NASA photo
NASA/JPL-Caltech/SETI Institute

The search for alien life has recently taken a surprise twist away from Mars and toward Europa, an ice ball of a moon in orbit around Jupiter. To understand why, you just need to look at these three numbers:

  • Zero

  • 1.33 billion

  • 3 billion

The first is the volume of known water on Mars (sorry, permafrost and billion-year-old riverbeds don’t count). The second is the volume of water on Earth, measured in cubic kilometers. The third is the inferred volume of water sloshing around just beneath Europa’s frozen surface. Sure, Mars may have had oceans billions of years ago, but Europa has them right now—and they are more than twice as large as all of Earth’s oceans combined.

Everything we know about life says that it needs water. Conversely, every place on Earth where water exists, life does too. The conventional thinking, then, is that if you want to find alien life, the first thing you look for is alien water. Europa is the wettest known world in the solar system. Life also needs food and energy. Europa scores there too: Its ocean might be nourished by a drizzle of organic chemicals and stirred by volcanic vents like the ones dotting the mid-Atlantic ridge. If any place in the solar system holds the answer to the “Are we alone?” question, it’s a good bet that Europa, not the Red Planet, does.

Which is not to say that getting the answer will be easy—not by a long shot. To give you a sense of exactly how hard it will be, consider three more numbers: 600 million—the average flight distance, in miles, from Earth to Europa, meaning that the journey there could take at least six years; 500—the average radiation dose, in rem per day, on Europa’s surface, enough to fry unprotected spacecraft electronics within a matter of days; and 10—the average estimated thickness, in miles, of Europa’s ice shell, more than four times as thick as the glaciers covering Antarctica. Overcoming those numbers will test the limits of human ingenuity. But a growing chorus of scientists has argued that we must try.

500: the average radiation dose, in rem per day, on Europa’s surface—enough to fry unprotected spacecraft electronics within a matter of days

This past May, NASA finally agreed and began the development of a probe to visit Europa sometime in the next decade. Many details of the mission, including its name, are still up in the air. But NASA has selected the nine scientific instruments that will ride aboard the craft to collect data, and Congress has put up the cash to get the potentially $2 billion project underway.

For the Europa faithful, this news has prompted celebratory toasts, along with more than a little giddy disbelief. “We understand how special Europa is. It’s worth the investment. It’s worth the risk,” says Louise Prockter, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory. Prockter has already made that investment herself, having spent half her career studying Europa’s unique, frosty terrain. “I just hope we can get something there while I’m alive.”


Sometimes great discoveries begin with what you do not see. Such was the case when Voyager 2­ flew past Europa in 1979. It radioed home images revealing a world as white as a glacier, slashed with enigmatic brown streaks, and utterly flat. “No craters. It just looked like a huge ice pack. That was a big surprise,” recalls Ed Stone, Voyager’s long-serving project scientist. A lack of craters indicated a fast-changing surface that quickly erases the scars of asteroid impacts; that, in turn, implied that there must be some unrecognized energy source driving the activity. We know now that Europa’s gravitational interactions with Jupiter’s other large moons stretch and squeeze its interior, producing frictional heat (akin to rubbing your hands together) and creating a vast, warm ocean. The ice pack is merely frosting on top.

Three years after Voyager’s flyby, the writer Arthur C. Clarke was so captivated by the discovery that he set his novel 2010 on Europa. He imagined it as a world inhabited by primitive aquatic creatures and guarded by the monoliths from 2001: A Space Odyssey. They issued a warning to humans: “All these worlds are yours—except Europa. Attempt no landing there.” Among planetary scientists, these words have become both an in-joke and a taunt.

A modernist painting

Not a modernist painting

This close-up of Europa, taken by Galileo in 1997, has been color-enhanced to reveal surface features. Blue-white terrain shows relatively pure water ice; reddish stripes may contain salts from an ocean.

In 1995, NASA’s Galileo spacecraft went into orbit around Jupiter, and Europa came into sharper focus—up to a point. A series of new, much-closer images showed the moon’s streaks appear to be glacial fissures that have been flooded from below. Other regions resembled sea ice that has broken apart and refrozen. But Galileo was hobbled by a faulty radio antenna that restricted its data transmissions to a trickle. Over much of Europa it was unable to capture any details less than roughly 1 mile wide, and the probe left some regions almost entirely unmapped. No other spacecraft has gone to visit since the Galileo mission concluded in 2003.

