trap fluid as it freezes, but it has not tested whether biological material would be captured or survive. The authors suggest the surrounding mineral could help shield anything inside from radiation.

There is a caveat about what the pockets would hold. The lab solution was about five times more concentrated than estimates from Cassini data, to grow crystals big enough to study. In a more dilute, ocean-like mix, ice would probably form first and concentrate the remaining brine, so natron would trap a saltier liquid. Either case could still tell scientists about the ocean and how freezing changed it.

If you want to see Saturn for yourself, our beginner telescope guide explains what kind of scope shows its rings, and this week's Orionid meteor shower is a good excuse to get outside.