Home Science News | Table Salt Compound Spotted on Europa – Jet Propulsion Laboratory

News | Table Salt Compound Spotted on Europa – Jet Propulsion Laboratory

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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 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
from the W. M. Keck Observatory on the dormant volcano Maunakea in Hawaii 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.

News Media Contact

Gretchen McCartney / Jia-Rui Cook
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-6215 / 818-354-0724
gretchen.p.mccartney@jpl.nasa.gov / jccook@jpl.nasa.gov

JoAnna Wendel
NASA Headquarters, Washington
202-358-1003
joanna.r.wendel@nasa.gov

Robert Perkins
Caltech, Pasadena, California
626-395-1862
rperkins@caltech.edu

2019-112

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