Maunakea, Hawaii – A familiar ingredient has been hiding in plain sight on the surface of Jupiter’s moon Europa. Using the Hubble Space Telescope to conduct a visible-light spectral analysis along with data taken from W. M. Keck Observatory on Maunakea in Hawaii, 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.
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. The finding was published by Science Advances on June 12.
Flybys from NASA’s Voyager and Galileo spacecraft have led scientists to conclude that Europa is covered by a layer of salty liquid water encased in an icy shell. Galileo carried an infrared spectrometer, an instrument scientists use to examine the composition of a surface they’re studying. Galileo’s spectrometer found water ice and a substance that appeared to be magnesium sulfate salts (like Epsom salts). 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.
“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,” said Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech and coauthor 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.
That all changed when new, higher spectral resolution data taken with Keck Observatory’s upgraded Near-Infrared Spectrograph (NIRSPEC) suggested that the scientists weren’t actually seeing magnesium sulfates on Europa. Most of the sulfate salts considered previously possess distinct absorptions, which serve as fingerprints for compounds, 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,” Brown said.
Meanwhile, JPL scientist Kevin Hand had used sample ocean salts, bombarded by radiation in a laboratory under Europa-like conditions, and found that several new and distinct features arose in sodium chloride after irradiation. He discovered that they 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,” said Hand.
By taking a close look with the NASA/ESA Hubble Space Telescope, the research team was 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 said. “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 the ocean floor is hydrothermally active,” Trumbo said. “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.” This research was supported by the NASA Earth and Space Science Fellowship Program, the Space Telescope Science Institute, and JPL, which is managed by Caltech for NASA.
The Near-Infrared Spectrograph (NIRSPEC) is a unique, cross-dispersed echelle spectrograph that captures spectra of objects over a large range of infrared wavelengths at high spectral resolution. Built at the UCLA Infrared Laboratory by a team led by Prof. Ian McLean, the instrument is used for radial velocity studies of cool stars, abundance measurements of stars and their environs, planetary science, and many other scientific programs. A second mode provides low spectral resolution but high sensitivity and is popular for studies of distant galaxies and very cool low-mass stars. NIRSPEC can also be used with Keck II’s adaptive optics (AO)system to combine the powers of the high spatial resolution of AO with the high spectral resolution of NIRSPEC. Support for this project was provided by the Heising-Simons Foundation. Learn more at www.heisingsimons.org.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.