Time & Life Pictures / Getty Images Jupiter's moon 'Europa'

If you went by news coverage alone, you’d think there’s only one world in the solar system aside from Earth worth studying—and that, of course, is Mars. NASA’s Curiosity rover is inching its way across the Red Planet’s Gale Crater; the Opportunity rover has entered its tenth year of exploration in a region some 5,000 miles (8,000 km) away; and a new rover, named InSight, is on the schedule for a 2016 launch. Why all that attention? In a word, water. Mars had plenty of it once, enough that life might have been able to take hold and could still, in theory, be hanging on in still-wet pockets below the surface.

But another world in our Solar System doesn’t have to look to the past for its maritime days. Jupiter’s moon Europa not only had water, it has it—likely a vast, globe-girdling ocean, 60 mi. (96 km) deep, just beneath a comparatively thin, 2-mi. (3.2 km) rind of ice. Gravitationally plucked by the tidal tugging of its sister moon Io and Jupiter itself, Europa retains a hot interior, which keeps the water comparatively warm and even pulsing. If that doesn’t sound like a place that could cook up life, nothing does. The only ingredients missing to make Europa’s ocean a potential home to living things have been salt and organic compounds. Now, according to a study about to be published in The Astronomical Journal, they’re not missing anymore. A dip in the waters of Europa, the paper concludes, could be very much like a dip in our oceans, perhaps with all the biology that implies.

Long before astronomers could know Europa’s composition for sure, they suspected that it might be covered in ice. It’s brighter in color than most of the Solar System’s other moons, and in the early 1970’s, telescopes detected what was interpreted as frost on the surface. In 1979, Voyager 2 spotted what appeared to be a network of cracks on Europa’s surface as it whizzed past Jupiter—stronger evidence that it wasn’t just frost, but almost certainly real ice.

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When the Galileo probe showed up about two decades later, it became clear that Europa’s ice coating was thick—but more important, the cracks, now clearly evident, meant the ice was floating, forever being fractured and re-fractured by the movement of the ocean below and the flexing of the moon itself. Neighboring Io is continually squeezed the same way, but there isn’t much water there, so the internal heating leads instead to sulfur-spewing volcanoes.

None of this meant Europa had the ingredients for life: you could keep a tank of sterile water warm and churning for 4.5 billion years and at the end, all you’d have would be the same tank of sterile water. Finding evidence of the organics and salt was the key, and that has at last been provided, thanks to a set of observations by the giant Keck II telescope in Hawaii.

The work was conducted by Caltech planetary scientist Mike Brown, who first used the Keck to study Europa 15 years ago and found evidence of salt straightaway—sort of. Those observations showed that Europa has a thin atmosphere containing sodium atoms—which is a sign of salt, but what kind? If it’s sodium chloride—plain table salt and ocean salt—the odds of life might be boosted considerably. With the optics the Keck had at the time, that was a hard thing to determine; it was even hard for Galileo spacecraft, despite its eight years in Jovian orbit.

“The big problem with any spacecraft,” says Brown, “is that they’re already old when you launch them,” meaning the design is locked in at the beginning of the years-long assembly process. “They started building [Gaileo] in 1977, but it wasn’t done for another 15 years.”

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New optics on the Keck have sharpened the telescope’s vision considerably, and when Brown and his colleague Kevin Hand, of NASA’s Jet Propulsion Laboratory, looked at Europa anew, they were at last able to tease out information showing that the salt on the surface is magnesium sulfate. The naïve—and disappointing—assumption would be that the material must have bubbled up from the water below, and that the oceans are rich in sulfur. The picture, however, was more complex than that. Magnesium sulfate is found only on one side of the moon—the side that faces Io, which means that the sulfur is coming from that moon’s volcanic exhaust, and is not native to Europa at all.

That doesn’t suggest the Europan ocean is salt-free. Indeed, it must be salty because the moon has a magnetic field, which means it has to be electrically conductive, something that fresh water isn’t and salt water is. And since magnesium sulfate salt is ruled out, that leaves sodium chloride—good old-fashioned sea salt—as the next best choice. At least that’s the inevitable—if yet unproven—deduction. “We haven’t actually detected either chlorine or sodium chloride,” says Brown, “so this is still a speculation.”

Still, it’s a speculation with big implications. The fractures on the surface have always suggested that the water in the ocean is not entirely trapped by the crust, but instead bubbles up and back down, with the chemistry of the ice above and the water below commingling. It’s statistically inevitable that Europa has been bombarded by many comets during its long lifetime, and since comets are known to contain carbon-based organic compounds, the oceans would be laced with the stuff too, rounding out the recipe for biology.

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“I’m not an expert on life,” says Brown. “But I do know that if you dip a net in the ocean here, you’re bound to pick up something.” Even if you could not get your net two miles deep into the Europan ocean, simply sampling the surface ice would tell you a lot. “You could just land on the surface, dig up a scoop, and know what the chemistry of the ocean really is,” says Brown.

That kind of hands-on study is not likely to happen soon; even a robot lander would be too ambitious (read: too expensive) for the current NASA. Instead, the agency is thinking about a probe called the Europa Clipper, which would orbit Jupiter and make flybys as little as 10 miles above the Europan surface. Armed with far better instruments than Galileo’s vintage electronics, it would nail down the chemistry on the moon’s surface. If that chemistry is life-friendly, the case for a lander would be much stronger—and perhaps irresistible. We’ve never before encountered seawater, after all, that didn’t have at least a little something swimming around in it.

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