Alien life may flourish in subsurface oceans on Jupiter's Europa, but another of the icy moon's secrets is displayed in plain view: a mysterious “brown gunk” filling many of the fissures, fractures and craters that crisscross its face. “That is our state-of-the-art term for it—brown gunk,” says NASA'S Curt Niebur, who explained at a recent conference that the unknown substance most likely is carried to the surface by water erupting from Europa's depths. “If we can determine what that brown gunk is,” Niebur explains, “we can then understand what is in the water, what is in the oceans of Europa.” Those insights could be crucial for learning whether the satellite harbors life.

Two NASA planetary scientists, Kevin Hand and Robert Carlson, have a lead on the case: the gunk may be simple sea salt, just like that in Earth's oceans but baked by radiation. They came to that conclusion after simulating the harsh environment on the moon with a laboratory “Europa-in-a-can,” a cryo-cooled vacuum chamber bathed in electron beams. Inside, samples of common table salt turned a yellowish-brown and developed spectroscopic features resembling those observed for Europa's brown gunk. The findings were published in May in Geophysical Research Letters.

If irradiated sea salt is indeed the identity of the gunk, that would mean the underlying ocean, like Earth's, is in direct contact with rock and enriched with potentially life-nurturing amounts of minerals. And because the experimental sea salt grew darker the longer it was exposed to the chamber's conditions, in the future scientists might seek out upwellings from the hidden ocean simply by locating the lightest-colored gunk. It won't be long before NASA starts exploring: this spring the space agency announced it will send a mission to Europa in the 2020s.