Prockter has made the best of a difficult situation, meticulously knitting together the two-decade-old imagery to show that Europa’s surface ice circulates down to the warmer layers below and back up—a colder version of Earth’s plate tectonics. Britney Schmidt, an astrobiologist at Georgia Tech, has found evidence of Lake Erie-size bodies of water embedded within Europa’s crust that could act as conduits between ocean and surface. Taken together, these discoveries point to an intriguing model. As the crust slowly churns, the surface ice could transport oxygen, minerals, and organic chemicals deposited by comets into the ocean’s depths. Meanwhile, upwelling ice or rupturing lakes might carry evidence of life to the surface. “If we can really understand how the ice shell works, that will tell us about Europa’s ability to support life, about where to look,” Schmidt says.

Recent long-range studies have added to Europa’s mystique. Nearly two years ago, researchers working with the Hubble Space Telescope sighted a huge vapor cloud hovering over Europa’s southern hemisphere. Evidently liquid water is able to break through the crust and blow into space, meaning that either there is water close to the surface or there are very deep cracks in the ice. Also, this past May, a team at NASA’s Jet Propulsion Laboratory (JPL) reported on experiments that reproduced the red-brown color of Europa’s streaks: The markings seem to be the result of oceanic salts that reached the surface and were discolored by Jupiter’s radiation. A salty ocean is just what you’d expect if water interacts vigorously with a rocky seabed, picking up dissolved salts. And well-stirred mineral-rich waters bode well for life.

If any place in the solar system holds the answer to “ Are we alone?” it’s a good bet Europa does.

All of which would make Europa a fascinating destination even if it were a freaky outlier, but it isn’t. Broadly similar icy worlds—including moons, dwarf planets, and giant asteroids—are the norm in the vast outer zone of the solar system. According to the latest research, at least nine of these bodies have inner oceans too. Even Pluto might be wet on the inside, a suspicion bolstered by the 11,000-foot ice mountains and other dramatic surface geology recently found by the New Horizons probe. Put another way, most of the liquid water in the solar system is found not on the surface of rocky worlds like Earth but inside icy bodies like Europa. That raises the stakes for NASA’s upcoming mission. If we find evidence of life on Europa, it would point to a whole new class of habitable worlds across the solar system, and probably across the universe.


Pretty much from the moment Galileo reached Jupiter 20 years ago, Europa proponents have been thinking about how to go back and study the moon in proper detail. Along the way, they have worked on three separate mission concepts that NASA initiated and then canceled. At this point, they know the challenges of Europa backward and forward.

Distance is the simplest one to address because it has a straightforward, brute-force solution. Using an off-the-shelf Atlas V rocket, the voyage to Europa would take at least six years—a painfully long time for academics and political supporters alike. The current Europa concept therefore calls for hitching a ride on NASA’s upcoming rocket, the giant Space Launch System (SLS). In its initial configuration, SLS will be 321 feet tall and pack 8.4 million pounds of thrust. That’s good enough to potentially cut the travel time to Europa at least in half. In NASA’s current schedule, the first SLS will be ready for a test flight in 2018—plenty early for a Europa mission, which is provisionally slated for launch in 2022.

Probing Europa

Probing Europa

The Europa probe will carry a radar that emits both short and long wavelengths. These will penetrate the moon’s shell to measure the ice and detect pockets of water.

Dealing with radiation is trickier. An earlier Europa mission concept included a heavily shielded orbiter that was designed to survive the nonstop barrage of energetic particles near Jupiter. Partly for that reason, the probe was estimated to cost a hefty $4 billion-plus. A team including Robert Pappalardo of JPL came up with a simpler, cheaper solution: Ditch the idea of orbiting Europa and instead settle into a straightforward orbit of Jupiter as a whole. As the probe whirls around the giant planet, it will fly past Europa about 45 times, skimming as close as 15 miles above the surface. After each pass, it will quickly retreat to a more distant part of its orbit, where radiation levels are drastically lower. The team estimates that the cut-and-run approach will easily allow the probe’s electronics to survive the intended three-year duration of the mission.

But radiation creates an informational challenge in addition to the physical one. Even if there is life on Europa, and even if circulation of the ice sometimes carries ocean organisms to the surface, Jupiter’s magnetic field blasts them with energetic particles as soon as they are exposed. Those particles break down organic molecules, so it would be difficult to detect intact traces of alien microbes lying on Europa’s surface. (A beached Europan whale? Maybe. But nobody is banking on that.)

Any bits of ocean or organisms will hit the probe at six times the speed of a bullet from an AK-47.

Fortunately, Europa itself has provided a solution. The icy plumes might eject water samples as high as 125 miles above the surface—well within the probe’s flight path. A group of scientists recently convened a workshop called “Potential for Finding Life in a Europa Plume” to figure out how to take advantage of this discovery. Forget about finding intact microbes, they concluded. Even if Europa’s ocean waters are as rich with life as Earth’s, the odds of scooping up a living cell are minuscule. Oh, and the craft will be moving about 10,000 mph relative to the plume; any bits of ocean, and any organisms in it, will hit at six times the speed of a bullet from an AK-47.

