The dark material coating some of the geologic features of Jupiter's moon Europa could be sea salt, according to NASA laboratory experiments. The sea salt could be from a subsurface ocean, darkened from exposure to radiation. The potential for a subsurface ocean is an important consideration in determining if the icy moon could support 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, an Outer Planets Program scientist at NASA Headquarters in Washington, according to a press release. "Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa's ice shell."
For more than a decade, scientists have wondered about the nature of the dark material that coats long, linear fractures and other relatively young geological features on Europa's surface. Its association with young terrains suggests the material has erupted from within Europa, but with limited data available, the material's chemical composition has remained elusive.
"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, Calif., according to the press release.
One certainty is that Europa is bathed in radiation created by Jupiter's powerful magnetic field. Electrons and ions slam into the moon's surface with the intensity of a particle accelerator. Theories proposed to explain the nature of the dark material include this radiation as a likely part of the process that creates it.
Previous studies using data from NASA's Galileo spacecraft, and various telescopes, attributed the discolorations on Europa's surface to compounds containing sulfur and magnesium. While radiation-processed sulfur accounts for some of the colors on Europa, the new experiments reveal that irradiated salts could explain the color within the youngest regions of the moon's surface.
To identify the dark material, Hand and his co-author Robert Carlson, also at JPL, created a simulated patch of Europa's surface in a laboratory test apparatus for testing possible candidate substances. For each material, they collected spectra - which are like chemical fingerprints - encoded in the light reflected by the compounds.
"We call it our 'Europa in a can,'" Hand said, according to the press release. "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."
"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, according to the press release.
Additionally, the longer the samples were exposed to radiation, the darker the resulting color. Hand thinks scientists could use this type of color variation to help determine the ages of geologic features and material ejected from any plumes that might exist on Europa.
