In order to make the confirmation, experts turned the Hubble Space Telescope to Europa, and the telescope was able to confirm that the yellows on the surface "were giving off a chemical signal that represented the irradiated table salt".
The evidence for a subsurface ocean on Europa first began to emerge when scientists analyzed data collected by the Voyager and Galileo spacecrafts. Using a visible-light spectral analysis, planetary scientists at Caltech and NASA's Jet Propulsion Laboratory in Pasadena, California, have discovered that the yellow color visible on portions of the surface of Europa is actually sodium chloride, a compound known on Earth as table salt, which is also the principal component of sea salt.
"For a long time after Galileo, it was thought that magnesium sulfate salts might be the most important". That is, it doesn't exhibit any easily identifiable spectral features at infrared wavelengths. Since the icy shell is geologically young and features abundant evidence of past geologic activity, it was suspected that whatever salts exist on the surface may derive from the ocean below. "It's just that nobody thought to look", Mike Brown, a co-author of the study and planetary scientists at Caltech, said in a statement. "It just had a near-infrared spectrometer, and in the near-infrared, chlorides are featureless", said Caltech graduate student Samantha Trumbo, lead author of the paper.
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. The discovery, reported in the journal Science Advances, suggests that Europa's underground ocean may chemically resemble Earth's oceans more than previously thought. But, the spectra of these areas "expected to reflect the internal composition lacked any of the characteristic sulfate absorptions", according to the study.
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.
They discovered that this white salt was turning into a yellowish shade which, you guessed it right - was the very same shade of yellow that's been spotted by NASA's Galileo spacecraft on its imaging missions between 1995 and 2003.
This time around, scientists detected sodium chloride in Europa's "chaos" regions, or areas on its surface that are geologically young and contain material absorbed from the oceans below. "Before irradiation you can't tell it's there, but after irradiation the color jumps right out at you", said Hand.
"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years".
Many experts believe the hidden ocean surrounding Europa, warmed by powerful tidal forces caused by Jupiter's gravity, may have conditions favourable for life.
"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.
The study is titled "Sodium chloride on the surface of Europa".