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Europa’s Crust Conceals a Most Earthlike Feature

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.”

Discovering Europa

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.

A translation of the key passages of Galileo Galilei's journal detailing his discovery of four moons orbiting Jupiter. (Image and caption, NASA)
A translation of the key passages of Galileo Galilei’s journal detailing his discovery of four moons orbiting Jupiter. (Image and caption, NASA)

“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.

Europa during Voyager 2's closest approach on July 9, 1979. (NASA/JPL)
Europa, seen during Voyager 2’s closest approach on July 9, 1979. (NASA/JPL)

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 large, ruddy smear in the middle of this image from the Galileo spacecraft is an example of a dilational band, where new ice is being pushed through cracks in Europa's crust. (NASA/JPL-Caltech/SETI Institute)
The large, ruddy smear in the middle of this image from the Galileo spacecraft is an example of a dilational band, where new ice is being pushed through cracks in Europa’s crust. (NASA/JPL-Caltech/SETI Institute)

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).

False-color image of Europa’s trailing northern hemisphere, where subduction zones are hypothesized to exist. (Caption and credit, NASA/JPL/University of Arizona)
False-color image of Europa’s trailing northern hemisphere, where subduction zones are hypothesized to exist. (Caption and credit, NASA/JPL/University of Arizona)

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.

Europa, as seen by the Galileo spacecraft in 1996. On the left, a true color image. On the right, a color-enhanced version. (NASA)
Europa, as seen by the Galileo spacecraft in 1996. On the left, a true color image. On the right, a color-enhanced version. (NASA)

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.

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A New View of Europa

Europa is like a spherical ice cube that has partially melted. In fact, if you shook this small moon of Jupiter, you might hear a sloshing sound.

That’s because a deep, global ocean lies beneath the moon’s frozen, criss-crossed crust – an ocean that might be as much as 100 kilometers deep. In fact, Europa’s ocean is so vast that it contains between two and three times as much water as all the oceans on Earth, combined.

What swims in that alien sea? We don’t yet know. Maybe nothing. But astrobiologists have placed Europa at – or near – the top of their visitation wish-list for decades. That massive ocean, which is likely filled with minerals from the alien ocean floor, plus the possibility of hydrothermal sea vents, makes this small moon one of the best places to look for life beyond Earth in the solar system.

Piercing that icy crust and sinking a spacecraft into the ocean is no trivial matter, though. But not to worry: Scientists are working on solving that problem. And maybe in the next decade or so, Earth will send a spacecraft to this faraway moon, a probe tasked with sniffing around and possibly scouting out landing sites for future spacecraft.

It wouldn’t be the first time a robotic emissary had the moon in sight, though. In the 1990s, the Galileo spacecraft zipped through the Jovian system and studied the solar system’s most massive world. Part of that campaign included taking pictures of the many moons in Jupiter’s gravitational clutches.

If you thought you’d seen all the images Galileo had to offer, you’d be wrong. NASA released a new view of Europa today. It’s a reprocessed version of images taken on November 6, 1997, and spans an area that’s roughly 160 square kilometers. Those red streaks that look like highways are formed by sulfur-containing compounds oozing through the ice. And if the streaks are anything like what we see on Earth – up in the Canadian Arctic – then maybe they’re the mark of life beneath the surface.

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Cassini Celebrates a Decade in Orbit Around Saturn

Saturn is the jewel of the solar system. The giant, pastel-colored planet is surrounded by huge, iconic rings – and dozens of sparkling, icy moons.

Ten years ago today, NASA’s Cassini spacecraft gently pulled into orbit around the ringed planet. The journey to Saturn took an arduous seven years and covered 3.5 billion kilometers. But the trek was essentially a warm-up for what would become a tireless, gymnastic exploration of the Saturnian system. (I’m in Washington, D.C. today for a National Geographic live event commemorating Cassini’s accomplishments.)

For the last decade, Cassini has been continually looping through the system, flying high above the planet, swooping low over its satellites, and swiveling to stare at the next enigmatic target. It even dropped the European Space Agency’s Huygens lander onto the surface of Titan, Saturn’s largest moon. The spacecraft has collected hundreds of gigabytes of data and snapped thousands of photos (a few of my favorites are in the gallery above), and beamed all of its observations back to eager scientists on Earth. Some of Cassini’s photos have even included Earth itself – visible as a pale blue smudge tucked into the spaces between the planet’s frigid rings.

