A new study by researchers at Caltech and NASA's Jet Propulsion Laboratory in Pasadena, California, indicates that the subterranean world ocean on Jupiter's moon Europa may be more Earthlike than scientists believed. Visible-light spectrographic analysis show that yellow stains on Europa's icy crust are common table salt, suggesting that the Europan ocean may be similar to its terrestrial counterparts.
When signs of an ocean on Europa were sent back to Earth by NASA's unmanned Voyager and Galileo spacecraft, it boosted hopes that it might be home to some form of extraterrestrial life. Unfortunately, this was tempered by the fact that the infrared spectrometer carried by the Galileo probe showed that the water ice on the surface was laced with magnesium sulfate salts, which are better known to most people as Epsom salts.
Because the crust of Europa is young from a geological point of view, it meant that these salts may have come from the buried ocean. If that was the case, then the odds of any life in those deep waters were drastically reduced.
However, the NASA team found flaws in these conclusions because of the reliance on the near-infrared band of the spectrum and the poor resolution of the equipment carried by Galileo. Because of this, chloride compounds were largely invisible from detection. To overcome this, data from higher resolution spectrometers operating in the visible band was needed.
This has come from the W. M. Keck Observatory atop Mauna Kea in Hawaii. By looking at high-resolution spectrograms, the team found that the signatures of magnesium salts disappeared, indicating that the previous findings were in error.
At the same time, JPL scientist Kevin Hand conducted an experiment where ocean salt samples were irradiated under conditions similar to those found on Europa. This irradiation altered the properties of the table salt, also known as sodium chloride, causing it to match the visible spectra seen by the Keck Observatory, as well as being the same shade as yellow regions found on the surface of Europa in a large, geologically young area called "Tara Regio."
"Sodium chloride is a bit like invisible ink on Europa's surface," says Hand. "Before irradiation you can't tell it's there, but after irradiation the color jumps right out at you."
According to NASA, the findings were confirmed by the NASA/ESA Hubble Space Telescope, which conducted a spectral scan in the visible band at 450 nanometers, producing an exact match with the spectra of the irradiated salt, especially in Tara Regio. The irony is that the Hubble could have made this discovery 20 years ago, if anyone had thought to look.
Though the news is encouraging, the space agency stresses that this is not direct evidence of sodium chloride in the Europan ocean, but may simply be a result of stratification.
"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," says Caltech graduate student Samantha Trumbo. "That would mean Europa is a more geologically interesting planetary body than previously believed."
The research was published in Science Advances.
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