Europa Has A Salty Subsurface Sea That May Be Hydrothermally Active

The ocean of Europa may be a lot more similar to Earth’s salty seas than previously thought, challenging decades of thought about the Jupiter moon.

Scientists from Caltech and NASA’s Jet Propulsion Laboratory have used the Hubble Space Telescope to do a visible light spectral analysis of huge subsurface ocean of Europa and discovered that the yellow colour is plain old run-of-the-mill table salt.

"We've had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years," said Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech and co-author of the study, in a statement. "It's just that nobody thought to look."

Tara Regio is the yellowish area to left of center, in this NASA Galileo image of Europa’s surface. This region of geologic chaos is the area researchers identified an abundance of sodium chloride.
(Credit: NASA/JPL/University of Arizona)

NASA/JPL/University of Arizona

Europa has been studied many times before, both by telescopes and flybys from the Voyager and Galileo spacecrafts. But Galileo’s spectrometer viewed the icy moon in infrared, finding water ice and magnesium sulfate salts similar to the Epson salts found on Earth.

When the WM Keck Observatory on Mauna Kea, Hawai’i followed that analysis up with a higher spectral resolution survey, it found that those sulfate salts might not be Epsom at all.

"We thought that we might be seeing sodium chlorides, but they are essentially featureless in an infrared spectrum," explained Brown.

"People have traditionally assumed that all of the interesting spectroscopy is in the infrared on planetary surfaces, because that's where most of the molecules that scientists are looking for have their fundamental features," he added.

Over at Jet Propulsion Labs, scientist Kevin Hand had irradiated ocean salts under Europa-like conditions in the laboratory and found that distinct features showed up in the visible portion of the spectrum.

"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 Hand, co-author of the Science Advances paper.

With Hubble, the team was able to identify a distinct absorption in the visible spectrum at 450 nanometers, which was a perfect match to the irradiated sea salt.

It’s not a dead cert that the sodium chloride on Europa comes from the subsurface ocean, there’s still the possibility that it’s actually stratified in the moon’s icy shell. But it definitely warrants further investigation.

"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," said Caltech graduate student Samantha Trumbo, the lead author of the paper. "That would mean Europa is a more geologically interesting planetary body than previously believed."

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