The surface of Jupiter’s moon Europa mostly consists of water ice mixed with a variety of salts that ultimately originate from its subsurface ocean. New experiments by Murthy Gudipati from NASA’s Jet Propulsion Laboratory and colleagues indicate that the presence of those salts on the surface may result in a subtle, unexpected effect: the night side of Europa might glow.

Jupiter’s intense magnetic field constantly bombards Europa with ionizing radiation. When this radiation interacts with salt molecules, it can break them into their constituent ions. Gudipati and colleagues irradiated mixtures of water ice and different salt compounds to show that these released ions give off photons whose energy can be correlated with their elemental composition. When irradiated, sodium, for example, gives off somewhat dimmer light than pure water ice, while ice containing magnesium sulfate salts glow brighter. The researchers estimated that on the unlit side of Europa, the light produced by this radiation could be similar to standing on a beach during a full moon. If this night-side glow can be detected by future probes such as the upcoming Europa Clipper mission, it might be possible to estimate the distribution of different salts across Europa’s surface based on the intensity of the glow at different visible wavelengths. This could in turn provide hints as to the evolution of Europa’s ice shell, as well as the history and rates of chemical exchange between the surface and interior ocean. READ MORE