Jupiter might have its own little night light, in the form of an icy moon that glows in the dark.
Like Earth’s lunar companion, the Jovian moon Europa shines brightly on its sunlight side. But our rocky lunar neighbor is swathed in darkness on its far side, which is part of why we see the moon wax and wane and even seem to vanish from sight. Now, laboratory experiments suggest that ice on the far side of Europa might instead give off a visible greenish or bluish white light, driven by the constant bombardment of radiation from Jupiter’s intense magnetic field.
“It would be a nice fairytale-like thing to stand on Europa and see,” says Murthy Gudipati, an astrophysicist at NASA’s Jet Propulsion Laboratory and lead author of the new study, published today in Nature Astronomy.
The team tested ice laced with a variety of different salts thought to linger on Europa’s surface, discovering that the precise composition influences the intensities of colors that make up the glow. That means if Europa’s surface does glow, future missions could study the light from the moon’s nighttime side to help decipher its complex chemistry. That in turn could provide clues to the composition of the deep—and potentially habitable—ocean thought to exist below Europa’s icy crust.
In the next few years, two ventures will start the lengthy trip to get a closer look: NASA’s Europa Clipper and the European Space Agency's Jupiter Icy Moons Explorer (JUICE). This new study adds “another tool in our toolbox” to help future scientists work out Europa’s complex surface chemistry, says Curt Niebur, the Europa Clipper Program Scientist with NASA, who was not part of the study team.
“But also,” he says, “if the surface glows, that’s just cool."
A salty surprise
Scientists have known since the 1950s that pure water ice glows when exposed to radiation, says Anna Pollmann, an astroparticle physicist at the University of Wuppertal. When electrons collide with molecules in the ice, they impart a small amount of energy. These “excited” molecules can’t stay that way for long, quickly releasing the energy in the form of light.
Pollmann, who was a reviewer of the new study, uses these faint winks of light in Antarctic ice to search for exotic cosmic particles thought to rain down on our planet. But Earth’s thick atmosphere and magnetic bubble shield our planet from much incoming radiation, making these streaks of luminescence scarce.
By contrast, Europa has almost no atmosphere and is caught up in the maelstrom of radiation from Jupiter’s fierce and enormous magnetic field. There’s so much radiation scouring the moon’s surface that if you stood on Europa unprotected, you would die within 10 to 20 minutes, Niebur says.
Gudipati and his colleagues initially had planned to study how this radiation might affect the properties of Europa’s icy crust, which is critical to understand if we ever send landers to the alien moon. They built an instrument that allowed them to bombard chunks of ice with a stream of electrons and track what happens from a safe distance. Gudipati dubbed the setup ICE-HEART, or Ice Chamber for Europa High-Energy Electron and Radiation-Environment Testing.
When they shone the electron beam onto a frozen block of pure water, the icy glow caught their attention. Then, they switched to sodium chloride ice, and the light was very faint. Thinking something was wrong, Gudipati and his colleagues tried it again—still no glow.
“That was the ah-ha moment for us,” Gudipati says.
He and his team tested a variety of salts that studies have hinted could linger on Europa’s surface. Some salts, like carbonates, seem to quench the glow. But others, like magnesium salts, give it a boost. The intensities of the colors that make up the glow also changed. For example, sodium chloride staunches green light, while sulfate salts add slightly more red to the mix.
The results suggest that the presence of different salts will influence any potential gleam coming from the ice moon’s surface—a feature that could perhaps “help us look at Europa through different eyes,” he says.
Gleaning new information
Gudipati and his team’s calculations suggest that the moon's radiation likely generates enough ice glow to be seen by the Europa Clipper’s camera. That doesn’t mean it’s a surefire thing, Niebur cautions—“that camera is still being built.”
But it does provide some tantalizing hints at what we might be able to learn if Europa does produce its own eerie glow. Such a method could also be useful for studying other icy Jovian moons, such as Ganymede, says Ines Belgacem, a planetary scientist at the European Space Agency who specializes in studying the surfaces of icy worlds and is not part of the study team.
Figuring out which compounds are embedded in the icy crust could help give clues to the chemistry of the likely subsurface ocean. Smooth expanses of ice along Europa’s face and tantalizing hints of water plumes suggest any liquid body below is seeping upward on a geologic time scale. Meanwhile, parts of the crust might also slowly sink downward into the subsurface sea. So, learning the surface composition could be one key to figuring out if and how life could eek out an existence in the deep.
“There’s a lot—a lot—we still need to learn about the moon,” Belgacem says.
A spacecraft hasn't studied Europa in detail since the Galileo mission in the 1990s, making it difficult to fill in our knowledge of this icy world. But answers might be coming soon from Clipper and JUICE, and the new study only adds to that potential.
“The more we know before we get there," she says, "the better it will be for the science we can get."