The results from a new NASA experiment suggest that the dark material seen coating geological features on Europa may well be sea salt. Originating from a subsurface ocean, the material is thought to have been discolored by exposure to radiation.
The nature of the dark material that coats the long fractures spreading across the moon's surface have puzzled scientists for more than a decade. It's long been thought that the material had erupted from within the moon, but with limited data available, researchers been unable to determine its chemical makeup.
The reason for the discoloration is generally attributed to the radiation emitted by Jupiter's magnetic field, with ions and electrons hitting the surface of the moon with the intensity of a particle accelerator. Though past studies have suggested that it could be due to radiation-processed sulfur, the new observations focus on irradiated salts as the cause.
In the hope of shedding some light on the enduring mystery, a team of NASA researchers turned to the laboratory, working to create a simulated patch of the moon's surface.
The team tested samples of sodium chloride and mixtures of salt and water in a vacuum chamber, with the setup designed to mimic the temperature (minus 280° F or minus 173° C), pressure and radiation levels found on Europa. An electron beam was used to simulate the radiation.
The samples were left for tens of hours in the harsh environment, simulating around a century on the moon. When observed, the salt – which was initially white – had turned a yellowish-brown color, similar to that observed in the moon's dark material.
The team collected spectra from each sample, which are essentially chemical fingerprints encoded in reflected light. This information was then compared to data recorded by the telescopes and the Galileo mission, with the results showing a strong resemblance to Europa's features.
"This work tells us the chemical signature of radiation-baked sodium chloride is a compelling match to spacecraft data for Europa's mystery material," says research lead Kevin Hand.
Furthermore, the longer the materials were left in the simulated environment, the darker they became. In light of this, the team believes that studying the color of certain geological features on the moon could allow for the determination of their age.
While the results of the study strongly suggest that Europa's dark material is sea salt, the quality of currently available data isn't sufficient to identify the substance with complete certainty. It's hoped that future spacecraft will be able to make observations that will put the mystery to bed for good.