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    Jupiter’s moon Europa may glow in the dark

    In news that is sure to delight every kid with stickers of stars and planets on their ceiling, planetary scientists have discovered that the night-side of Jupiter’s moon Europa may glow in the dark.

    The illuminating lab simulations, performed by NASA’s Jet Propulsion Laboratory (JPL), could help future spacecraft peer deep into the moon’s sub-surface oceans.

    Europa is one of the most exciting places in the Solar System because of its potential to harbour extraterrestrial life. Though only the size of our own Moon, it’s thought to have a vast subsurface sea with twice as much water as all of Earth’s oceans combined, covered by a thick frozen crust.

    Jupiter’s massive gravitational influence stretches and squeezes this icy world as it orbits, generating heat in the interior through friction – so even though it’s more than 700 million kilometres from the Sun, Europa could be habitable for life.

    A 12-frame mosaic image of Europa. Credit: NASA / JPL / University of Arizona

    The outer surface of the moon is bombarded by high levels of radiation, channelled by Jupiter’s powerful magnetic field. According to the new study published in Nature Astronomy, this radiation might prove key to studying the moon’s composition.

    Led by JPL scientist Murthy Gudipati, the research team simulated the interactions between high-energy charged particles and the surface of Europa by irradiating salted ice with energetic electrons. This, they discovered, causes the ice to emit a visible greenish glow – a phenomenon called electron-stimulated luminescence.

    “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 authors write in their paper.

    “In some ways, you could almost think of this like an ice aurora,” comments Jonti Horner, an astronomer at the University of Southern Queensland in Australia, who was not involved in the study. “The ice and material in it is being bombarded by high energy electrons, and that makes it glow – with different chemical species giving off different colours.”

    The intensity of the glow depends on the composition of the ice: the presence of sodium chloride and carbonate produced a fainter light, while epsomite produced a brighter light.

    The authors suggest that “these emission characteristics could be used to determine the chemical composition of Europa’s surface during night-time low-altitude fly-bys of spacecraft such as the Europa Clipper.”

    NASA’s Europa Clipper mission is slated for launch in the mid-2020s and aims to conduct a detailed survey of Europa, from the thin atmosphere to the subsurface ocean to the deep interior. Alongside taking measurements and images, it will sample molecules in the atmosphere and scout out potential spots for a future lander.

    The mission could also take advantage of the “ice aurora” predicted by this study in order to work out what minerals are common on the surface of Europa’s ice. By observing the intensities of radiation-induced light emitted by the surface, Europa Clipper could build a composition map of the moon.

    “It sounds like a really clever way to take advantage of a natural phenomenon to do science – something astronomers and planetary scientists are great at,” says Horner.

    Earth-bound telescopes have observed Europa in the visible spectrum before, including the Keck Observatory and the Hubble Space Telescope, but no significant emissions were detected. However, the large distances between the Earth and Europa may be partially to blame. From low altitude – just 50 kilometres above the surface – the Europa Clipper mission has a better chance.

    These measurements could give us a better understanding of the sub-surface ocean, placing constraints on properties such as salinity. Looking further forward, they could also help identify the most interesting places on Europa to visit.

    Horner explains: “It’s going to be really hard for us to ever land on Europa, much harder than Mars, but people are really keen to try it one day – and these kinds of observations might really help focus the decisions on where such a lander should go.”

    This research may also be relevant to other celestial bodies exposed to high doses of ionising radiation, such as two of Jupiter’s other moons, Io and Ganymede.

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