PASADENA, Calif.—In an earthquake, ground motion is the result of waves emitted when the two sides of a fault move—or slip—rapidly past each other, with an average relative speed of about three feet per second. Not all fault segments move so quickly, however—some slip slowly, through a process called creep, and are considered to be "stable," or not capable of hosting rapid earthquake-producing slip.  One common hypothesis suggests that such creeping fault behavior is persistent over time, with currently stable segments acting as barriers to fast-slipping, shake-producing earthquake ruptures. But a new study by researchers at the California Institute of Technology (Caltech) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) shows that this might not be true.

"What we have found, based on laboratory data about rock behavior, is that such supposedly stable segments can behave differently when an earthquake rupture penetrates into them. Instead of arresting the rupture as expected, they can actually join in and hence make earthquakes much larger than anticipated," says Nadia Lapusta, professor of mechanical engineering and geophysics at Caltech and coauthor of the study, published January 9 in the journal Nature.

She and her coauthor, Hiroyuki Noda, a scientist at JAMSTEC and previously a postdoctoral scholar at Caltech, hypothesize that this is what occurred in the 2011 magnitude 9.0 Tohoku-Oki earthquake, which was unexpectedly large.

Fault slip, whether fast or slow, results from the interaction between the stresses acting on the fault and friction, or the fault's resistance to slip. Both the local stress and the resistance to slip depend on a number of factors such as the behavior of fluids permeating the rocks in the earth's crust. So, the research team formulated fault models that incorporate laboratory-based knowledge of complex friction laws and fluid behavior, and developed computational procedures that allow the scientists to numerically simulate how those model faults will behave under stress.

"The uniqueness of our approach is that we aim to reproduce the entire range of observed fault behaviors—earthquake nucleation, dynamic rupture, postseismic slip, interseismic deformation, patterns of large earthquakes—within the same physical model; other approaches typically focus only on some of these phenomena," says Lapusta.

In addition to reproducing a range of behaviors in one model, the team also assigned realistic fault properties to the model faults, based on previous laboratory experiments on rock materials from an actual fault zone—the site of the well-studied 1999 magnitude 7.6 Chi-Chi earthquake in Taiwan.

"In that experimental work, rock materials from boreholes cutting through two different parts of the fault were studied, and their properties were found to be conceptually different," says Lapusta. "One of them had so-called velocity-weakening friction properties, characteristic of earthquake-producing fault segments, and the other one had velocity-strengthening friction, the kind that tends to produce stable creeping behavior under tectonic loading. However, these 'stable' samples were found to be much more susceptible to dynamic weakening during rapid earthquake-type motions, due to shear heating."

Lapusta and Noda used their modeling techniques to explore the consequences of having two fault segments with such lab-determined fault-property combinations. They found that the ostensibly stable area would indeed occasionally creep, and often stop seismic events, but not always. From time to time, dynamic rupture would penetrate that area in just the right way to activate dynamic weakening, resulting in massive slip. They believe that this is what happened in the Chi-Chi earthquake; indeed, the quake's largest slip occurred in what was believed to be the "stable" zone.

"We find that the model qualitatively reproduces the behavior of the 2011 magnitude 9.0 Tohoku-Oki earthquake as well, with the largest slip occurring in a place that may have been creeping before the event," says Lapusta. "All of this suggests that the underlying physical model, although based on lab measurements from a different fault, may be qualitatively valid for the area of the great Tohoku-Oki earthquake, giving us a glimpse into the mechanics and physics of that extraordinary event."

If creeping segments can participate in large earthquakes, it would mean that much larger events than seismologists currently anticipate in many areas of the world are possible. That means, Lapusta says, that the seismic hazard in those areas may need to be reevaluated.

For example, a creeping segment separates the southern and northern parts of California's San Andreas Fault. Seismic hazard assessments assume that this segment would stop an earthquake from propagating from one region to the other, limiting the scope of a San Andreas quake. However, the team's findings imply that a much larger event may be possible than is now anticipated—one that might involve both the Los Angeles and San Francisco metropolitan areas.

"Lapusta and Noda's realistic earthquake fault models are critical to our understanding of earthquakes—knowledge that is essential to reducing the potential catastrophic consequences of seismic hazards," says Ares Rosakis, chair of Caltech's division of engineering and applied science. "This work beautifully illustrates the way that fundamental, interdisciplinary research in the mechanics of seismology at Caltech is having a positive impact on society."

