A mysterious dark material coating a number of geographical features on the surface of Jupiter's moon Europa is probably sea salt rising from a subsurface ocean under the icy surface, a NASA experiment suggests.
The dark color of the deposits is likely the result of exposure to radiation created by Jupiter's powerful magnetic field, researchers report.
The space agency has conducted laboratory experiments that point to the ocean interacting with its mineral-rich sea floor, a finding that may help determine whether frozen Europa might support life, they say.
"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," says Curt Niebur, Outer Planets Program scientist at NASA headquarters in Washington, D.C. "Once we have those answers, we can tackle the bigger question about life in the ocean beneath Europa's ice shell."
Scientists have been attempting for the better part of decade to understand the material seen coating long, straight fracture lines and other apparently young surface features on the moon.
The dark material's presence on those younger features suggests it has come to the surface from the moon's interior, but its exact composition has remained a mystery.
The experiment by NASA, dubbed "Europa-in-a-can," was designed to recreate the conditions of a near vacuum, cold temperatures and heavy radiation on the moon's surface.
In their experiment, the researchers created a simulated bit of Europa's surface to test candidates for the dark material.
For each selected material tested they collected spectra contained in the light reflecting from the various compounds, looking for one that would match the dark hue observed on Europa.
When samples of common table salt, or salt mixed with water, were tested - in minus 280 degrees Fahrenheit temperatures and subjected to intense radiation - after several hours in the simulated environment the samples took on a yellow-brown coloration that closely matched what is seen on the frigid surface of the moon, the experimenters reported in Geophysical Research Letters.
"This work tells us the chemical signature of radiation-baked sodium chloride is a compelling match to spacecraft data for Europa's mystery material," says study leader Kevin Hand, a planetary scientist at the space agency's Jet Propulsion Laboratory in Pasadena, Calif.
The longer the samples spent being exposed to radiation the darker they became, Hand noted, suggesting color variations or changes could help determine the age and composition of material observed on Europa.
"If it's just salt from the ocean below, that would be a simple and elegant solution for what the dark, mysterious material is," he says.