Radiation Might Make Jupiter’s Salty, Icy Moon Europa Glow

Europa is one of Jupiter’s four largest moons and a prime candidate for finding life beyond Earth

An illustration of a glowing blue circle on a black background
Laboratory tests reveal that ice blocks containing different salts glow differently after being exposed to radiation. NASA/JPL-Caltech

Jupiter sits in the hole of a giant, doughnut-shaped magnetic field swirling with charged particles that create intense radiation belts. The planet’s many moons are caught in the waves of radiation—and that might even make one of them glow, according to new research published on November 9 in the journal Nature Astronomy.

Researchers mimicked the cold, salty surface of Jupiter’s fourth-largest moon, Europa, using ice. When they exposed their frozen faux-Europa sample to radiation, it lit up, reports Science News' Maria Temming.

Our moon appears bright in the night sky because it’s hit by sunlight, which it reflects down to Earth. The side without sunlight is dark. Europa, which is just a bit smaller than Earth’s moon, also has a sunlit side. But the other side might glow in the dark because of Jupiter’s radiation.

Scientists from NASA’s Jet Propulsion Laboratory figured this out because they created a device that emits the same kind of radiation that surrounds Jupiter. When they hit ice samples with 20 seconds of radiation, the samples glowed. And the glow changed depending on the chemical makeup of the ice block. The team hopes that spacecraft missions to Jupiter might be able to use this information to map the salts on the surface of Europa.

“I was doing some back of the envelope calculations [of] what would be the brightness of Europa, if we were to be standing on it in the dark,” says JPL physical chemist Murthy Gudipati to Science News. “It’s approximately … as bright as me walking on the beach in full moonlight.”

The team didn’t set out to study glowing ice. They were busy firing radiation at ice samples because they were curious whether Jupiter’s magnetic field might change the chemistry of Europa’s surface, and any organic materials they might have found there, per a statement.

The scientists cooled a pure water ice core to the surface temperature of Europa, about minus 280 degrees Fahrenheit, and shot it with Jupiter-like radiation when they noticed that it lit up.

Then, they performed the same experiment with ice that had table salt mixed in. It glowed, but dimmer than the last one.

“That was the ah-ha moment for us,” Gudipati tells National Geographic’s Maya Wei-Haas. They then targeted other kinds of salty ices with radiation and measured the specific wavelengths of light that the glowing ice cores emitted.

Table salt and another kind of salt called carbonates dimmed the post-radiation glow. But ice with Epsom salt mixed in glowed brighter.

The discovery puts “another tool in our toolbox” for studying Europa’s surface, says NASA planetary scientist Curt Niebur, who wasn’t involved with the new study, to National Geographic. Niebur is also the program scientist assigned to NASA’s upcoming Europa Clipper mission, a Europa-bound spacecraft that will launch sometime in the next decade.

Europa Clipper’s gear is still being built, but Gudipati tells Science News that the camera on board will likely be sensitive enough to spot Europa’s glow. If the lab experiments hold true, then the camera could map dark regions as rich in sodium, and bright areas as rich in magnesium.

But Europa Clipper, or the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), could also find something different on Jupiter’s icy moon. Planetary scientist Roger Clark of the Planetary Science Institute, who wasn’t involved with the new study, tells Science News that some scientists interpret observations of Europa’s surface as acidic, instead of or in addition to its saltiness.

“What [the researchers] need to do next is irradiate acids … to see if they can tell the difference between salt with water ice and acids with water ice,” says Clark to Science News.

The reality of Europa’s surface is also a far cry from lab experiments. Europa is probably about 4.5 billion years old, and formed from material left behind after Jupiter condensed into the gas giant we know today. It’s been battered by a lot more than 20 seconds of radiation, and the radiation is so strong that it would kill a person in under 20 minutes if they stood in it unprotected.

“But also, if the surface glows, that’s just cool," Niebur tells National Geographic.

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