Saturn in silhouette, glimpsed by Cassini last year. Earth is a speck near the planet's lower right. (NASA/JPL/Space Science Institute)
Saturn in silhouette, glimpsed by Cassini last year. Earth is a speck near the planet’s the lower right. (NASA/JPL/Space Science Institute)

As Cassini swung through the picturesque planetscape, dynamic — sometimes violent — stories began to emerge.

Cassini saw that the planet’s rings, made of small, icy particles, are constantly shifting. Ghostly spokes and braided patterns swim through parts of the disk. One of these patterns, spied in Saturn’s C-ring, is the twisted fingerprint of a comet that smashed into the rings around the time the Black Death was running around Europe (scientists figured this out by mathematically unwinding the twisty pattern – kind of the equivalent of a planetary ring rewind).

Cassini dropped the Huygens lander onto Titan's surface in 2005.
Cassini dropped the Huygens lander onto Titan’s surface in 2005. (ESA/NASA/JPL/University of Arizona)

The rings hide more than relics of ancient impacts. Surfing through the particulate sea are tiny moonlets, perhaps hundreds of them, that are invisible except for the propeller-shaped wakes they create. Small shepherd moons, like potato-shaped Prometheus and Pandora, prune the rings, sometimes creating gaps. And there are strange, 2.5-kilometer tall features that cast shadows over the super-flat disk, which average about 30 meters thick.

And then there’s Saturn itself. When Cassini arrived in 2004, the planet’s northern hemisphere was emerging from the throes of winter, its pole a deep, otherworldly blue. Now, one-third of a Saturn year later, summer is arriving in the north. There, the planet’s jetstream churns away and imprints a curious hexagonal shape around the planet’s crown. Simultaneously, the planet’s poles have been lit by beautiful, strange auroras – the likes of which aren’t seen anywhere else in the solar system. Under Cassini’s watchful eye, the clouds crossing Saturn’s enormous face have morphed and swirled. In late 2010, an enormous storm began brewing; it started as a great white spot and eventually wrapped itself around the entire planet (complete with a massive amount of lightning).

But if the planet is remarkable, its moons, arguably, are more so. Many of these tiny worlds are so different from one another, and so bizarre, that scientists puzzle over how they could have formed — how did bright white, pockmarked Tethys grow from the same building blocks that produced hazy, orange, oily Titan? It’s the same kind of question astronomers ask when they consider how such different planets emerged from the same disk of material swirling around an infant sun…just on a smaller scale.

Some of the moons likely formed during giant, cataclysmic collisions early in the planet’s history. Others, like dark, far-flung Phoebe, are probably interlopers – objects from the outer-outer solar system that scientists suspect have been captured by Saturn’s gravity. Some moons offer few clues and many questions – like Iapetus, with its two-toned coloration, enormous landslides, and strange, equatorial mountain range. Rhea might have once had a ring (or not), Dione a tenuous oxygen atmosphere (and a massive series of canyons gouged through one hemisphere). Mimas, with its huge, carved out crater, improbably looks like the Death Star.

And, the biggest mystery of all: Some of the moons might not be dead, lifeless worlds, but places where alien microbes could be stirring and slicing up molecules to produce energy.

Now, thanks to data from Cassini, we know that two of Saturn’s moons are among the best places to look for extraterrestrial life in the solar system.

Tiny Enceladus spews warm, salty water from giant fractures in its southern hemisphere; Cassini observations suggest that all the ingredients needed to build life are right there, tucked beneath the moon’s icy shell and wafting into outer space. Enormous Titan, the largest of the moony contingent, clings to a dense atmosphere that might be older than Saturn itself, and has a surface very similar Earth’s – except that where there’s water on Earth, there are liquid hydrocarbons on Titan. If scientists find life on Titan’s windswept surface, it will be in a form that’s vastly different from what we know on Earth. It will be unmistakably alien.

These chapters of Cassini’s adventures in the Saturnian system have been an incredible odyssey to follow. And the good news is, the book isn’t completely written. If all goes well, Cassini could continue to twirl around the system for another three years. Then, when its fuel supply runs out in 2017, the spacecraft will plunge toward Saturn’s surface, desperately beaming data back to Earth until it crumples beneath the weight of the planet’s atmosphere.

