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As Europa, the icy, ocean-filled moon, orbits Jupiter, it resists a relentless blast of radiation. Jupiter strikes the surface of Europa day and night with electrons and other particles, bathing it in high-energy radiation.
But as these particles hit the surface of the moon, they may also be doing something otherworldly: making Europa glow in the dark.
New research by scientists at NASA’s Jet Propulsion Laboratory in Southern California details for the first time what the glow would look like and what it might reveal about the composition of ice on Europa’s surface. Different salty compounds react differently to radiation and give off their own unique glow. To the naked eye, this glow would appear sometimes slightly green, sometimes slightly blue or white and with varying degrees of brightness, depending on the material it is.
Scientists use a spectrometer to separate the light into wavelengths and connect the different “signatures,” or spectra, to different ice compositions. Most observations using a spectrometer on a moon like Europa are taken using sunlight reflected from the moon’s day side, but these new results illuminate what Europa would look like in the dark.
“We were able to predict that this nighttime ice glow could provide additional information on the composition of Europa’s surface. The way that composition varies could give us clues as to whether Europa harbors suitable conditions for life,” said Murthy Gudipati of JPL, author. main work published in November. 9 in Astronomy of Nature.
That’s because Europa has a massive global inland ocean that could seep to the surface through the moon’s thick ice crust. By analyzing the surface, scientists can learn more about what’s underneath.
Shining a light
Scientists have inferred from previous observations that Europa’s surface could be made of a mixture of ice and salts commonly known on Earth, such as magnesium sulfate (Epsom salt) and sodium chloride (table salt). The new research shows that the incorporation of these salts into water ice under conditions similar to those in Europa and the explosion of radiation produces a glow.
That was not a surprise. It’s easy to imagine an irradiated surface glowing. Scientists know that glow is caused by energetic electrons penetrating the surface, energizing the molecules below. When those molecules relax, they release energy as visible light.
“But we never imagined that we would see what we ended up seeing,” said Bryana Henderson of JPL, a co-author of the research. “When we tried new ice compositions, the gloss looked different. And we all looked at it for a bit and then said, ‘This is new, right? It’s definitely a different gloss?’ So we pointed it with a spectrometer, and each type of ice had a different spectrum. “
To study a lab mockup of Europa’s surface, the JPL team built a unique instrument called the Ice Chamber for Europa’s High-Energy Electron and Radiation Environmental Testing (ICE-HEART). They brought ICE-HEART to a high-energy electron beam facility in Gaithersburg, Maryland, and began the experiments with a completely different study in mind: to see how the organic material beneath Europa’s ice would react to blasts of radiation.
They didn’t expect to see variations in the brightness itself linked to different ice compositions. It was, as the authors called it, a fluke.
“Seeing the sodium chloride brine at a significantly lower brightness level 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 National Institute of Standards and Technology’s Medical Industrial Radiation Facility in Maryland.
A moon that is 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 shines continuously, even on its night side, the side opposite the Sun.
“If Europa weren’t under this radiation, it would look like our moon looks to us: dark on the shaded side,” Gudipati said. “But because it is bombarded by radiation from Jupiter, it glows in the dark.”
NASA’s next flagship Europa Clipper, launching in the mid-2020s, will observe the moon’s surface in multiple flybys as it orbits Jupiter. Mission scientists are reviewing the authors’ findings to assess whether the spacecraft’s science instruments could detect a glow. It’s possible that the information collected by the spacecraft could match measurements in the new research to identify salty components on the moon’s surface or reduce what they could be.
“It’s not often that you’re in a lab and say, ‘We could 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 all about.”
Missions such as Europa Clipper help contribute to the field of astrobiology, interdisciplinary research on the variables and conditions of distant worlds that could host life as we know it. While Europa Clipper is not a life-detection mission, it will conduct a detailed reconnaissance of Europa and investigate whether the icy moon, with its subsurface ocean, has the ability to support life. Understanding the habitability of Europa will help scientists better understand how life developed on Earth and the potential to find life beyond our planet.
You can find more information about Europa and Europa Clipper here: europa.nasa.gov
Astrobiology
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