Jupiter’s moon Europa is considered to be the most promising candidate for extraterrestrial life in our solar system. So far, however, the chemical composition of its ice crust and subglacial ocean is only partially known. But an unusual property of Jupiter’s moon could help to find out more about it soon: Europe’s night side is glowing. This glow is caused by the constant bombardment of the ice surface with high-energy particles from nearby Jupiter, as experiments suggest. And the spectrum of this fluorescence reveals which molecules are present in the ice.
At first glance, the ice moon Europe seems cold and cold. Because its entire surface is covered with ice, there is no protective atmosphere and the temperature is around minus 150 degrees Celsius. But ten to 15 kilometers below the icy surface of Jupiter’s moon lies a completely different world. Because there is an ocean of liquid salt water that is probably up to 100 kilometers deep, as data from space probes suggest. This water is kept warm by the tidal forces of Jupiter: The force of gravity, which changes depending on the position of the moon in the orbit, compresses and expands the crust and core of the moon and warms them. At the same time, this creates currents under the ice and repeatedly cracks open the ice crust through which water vapor penetrates to the outside.
Electron bombardment makes molecules glow
So far, however, planetary researchers can only speculate about the chemical conditions of the water in Europe’s subglacial ocean and whether they are life-friendly. For this question, the chemical composition of the water and the overlying ice crust is decisive. “Knowing the inorganic composition of Europe’s surface is important, for example, to be able to check the salinity of the ocean and models for the exchange between ocean and surface,” explain Murthy Gudipati from NASA’s Jet Propulsion Laboratory in Pasadena and his colleagues. This is one of the reasons why the Europa Clipper space probe is scheduled to launch to Jupiter’s moon in the next few years. As the researchers have now determined, the probe could benefit from an unusual property of Jupiter’s moon in deciphering its chemistry: the night side of the moon glows.
Although this fluorescence is too weak to be seen by earth-based telescopes, the scientists say that this glow would be clearly visible when the Europa Clipper flies close by. The cause of this appearance is the constant bombardment of Jupiter’s moon with charged particles from Jupiter’s magnetic field. When these particles hit molecules and atoms on the surface of Europe, they give off energy to them. When these excited atoms then return to their basic state, they give off the energy in the form of photons – they glow. “Such a light emission induced by electron bombardment, also known as electron-stimulated luminescence, has already been observed in several studies with pure water ice,” report Gudipati and his team. Salty ice also emits characteristic light spectra. The researchers therefore consider it very likely that such a glow will also occur in Europe.
Spectral signature reveals composition
For their study, Gudipati and his colleagues have now investigated in more detail what the glow on the night side of Europe could look like in concrete terms and what spectral signatures the various molecules that may be present in the ice of Jupiter’s moon leave behind in it. To do this, they bombarded various ice mixtures with electrons whose energy was up to 25 megaelectron volts (MeV) – this corresponds to the energy range that the particle streams hitting Europe’s night side have according to calculations. And indeed: when the electrons were bombarded, the ice began to glow. “The analogues to Europe’s ice emit characteristic spectral signatures in the visible range when they are exposed to high-energy electron beams,” the scientists report. “The spectra for water ice have three characteristic emission bands.” One of these peaks is in the violet range at 330 nanometers, the second in the green-blue range at 440 nanometers and the third, particularly strong, has its maximum in the yellow-green color range at 525 nanometers.
As the further tests showed, subtle changes in this basic spectrum can reveal whether salts are mixed with the water ice and which ones. The addition of sodium chloride (NaCl) significantly reduced the emission peak at 525 nanometers, while the presence of magnesium sulfate led to a slight shift of this peak to a wavelength of 560 nanometers. “Sulphate-doped ice spectra also showed a broad emission shoulder in the red area that was not present in any of the other types of ice,” report Gudipati and his colleagues. According to their calculations, the glow and its spectral signatures would have to be strong enough to be captured and read by the Europa Clipper space probe during a flyby at an altitude of around 50 kilometers. “Nocturnal passages could therefore be used to obtain a map of the composition of Europe’s ice surface,” said the scientists. This could help to clarify whether the crust and ocean of Jupiter’s moon have the chemical requirements for life.
Source: Murthy Gudipati (NASA Jet Propulsion Laboratory, Pasadena) et al., Nature Astronomy, doi: 10.1038 / s41550-020-01248-1