On Earth’s surface, sodium and chloride can crystallize into two minerals: table salt and hydrohalite. These minerals are so mundane and well studied that their combination of atoms is incapable of surprising scientists. Or so we thought.
A team of researchers recently discovered two hydrates, or water-containing salts, of sodium chloride previously unknown to science. Although unstable at Earth conditions, the new salts are perfectly stable in the extreme environments of icy moons orbiting Jupiter and Saturn, and they may finally explain puzzling spectroscopic measurements of the surface of one of those moons.
The surface of Europa, one of Jupiter’s moons, is covered in a shell of water ice 15 to 25 kilometers (9 to 16 miles) thick. Beneath that ice shell is a salty ocean; it is nearly 14 times deeper than the deepest part of Earth’s ocean and is one of the most promising places to find extraterrestrial life in our solar system.
All that water creates enormous pressure in Europa’s crust and ocean depths. The high pressure—up to 500 atmospheres—and extremely cold temperatures would allow the novel salts to crystallize in Europa’s outer ice shell. The salts may also form in the high-pressure environments beneath the oceans of other Jovian moons like Ganymede and Callisto or Saturnian moons Enceladus and Titan, said Baptiste Journaux, a planetary scientist from the University of Washington who led the study, which was published in the Proceedings of the National Academy of Sciences of the United States of America.
Journaux and a team of international scientists formed the new sodium chloride hydrates by simulating the interiors of icy moons in the lab. The researchers contained a sodium chloride brine in a vessel about as thick as a human hair and wedged it between two diamonds inside a diamond anvil cell. Inside the cell, the diamonds are pushed together with a piston, exerting high pressure that can be sustained for weeks without breaking. Meanwhile, a cryostat can crank the temperatures to nearly −123°C (−190°F).
Under such high pressure, crystals unexpectedly formed, which the team identified by measuring how the direction of X-rays changed as they bounced off the crystal’s atoms. The resulting X-ray diffraction pattern is unique to each crystal structure, creating a kind of fingerprint useful for characterizing crystal structures. When their crystals’ fingerprints did not match entries in any databases, the researchers knew that they had something novel.
“It was kind of a eureka moment,” said Journaux.
Additional tests revealed that the new crystals were “hyperhydrated” sodium chloride, meaning that water molecules completely separate the sodium and chloride ions. This structure differs from the sodium and chloride ions in hydrohalite, which are situated closer together. The crystals’ ratios of water to sodium chloride were also more than 4 times higher than that of hydrohalite.
Has a Missing Puzzle Piece Crystallized?
Journaux suspects that hyperhydrated sodium chloride is the missing piece in the puzzle of reconciling satellite images of Europa’s surface. Some scientists think that reddish-brown bands and cracked, yellow terrain on parts of Europa’s surface match the color of table salt irradiated by particles from Jupiter’s magnetic field. These surface features also absorb ultraviolet light in a manner that suggests table salt is present, but studies using infrared suggest that more hydrated magnesium-containing chlorinated salts are a better match.
Perhaps the more hydrated sodium chloride salts “can explain both the presence of sodium chloride and the interpreted signatures of more hydrated salts in the infrared data,” suggested Samantha Trumbo, a planetary astronomer from Cornell University not involved in the study.
However, scientists will need the new salts’ absorbance and reflectance spectra to be certain, said Trumbo. No spectral data exist for the new salts yet because of technical challenges in acquiring the data.
If hyperhydrated sodium chloride does prove a good match to Europa’s surface, it “would be even stronger evidence that sodium chloride is a major constituent of the ocean,” said Trumbo. “The only explanation we have for why [salt] is on the surface, where it is on the surface, is that it came from the interior,” she added.
The salts could also reveal areas where material recently upwelled from deep in Europa’s ice crust and ocean because “hyperhydrated species are not necessarily stable in conditions of zero pressure” such as Europa’s surface, said Federico Tosi, a research scientist at Italy’s National Institute for Astrophysics and a coordinator with the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission. Tosi was not involved in the new research. Although the salts could indicate upwelling, Tosi said, “we also need to [better] understand at what timescales the hyperhydrated species [decay].”
Two upcoming missions to the Jovian moons, ESA’s JUICE and NASA’s Europa Clipper, could aid in finding the hyperhydrated salts and determining their distribution across the surface by measuring the chemistry of the moons’ thin atmospheres and obtaining higher-resolution spectra of their surfaces, said Tosi.
—Derek Smith (@djsmitty156), Science Writer