Cassini observed enormous geysers erupting from the southern hemisphere of tiny moon Enceladus. (NASA/JPL/Space Science Institute)
Cassini observed enormous geysers erupting from the southern hemisphere of tiny moon Enceladus. (NASA/JPL/Space Science Institute)
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Saturn’s Moon Wears the Weirdest Mountain Range in the Solar System

Of all the moons in the solar system, Iapetus has to be among the weirdest. Named after a spear-wielding Titan, the strange Saturnian satellite is less than half the size of Earth’s moon. But it’s a cluster of enigmas: Squished at its poles, the moon is walnut-shaped, has a face as black as coal and a bright white backside, and wears a big, spiky mountain range as a belt.

Even its orbit is weird: Iapetus is roughly three times farther from Saturn than its closest neighbor, Titan. And the path it takes around the planet is tilted, meaning it swings up and down as it orbits, rather than staying in the plane of Saturn’s rings like the rest of the “normal” satellites.

In other words, it’s kind of like the rebel of the Saturnian system, a moon who’d prefer to hang out behind the dumpster and cut class rather than play ball with the other kids.

Among the strangest of Iapetus’ unsolved mysteries is its super-chic, spiky mountain range. Running straight as an arrow along three-quarters of the moon’s equator, the thing is huge: Roughly 20 kilometers tall and up to 200 kilometers wide. (The peak of Mt. Everest, in comparison, rises only 8.85 kilometers above sea level.)

There’s nothing else like it in the solar system.

Close-up of the jagged ridge along Iapetus. (NASA/JPL/SSI)
Close-up of the jagged ridge along Iapetus. (NASA/JPL/SSI)

Scientists first spotted the ridge in 2004,  and since then, they’ve been trying to figure out how such a thing formed. Early theories suggested geologic activity within the moon itself – maybe something akin to Earth’s plate tectonics or volcanism had forced the ridge to rise up along the equator. But that didn’t make a lot of sense. The moon’s crust wasn’t spongy when the ridge formed, the evidence for active geology tepid.

Then, scientists thought maybe the ridge had formed as a result of the moon’s rotation period abruptly slowing down. Some early simulations suggest a day on the moon used to last for a mere 16 hours. Now, though, a day on Iapetus lasts 79 Earth-days – the same amount of time it takes the little guy to shuffle once around Saturn (the moon is tidally locked, meaning it’s plowing through space with the same face forward, always).

Maybe, teams said, a giant impact had knocked Iapetus into its current rotation state, and the resulting braking action caused the crust to buckle.

But most of these theories also predict other strange geologic features (which aren’t observed), or hinge upon the crust being a certain thickness (which it may not be).

In 2010, a new theory emerged. Perhaps the ridge is the remains of a former moon – a moon-moon, suggested Andrew Dombard of the University of Illinois at a meeting of the American Geophysical Union. Sometime during the evolution of the Saturnian system, he said, Iapetus may have had a little friend, roughly 100 kilometers in diameter. Whether the moon captured the moonlet, or the moonlet formed from the debris ejected by a giant impact (this is how Earth’s moon formed) isn’t known.

But eventually, that little friend wandered too close to cranky Iapetus and ended up being shredded by the planet’s gravity.

As the moonlet broke up, Dombard said, its pieces formed an ephemeral ring around Iapetus’ equator. The ring eventually rained down upon the satellite and deposited the giant ridge.

In 2011, another team suggested something similar, this time with a giant impact forming both a ring and a moonlet. The ring would go on to form the mountain range, while the moonlet would smash into Iapetus and create one of its many large impact basins.

Recent evidence, gleaned from the shape of the mountain ridge itself (steep and triangular), suggests that pieces falling from on high could make total sense. It’s kind of the same shape you get when you take a handful of sand and slowly sprinkle it into a pile.

Why the ridge only runs along three-quarters of the equator isn’t explained by this scenario, though.

In short, we still don’t know how Iapetus grew its monstrous mountains. But the idea of a moon with a moon, or a moon with a ring, is strangely compelling. Too bad Iapetus had to go and tear its little friend to bits.

The remains of a moonlet? (NASA/ESA/SSI)
The remains of a moonlet? (NASA/ESA/SSI)
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The Absolute Weirdness of Miranda

Miranda looks like it’s been Frankensteined together. The small, lumpy moon orbits Uranus and has a surface covered by patches of intersecting ridges, weirdly bumpy terrain and pockmarked plains, and dark, irregular canyons. It’s kind of like a badly crafted moon-quilt, except there’s nothing warm and fuzzy about a barren chunk of icy rock with grooves that make the Grand Canyon look like a paper cut.

