Europa. Image: Stocktrek via Getty
Europa, an ice moon that orbits Jupiter, is considered one of the most likely places in our solar system to host life, because scientists think a voluminous ocean of liquid water lurks under its thick crust.
Now, a team of planetary scientists has discovered that this tantalizing world may also glow in the dark, a characteristic that “could enable more precise surface characterization and provide unique night-time views” for future space probes, according to a paper published on Monday in Nature Astronomy. This would allow Europa missions to learn even more about the conditions on the mysterious moon, including whether it could (or does) host alien life.
The researchers predicted the existence of this “night-side surface ice glow” by simulating the intense radiation that Europa receives from Jupiter on salted ice samples in the laboratory. The results reveal that the flux of charged particles from Jupiter could produce radiant spectral signatures on Europa.
“The emission we observed is very broad,” said Bryana Henderson, a planetary scientist at NASA’s Jet Propulsion Laboratory who co-authored the study, in an email. The emission peaks at a wavelength of 525 nanometers, which corresponds to the color green, but Henderson expects that “it would actually appear whitish in most cases” because of the “breadth of the spectrum.”
While the presence of the ice glow has not been confirmed by direct observations of Europa, NASA is planning to launch an orbiter called the Europa Clipper to the moon by the mid-2020s. If the glow exists, the Clipper would be able to spot it on the dark side of the moon with its wide-angle camera, according to the study.
“Owing to the unique radiation environment and rich geological and compositional diversity on its surface, the night-time ice glow occurring on Europa may be very unique and unlike any other phenomenon in our Solar System,” the team said in the study.
Europa is tidally locked to Jupiter, meaning that the same face is always pointed toward the planet. Even so, the moon is fully bathed by the strong radiation belts that slosh around the gas giant; as a result, Europa is constantly pelted by electrons, positrons, and ions—virtually eliminating the odds of life at the surface.
To simulate this effect, Henderson and her colleagues exposed salted ice to electrons with energies of about 10-25 Mega electron-volts (MeV), which is within the range of what Europa receives from Jupiter. The experiment produced an effect known as “electron-stimulated luminescence”—in other words, the ice glow—that would be visible in the absence of the Sun’s glare.
“The entire surface [of Europa] is exposed to electron radiation, but we would only be able to see the night glow on the side facing away from the Sun,” said Henderson.
“Another interesting note is that reflected light from Jupiter and from nearby moons could also interfere!” she added. “So the best viewing conditions here could actually vary depending on the positions of these bodies.”
This eerie moon-shine would be a phenomenal sight, but it could also reveal useful information about Europa to scientists hoping to understand this compelling world, including its potential to host life. Because the intensity of the glow depends on surface composition and temperature, these spectral signals could help scientists map the moon’s contours and understand its geology.
The glow could also help assess the potential habitability of Europa, but only as a backup for more relevant observations that NASA plans to capture with the Clipper, especially data from its thermal imaging instrument.
“The main thing that our work would provide is complementary information about the surface composition,” Henderson said. “All of these results taken together (along with other models and laboratory work) could shed light on whether the moon might be habitable.”
Given that the Clipper will not arrive at Jupiter until the 2030s, Henderson and her colleagues have plenty of time to build on the new study.
“We have only studied 10-25 MeV electrons so far, but Europa sees a much broader range of electron energies and we don’t yet know how the higher or lower energies would change the glow,” she said. “Could we see this effect even for 1 keV electrons? Are 100 MeV electrons 10x the intensity of 10 MeV electrons? Where does this relationship break down?”
The team also hopes to get a better grip on how the glow spectrum looks across many wavelengths, including infrared light, and how high-energy radiation might affect other materials on planetary bodies and surfaces.
“We’ve only scratched the surface as far as different compositions go,” Henderson concluded.
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