Diamond Light Source scientists identify natron as a key target for future Enceladus missions

Scientists at Diamond Light Source have uncovered new evidence that could help with the search for signs of life on Saturn’s icy moon, Enceladus. 

Using Diamond’s DIAD beamline, they observed, for the first time, fluid inclusions preserved within a cryogenic salt mineral known as natron. The findings, published in Communications Earth & Environment, suggest that natron could trap and preserve material from an ocean beneath Enceladus’s icy crust, potentially including chemical signatures associated with life. 

Beneath Enceladus’s frozen surface lies a subsurface ocean, with jets of material which erupt from its south polar region into space. These plumes provide a natural way to access material originating from the moon’s interior without having to drill through kilometres of ice. 

The new research shows that, as an Enceladus-like salty solution freezes, sodium carbonate decahydrate – natron – can crystallise and trap pockets of the original fluid inside the growing mineral. 

Lead author and Diamond beamline scientist Liam Perera said: “Enceladus is one of our most exciting targets in our search for whether life exists beyond Earth. This is because it has a global subsurface ocean, fed by hydrothermal activity, that erupted onto the surface through massive cryovolcanic plumes. Because of its potential habitability, both NASA and ESA are planning two separate flagship missions to remotely observe and directly sample the surface. These missions will need to look for geochemical signatures that represent fresh, unprocessed material from the subsurface. 

Our work shows for the first time, that natron, a carbonate mineral that forms during freezing, forms fluid inclusions within its crystal structure. These inclusions act as microscopic capsules, preserving samples of the fluid from which the crystal formed. If life is present within Enceladus, these crystals have the potential to trap and preserve biological material. Future missions should have the capability to identify this mineral remotely and surface landers should prioritise sites with signatures of natron.” 

Dr James Le Houx, Senior Lecturer, University of Greenwich, part for the London and South East University Group, said: "Enceladus's ocean is buried under 20km of ice, far too deep to drill. Our experiments suggest the moon may be bringing samples of it up to the surface for us, sealed inside salt crystals. That gives a future lander something very specific to look for." 

The team, a collaboration between Diamond and ISIS Neutron and Muon Source, used synchrotron X-ray computed microtomography (μCT) alongside X-ray diffraction (XRD) at Diamond’s DIAD beamline to watch the solution freeze and identify the minerals forming within it. DIAD’s two independently configurable X-ray beams allow researchers to combine spatially registered imaging and diffraction measurements, providing both a detailed view of the sample’s internal structure and information about its mineral composition. 

Dr Perera said: “DIAD’s unique capabilities allowed us to image and identify these salts non-destructively as they are 100s of microns in size and form within ice at low temperatures (205K). Without X-rays, we would be unable to see them. DIAD’s X-rays allow us to see through the ice and furthermore into the crystals themselves to see the pockets of frozen fluid. The brightness of the synchrotron source means that we could also watch the crystallisation of these salts in real time as we froze the Enceladus-type solution.” 

A potential target for future missions 

Fluid inclusions are important because they can preserve fluids at the moment a mineral forms. On Earth, fluid inclusions in salt minerals have been used to reconstruct the chemistry of ancient environments, and some salt-hosted inclusions have been found to contain organic biological signatures and microorganisms. 

Natron is therefore an important target for future missions to Enceladus, as the team of scientists suggest. If biological material or other biosignatures are present in the ocean, natron could potentially trap and preserve them within its crystal structure. The team have identified a mechanism that could potentially preserve such signatures if they exist. 

The team suggests that future spacecraft should be equipped to identify crystalline natron and other hydrated carbonate minerals at the surface. In particular, the researchers recommend looking for natron in fresh deposits close to Enceladus’s plume sources, where larger salt-rich particles may be more likely to contain inclusions. Near-infrared spectroscopy could provide a way of identifying hydrated carbonate deposits and helping select promising locations for future sampling. 

Seeing inside an icy-world analogue 

The experiment used a sodium chloride–sodium carbonate–water solution as an analogue for Enceladus material. The sample was cooled while tomography and diffraction measurements were collected, allowing the researchers to follow the evolution of the ice and salt phases in real time. 

The final frozen sample contained three major phases: hexagonal ice, a eutectic mixture containing ice, natron and hydrohalite, and individual natron crystals. Natron accounted for around 21% of the imaged volume. 

The ability to correlate the mineral phases with their three-dimensional microstructure was crucial to identifying the fluid inclusions. The study reports that this combination of μCT and XRD provided a non-destructive method for identifying the inclusions in situ. 

The research highlights the wider potential of synchrotron science for planetary research. Understanding how minerals form and preserve fluids at extremely low temperatures can help scientists interpret samples from icy worlds and determine which materials may be most valuable for future exploration. 

The researchers conclude that natron should be considered a mineral of astrobiological importance and that future missions to Enceladus and other carbonate-bearing icy bodies should have the capability to characterise surface mineralogy and identify potential natron-rich sampling sites.