Earlier this week, I asked a bunch of scientists to share what they’ve been the most surprised by in the solar system. “The absolute weirdness of Miranda,” was one of the responses from planetary scientist Bill Bottke of the Southwest Research Institute. There are a number of bizarre satellites in the solar system, so Bottke pointing to Miranda meant it was worth a closer look.

Miranda was spotted in 1948 by Gerard Kuiper, but it wasn’t until Voyager 2 swung by the solar system’s most unfortunately named planet in 1986 that we got a good look at its little moon. Miranda is only 500 kilometers across, or about one-seventh the size of our moon.

Basically, Miranda appears as though it’s made out of pieces that don’t quite fit together properly, sort of like poor, lurchy Frankenstein. How the moon came to be like this is still a mystery. One theory suggests that in its first incarnation, Miranda was a less-grotesque, more-normal version of itself — until a giant impact or five came along and blew the moon apart. The pieces eventually reassembled, but not in a way that made much sense. Another hypothesis suggests that meteorite impacts locally melted the moon, and slush rising to the surface formed the giant, ridged patches scientists call “coronae.” Other theories have thrown in a little icy volcanism and internal heating caused by gravitational interactions with Uranus and its other moons. Or, Miranda could have begun to differentiate, with its internal layers separating into something like a core, mantle, and crust — but froze before it finished the job.

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These Are Some of the Solar System’s Biggest Surprises

The universe is full of surprises, but two discoveries in the outer solar system are dominating astronomy news this week.

First, astronomers reported yesterday that they have found a distant, tiny world – a small, icy body that lives in the darkness far beyond the orbit of Neptune. Called 2012 VP113, the world’s existence challenges theories describing the infant solar system, and flames speculation that a large planet hides on the fringes of detection. 2012 VP113 and its sibling Sedna are now the two farthest-flung, roundish objects we’ve spied whose gravitational allegiance lies with the sun.

Next, we learned of an asteroid-like body with rings. Called Chariklo, the ringed world is a Centaur – an icy, rocky object that lives between the orbits of Jupiter and Neptune. Chariklo is the first not-planet in the solar system known to have a ring system. And it isn’t just any old ring system – there are two bright, glimmering icy rings encircling the enigmatic, 248-kilometer-wide world. For decades, scientists had thought a small body’s gravity would be too weak to cling to rings like Saturn’s. “When it appeared, it was a complete surprise,” said Felipe Braga-Ribas, a planetary scientist at the National Observatory in Brazil, who discovered Chariklo’s rings hiding in a few seconds of observational data. “We started trying to understand it.”

While discussing the discovery with various planetary scientists, it became apparent that such surprises are the norm in planetary science. It seems the solar system has no shortage of unexpected offerings. “It’s one of these discoveries that you just don’t expect to have happen. But that’s the story of planetary science,” said Joseph Burns, a planetary scientist at Cornell University, while talking about Chariklo. “We go out and see what nature says.”

In honor of this week’s outer solar system double feature, I thought it would be fun to ask some scientists which discoveries they consider the most surprising. Whether recent or decades-old, nearby or faraway, groundbreaking or just “wow,” the discoveries described below invoke a delightful tour through the solar system’s many eclectic treasures.

And please, whether scientist or not, feel free to comment on this post and share your most surprising solar system moment – what has our planetary neighborhood surprised you with?


Q: Which discovery (or discoveries) in the solar system has most surprised you, and why? (responses have been lightly edited for length and clarity)

Ryan Anderson, astrogeologist, U.S. Geological Survey

For me, the biggest recent surprise was the discovery of plumes at Europa. A lot of times the big “surprises” make perfect sense in retrospect. We think Europa has liquid water under the ice and that the ice is shot through with fractures, so it makes sense that it might have plumes much like Enceladus at Saturn. But still, making sense of the discovery in retrospect doesn’t change the fact that it’s surprising and exciting when you first hear about it.

Erik Asphaug, planetary scientist, Arizona State University

My biggest surprise was seeing a comet split up into a dozen pieces in 1993, with the discovery of Shoemaker-Levy 9 when I was graduating from Arizona. Wow! I remember thinking, “How many comets get made in this way?” Twenty years later we’ve all been blown away by the sequential space missions to Wild 2, Tempel 1, and Hartley 2. They are all so different. After all this time we still know basically nothing about comets as geologic bodies, which makes them the most fun and rewarding objects of investigation.

