Nav: Home

Table salt compound spotted on Europa

June 12, 2019

A familiar ingredient has been hiding in plain sight on the surface of Jupiter's moon Europa. Using a visible light spectral analysis, planetary scientists at Caltech and the Jet Propulsion Laboratory, which Caltech manages for NASA, 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.

The discovery suggests that the salty subsurface ocean of Europa may chemically resemble Earth's oceans more than previously thought, challenging decades of supposition about the composition of those waters and making them potentially a lot more interesting for study. The finding was published in Science Advances on June 12.

Flybys from the Voyager and Galileo spacecrafts have led scientists to conclude that Europa is covered by a layer of salty liquid water encased by an icy shell. Galileo carried an infrared spectrometer, an instrument scientists use to examine the composition of the surface they're examining. Galileo's spectrometer found water ice and a substance that appeared to be magnesium sulfate salts--like Epsom salts, which are used in soaking baths. 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. As such, scientists have long suspected an ocean composition rich in sulfate salts.

That all changed when new, higher spectral resolution data from the W. M. Keck Observatory on Maunakea suggested that the scientists weren't actually seeing magnesium sulfates on Europa. Most of the sulfate salts considered previously actually possess distinct absorptions that should have been visible in the higher-quality Keck data. However, the spectra of regions expected to reflect the internal composition lacked any of the characteristic sulfate absorptions.

"We thought that we might be seeing sodium chlorides, but they are essentially featureless in an infrared spectrum," says Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech and co-author of the Science Advances paper.

However, Kevin Hand at JPL had irradiated ocean salts in a laboratory under Europa-like conditions and found that several new and distinct features arise after irradiation, but in the visible portion of the spectrum. He found that the salts changed colors to the point that they could be identified with an analysis of the visible spectrum. Sodium chloride, for example, turned a shade of yellow similar to that visible in a 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," says Hand, scientist at JPL 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," says Caltech graduate student Samantha Trumbo, the lead author of the paper.

"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," Brown says.

By taking a close look with the Hubble Space Telescope, Brown and Trumbo were able to 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 capacity to do this analysis with the Hubble Space Telescope for the past 20 years," Brown says. "It's just that nobody thought to look."

While the finding does not guarantee that this sodium chloride is derived from the subsurface ocean (this could, in fact, simply be evidence of different types of materials stratified in the moon's icy shell), the study's authors propose that it warrants 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 that the ocean floor is hydrothermally active," Trumbo says. "That would mean Europa is a more geologically interesting planetary body than previously believed."
The study is titled "Sodium chloride on the surface of Europa." It can be found online at This research was supported by the NASA Earth and Space Science Fellowship Program, the Space Telescope Science Institute, and JPL.

California Institute of Technology

Related Hubble Space Telescope Articles:

New way to weigh a white dwarf: Use Hubble Space Telescope
For the first time, astronomers have used a novel method to determine the mass of a type of star known as a 'white dwarf' -- the shrunken corpse of a dead star that used to be like our sun.
NASA's James Webb space telescope completes acoustic and vibration tests
At NASA's Goddard Space Flight Center in Greenbelt, Maryland the James Webb Space Telescope team completed the acoustic and vibration portions of environmental testing on the telescope.
Probing seven worlds with NASA's James Webb Space Telescope
With the discovery of seven earth-sized planets around the TRAPPIST-1 star 40 light years away, astronomers are looking to the upcoming James Webb Space Telescope to help us find out if any of these planets could possibly support life.
NASA restarts rigorous vibration testing on the James Webb Space Telescope
Testing on the James Webb Space Telescope successfully resumed last week at NASA's Goddard Space Flight Center, in Greenbelt, Md.
Robot would assemble modular telescope -- in space
A new concept in space telescope design uses a modular structure and an assembly robot to build an extremely large telescope in space, performing tasks in which astronaut fatigue would be a problem.
More Hubble Space Telescope News and Hubble Space Telescope Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Teaching For Better Humans
More than test scores or good grades — what do kids need to prepare them for the future? This hour, guest host Manoush Zomorodi and TED speakers explore how to help children grow into better humans, in and out of the classroom. Guests include educators Olympia Della Flora and Liz Kleinrock, psychologist Thomas Curran, and writer Jacqueline Woodson.
Now Playing: Science for the People

#535 Superior
Apologies for the delay getting this week's episode out! A technical glitch slowed us down, but all is once again well. This week, we look at the often troubling intertwining of science and race: its long history, its ability to persist even during periods of disrepute, and the current forms it takes as it resurfaces, leveraging the internet and nationalism to buoy itself. We speak with Angela Saini, independent journalist and author of the new book "Superior: The Return of Race Science", about where race science went and how it's coming back.