Jupiter’s icy moon Europa is a fascinating object. Below its frozen surface, there is a liquid ocean that could potentially host life. Researchers have not found evidence of that yet, but they have just discovered something else very interesting. The moon has a large quantity of sodium chloride, the main component of sea salt, also known as table salt.
The discovery, reported in Science Advances, comes from observations conducted by the Hubble Space Telescope. Researchers from Caltech and NASA's Jet Propulsion Laboratory discovered that the yellow features on the moon’s surface are an abundance of common salt. This strongly suggests that the salt comes from the ocean below, which would make it similar in composition to our own oceans.
Past observations of the moon from the Galileo spacecraft that orbited Jupiter found water ice on Europa as well as a substance thought to be magnesium sulfate salts (like Epsom salts), but the new observations have shown a dramatically different picture. And this has important geological implications.
"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," lead author Samantha Trumbo, a graduate researcher at Caltech, said in a statement. "That would mean Europa is a more geologically interesting planetary body than previously believed."
Evidence of hydrothermal activity was also found on Saturn’s icy moon Enceladus. This is important because hydrothermal activity leads to complex chemistry and heat, two crucial ingredients for life.
The difference in observations comes from the range of the spectrometer used. By breaking down the light reflected off an object it is possible to see the elements that cover it.
"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," Trumbo explained.
You might be wondering why the table salt is yellow-ish on Europa, while it remains white on Earth (unless you use a fancy type). The reason is irradiation. The exposure to harsh light in a thin atmosphere leads to certain changes in sodium chloride. One of them is turning a more yellow hue. They'd tested the sodium chloride in the lab under Europa-like conditions, and observed a distinct absorption in the visible spectrum at 450 nanometers that matched the yellow color of Tara Regio, a geologically young region on the moon, confirming the salt's presence.
"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 co-author Kevin Hand of NASA's Jet Propulsion Laboratory.
There are two missions set to reach Europa and study it in more depth over the next decade – NASA’s Europa Clipper is launching in 2023, and ESA’s Jupiter Icy Moons Explorer, planned for a 2022 launch – where we will learn more about this icy moon and its potential for harboring life