Fran Bagenal, planetary scientist, University of Colorado

Volcanoes on Io, which revealed the most geologically active object in the solar system, and moons around asteroids (how did they get there?).

Michele Bannister, postdoctoral fellow, University of Victoria

How active so many of the icy worlds (moons, dwarf planets) of the Solar System are. Not only active in the great, slow past of geological time, but active now: We can see their surfaces change in our lifetime. Geysers on Enceladus, possible plumes on Ceres and Europa, suggestive geyser-features on Triton, storms and rivers on Titan…we’re living in a Solar System that is changing and dynamic.

Bill Bottke, planetary scientist, Southwest Research Institute

The Nice model, where the giant planets possibly started in a very different configuration than they have today; the ~200 km diameter naked iron core represented by the asteroid Psyche; the discovery of the Kuiper belt and more recently, Sedna and its brethren; not quite a discovery, but the paucity or absence (depending on who you believe) of ancient rocks on Earth that are older than 4 billion years; ice within the permanently shadowed craters on the Moon and Mercury; the prediction that many icy moons have deep oceans; the equatorial ridge around Iapetus, and the absolute weirdness of Miranda; how much our view of the solar system has fundamentally changed since the advent of fast computers and efficient numerical integration codes.  (Exoplanets!)

Mike Brown, astronomer, Caltech

Since the story on the second Sedna body is coming out, I am reminded just how incredibly surprised we were when we discovered Sedna. So surprised that we didn’t believe it for about a month until we got multiple confirmations and ruled out every other possibility. It was the only thing ever found so far away and it lived in an area of space where nothing should have been. I always said, at the time, that this is the best part of doing science, because when you find something that is not supposed to be there, you have learned something new about how the solar system works.

Joseph Burns, astronomer, Cornell University

If you go back long enough, a big [surprise] is just the nature of natural satellites. When I was growing up as an academic, satellites were supposed to be just bombarded, barren, cratered objects. Uninteresting. Why would you want to look at them? And then we got in orbit around Mars and saw Phobos and Deimos, and they were pretty bizarre-looking. And then Voyager went out – and suddenly you see Io. And it’s got volcanoes and sulfur, it looks like a pizza and has some sort of bizarre surface. And it turns out, when you go to every system, every one of the satellites is unique – and now, three of the possible abodes for life in the solar system are on moons.

Athena Coustenis, planetary scientist, Observatoire de Paris-Meudon, CNRS

For me, the most surprising discovery were the jets at Enceladus, because it demonstrated that we could find liquid water under the surface of the icy moons at 10 astronomical units, and challenges all the conventional habitable zone models…

Luke Dones, planetary scientist, Southwest Research Institute

We went in a decade from, “only Saturn has rings” to ring systems around all four giant planets. The ring arcs of Neptune were particularly surprising. Also, the very complicated orbital structure of the Kuiper Belt/trans-Neptunian region/inner Oort Cloud/whatever you want to call it. Most of the populations beyond Neptune could have formed closer to the Sun and moved out to their current locations, but there’s a “cold classical” population that seems like it’s always been where it is now.

And, the first extrasolar planets were found around a pulsar, of all places.

Lindy Elkins-Tanton, director of the Department of Terrestrial Magnetism, Carnegie Institution for Science

I was really surprised when Mercury’s magnetic field was found to be offset to the north; we’re used to a magnetic field whose pole can wander relative to the spin pole of the planet, but what about one whose magnetic equator is north of the planet’s equator? This surprise gives me the exciting idea that we may not understand magnetic dynamos very well after all!

Jay Farihi, astronomer, University College London

Comets in the main asteroid belt – they’re like comet-spies in the inner system where they don’t belong. Some main belt asteroids sometimes exhibit characteristics of comets – tails, outflows, etc. They are still poorly understood; it’s not clear if ice and volatiles are evaporating or being released from collisions or rotational break up.

David Grinspoon, astrobiologist, U.S. Library of Congress

I have been repeatedly surprised by the level of activity in small worlds that we “knew” should be old and dead, before we started exploring the outer solar system. Volcanoes on Io, geysers on Enceladus and now evidence of surface water on Asteroid Vesta?  The apparent prevalence of liquid water environments far beyond the sunny inner solar system is a delightful, ongoing set of surprises.

