Astronomers using the NASA/ESA Hubble Space Telescope have discovered that the yellow color visible on portions of the surface of Europa, the second Galilean satellite outward from Jupiter, is actually sodium chloride (table salt). The discovery, reported in the journal Science Advances, suggests that Europa’s underground ocean may chemically resemble Earth’s oceans more than previously thought.

Beneath its icy crust, Europa has a salty, liquid-water ocean in contact with a rocky seafloor, making it an exciting place to explore habitability in the Solar System. However, the ocean’s potential to support life relies heavily on its composition.

Currently, best window to understanding Europa’s ocean chemistry is to study the composition of its geologically young and active surface.

Flybys from NASA’s Galileo spacecraft have led scientists to conclude that Europa is dominated by three chemical materials: water ice, sulfuric acid hydrate, and an additional non-ice material interpreted as magnesium sulfates from the subsurface ocean.

That all changed when new, higher spectral resolution data from the W. M. Keck Observatory suggested that the researchers weren’t actually seeing magnesium sulfates.

Most of the sulfate salts considered previously actually possess distinct absorptions that should have been visible in the higher-quality Keck data. However, the spectra of regions expected to reflect the internal composition lacked any of the characteristic sulfate absorptions.

“We thought that we might be seeing sodium chlorides, but they are essentially featureless in an infrared spectrum,” said Caltech Professor Mike Brown.

However, the scientists had irradiated ocean salts in a laboratory under Europa-like conditions and found that several new and distinct features arise after irradiation, but in the visible portion of the spectrum.

They found that the salts changed colors to the point that they could be identified with an analysis of the visible spectrum.

Sodium chloride, for example, turned a shade of yellow similar to that visible in a geologically young area of Europa known as Tara Regio.

“Sodium chloride is a bit like invisible ink on Europa’s surface. Before irradiation, you can’t tell it’s there, but after irradiation, the color jumps right out at you,” said Dr. Kevin Hand, from NASA’s Jet Propulsion Laboratory.

“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.

By taking a close look with Hubble, the team was able to identify a distinct absorption in the visible spectrum at 450 nm, which matched the irradiated salt precisely, confirming that the yellow color of Tara Regio reflected the presence of irradiated sodium chloride on the surface.

“Magnesium sulfate would simply have leached into the ocean from rocks on the ocean floor, but sodium chloride may indicate that the ocean floor is hydrothermally active,” Trumbo said.

“That would mean Europa is a more geologically interesting planetary body than previously believed.”

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Samantha K. Trumbo et al. 2019. Sodium chloride on the surface of Europa. Science Advances 5 (6): eaaw7123; doi: 10.1126/sciadv.aaw7123