A familiar ingredient has 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 most important component of sea salt.
The discovery suggests that the salty sea surface in Europe can look more to chemical parts than the previously thought, challenging decades of the assumption of the composition of these waters and make them potentially much more interesting for studies. The finding was published in Science Advances on June 1
Flybys from Voyager and Galileo spacecraft have led researchers 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 could see sodium chlorides, but they are essentially feature free in an infrared spectrum," said Mike Brown, Richard and Barbara Rosenberg, professor of planetary astronomy at Caltech and co-author of Science Advances .
However, Kevin Hand at the JPL had irradiated sea salts in a laboratory under European-like conditions and found that several new and distinct properties arise 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, became 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 color on the surface of Europe. Before irradiation you cannot tell it is there, but after irradiation, the color jumps right on you," says Hand, researcher at JPL and co-author of Science Advances
"No one has taken visible wavelength spectra of Europe before it had this type of space and spectral resolution. Galileo spacecraft did not have a visible spectrometer. It only had an infrared spectrometer, says Caltech research student Samantha Trumbo, the lead author of the paper.  "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," Brown says.
Taking a closer look at the white Hubble Space Telescope, Brown and Trumbo could identify a distinct absorption in the visible spectrum at 450 nanometers, which matched the irradiated salt precisely, confirming that the yellow color of 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 says. "It's just what no one thought to look at."
Although there is no guarantee that this sodium chloride originating from the submarine (there can actually only be evidence of different types of materials stratified in the lunar shells of the moon), the study's author suggests that it guarantees a re-evaluation of geochemistry for Europe.
"Magnesium sulfate would simply have leached into the sea from rocks on the bottom of the sea, 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 was previously believed."
This research was supported by NASA Earth and the Space Science Fellowship Program, the Space Telescope Science Institute and the JPL.