A familiar ingredient has been hiding on the surface of Jupiter's moon Europa. Using visible-light spectral analysis, Caltech's planetary scientists and the Jet Propulsion Laboratory, which manages Caltech for NASA, have discovered that the yellow color visible on parts of the surface of Europe is actually sodium chloride, a compound found in the US Earth is known as table salt, which is also the main constituent of sea salt.
The discovery suggests that Europe's salty sub-ocean may be more chemically similar to Earth's oceans than previously thought, which calls into question decades of conjectures about the composition of these waters and potentially makes them more interesting for study purposes. The result was published June 12 in Science Advances.
Flyby's the spaceships Voyager and Galileo have led scientists to conclude that Europe is covered by a layer of salty liquid water surrounded by an icy shell. Galileo used an infrared spectrometer to study the composition of the surface under investigation. Galileo's spectrometer found water ice and a substance that appeared to be magnesium sulfate salts like bittersalts used in soaking baths. Since the icy shell is geologically young and contains abundant evidence of previous geological activity, it has been suggested that the salts present on the surface may originate from the sea. Therefore, scientists have long suspected that the composition of the oceans is rich in sulfate salts.
All this changed when new data with higher spectral resolution from the W. M. Keck Observatory on Maunakea indicated that the scientists on Europe did not see magnesium sulphates. Most of the previously considered sulfate salts actually have significant absorptions that should have been visible in the higher-order Keck data. However, in the spectra of the regions expected to reflect the internal composition, one of the characteristic sulfate absorptions was absent.
"We thought we could see sodium chlorides, but they are essentially imperceptible in the infrared," says Mike Brown, Richard and Barbara Rosenberg, Caltech planetary astronomy professors and co-author of Science Advances.
Kevin Hand of JPL, however, had irradiated ocean salts in a laboratory under Europe-like conditions and found that after irradiation, several new and unique features appear, but they are in the visible part of the spectrum. He noted that the salts changed color to such an extent that they could be identified by analysis of the visible spectrum. Sodium chloride, for example, turned yellowish, similar to a geologically young area in Europe known as Tara Regio.
"Sodium chloride is a bit of an invisible ink on Europe's surface, and you can not say it's there before it's irradiated, but after it's irradiated the color will jump right out to you," says Hand, a scientist at JPL and co-author of Science Advance -Papiers.
"No one has previously recorded visible wavelength spectra from Europe that had such spatial and spectral resolution. The Galileo spaceship had no visible spectrometer. It only had a near-infrared spectrometer, "says Caltech student Samantha Trumbo, the lead author of the paper.
"People have traditionally assumed that all interesting spectroscopy on planetary surfaces is in the infrared, because most of the molecules that scientists are looking for have their basic characteristics," says Brown.
Upon closer inspection with the Hubble Space Telescope, Brown and Trumbo were able to identify significant absorption in the visible spectrum at 450 nanometers, which coincided exactly with the irradiated salt. This confirmed that the yellow color of Tara Regio indicates the presence of irradiated sodium chloride on the surface.
"We've had the ability to do this analysis over the last 20 years with the Hubble Space Telescope," says Brown. "It's just that nobody thought about it."
While the finding does not guarantee that this sodium chloride comes from the subterranean ocean (this could indeed be merely an indication of different types of materials layered in the ice shell of the moon), the authors of the study suggest that a reassessment of the chloride is justified geochemistry of Europe.
"Magnesium sulfate would simply be leached from rocks on the seafloor into the ocean, but sodium chloride could indicate the ocean floor is hydrothermally active," says Trumbo. "That would mean that Europe is a geologically more interesting planetary body than previously thought."
The study is titled "Sodium chloride on the surface of Europe". It can be accessed online at https://advances.sciencemag.org/content/5/6/eaaw7123. This research was supported by NASA's Earth and Space Science Fellowship Program, the Space Telescope Science Institute and JPL.
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