Avi Loeb, astrophysicist, Harvard University

The discovery of ice on Jupiter’s moon Europa – with potentially liquid water under it – was most surprising to me because this environment might host life (in other words, there might be fish in that water). Another surprising discovery was of Sedna, which is a Pluto-size object on an eccentric orbit extending thirty times farther than Neptune’s distance from the Sun. 

Ralph Lorenz, planetary scientist, Johns Hopkins University Applied Physics Laboratory

I wrote a paper in 1995 anticipating that we would not find sand dunes on Titan, which may be one of the most outstandingly bad predictions in planetary science (for interesting reasons) since some 15 percent of Titan turned out to be covered in giant dunes. Bigger picture, Titan turned out to be much more diverse than anyone remotely expected.  Pre-Cassini, everyone thought in one-dimensional terms – Titan is the same everywhere. We thought it would be wet (no dunes), and yet it has vast deserts.  It turns out climate (latitude) controls a lot – it’s wet too, just around the north polar regions. And nobody, not even us, expected us to be able to see the bottom of Ligeia Mare, all the way down to 170 meters. Titan’s seas must be amazingly clear.

Franck Marchis, planetary astronomer, Carl Sagan Center, SETI Institute

Asteroids are mini geological worlds with complex surface activity, differentiated interiors, complex evolution histories, and moons. Also, the satellites of Saturn – their shape, structure and evolution (captured or formed from the ring?) – and the complex interactions between Saturn, its ring system and its satellites. Io (with its outburst eruptions) and Europa and Jupiter’s magnetic field. There is still a lot to learn about the interaction of satellites and the giant planet’s magnetosphere — maybe a future promising way to detect moons around exoplanets?

Sarah Milkovich, planetary geologist, NASA’s Jet Propulsion Laboratory

I’d say three things: The plume at Enceladus, because this tiny moon is spitting out enough water to form the E-ring around Saturn! The plumes at Europa, because we didn’t see them from the Jovian system with the Galileo spacecraft, but from Earth with Hubble. The recurring slope linea (RSL) at Mars, because the idea of any kind of liquid water-related activity on Mars today is rather mind-boggling.

Catherine Neish, planetary scientist, Florida Institute of Technology

I think my top three are: 1. Ice on Mercury’s poles. Although I was only 11 when this was discovered using ground-based radar, I still think this stands out as one of the neatest discoveries in planetary science. The fact that there is water ice on the closest planet to the Sun just seems so incongruous (even though in retrospect it makes perfect sense). 2. Plumes on Enceladus. Water jetting directly into space! How neat is that? 3. Titan: The desert planet. Prior to the Cassini mission, a lot of people thought that Titan would be covered by a sea of liquid hydrocarbons. The Cassini-Huygens mission revealed it to be much more of a desert planet, with vast sand dunes near the equator, and only smaller lakes and seas near the poles.

Alex Parker, planetary astronomer, University of California, Berkeley

In recent memory, the most exciting and surprising series of confirmed discoveries were the detection of Pluto’s packed system of four small moons outside its very large moon Charon. They were unexpected, are in a surprisingly delicate configuration, and their origin and survival remains challenging for theorists to explain. They’re also particularly exciting in light of the fact that we now get to explore them up close when New Horizons visits the Pluto system next year!

Carolyn Porco, planetary scientist and Cassini imaging lead, Space Science Institute

I was part of the Voyager mission to the outer solar system.  Every stop was packed with surprises. But the best surprise of all was the spectacular geysering activity we on Cassini have found at the south pole of Enceladus.  We suspected that moon might have geysers of some sort.  But never did we imagine they’d be the phenomenally dramatic and huge things they turned out to be. It all comes down to a failure of imagination and our inability to divine the variety and spectacle of the phenomena that Mother Nature can create.

Christopher Russell, Geophysicist, Dawn mission principal investigator, UCLA

I will give you three answers, all from the moons of the outer solar system: The magnetic field of Ganymede (which seems to be generated by a magnetic dynamo similar to that in the Earth – no other moon is like that today, although our moon once was), the plume of Enceladus and the lakes on Titan. These moons were like small planets.

Mark Showalter, planetary astronomer, SETI Institute

I’ll vote for Saturn’s F ring as first imaged by Voyager 1. It showed the so-called “braids” (which aren’t really braids, of course) and it was the first time we all realized that a ring didn’t have to be circular and uniform.

