“All these worlds are yours, except Europa. Attempt no landing there.”
Most of us are familiar with this famous final message sent from Hal-9000 to all humanity. The movie 2010, based on the writings of Arthur C. Clarke, tantalized us with the possibility of what could be hiding under the icy crust of Europa. But our curiosity didn’t stop with mere images on a screen.
Last September, I told you all about a new mission in the works at NASA/JPL, called the Europa Clipper. Though the mission isn’t set to launch until sometime around 2022-2025, this hasn’t slowed the science being conducted in order to learn more about the frozen Jovian satellite.
The fourth largest moon of Jupiter was found to have liquid water under its icy surface back in the 1990s by the Galileo probe. These findings were confirmed when the Hubble Space Telescope spotted plumes of water shooting out into space in 2014.
On June 12th of this year, however, findings of something even more interesting were published in Science Advances.
If you’ve ever looked at pictures of Europa, you’ll have noticed that the surface is marked by deep lines and dark yellow (almost rust colored) features. And indeed, using visible-light spectral analysis, planetary scientists at Caltech and NASA’s JPL have discovered that the yellow color that is visible on the surface is composed of sodium chloride. This is the compound known on Earth as table salt and is the principal component of sea salt.
This is exciting because it suggests that the salty subsurface of the Europa ocean may be more like those of oceans here on Earth than scientists previously thought.
Back when the Galileo probe was in the area, it carried an infrared spectrometer to examine the composition of the moon’s surface. This spectrometer found water ice and a substance that appeared to be magnesium sulfate salts (like Epsom salts). But no one thought to look at the visible light spectrum for analysis.
“People have traditionally assumed that all of the interesting spectroscopy is in the infrared on the planetary surfaces, because that’s where most of the molecules that scientists are looking for have their fundamental features,” said Mike Brown, Professor of Planetary Astronomy at Caltech and co-author of the Science Advances paper.
“No one has taken visible-wavelength spectra of Europa before that had this sort of spatial and spectral resolution. The Galileo spacecraft didn’t have a visible spectrometer. 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.
So how did scientists go from magnesium sulfate to sodium chloride?
Higher spectral resolution data from the Keck Observatory on Maunakea suggested that scientists weren’t actually seeing magnesium sulfates on Europa because the spectra of regions expected to reflect the internal composition lacked any of the characteristic sulfate absorptions. Most sulfate salts possess distinct absorptions, and these serve as fingerprints for compounds that should have been visible in the higher-quality Keck data.
“We thought that we might be seeing sodium chlorides, but they are essentially featureless in an infrared spectrum,” Brown said.
To test this idea, JPL scientist Kevin Hand used sample ocean salts, bombarded by radiation to simulate Europa-like conditions. What he found was that several new and distinct features arose in the sodium chloride after the irradiation.
In fact, they changed colors to the point that they could be identified with an analysis of the visible spectrum, and sodium chloride turned a shade of yellow similar to what is visible in the geologically young area of Europa known as Tara Regio.
“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.
Taking a closer look with the Hubble Telescope, the team was able to identify distinct absorption in the visible spectrum that matched the irradiated salt precisely. This confirmed that the yellow color of Tara Regio reflected the presence of irradiated sodium chloride on the surface.
This finding doesn’t guarantee that sodium chloride is derived from the subsurface ocean, but the study’s authors propose that it does warrant a reevaluation of the geochemistry of Europa.
“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. “That would mean Europa is a more geologically interesting planetary body than previously believed.”
In the end, a familiar ingredient seems to have been hiding in plain sight on the surface of Europa for a long while now. Knowing about its existence makes studying this fascinating natural satellite of Jupiter even more compelling than it was already.
“We’ve had the capacity to do this analysis with the Hubble Space Telescope for the past 20 years,” Brown said. “It’s just that nobody thought to look.”