They compared these observations with previous data gathered by the Near Infrared Mapping Spectrometer on the Galileo spacecraft in 1998, which indicated that the moon may contain table salt on its surface.
"The potential habitability of Europa's subsurface ocean depends on its chemical composition, which may be reflected in that of Europa's geologically young surface", the researchers write. Scientists studying the planet are intrigued by its massive ocean of liquid water hiding beneath a thick icy shell, and it's possible that life is lurking there, too. Galileo carried an infrared spectrometer, an instrument scientists use to examine the composition of a surface they're studying.
"People have traditionally assumed that all of the interesting spectroscopy is in the infrared on planetary surfaces, because that's where most of the molecules that scientists are looking for have their fundamental features", said Mike Brown, study co-author and the Richard and Barbara Rosenberg Professor of Planetary Astronomy at the California Institute of Technology, in a statement.
"No one has taken visible wavelength spectra of Europa before that had this sort of spatial and spectral resolution", said Caltech graduate student Samantha Trumbo.
"The Galileo spacecraft didn't have a visible spectrometer". Most of the sulfate salts previously considered have distinct absorptions that should have been easily seen with the high spectral resolution data. However, the spectra of regions expected to reflect the internal composition lacked any of the characteristic sulfate absorptions.
Even though this finding isn't a set guarantee that the sodium chloride comes from Europa's subsurface ocean, the scientists say it could kickstart a re-evaluation of Europa's geochemistry.
By conducting further observations with the NASA/ESA Hubble Space Telescope, scientists identified a distinct absorption in the visible spectrum at 450 nanometers, which correlated with the irradiated salt, demonstrating that "Tara Regio's" yellow color confirmed the presence of irradiated sodium chloride on Europa's surface. He discovered that these sample ocean salts changed enough to be identified by color.
This was similar to an area on Europa called Tara Regio, which has a distinct yellow color. "Before irradiation, you can't tell it's there, but after irradiation, the color jumps right out at you".
"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years", Brown said.
"Sodium chloride is a bit like invisible ink on Europa's surface", NASA's Kevin Hand stated in a press release. Such a mission would be costly, however, and while there's plenty of interest among scientist searching for life outside of Earth, NASA and other groups are largely focused on trips to the Moon and Mars in the immediate future.
"Magnesium sulfate would simply have leached into the ocean from rocks on the ocean floor, but sodium chloride may indicate the ocean floor is hydrothermally active", Trumbo said.
With these findings, the researchers say they're confident that sodium chloride is present in Europa's oceans, but it's unclear whether it dominates the subterranean waters, or if sulfate salts reside there as well.
Many experts believe the hidden ocean surrounding Europa, warmed by powerful tidal forces caused by Jupiter's gravity, may have conditions favourable for life.