Let me pass along a quick anecdote. At the moment that first image came down, one of the Imaging team scientists was doing an on-camera interview, which I was watching. Somewhat befuddled, he blurted out, “This is high on the list of things we didn’t expect to see!” I always wondered what else was on that list.

Linda Spilker, Cassini project scientist, Jet Propulsion Laboratory

For me, the discovery of the icy jets spewing out of the south polar region of Saturn’s tiny moon Enceladus was a big surprise. Enceladus is only 500 km in diameter and should have frozen solid long ago yet today it lofts icy particles and gas into space, creating Saturn’s diffuse E ring. The discovery of liquid methane lakes and seas at the north pole of Saturn’s giant moon Titan was another big surprise. Titan has a thick nitrogen atmosphere and methane plays the role on Titan, with clouds and rain, that water plays on Earth, creating river channels and filling lakes and seas which contain more than 100 times as much hydrocarbons than all of the oil and gas on Earth.

Alan Stern, New Horizons principal investigator, Southwest Research Institute

River valleys on Mars, volcanoes on Io, and the discovery that dwarf planets dominate the population of planets in our solar system. More generally, we should not be surprised at being surprised at the richness of nature — that’s a hallmark of planetary science.

Anne Verbiscer, planetary scientist, University of Virginia

I know this sounds awfully self-serving, but the discovery that surprised me the most was that of Saturn’s Phoebe Ring!  Yes, we planned our Spitzer observations with the intent of finding a ring, but we were really surprised (and delighted) to find that it was there!

Orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. Jupiter, Saturn, Uranus and Neptune are in purple solid circles. The Kuiper Belt (including Pluto) is the dotted light blue region just beyond the giant planets. Sedna's orbit is shown in orange, and 2012 VP113's orbit is shown in red. (Scott Sheppard, Carnegie Institution for Science)
New orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. Jupiter, Saturn, Uranus and Neptune are in purple solid circles. The Kuiper Belt (including Pluto) is the dotted light blue region just beyond the giant planets. Sedna’s orbit is shown in orange, and 2012 VP113’s orbit is shown in red. (Scott Sheppard, Carnegie Institution for Science)

This post has been updated to include more responses

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Saturn’s Largest Moon Would Host Really, Really Weird Life

Ah, Titan. Saturn’s largest, haziest moon had a brief starring role in last night’s Cosmos: A Spacetime Odyssey. Toward the end of the episode, Neil DeGrasse Tyson eases his spaceship into one of the moon’s dark, oily seas. He wanted to see what was down there—more specifically, what kind of life might be down there.

After spending most of an hour describing the evolution of life on Earth, it was time to turn toward alien terrains and chemistries—to a place that, while not so very far away, could host some very, very strange lifeforms.

There’s a world I want to take you to, a world far different from our own, but one that may harbor life. If it does, it promises to be unlike anything we’ve ever seen before,” Tyson says, in the episode.

Titan is deceptively Earth-like. It has a thick, nitrogen atmosphere. Seasonal rainstorms produce wet patches that are visible from orbit. It has lakes. In fact, Titan is the only place in the solar system, besides Earth, with stable liquids on its surface. Those liquids flow through rivers and streams, pool into lakes and seas, sculpt shorelines and surround islands, just like on Earth.

But Titan’s puddles aren’t filled with water—the moon is soaked in hydrocarbons. Methane and ethane, compounds that are gassy on Earth, are liquid on Titan’s frigid surface. Here, temperatures hover around -179 Celsius (or -290 Fahrenheit). It’s so cold that water ice is rock-hard—in fact, the rocks littering the moon’s surface are made from water. Water is everywhere on Titan, but it’s locked in a state that’s inaccessible for life-sustaining chemistries.

Ask an astrobiologist about the prospect of finding life on Titan, and they’ll say the shrouded, orange moon is the place to go if you’re looking for bizarre life. Life that’s not at all like what we know on Earth. Life that, instead of being water-based, uses those slick, liquid hydrocarbons as a solvent. Life that, if we find it, would demonstrate a second genesis—a second origin—and be suggestive of the ease with which life can populate the cosmos.

Life that’s worth taking a chance to find?

“We will never know if liquid water is the only special solvent in which life can form and propagate unless we go and sample these damn lakes and seas,” planetary scientist Jonathan Lunine of Cornell University said during a recent astrobiology conference. Lunine has spent years studying Titan; at one point, he and his colleagues designed a spacecraft that could land on the moon and float in one of its hydrocarbon seas [pdf].

