Europa is known to have an ocean under its icy surface and, as a result, is often the location of extra-terrestrial life in science fiction. What scientists have recently discovered could be mistaken for such fiction, but Europa is truly a glowing, colour-changing moon.
Astronomers and scientists have previously used spectroscopy to show that the surface of Europa is a mixture of ice and salts. Spectroscopy is a commonly used technique that studies the absorption and emission of radiation, such as light. Primarily, it measures the wavelength of the radiation to investigate the structure of the sample. The majority of spectroscopy experiments in astrophysics are done on the dayside using the light reflected from the Sun; however, this new study sheds light onto the dark side of Europa. Just last month, scientists at NASA’s Jet Propulsion Laboratory (JPL) in California recreated the Jovian moon’s environment and showed that the surface glows on the side facing away from the Sun – called the nightside. Most of the surface is made of bright white water ice; however, regions of yellow, brown, and red exist. Some of the salts responsible for this are common to us here on Earth and include sodium chloride (table salt) and magnesium sulphate (Epsom salt). The unique instrument responsible for this discovery was the Ice Chamber for Europa’s High-Energy Electron and Radiation Environment Testing (ICE-HEART).
Jupiter pelts Europa with high-energy radiation – namely electrons, photons and ions. Without this radiation, it would be expected that the shadowed side is dark, much like our own moon. But thanks to the various salts within the surface of the satellite, it actually glows. The science of this is well established: fast-moving particles excite molecules that release energy in the form of visible light when they relax. What was unexpected about the discovery, however, was that the colour of the resulting glow was different depending on the salt composition. The colours ranged from white, to green and blue and, coupled with different brightness across the surface, showed that the various salty compounds react to the radiation differently. Spectroscopy then allowed the different spectra emitted to be linked to the different ice-salt mixtures. This revelation will reveal new information on the exact architecture of Europa’s ice, which directly affects the true appearance of the satellite.
The discovery was a result of bombarding high-energy electrons at varying mixtures of the salts present in Europa’s surface in a sub-zero ice chamber. This simulates the effect Jupiter’s magnetosphere has on its second-closest moon. The powerful magnetic field, second only to that of the Sun, is generated due to the presence of metallic hydrogen in the high-pressure core. So powerful, in fact, that even the simulated radiation in this experiment’s model had to be viewed through cameras. They saw that whenever the electron beam was on, the ice-salt mixture glowed, and the glow disappeared when the beam was switched off. However, the discovery was serendipitous; the team actually set out to study how the radiation affected the organic compounds within the ice shell.
The regions of hydrated salts and non-ice material are correlated with the geologically young aspects of Europa’s surface – the ridges, large impact structures, and disrupted terrain. These structures are able to bring to the surface material from the well-mixed subsurface layer. Additionally, the colour of the surface is dependent on longitude. The leading hemisphere is more highly coloured as it receives the most dust. One example of the obstacles within its orbit is yellow sulphur resulting from Io, Jupiter’s largest moon. The trailing hemisphere receives more of Jupiter’s radiation and has a lower salt concentration. The combination of the subsurface salts reaching the surface through the formation of geologically active features reacting with the radiation and sulphur causes the moon to change colour depending on the location around the surface.
The scientists that discovered this phenomenon have stated that the night-time glow of Europa is thought to be unique and unlike anything else occurring in our Solar System. The variation in colour and intensity will allow further research into the composition of Europa in much more detail. This is important as scientists have identified Europa to be one location within the Solar System that could be home to microbial extra-terrestrial life. NASA planetary scientist Mohit Melwani Daswani went so far as to name Europa as having the best chance within the Solar System of containing life. The composition of the surface will play a big part in deciding if the conditions are habitable. The launch of Europa Clipper no later than 2025 will verify the findings. This mission will allow the spacecraft to further study the satellite’s surface and atmospheric composition and see this spectacle up close. Additionally, it will be the first dedicated and detailed study of an ocean world not on Earth. NASA scientists on the mission are now using this newfound information to decide if the instruments planned to be aboard the spacecraft will be able to detect the glow.
- Gudipati, M.S., Henderson, B.L. & Bateman, F.B. Laboratory predictions for the night-side surface ice glow of Europa. Nat Astron (2020). https://doi.org/10.1038/s41550-020-01248-1
- Howell, S.M., Pappalardo, R.T. NASA’s Europa Clipper—a mission to a potentially habitable ocean world. Nat Commun 11, 1311 (2020). https://doi.org/10.1038/s41467-020-15160-9
Image - An artist’s illustration of Europa showing the moon’s glow on the nightside (the side facing away from the Sun). Credit: NASA/JPL-Caltech