For centuries, the icy moons of the outer solar system hovered beyond Earth’s grasp. These were cold, sterile worlds – small, frozen spheres with brittle surfaces that didn’t do much except reflect shards of distant sunlight.
Now, of course, we know those moons are anything but dead. One of them – Jupiter’s moon Europa – harbors a rather Earthlike feature, scientists reported this week in Nature Geoscience. Like Earth’s, Europa’s crust might be broken up into a patchwork of tectonic plates. On Earth, the shifting of tectonic plates is responsible for the movement of continents, violent earthquakes, and volcanic eruptions.
Europa is the only other body in the solar system that shows strong evidence for plate tectonic activity – and the implications for life beneath its surface are intriguing.
“It’s a bit like the Earth, which gets everyone excited,” says planetary scientist William McKinnon of Washington University in St. Louis. “But this is a very exotic realm. We’re not talking about rock, we’re talking about ice.”
Galileo described the first cluster of icy satellites, the four largest around Jupiter, in the early 1600s. Night after wintry night, he watched as three strange “stars” swirled around the giant planet. The stars weren’t behaving as he’d expected, and traced odd patterns in the sky. Gradually, Galileo realized that his stars swore allegiance to great old Jupiter rather than the glittering black backdrop. After several months, Galileo concluded that he was not, in fact, seeing stars. He was watching planetary bodies moving around giant Jupiter – and there weren’t just three of them, but four.
“I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times,” Galileo wrote in his Sidereus Nuncius. “I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day.”
Just like that, the Galilean moons were described. They wouldn’t be known as Ganymede, Callisto, Io, and Europa for another 250 years. Those were not the names proposed by Galileo, who instead called them the “Medicean planets,” after the powerful Medici family, whose influence had spread throughout Europe. Rather, it was German astronomer Johannes Kepler who suggested naming the quartet after Jupiter’s collection of lovers.
Coming Into Focus
It wasn’t until the mid-1900s, when the first of Earth’s robotic explorers visited the realm of the giant planets, that these distant moons stepped into the spotlight. Far from Earth and far from familiar, the former stars were anything but ordinary.
Little Io, which makes its home around Jupiter, proved to be the most volcanic body in the solar system. Ganymede is the largest moon in the solar system. And Europa, the smallest of Galileo’s moons and the sixth-largest moon of any, wasn’t just another spherical ice cube.
The 3,100-kilometer wide moon had a perplexingly young surface that suggested some kind of active replenishing of the frozen crust. Crisscrossed with rusty red lines and patches, Europa’s shell was anything but polished. It contained patches of jumbled icy blocks, now known as “chaos terrain,” and long fractures suggestive of geologic fault lines.
What’s more, observations of the moon’s magnetic field by the Galileo spacecraft strongly suggested the frozen crust encapsulated an immense global ocean – a large body of water, containing as much as three times the amount of liquid in all of Earth’s oceans combined. Heated by the gravitational squeeze of Jupiter and shielded from the planet’s intense radiation belts, the ocean, scientists quickly realized, might be capable of incubating extraterrestrial life.
Before long, revisiting Europa was at the top of every astrobiological wish-list. But that hasn’t happened yet.
Now, recent observations suggest the little moon’s crust hides a very Earthlike characteristic – a global system of tectonic plates. Scientists suspect that just like on this planet, those plates slip past and dive beneath one another, producing areas with telltale signs of geologic activity.
Such a system helps solve a long-standing conundrum at Europa: Put simply, scientists needed to figure out how the moon stays the same size.
Europa’s surface is estimated to be between 40 and 90 million years old. That’s much, much younger than its 4-billion-year age. In other words, something must be resurfacing the moon and erasing the cratered evidence of bombardment that would ordinarily pile up, as it does on Europa’s pockmarked neighbors.
Finding that special something wasn’t too hard. When scientists looked at Galileo spacecraft images of Europa, they found areas where new ice had recently appeared. Called dilational bands, these were stretches where the crust looked as though it had cracked and been pushed apart by new ice welling up from beneath. And those bands weren’t small.
“This can be in excess of 20 or 30 kilometers,” says University of Idaho planetary scientist Simon Kattenhorn, an author of the paper describing plate tectonics on Europa. “These are very wide zones, where the two sides moved apart like rigid plates.”
The trouble was, scientists studying the Europan surface couldn’t find the opposite: Places where old crust was destroyed. Without those areas, the moon would just keep growing and growing. Which it isn’t.
“If you’re going to open up the ice, and the moon isn’t expanding like a balloon, then what happens at the other end?” McKinnon says.
A simple explanation is that as new material emerges, portions of the moon crinkle or fold in response. There’s a little bit of evidence for that.
Another possibility, which has been investigated for years, is that subduction zones between tectonic plates in the moon’s crust are helping remove material from the surface. As one plate dives beneath the other, old bits of surface are being recycled.
