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By Jupiter! There's Sodium in the Air


One evening last June at Tucson's Steward Observatory, astronomer Michael Brown set up his equipment to test out an idea about the rings of Saturn.

With six hours to kill before Saturn would become visible, Brown pointed the telescope toward Europa, one of Jupiter's moons, where a thin oxygen atmosphere had been discovered a few months earlier.

"It was completely whim," said Brown, now a researcher in Caltech's geological and planetary sciences department.

That whim, however, led to the discovery of trace amounts of the element sodium in Europa's atmosphere. (Sodium constitutes half of salt molecules and generates the sallow, yellow light of many street lamps.)

Brown, along with Richard E. Hill of the Lunar and Planetary Laboratory at the University of Arizona, reported the finding in a recent issue of the journal Nature.

Besides demonstrating the central role that good old-fashioned luck still plays in many scientific inquiries, the discovery was surprising because scientists did not know of any sodium on Europa. As far as they can tell, Europa's surface is smooth, pure water ice.

The sodium could offer them a tool for deciphering the dynamics of Europa's atmosphere.

"I was delightfully surprised it was there," said William H. Smyth, director of planetary sciences at Atmospheric and Environmental Research, a private research firm in Cambridge, Mass. "It was intriguing and opens up a whole new series of interesting questions and problems."

Sodium is particularly efficient at absorbing one wavelength of yellow light and then shining it back out--a clear, bright beacon that Brown detected in examining Europa's visual spectrum. Just as radioactive tracers help doctors track what's going on inside patients, the sodium light could "tip the hand" of the dynamics of the rest of the moon's atmosphere, Smyth said.

As to where the sodium came from, Brown points the finger at the moon next door: Io.

Of Jupiter's four main moons, red-hued Io orbits the closest, at a distance of 262,000 miles. There, giant volcanoes spew out bountiful amounts of sodium-rich gases.

Brown hypothesizes that high-speed charged particles from space knock the airborne sodium atoms into space while knocking off one or more of the atoms' electrons to create ions.

The ions then accelerate along Jupiter's magnetic field lines, some of which intersect the path of Europa, which lopes around Jupiter at a distance of nearly 420,000 miles.

After journeying less than 10 days through the Jovian system, the sodium slams into Europa's icy surface at 200,000 mph, burrowing a 10th of a millimeter into the ice.

Eventually, the sodium succumbs to the constant stream of particles raining on Europa. Those particles, which include oxygen and sodium ions, gradually wear away the ice and hit the embedded sodium, sending it upward into the atmosphere at a brisk 4,000 mph.

The force is so great that the sodium is shot as high as 18,000 miles above Europa's surface.

"This is the first time we've found anything like this [where one moon feeds the atmosphere of another] in the universe," Brown said. "The thing I was trying to find--sodium in the rings of Saturn--was totally not there. But this was [on Europa]."

Scientists believe Jupiter's moons offer clues about the solar system.

"Their surfaces are for the most part frozen," preserving much of the moons' original structure and composition, said Doyle Thomas Hall, a planetary scientist at Johns Hopkins University in Baltimore. "The surfaces and what's above the surfaces should kind of reflect the history of those bodies."

The presence of sodium, Hall said, "gives us a means for investigating the tenuous atmosphere on Europa from ground-based means."

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