The icy Jupiter moon Europa is an astrobiological beacon, quite literally glowing in the deep darkness far from the sun, a new study suggests.
Jupiter's intense radiation environment likely lights up Europa's icy shell, which overlies a huge, potentially habitable ocean of salty liquid water, researchers have found.
"If Europa weren't under this radiation, it would look the way our moon looks to us — dark on the shadowed side," study lead author Murthy Gudipati, a scientist at NASA's Jet Propulsion Laboratory (JPL) in Southern California, said in a statement. "But because it's bombarded by the radiation from Jupiter, it glows in the dark."
Gudipati and his team set out to study how organic molecules in Europa's ice shell might be affected by the charged particles that zoom around Jupiter at tremendous speeds, trapped and accelerated by the giant planet's powerful magnetic field.
The researchers built an instrument called Ice Chamber for Europa's High-Energy Electron and Radiation Environment Testing, which they took to an electron-beam facility in Maryland. They tested the effects of radiation on simulated Europa surfaces composed of water ice and various salts suspected to be there, including sodium chloride and magnesium sulfate.
The radiation caused the samples to glow. This was not terribly surprising, the researchers said. The phenomenon is well understood: Fast-moving particles penetrated into the sample, exciting molecules in the near subsurface and generating a glow.
"But we never imagined that we would see what we ended up seeing," study co-author Bryana Henderson, also of JPL, said in the same statement. "When we tried new ice compositions, the glow looked different. And we all just stared at it for a while and then said, 'This is new, right? This is definitely a different glow?' So we pointed a spectrometer at it, and each type of ice had a different spectrum."
This nightside glow — it won't be visible on Europa's sun-illuminated dayside — has more than just gee-whiz appeal. Its color and intensity could reveal key details about the composition of the moon's icy shell, study team members said.
And, because water from Europa's buried ocean probably makes its way to the moon's surface in places, "how that composition varies could give us clues about whether Europa harbors conditions suitable for life," Gudipati said.
The color variation likely ranges from greenish to bluish to whitish, depending on the surface composition, team members said.
Scientists might be able to observe the glow up close relatively soon, thanks to NASA's Europa Clipper probe, which is scheduled to launch in the mid-2020s. Clipper will orbit Jupiter but scrutinize Europa on dozens of flybys, gathering data that will help researchers assess the moon's habitability and plan out a life-hunting Europa lander mission. (The lander has been mandated by Congress, but it remains a concept at the moment, not a full-fledged NASA mission. It will launch sometime after Clipper does.)
The Clipper team is looking at the results of the new study, which was published online Monday (Nov. 9) in the journal Nature Astronomy, to determine if Europa's glow could be detectable by the spacecraft's instruments, NASA officials said in the same statement.
"It's not often that you're in a lab and say, 'We might find this when we get there,'" Gudipati said. "Usually it's the other way around — you go there and find something and try to explain it in the lab. But our prediction goes back to a simple observation, and that's what science is about."
Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.