“Follow the water.” That’s been a guiding principle in the modern search for extraterrestrial life, based on the overwhelming evidence that all living things on Earth — no matter how exotic or extreme — require water to survive. For the past two decades, this emphasis on water has focused attention on Mars, where NASA has intently sought evidence of ancient rivers or modern trickles. Each eroded pebble and layer of sediment there has been heralded as an important clue.

Lately, though, the celestial dowsing process has been pointing in a sharply different direction, away from the majestic deserts of the Red Planet and toward a motley assortment of small, frozen bodies. The shift began in the late 1990s, when the Galileo spacecraft gathered evidence that Jupiter’s moon Europa has a thick layer of water beneath its icy crust. No need to mince words: “Thick layer of water” is another way of saying “a vast, global ocean,” one that just happens to be sequestered underground (or rather, under ice). Europa may hide twice as much water as all Earth’s oceans put together.

Any thought that Europa was an anomaly soon evaporated with the discovery of geysers on Enceladus, a 300-mile-wide satellite of Saturn. Then the, er, floodgates opened as scientists began reporting evidence of water sloshing around inside Jupiter’s moon Ganymede, Saturn’s Titan and Mimas, and perhaps Neptune’s Triton. There may be water layers inside Pluto and the dwarf planet Ceres, too.

“Instead of being the exception, maybe it’s normal to have an ocean in an icy body,” says Louise Prockter, a Europa expert at Johns Hopkins University. “And there are so many icy bodies we haven’t even looked at yet.” Taken together, the evidence suggests that little ice worlds contain much, perhaps most, of the warm, wet real estate in the solar system. That epiphany inevitably leads to a pair of captivating questions. Could organisms really eke out a living in the eternal darkness of a subterranean ocean? And if so, does that mean our Mars-obsessed space program has been diligently looking for life in all the wrong places?

Europa Union

As soon as I mention life, Prockter pauses and offers a gentle but firm disclaimer that she is not looking for “bugs” on Europa. “I’ve been horribly misquoted in the past,” she sighs.

True enough, but her work is central to addressing those existential questions. If anything lives on Europa, it probably needs a way for fresh materials to circulate from the surface down into the ocean. And if we want to find that alien life, it will be a lot easier if some stuff from the oceans migrates back up to the surface. The underlying issue, then, is whether the icy crust of Europa is dynamic and vital, or if it is more of an immobile casket. That’s where Prockter comes in.

About two years ago, she sat down with Simon Kattenhorn, a friend and colleague at the University of Idaho, and mapped out an enigmatic region where Europa’s landscape resembles a jigsaw puzzle. When the two researchers fit the pieces together, re-creating the original landscape, they found “a big hole in the middle” — a place where the original crust went missing. It almost surely ended up below the surface, shoved by the pieces’ motion, where it got recycled into the moon’s interior.

If old crust is disappearing in some places, then new crust must be forming elsewhere. That fits with other evidence that the cracks on Europa’s surface are slowly spreading, presumably pulling up stuff from below in the process. “There is material on the surface that’s quite likely ocean material,” Prockter says.

Bolstering that view, the Hubble Space Telescope recently detected large, intermittent clouds of water vapor hovering around Europa’s southern hemisphere. The strong implication is that liquid water lurks just below the surface, perhaps in glacial lakes akin to Lake Vostok in Antarctica. During local high tide, fissures in the ice crack open and water squirts out, forming a plume. That discovery parallels the current interpretation of the jets on Enceladus, driving home Prockter’s larger point: “We think of Europa as the poster child for icy satellites. If we can understand Europa, we’ll make great leaps toward understanding these other bodies.”

Europa and Enceladus seem to represent one extreme of the water worlds, places where the ice is thin and the ocean is just barely out of reach. With geothermal heat welling up from below and energetic compounds raining down from above, it’s not hard to imagine how life might gain a foothold on those moons. For Ganymede, Mimas and the rest, the liquid layer is probably locked away deeper, dimming prospects for life. Titan is a special case because it has hydrocarbon lakes on the surface in addition to a likely ocean underneath, making it a doubly liquid world that is literally awash in organic chemicals.

“There’s still a lot to be discovered on Titan; I hope there will be repeat missions, not just one,” muses Caltech’s Ed Stone. As lead scientist on the historic Voyager project, he oversaw the original reconnaissance of Titan in the 1980s. But in 2012, NASA rejected a proposed spacecraft mission to Titan’s lakes in favor of another trip to — you guessed it — Mars.

Take Me to the Ocean

More broadly, the efforts to learn more about the hidden water worlds have proceeded at an agonizing half-pace. True, this year Ceres and Pluto will get robotic visitors, the Dawn and New Horizons probes, respectively. The results should clarify those dwarf planets’ inner structures, but they’re unlikely to reveal much about life. Meanwhile, Europa, the archetype of the breed, hasn’t received a second look since Galileo’s termination 12 years ago.

NASA’s Europa Clipper mission will offer an unprecedented look at Jupiter’s fractured satellite. The probe’s long, looping orbit around the planet and moon will find out if the watery Europa is capable of hosting life. (Credit: NASA/JPL-Caltech/Johns Hopkins University/APL)

The obstacles are both practical and political. Europa has the best-understood and most-accessible of the buried oceans, but even a probe on the surface would have a hard time figuring out what is happening beneath its ice. At the same time, NASA’s obsession with Mars means little money is left over for a Europa mission — and the money needed would be substantial. Not only is Europa much farther from Earth, but it also orbits within Jupiter’s fierce radiation belts, so any probe there would need protection against destructive particle blasts. A meaningful exploration of Europa would probably cost at least $2 billion.

But wait! Help is on the way — from Congress of all places. Support there is steadily growing for the Europa Clipper, a concept that NASA has been nurturing and refining for the past few years. Prockter, who helped define the spacecraft’s scientific goals, is cautiously optimistic that this mission will really happen. NASA is currently evaluating proposals for specific instruments.

The subsurface ocean of Europa contains more than twice as much water as Earth’s oceans. That’s a lot of real estate for possible Europan life, with energy provided by geothermal heat. | JPL

In its latest guise, the Europa Clipper would make repeated, rapid swoops past Europa before retreating to safer distances from Jupiter. Here at last is where the real answers could begin. Flying through the moon’s liquid plumes, the Clipper could map their sources and get a direct sample of Europa’s ocean, tasting for hints of biochemistry. Further ahead, some researchers envision submarines somehow breaching the Europan ice to take a direct look, or radiation-hardened rovers conducting life-science experiments at the edges of the geysers. But these are all distant possibilities, decades away at least.

“There might be a massive, well-developed ecosystem under the ice that we’ll never get to see — at least not anytime in the near future,” Prockter says wistfully.

It reminds me of President Kennedy’s words in 1962: “We choose to do these things not because they are easy, but because they are hard.” I’m also thinking about the Space Age lesson we’ve learned over and over since then: Each time we summon our resources and tackle the hard things, we achieve a new and deeper relationship with the universe.