Jupiter’s icy moon Europa could give the word “moonlight” a whole new meaning. New laboratory experiments suggest that the night side of this moon glows in the dark.
The surface of Europe, which is believed to consist mainly of water ice mixed with various salts, is constantly bombarded with energetic electrons by Jupiter’s intense magnetic field (SN: May 19, 2015). When researchers simulated this interaction in the laboratory by shooting electrons at salty ice samples, the ice glowed. The brightness of this glow depended on the type of salt in the ice, researchers report online November 9 in Natural astronomy.
If the same interaction on Europe creates this never-before-seen type of moonlight, a future mission there, such as the NASA-planned Europa Clipper spacecraft, may possibly use this ice light to map the surface composition of Europe. This, in turn, could provide insight into the salinity of the ocean lurking beneath Europe’s icy crust (SN: 06/14/19).
“That affects the temperature of this liquid water – the freezing point; it affects the thickness of the ice shell; This has an impact on the habitability of this liquid water, ”says Jennifer Hanley, a planetary researcher at Lowell Observatory in Flagstaff, Arizona who is not involved in the new work. The subterranean ocean of Europe is considered to be one of the most promising places to look for extraterrestrial life in the solar system (SN: April 8th, 2020).
The discovery of the potential ice glow in Europe was “accidental,” says Murthy Gudipati, who studies the physics and chemistry of ice at NASA’s Jet Propulsion Laboratory in Pasadena, California. Gudipati and colleagues originally wanted to investigate how electron bombardment can change the chemistry of ice Europe’s surface ice. However, in video recordings of their first experiments, the team noticed that ice samples pelted with electrons gave off an unexpected glow.
Fascinated, the researchers aimed their electron beam at samples of pure water ice as well as water ice mixed with various salts. Each ice core was cooled to the surface temperature of Europe (approx. –173 ° Celsius) and showered with electrons that had the same energies as those that hit Europe. Over 20 seconds of irradiation, a spectrometer measured the wavelengths of light or spectrum given off by the ice.
The ice samples all radiated whitish because they emitted light with many different wavelengths. However, the brightness of each ice sample depended on its composition. Ice that contained sodium chloride, also known as table salt or sodium carbonate, appeared to be weaker than plain water ice. Ice cream mixed with magnesium sulfate, on the other hand, was lighter.
“I’ve done some envelope calculations [of] What would the brightness of Europe be if we stood on it in the dark, ”says Gudipati. “It’s about … as bright as I am when I walk the beach when the moon is full.”
Based on the specifications proposed for a camera to fly on the Europa Clipper mission, Gudipati and colleagues estimate that the spaceship could see Europa glowing ice during a flyby on the dark side of the moon. Dark spots in Europe could reveal regions rich in sodium, while lighter areas could be rich in magnesium.
The glow of ice in the laboratory doesn’t necessarily mean that it will happen the same way in Europe, warns Hanley. Jupiter’s icy moon was blocked by high-energy electrons for much longer than 20 seconds. “Is there ever a point where you could break down the salts and this glow stop happening?” She wonders.
Other planetary researchers are not convinced that the surface of Europe is very salty. These researchers, including Roger Clark of the Planetary Science Institute in Lakewood, Colorado, believe that the obvious evidence of salts on Europe is actually created by acids like sulfuric acid. The surface of Europe can be coated with both salts and acids, says Clark. “What [the researchers] Next, acids need to be irradiated … to see if they can tell the difference between salt with water ice and acids with water ice. “