NASA / JPL / University of Arizona / The Atlantic

Updated at 6:53 p.m. on November 10, 2020.

One of Jupiter’s moons might be glowing in the dark.

At first glance, this is perhaps unsurprising. Our own moon glows in the dark, reflecting the light of the sun. Jupiter is far away from here, but our star still illuminates the planet and its many moons, including the moon Europa.

But Europa is different from the others. According to new research, Europa might glow even on its night side, producing an ethereal glimmer without the help of the sun.

The glow arises from the special nature of Europa’s cosmic home. Jupiter’s magnetic field is the largest of any other planet in the solar system, and the radiation within its boundaries is many millions of times more intense than the radiation near Earth. The high-energy particles constantly bombard Europa, a world slightly smaller than our moon, with a wispy atmosphere. And when those particles strike the moon’s ice-covered surface, a quirk of chemistry could make it glow in the dark.

Could, because scientists haven’t observed this mysterious light show on Europa itself, but only here on Earth, in a lab chamber that simulates the environment around the distant moon. Murthy Gudipati, a scientist at NASA’s Jet Propulsion Laboratory (JPL), and his team created miniature versions of Europa’s icy surface, based on astronomy’s current understanding of its composition, and then bombarded them with electron beams. At the touch of radiation, the simulated surface shone.

Scientists will get a chance to look for the real thing in this decade. NASA is currently developing a robotic mission to Europa, named Clipper, set to launch in the mid-2020s. Clipper is designed to orbit Jupiter, but it will also carry out dozens of close passes of the icy moon, shifting its path each time so it covers new, frozen ground.

Europa is one of the most intriguing moons in the solar system. The surface is so cold that the ice is as hard as concrete. Observations by spacecraft and ground telescopes alike show that the terrain is sprinkled with chemical compounds such as sodium chloride and magnesium sulfate. On Earth, we know these as table salt and Epsom salt. Cynthia Phillips, a planetary geologist at the JPL who works on the Clipper mission and was not involved in this research, describes it in these extremely relatable terms: “It’s frozen water, a little bit salty, might be good in a margarita.”

The salts strewn across Europa’s surface are a necessary ingredient for the strange glow. Radiation from Jupiter has an analogous effect on these compounds as a big cup of coffee might have on a person. “If you’re hyper-caffeinated, you get very excited,” Gudipati told me. “Same thing happens with molecules and atoms.” But molecules and atoms can’t remain in this excited state for long, and they return to normal by giving off energy in the form of photons—visible light. When Gudipati and his colleagues simulated a Europa bathed in radiation, they produced a glow that ranged from green to bluish to neon white, depending on which salts they’d mixed in with the ice.

Outside the lab, the effect doesn’t occur naturally on our moon, or on Earth. The closest glow on our planet comes from the northern lights, which occur when particles drifting away from the sun meet the particles trapped within our planet’s magnetic field, sending them into a frenzy.

Gudipati suspects that this glowing effect occurs all across Europa, but is likely too dim to spot in the sun’s glare on the moon’s dayside. But on the nightside, the moon casts its dreamy glow out into the dark of space.

The glow is more than a pretty light show; it could help scientists learn more about what lies beneath Europa’s icy crust. Since the Voyager missions first flew past Europa in the 1970s, scientists have believed that the moon has a salty ocean, kept warm by interior heat, stoked by Jupiter. As Europa swings around the planet on its elongated orbit, Jupiter’s gravity stretches and crunches the entire moon, creating heat that keeps the ocean liquid. Today, scientists suspect that Europa’s briny ocean could support microbial life-forms.

The detection of an ethereal glow on Europa could help identify the salts producing it, which in turn could tell scientists something about the watery world below. The leading theory posits that, over many millions of years, materials from Europa’s watery depths have risen to the surface, and vice versa. “The presence of salts on and within the surface ice of Europa could be a direct indication of ocean water coming up from below, delivering those salts and perhaps other materials, to the surface,” explains Kevin Hand, a planetary scientist at the JPL who was not involved in the new research.

Life on the surface of Europa can’t survive; the same radiation that produces the glowing effect would fry it out of existence. “We’re probably not going to find fish flopping around on the surface of Europa, but we might find some interesting chemistry that would help us understand what kind of chemical pathways could be available for life down in the ocean,” Phillips says.  

Hand has searched for signs of fluorescence on Europa using Hubble, the world’s top space telescope, but didn’t find anything. NASA scientists are now hoping that the glowing effect, if it indeed exists, is bright enough for the Clipper to capture it. The camera has yet to be built and tested, so scientists don’t know whether it will be sensitive enough to catch the gleam. But “it’s always exciting to discover something in the lab, and then have the opportunity to see if it translates into the real world,” Curt Niebur, a program scientist for the mission, told me. “And we don’t get that opportunity a lot in planetary science.”

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