Europa’s dark lines may be due to irradiated salt
NASA released data this week from recent laboratory experiments further indicating Jupiter’s moon Europa could contain a subsurface ocean and possibly even support life. Europa, the smallest of the four Galilean moons, contains surface features covered by an intriguing dark material. The data shows the enigmatic dark streaks are likely formed as a result of sea salt discolored from exposure to radiation. The presence of surface sea salt could mean the ocean is interacting with the seafloor, boosting the notion that it may be hospitable to life.
“We have many questions about Europa, the most important and most difficult to answer being is there life? Research like this is important because it focuses on questions we can definitively answer, like whether or not Europa is inhabitable,” said Curt Niebur, Outer Planets Program scientist at NASA Headquarters in Washington. “Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa’s ice shell.”
The nature of the dark material covering the linear surface fractures and other geological features on the Europa’s surface has puzzled scientists for well over a decade. The dark material covers younger features on Europa’s surface, and as such is thought to have erupted from inside the moon, but with very little data available, scientists could not determine the chemical composition of the dark material.
One thing scientists know for sure is that Europa is constantly bombarded by radiation from Jupiter’s mighty magnetic field. Electrons and ions are constantly slammed into the moon’s surface, much like particles slam together inside a particle accelerator such as the Large Hadron Collider (LHC). This process contributes to the dark color of the fractures on Europa.
Prior studies utilized data from NASA’s Galileo spacecraft, along with other telescopes, to hypothesize that the surface discolorations were attributed to sulfur and magnesium compounds. Results showed the irradiated sulfur compounds could be responsible for some of the discolorations; however, the new study indicated irradiated salts were behind the discoloration within Europa’s youngest surface features.
“If it’s just salt from the ocean below, that would be a simple and elegant solution for what the dark, mysterious material is,” said research lead Kevin Hand, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
So how did scientists figure out these dark surface stripes? Hand and his colleague Robert Carlson used what they call “Europa in a can”. Basically, they created a simulated Europa in their lab’s test apparatus. They tested possible candidate substances and collected spectra – a.k.a. chemical fingerprints – which are hidden in the light reflected by the individual compounds.
“We call it our ‘Europa in a can’,” Hand said. “The lab setup mimics conditions on Europa’s surface in terms of temperature, pressure and radiation exposure. The spectra of these materials can then be compared to those collected by spacecraft and telescopes.”
In this study, the team tested samples of common salt (sodium chloride), as well as mixtures of salt and water, within a vacuum chamber acclimated to Europa’s chilly minus 280 degrees Fahrenheit (minus 173 degrees Celsius) surface temperature. The team then blasted the salty samples with beams of electrons in order to simulate the radiation blasts the moon’s surface is subjected to.
In the lab, tens of hours of radiation exposure are roughly equivalent to a century on Europa. The salty samples started out white, just like table salt, but turned a yellow-brown color from the radiation. The spectral analysis showed that the colors produced in the lab were very similar to the colors of the surface features in the Galileo mission’s images.
“This work tells us the chemical signature of radiation-baked sodium chloride is a compelling match to spacecraft data for Europa’s mystery material,” Hand said.
The team discovered that the longer the samples were exposed to radiation, the darker the color of the surface features. The team thinks the color variation can help determine the age of the various geologic features as well as any material ejected from Europa’s surface plumes.
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