Titan's surface, snapped by the Huygens lander.NASA/ESA/JPL/University of Arizona
Titan’s surface, snapped by the Huygens lander. NASA/ESA/JPL/University of Arizona
Titan's surface, snapped by the Huygens lander. NASA/ESA/JPL/University of Arizona

Thinking about life on Titan isn’t new. In the 1970s, Carl Sagan and chemist Bishun Khare, then at Cornell University, were already publishing papers describing the organic chemistry that might be taking place on the Saturnian moon. At that point, though, the large bodies of liquid on the moon’s surface hadn’t yet been spotted, so Sagan and Khare were thinking about the types of reactions that might be taking place in the moon’s atmosphere (in 1982, Sagan and Stanley Dermott proposed that such lakes might exist). Later, Sagan and Khare would show it was possible to make amino acids using the elements found in the moon’s haze.

In the 1990s, the Hubble space telescope offered hints of a wet world, but it wouldn’t be until NASA’s Cassini mission that scientists got a good look at the moon. In 2004, the spacecraft began peering beneath Titan’s cloudy shroud; in 2005, Cassini sent the Huygens probe parachuting through the haze to a spot on Titan’s equator. Data sent back to Earth revealed a world that looks very much like ours—just with a completely different chemistry.

What that different chemistry means for the possibility of life is still speculative.

“Think about life on Earth—we’re all either in water or we’re fancy bags of water,” says astrobiologist Kevin Hand of the Jet Propulsion Laboratory. “On Titan, life in the lakes would be ‘bags’ of liquid methane and/or ethane. That 90[Kelvin] liquid would be the solvent and then whatever is dissolved into the lakes would be the material that’s used to build the other components needed for life, and to power metabolism.”

Powering metabolism is tricky at those temperatures, though, which is one of the reasons why some scientists are hesitant to focus on sending a probe to Titan. Nonetheless, astrobiologists are studying the reactions and pathways that life might use to gain some traction on Titan—including things like breathing hydrogen and eating acetylene.

“Which elements are easy and which elements are hard to access if you’re a ‘weird’ microbe living in Titan’s lakes?” Hand says. “At this point we don’t really know—work is ongoing.”

I had a few questions after watching the Cosmos depiction of Titan’s alien seas. First, if I were a weird life form on Titan, would I be able to see Saturn through Titan’s hundreds of kilometers of haze? Or would the most spectacular planetscape in the solar system be hidden behind that smoggy curtain?

“Even with the human eye, Saturn would be visible as a faint, bright-ish blob in the nighttime haze,” Lunine says. “And if you have eyes that extend even a bit beyond human sight into the nearest part of the infrared, the ringed world would be clearly seen floating ethereally in the skies of Titan.”


Second, the scene with Tyson in the spacecraft shows a craggy, chaotic seafloor, with things that look like hydrothermal vents. How much do we really know about Titan’s seafloors?

Large bodies of liquid in Titan's northern hemisphere, mapped by Cassini in 2006. NASA/JPL-Caltech/USGS
Large bodies of liquid in Titan’s northern hemisphere, mapped by Cassini in 2006. NASA/JPL-Caltech/USGS
Large bodies of liquid in Titan's northern hemisphere, mapped by Cassini in 2006. NASA/JPL-Caltech/USGS

Turns out, we know quite a lot about Titan’s seashores, and slightly less about its seafloors. Until now, scientists had mostly used seashore shapes and surrounding topography to infer what the seafloors might be like. But in May 2013, the Cassini spacecraft aimed its radar at the depths of Ligeia Mare, the second largest sea on Titan (Kraken Mare, which Tyson took a swim in, is the largest). Using the radar data, the team created a map of the sea’s floor—its bathymetry—and saw that Ligeia Mare plunges to a depth of 160 meters (524 feet). The northern seabed is gentler and smoother than the southern, which is riven with flooded valleys and punctuated by steep peaks.

Getting the depth profile meant that scientists could estimate how much liquid hydrocarbon rests in Ligeia Mare: As much as 100 times more than the oil and gas reserves on Earth combined.

Next up? Peering into the depth of Kraken Mare, which covers an area of at least 400,000 square kilometers, or approximately equal to the size of Germany. “Kraken appears to consist of no fewer than three distinct basins, each about the size of Ligeia Mare,” Lunine says. “So there’s a lot of sea to see on Titan.”