NASA's Hubble Space Telescope spotted the presence of sodium chloride, also known as simple table salt, on the icy planet's surface.
Unlike on Earth, however, this ocean is deep enough to cover the whole surface of Europa, and being far from the sun, the ocean surface is globally frozen over.
"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.
Table salt, or sodium chloride is a seemingly unremarkable substance which is ubiquitous in our everyday lives. Now, astronomers may have to think differently about the composition of Europa's ocean.
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.
Planetary scientists did a visible-light spectral analysis of the yellow spots on Europa to find the salt.
"Table salt is white and doesn't stand out in a visible spectrum", explained Trumbo in an email to Gizmodo. Despite decades of research, scientists still don't know for sure how Europa's chaos terrain formed.
Hidden beneath an icy crust, most of what researchers know about that ocean is based on the moon's smooth, streaked surface.
The authors warn that their results do not provide concrete evidence that the sodium chloride they detected actually comes from the subsurface ocean.
"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years". 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.
Chaos regions are likely caused by materials rising up to the surface, causing the ice to break apart.
'If this sodium chloride is really reflective of the internal composition, then [Europa's ocean] might be more Earth-like than we used to think'. This revealed the electromagnetic fingerprints of various elements, also called spectral signatures. Lunine said he was "impressed by the extensive laboratory work that the authors performed, which made it possible to interpret the Hubble data in terms of sodium chloride".
It's a particularly important finding because of what it can tell us about the subsurface ocean chemistry.
Fran Bagenal, a professor of astrophysical and planetary sciences at the University of Colorado, liked the new study, saying the authors "have good data and they are thorough and honest".
Bagenal was impressed that the researchers managed to "get numbers out of a relatively weak signal", adding that it "does seem to look like sea salt on Europa's surface-but, as they admit, it is not definitive".
The good news is that the NASA Europa Clipper probe is due to launch in the 2020s and will make no fewer than 45 passes of Europa. The discovery of sodium chloride means Europa's ocean could look more like those on Earth and even possibly include salt. It was believed that the shell was similar in composition to the same ingredients that made up the ocean.
Indeed, as this new research suggests, this exotic frozen moon may have more in common with Earth than we ever imagined.