The researchers are optimistic all the same, for two reasons. First, finding chemical signatures of life is far easier than finding an intact alien bug. With that in mind, a pair of miniature labs aboard the probe will aim to measure the composition of the plume as it flies through. Second, it might be possible to spot flash-frozen, life-bearing seawater (or at least life-friendly chemistry) on the Europan surface before it gets irradiated beyond recognition. The probe will carry a radar to seek out regions of thin ice, where eruptions might have occurred in the recent past, and a device called an imaging spectrometer to scan the surface composition in detail.

In the end, though, Europa’s plumes might give merely circumstantial evidence. The better way to get answers—the one that puts a gleam into the researchers’ eyes—is to ignore Arthur C. Clarke’s monoliths and send a lander.


What we will find if we touch down on the surface of Europa is anyone’s guess. Humans have never landed on an ice world before. The closest analogs are the Earth’s Arctic and Antarctic regions, but in many ways the resemblance is only superficial. Pappalardo points out that Europa is far colder, with highs never rising above minus 210 degrees Fahrenheit. At those temperatures water is as hard as concrete and produces novel forms of ice geology. Radiation erodes the surface in unpredictable ways. In short, we have almost no idea what kind of surface we’d be landing on, or what we’d see once we got there.

Adam Steltzner, the JPL engineer whose team designed the insane-genius “skycrane” landing system for the Mars Curiosity rover, is ready to design a Europa lander all the same. He notes that the engineers didn’t know the surface landscape of Mars when the Viking landers descended in 1976, and they did just fine. Touching down is, literally, just a matter of rocket science. “It’s how much weight you can carry,” he says, “how much power, how big of a retro-rocket.” If you want the lander to last more than a few days, you also need a lot of radiation protection. The heavier the lander, the bigger the thrusters you need to go from 12,000 mph in orbit to 0 mph on touchdown. And you’d need the primary orbiter to scout out good sites before releasing the lander. Hard jobs, but there are no showstoppers from the engineering side.

Steltzner already has some clever ideas about how the lander might go about its scientific work. Extracting samples from the frozen Europan surface would not require a costly power drill, he notes; the lander could do the job more elegantly by using its onboard power supply to run a heater in one of its legs. A little warmth would be enough to melt and vaporize the underlying ice. Then the lander could suck the fumes up through the leg, draw them through a mini chemistry lab, and look for signs of life. As the lander settles it would perform a tiny excavation, burrowing through the irradiated surface into the more-intact ice below.

If we find evidence of life on Europa, it would point to a whole new class of habitable worlds across the solar system, and probably across the universe.

NASA has also invited the European Space Agency to submit a proposal for a Europa lander or a high-speed ice penetrator—basically a javelin thrown into the ice. Any of these ideas would have to go through a review process, and probably would not be formally approved until 2016 or 2017. There is also the matter of money: NASA has no funding for a Europa lander, although several members of Congress support the concept.

The great fear of the Europa believers is not that the challenges of visiting this ice moon are too daunting, but that the process of rising to them will simply be too costly. Mars is closer, more accessible, and more familiar; if you just crunch the budget numbers, that’s the place to go. But Europa is the world with the winning scientific numbers. If we invest the time and money, a mission to Jupiter’s ocean moon could yield the most exciting figure of all: 2, the number of places in the universe where we know that life exists.

This article was originally published in the September 2015 issue of Popular Science.

Our Watery Neighbors

Where there’s water, there’s the possibility of life. And a surprising amount of water can be found throughout the solar system. Scientists have now identified at least nine worlds other than ours that likely harbor warm inner oceans.

Callisto Planet


Under a 60-mile-thick crust, this Jovian moon may have an ocean deeper than any on Earth.
Ceres Planet


Nearly 25 percent of the dwarf planet is made up of water ice, and a fraction of that could be liquid.
Europa Planet


There is strong evidence of a salty, circulating ocean just beneath this Jovian moon’s ice shell.
Ganymede Planet


Jupiter’s biggest moon may have saltwater inside; a 2022 European probe plans to investigate.
Enceladus Planet


Jets of water seen on Saturn’s moon Enceladus seem to be fed by an ocean under its south pole.
Titan Planet


A salty ocean lurks inside Saturn’s largest moon; up top, there are lakes of liquid natural gas.
Mimas Planet


Another moon of Saturn, Mimas hides either a subsurface ocean or a football-shaped core.
Triton Planet


The features on Neptune’s moon Triton imply it may have had an ocean—but possibly no longer.
Pluto Planet


New Horizons spotted geologic activity on the dwarf planet that might be driven by an inner ocean.