A familiar ingredient has been hiding in plain sight on Jupiter’s moon Europa — table salt. Planetary scientists at Caltech and NASA’s Jet Propulsion Lab have discovered that the yellow patches on the moon’s surface are actually sodium chloride, the chemical name for table salt. Think about that the next time you salt your french fries. The scientists examined the light reflected from the patches with a spectrometer, an instrument that teases out the composition of materials at a distance by analyzing the light they reflect.
Europa is 1,940 miles (3,122 km) across — the smallest of the four bright Galilean moons — with a crust of water ice 12-15 miles (19-25 km) thick floating atop a subsurface ocean 40 to 100 miles (60-150 km) deep with a metallic iron core at its center.
No one has seen the ocean but its presence is implied from data gathered through close flybys by NASA’s Galileo satellite. Galileo discovered that Europa has a magnetic field created by an electrically-conductive material below its surface. A salty ocean fits the bill. Next, take a look at the surface itself — a jumble of ice sheets that have been tossed and tumbled about (then refrozen in place) probably due to subsurface water flows.
Galileo, which orbited within Jupiter system from 1995 to 2003, found both water ice and a substance that appeared to be magnesium sulfate salts similar to Epsom salts using its infrared spectrometer. Infrared red lies just beyond the red end of the rainbow spectrum. We can’t see it (mosquitos and snakes can) but instead sense it as heat. Many of the most interesting molecules on planets and moon reveal their presence through infrared light, so if you’re going to pack a spectrometer, an infrared one is a good choice.
Unfortunately, sodium chloride doesn’t show any features in infrared light so Galileo didn’t detect it. And so it sat until JPL scientist Kevin Hand bombarded samples of earthly ocean salts with radiation in a laboratory under Europa-like conditions. He discovered that the sodium chloride 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.
Flyover of the moon Europa using spacecraft imagery
But no one had ever taken a high resolution visible light spectrum of Europa, so astronomers pointed the Hubble Space Telescope at Europa and were able to identify a distinct absorption of light in the yellow part of the spectrum in Tara Regio that exactly matched the radiation-bombarded table salt sample. Bingo!
The discovery potentially could have made the discovery 20 years ago but no one thought to look! Funny how sometimes the most amazing things are right under our noses. The discovery suggests that Europa’s salty waters may be more similar to Earth’ oceans than we suspected and continues to make Europa one of the best places to look for potential life on future missions.