This process is what Kattenhorn and Louise Prockter, of The Johns Hopkins University Applied Physics Laboratory, think they’ve found evidence for. The pair spotted subduction zones by studying old Galileo spacecraft images of a surface patch on Europa’s northern hemisphere. Based on the patterns of fractures, folds, and streaks, they rewound the geologic clock and tracked the movements of different areas through time (in a fashion analogous to that which identified South America and Africa as once being joined).
When Prockter and Kattenhorn reassembled the Europan puzzle, they found they were missing a piece. In the not-so-distant past, an area roughly the size of Massachusetts had disappeared. The pair attributed that disappearance to a subduction zone near the reddish band known as Minos Linea.
“We were really surprised and excited,” Kattenhorn says. “It’s not like, ‘Wow, we’ve discovered something new!’ It’s more like, ‘Now it all makes sense.’ If you take portions of the surface away, and you make new material, over geologic time periods, the surface will recycle.”
On Europa, these tectonic plates live in the icy crust itself. They’re in the uppermost, brittle portion of the shell, rather than floating just above the ocean surface or riding atop the moon’s core. Whether the plates gradually slide past one another, without any major build-up of tension – or move quickly, with dramatic, shattering releases of tension like on Earth – is unclear.
“If the stress is building up and building up like on Earth – if that really does happen on Europa – then maybe the plate does shift downward suddenly to create a seismic release of energy,” Kattenhorn says. “Even on Earth, there’s still so much we don’t understand about the earthquake process. We know even less about ice.”
One marked disappointment is that the terrain above the subduction zone isn’t more spectacular – it’s pretty ordinary, and wouldn’t be noticeable except through the kind of detailed reconstruction the pair did. That’s why no one has found it until now.
“It’s an even, low-lying band of terrain,” McKinnon says. “It doesn’t have any of the hallmarks from the Earth, it’s not particularly suggestive of anything.”
The team isn’t sure yet if plate tectonic activity is associated with the recently discovered massive plumes of water vapor venting from the moon. Kattenhorn and his colleagues are working on determining that, and on verifying these initial observations. He and others would really like to see evidence for the same process at different sites on the moon – and would especially like some new images to peruse.
“If this could be convincingly demonstrated in another area or two, then that would cinch the story. Kind of like the plumes,” says the Jet Propulsion Laboratory’s Robert Pappalardo, pointing to the recent failure to spot those plumes again. “You see it once, and it’s exciting. But the scientific method says it’s got to be repeatable. So let’s go look again, or look somewhere else.”
Revisiting a Small, Icy Incubator
The astrobiological implications of tectonic plates on Europa are profound. Previously, scientists had struggled to explain how nutrients and oxygen might seed the moon’s subsurface ocean, locked beneath that icy crust. The most obvious source of these molecules is the moon’s surface, where radiation from Jupiter cleaves oxygen from water ice and produces such compounds as hydrogen peroxide and formaldehyde. These compounds are vital chemical reactants and energy sources for lifeforms living beneath the surface.
Giant, icy conveyor belts regularly moving chunks of surface ice into the interior could be an efficient way to deliver such energy and oxygen to the ocean below.
“That’s a big deal for getting at the chemical energy part of habitability. It’s one of those key ingredients for life,” Pappalardo says.
All of these data, scientists say, are excellent arguments for why we need to revisit Europa. It’s the place in the solar system most likely to host alien life today, and it’s a curious geological world that we’re just beginning to understand.
But though the National Academy of Science’s 2011 Planetary Decadal Survey placed a mission to Europa among its highest priorities, that mission keeps running into budget trouble. Over the last few years, teams designing the mission have cut the original US $4.7 billion mission proposal into half – and then into half again.
“NASA is looking at whether there’s anything that can be done for a billion dollars,” Pappalardo says. Though it sounds like a lot, that price tag is a challenge. Europa is far away, and any spacecraft spending any time around the moon will need heavy radiation shields, which add to launch costs.
Pappalardo and others are working on designing what’s called the Europa Clipper mission, which could launch as early as 2022. Rather than orbiting the moon, the spacecraft would perform a series of fly-bys, gathering data that would reveal information about the moon’s gravity and its ocean. “We’ll see what happens in the spring with the next budget cycle,” he says.
Additionally, the European Space Agency is eyeing a 2022 launch for its Jupiter Icy Moons Explorer mission, which would investigate the giant planet as well as Europa, Callisto, and Ganymede.
So perhaps Europa and its enigmatic ocean are squarely on the horizon.
Imagine what it would be like to zoom in for a close look at this watery moon and make a full-throttle attempt to unlock its secrets. My guess is, those initial observations would provoke an astonished response similar to Galileo’s, when he looked through his telescope and saw distant Europa for the first time.