New lab experiments at NASA’s Jet Propulsion Laboratory have simulated the environment of Jupiter’s storied ocean-moon, Europa, perhaps the most intriguing of Jupiter’s 79 moons, and found that the icy orb glows, even on its nightside as it undergoes a never-ending bath of high-energy radiation from its gas-giant host, perhaps mutating unknown species that dwell in the dark depths far below its chaotic surface.
As NASA scientists plan for upcoming exploration of Europa, they have to model where is the constant radiation bombardment most intense? How deep do the energetic particles go? How does radiation affect what’s on the surface and beneath – including potential chemical signs, or biosignatures, that could imply the presence of life. This radiation can destroy or alter material transported up to the surface, making it more difficult for scientists to know if it actually represents conditions in Europa’s ocean.
Intense Radiation Key to Understanding Europa
“If we want to understand what’s going on at the surface of Europa and how that links to the ocean underneath, we need to understand the radiation,” said Tom Nordheim, a planetary scientist and roboticist working in the Astrobiology and Ocean Worlds group at JPL in 2018. “When we examine materials that have come up from the subsurface, what are we looking at? Does this tell us what is in the ocean, or is this what happened to the materials after they have been radiated?”
2018 NASA Study –Radiation Varies by Location
Nordheim and his team looked closely at the electrons blasting the moon’s surface in their 2018 study, and found that the radiation varies by location. The harshest radiation is concentrated in zones around the equator, and the radiation lessens closer to the poles.
The New 2020 Research –Unique Glimmers
New 2020 research from scientists at NASA’s Jet Propulsion Laboratory details for the first time what the glow would look like on this “lunar Chernobyl” and what it could reveal about the composition of ice on Europa’s surface. Different salty compounds react differently to the radiation and emit their own unique glimmer. To the naked eye, this glow– caused by energetic electrons penetrating the surface, energizing the molecules underneath– would look sometimes slightly green, sometimes slightly blue or white and with varying degrees of brightness, depending on what material it is.
Scientists use a spectrometer to separate the light into wavelengths and connect the distinct “signatures,” or spectra, to different compositions of ice. Most observations using a spectrometer on a moon like Europa are taken using reflected sunlight on the moon’s dayside, but these new results illuminate what Europa would look like in the dark.
“We were able to predict that this nightside ice glow could provide additional information on Europa’s surface composition. How that composition varies could give us clues about whether Europa harbors conditions suitable for life,” said astrobiologist, JPL’s Murthy Gudipati, lead author of the work published Nov. 9 in Nature Astronomy, referring to the moon’s massive, global interior ocean that could percolate to the surface through the moon’s thick crust of ice. By analyzing the surface, scientists can learn more about what lies beneath.
Scientists have inferred from prior observations that Europa’s surface could be made of a mix of ice and commonly known salts on Earth, such as magnesium sulfate (Epsom salt) and sodium chloride (table salt). The new research shows that incorporating those salts into water ice under Europa-like conditions and blasting it with radiation produces a glow.
“But we never imagined that we would see what we ended up seeing,” said JPL’s Bryana Henderson, who co-authored the research. “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.”
The Ice Chamber
To study a laboratory mockup of Europa’s surface, the JPL team built a unique instrument called Ice Chamber for Europa’s High-Energy Electron and Radiation Environment Testing (ICE-HEART). They took ICE-HEART to a high-energy electron beam facility in Gaithersburg, Maryland, and started the experiments with an entirely different study in mind: to see how organic material under Europa ice would react to blasts of radiation. They didn’t expect to see variations in the glow itself tied to different ice compositions. It was – as the authors called it – serendipity.
The ‘Aha’ Moment
“Seeing the sodium chloride brine with a significantly lower level of glow was the ‘aha’ moment that changed the course of the research,” said Fred Bateman, co-author of the paper. He helped conduct the experiment and delivered radiation beams to the ice samples at the Medical Industrial Radiation Facility at the National Institute of Standards and Technology in Maryland.
A moon that’s visible in a dark sky may not seem unusual; we see our own Moon because it reflects sunlight. But Europa’s glow is caused by an entirely different mechanism, the scientists said. Imagine a moon that glows continuously, even on its nightside – the side facing away from the Sun.
“The Chernobyl Effect”
“If Europa weren’t under this radiation, it would look the way our moon looks to us – dark on the shadowed side,” Gudipati said. “But because it’s bombarded by the radiation from Jupiter, it glows in the dark.”
Set to launch in the mid-2020s, NASA’s upcoming flagship mission Europa Clipper will observe the moon’s surface in multiple flybys while orbiting Jupiter. Mission scientists are reviewing the authors’ findings to evaluate if a glow would be detectable by the spacecraft’s science instruments. It’s possible that information gathered by the spacecraft could be matched with the measurements in the new research to identify the salty components on the moon’s surface or narrow down what they might be.
“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.”
The Daily Galaxy, Sam Cabot, via NASA/JPL
Image credit: Jupiter’s moon Europa shows how the icy surface may glow on its nightside, the side facing away from the Sun. NASA/JPL-Caltech