Sea salt may stripe Europa’s surface

Brown deposits could come from water-rock interaction on the seafloor, a promising sign for a habitable environment

Europa

PINCH OF SALT  Colored bands and patches on the surface of Europa might come from irradiated sea salt deposited by a subsurface ocean.

JPL-Caltech/NASA, SETI Institute

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Salt from a subsurface sea on Jupiter’s moon Europa might scribble the rust-colored bands that crisscross the satellite’s icy surface.

The salt is probably deposited on the surface by liquid water that intermingles with a rocky seabed, researchers report May 15 in Geophysical Research Letters.  Deposits of NaCl — ordinary table salt — are normally white. But planetary scientists Kevin Hand and Robert Carlson, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., speculated that harsh radiation around Jupiter might add a dash of color. The planet’s magnetic field traps passing electrons, some of which smack into Jupiter’s moons.

To simulate Europa, the researchers blasted high-energy electrons at cold, vacuum-sealed salt samples — “Europa in a can,” Hand says. The electron shower turned the white salt crystals brown, similar to the color of the fractures on Europa.

The finding could help resolve a long-standing debate about the source of the color. Some researchers have argued that another type of salt, magnesium sulfate, could be responsible. But if Europa’s sea has a rocky bottom that is similar to Earth’s, the rock will absorb the magnesium salt, says Hand. That would leave behind an ocean dominated by NaCl.

“That’s kind of a leap,” says Tom McCord, a planetary scientist at the Bear Fight Institute in Winthrop, Wash. While NaCl is probably one of the salts in Europa’s ocean, he says, it may be neither dominant nor responsible for the cracks’ color. Part of the problem, he says, is that the researchers made their experimental observations in visible light, whereas much of the evidence for Europa’s salts comes from infrared data.

The presence of any kind of salt points to an interaction between water and rock. And that’s a promising sign for the moon’s potential habitability. “Liquid water is not enough for life,” says Hand. “It needs some elements and some energy.” Water rushing over rock, as it does on seabeds on Earth, provides both.

SIMULATING EUROPA  Experiments show how radiation may add a dash of color to Europa’s salt. Credit: Kevin Hand.

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