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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.
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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.
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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.