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
The discovery suggests that Europe's salty, subterranean ocean may be more chemically similar to the earth's oceans than previously thought, which challenges decades of conjectures about the composition of these waters and makes them potentially very powerful for the study. The discovery was published in Science Advances on June 1
Flyby's Voyager and Galileo ships have led scientists to conclude that Europe is covered by a layer of salty liquid water surrounded by an icy shell. Galileo carried an infrared spectrometer, with scientists investigating the composition of the surface to be examined. 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. Scientists have therefore long suspected a rich in sulfate salts ocean composition.
This changed when new data with higher spectral resolution from the W. M. Keck Observatory on Maunakea indicated that scientists on Europe actually saw no 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 intended to reflect the internal composition, one of the characteristic sulfate absorptions was missing.
"We thought we could see sodium chlorides, but they are essentially imperceptible in the infrared spectrum," says Mike Brown Richard and Barbara Rosenberg, professor of planet astronomy at Caltech and co-author of the publication Science Advances .
However, JPL's Kevin Hand had irradiated and found ocean salts in a laboratory under European conditions. After irradiation, some new and unique features appear, but 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 like in a geologically young area of Europe known as Tara Regio.
"Sodium chloride is a bit of an invisible ink on the surface of Europe – it's not possible before it's irradiated Say it's there, but after the irradiation, the color jumps right at you," says Hand, a scientist at JPL and co-author Work Science Advances .
"Nobody has recorded visible wavelength spectra from Europe before had such a spatial and spectral resolution The Galileo spacecraft had no visible spectrometer It only had a near-infrared spectrometer", says the Caltech PhD student Samantha Trumbo, the lead author of the work ,  "People have traditionally assumed that all interesting infrared spectroscopy takes place on planetary surfaces, because most of the molecules that scientists are looking for have their basic characteristics," says Brown.
Using the Hubble Space Telescope, Brown and Trumbo identified significant visible-spectrum absorption at 450 nanometers, which was exactly the same as the irradiated salt. This confirmed that the yellow color of Tara Regio reflected the presence of irradiated sodium chloride on the surface.
"We've been able to do this analysis with the Hubble Space Telescope for 20 years," says Brown. "It's just that nobody thought to look."
Although the finding does not guarantee that this sodium chloride comes from the subterranean ocean (this could in fact simply be evidence of different types of materials layered in the icy moon shell), the authors of the study suggest one Re-evaluate the geochemistry of Europe.
"Magnesium sulfate would simply be leached from rocks on the ocean floor into the ocean, but sodium chloride could indicate that the ocean floor is hydrothermally active," says Trumbo. "That would mean that Europe is a geologically more interesting planetary body than previously thought."
This research was supported by NASA's Earth and Space Science Fellowship Program, the Space Telescope Science Institute and JPL.