A familiar ingredient has been hidden in plain view of the surface of Jupiter's moon Europe. Using a visible light spectral analysis, planetary scientists at Caltech and Jet Propulsion Laboratory, which Caltech manages for NASA, have discovered that the yellow color visible on parts of the surface of Europe is actually sodium chloride, a compound known on earth as table salt, which is also is the main component of sea salt.
The discovery suggests that the salty sea surface in Europe may resemble chemical parts of the Earth's oceans more than previously thought, challenging decades of assumptions about the composition of these waters and making them potentially much more interesting for study. The result was published in Science Advances June 12.
Flybys from Voyager and Galileo spacecraft have led scientists to conclude that Europe is covered by a layer of salted liquid water enclosed by an icy shell. Galileo carried an infrared spectrometer, an instrument researcher uses to investigate the composition of the surface they are examining. Galileo's spectrometer found water ice and a substance that appeared to be magnesium sulfate salts – such as Epsom salts, used in soaking baths. Since the icy shell is geologically young and has ample evidence of the earlier geological activity, it was suspected that any salt present on the surface can be derived from the sea below. As such, researchers have long suspected a sea composition rich in sulfate salts.
Everything changed when new, higher spectral resolution data from the W. M. Keck Observatory at Maunakea suggested that scientists did not actually see magnesium sulfates in Europe. Most of the sulphate salts previously considered to possess actually separate absorptions that should have been visible in higher quality Keck data. However, spectra of regions that are expected to reflect the internal composition lacked any of the characteristic sulfate absorptions.
"We thought we might see sodium chlorides, but they are essentially featureless in an infrared spectrum," said Mike Brown, Richard and Barbara Rosenberg professor of planetary astronomy at Caltech and co-authors of Science Advances paper.
However, Kevin Hand that JPL had irradiated sea salts in a laboratory under European-like conditions and found that several new and distinct properties occur after irradiation, but in the visible part of the spectrum. He found that the salts changed colors to the point they could be identified with an analysis of the visible spectrum. Sodium chloride, for example, turned a shade of yellow similar to that seen in a geologically young area in Europe called Tara Regio.
"Sodium chloride is a bit like invisible ink on Europe's surface. Before irradiation you can't say it's there, but after irradiation, the color jumps right on you," says Hand, researcher at JPL and co-author of Science Advances paper.
"No one has seen visible wavelength spectra in Europe before it had this type of spatial and spectral resolution. Galileo spacecraft did not have a visible spectrometer. It only had an infrared spectrometer," said Caltech student Samantha Trumbo, the leading author of the magazine.
"People have traditionally assumed that all the interesting spectroscopy is in infrared on planet surfaces, because that's where most molecules that researchers are looking for have their basic properties," says Brown.
By looking at the Hubble Space Telescope, Brown and Trumbo were able to identify a distinct absorption in the visible spectrum at 450 nanometers, which matched the irradiated salt accurately and confirmed that the yellow color of the Tara Regio reflected the presence of irradiated sodium chloride on the surface.
"We've had the ability to do this analysis with the Hubble Space Telescope over the past 20 years," Brown said. "It was just what no one thought to look at."
Although the discovery does not guarantee that this sodium chloride originates from the underside of the sea (this may actually just be evidence of different types of material stratified by the lunar shells of the moon), the study's authors suggest that it guarantees a re-evaluation of geochemistry by Europe.
"Magnesium sulfate would simply have leached into the sea from rocks on the seabed, but sodium chloride may indicate that the seabed is hydrothermally active," says Trumbo. "It would mean that Europe is a more geologically interesting planetary body than previously thought."
This research was supported by NASA Earth and the Space Science Fellowship Program, the Space Telescope Science Institute and the JPL.