Now that they've been proven to qualitatively reproduce the behavior of the Tohoku-Oki quake, the models may be useful for exploring future earthquake scenarios in a given region, "including extreme events," says Lapusta. Such realistic fault models, she adds, may also be used to study how earthquakes may be affected by additional factors such as man-made disturbances resulting from geothermal energy harvesting and CO₂ sequestration. "We plan to further develop the modeling to incorporate realistic fault geometries of specific well-instrumented regions, like Southern California and Japan, to better understand their seismic hazard."

"Creeping fault segments can turn from stable to destructive due to dynamic weakening" appears in the January 9 issue of the journal Nature. Funding for this research was provided by the National Science Foundation; the Southern California Earthquake Center; the Gordon and Betty Moore Foundation; and the Ministry of Education, Culture, Sports, Science and Technology in Japan.

One of the most powerful computer clusters available to a single department in the academic world just got stronger.

The California Institute of Technology's CITerra supercomputer, a high-performance computing cluster of the type popularly known as a Beowulf cluster, was replaced this year with a faster and more efficient system. The new cluster capitalizes on improvements in fiber-optic cables and video chips—the kind found in many gaming devices and mobile phones—to increase processing capacity and calculation speeds. With access to this improved supercomputer, Caltech's researchers are able to use advanced algorithms to analyze and simulate everything from earthquakes to global climate and weather to the atmospheres of other planets.

The new $2 million supercomputer, which is administered by the Division of Geological and Planetary Sciences, performs with five times the computational power of the previous cluster while using roughly half the energy.  It has 150 teraflops of computing capacity, meaning it can perform 150 trillion calculations per second. The upgrade was made possible in part with the private support of many individuals, including members of GPS's chair's council, a volunteer leadership board.

So what does a faster, more energy-efficient supercomputer mean for Caltech's geoscientists and geophysicists?

"There is a whole new class of problems that we can now address," says Professor of Geophysics Mark Simons, who oversees the cluster. "We can not only solve a given problem faster, but because it takes less time to solve those problems, we can choose to work on harder problems."

Simons, for instance, is working to develop models to understand what happens underground after an earthquake—and what is likely to occur in the months and years after—by analyzing ground motion observed on the surface. In 2011, for instance, a Caltech research team led by Simons used data from GPS satellites and broadband seismographic networks to develop a comprehensive mechanical model for how the ground moved after Japan's 9.0 earthquake.

"Mark's team developed the framework allowing them to do millions of computations where seismologists had only been able to do hundreds before," says Michael Gurnis, the John E. and Hazel S. Smits Professor of Geophysics and director of the Seismological Laboratory. "The ability to routinely compute at a level that is so much higher than anyone else had previously done—to have the computational resources immediately available during the hectic days after a devastating earthquake—was an amazing advance for geophysics."

Simons is not alone in using advanced computation to unlock Earth's greatest mysteries. He and Gurnis—who studies the forces driving Earth's tectonic plates—are among a group of 15 GPS faculty members who, with their students, routinely use the cluster. The division is unique among universities in that it provides its faculty with access to such a large computational facility, giving almost any of its researchers the ability to number crunch when they need to—and for extended periods of time.

Research done using computations from the previous cluster led to more than 140 published papers, which crossed the fields of atmospheric science, planetary science, Earth science, seismology, and earthquake engineering.

One of the biggest users of the new cluster is Andrew Thompson, an assistant professor of environmental science and engineering, who uses it to simulate complex ocean currents and ocean eddies. Capturing the dynamics of these small ocean storms requires large simulations that need to run for weeks.

Thanks to the size of Caltech's cluster, Thompson has been able to simulate large regions of the ocean, in particular the ocean currents around Antarctica, at high resolution. These models have led to a better understanding of how changes in off-shore currents, related to changing climate conditions, affect ocean-heat transport toward and under the ice shelves. Ocean-driven warming is believed to be critical in the melting of the West Antarctic Ice Sheet.

"Oceanography without modeling and simulations would be really challenging science," says Thompson, who arrived at Caltech in the fall of last year. "These models indicate where we need improved or more frequent observations and help us to understand how the ice sheets might respond to future ocean circulation variability. It is remarkable to have these resources at Caltech. Access to the Caltech cluster eliminates some of the need to apply for time on federal computing facilities, and has allowed my research group to hit the ground running."

For over 150 years, geologists have debated how and when one of the most dramatic features on our planet—the Grand Canyon—was formed. New data unearthed by researchers at the California Institute of Technology (Caltech) builds support for the idea that conventional models, which say the enormous ravine is 5 to 6 million years old, are way off.

In fact, the Caltech research points to a Grand Canyon that is many millions of years older than previously thought, says Kenneth A. Farley, Keck Foundation Professor of Geochemistry at Caltech and coauthor of the study. "Rather than being formed within the last few million years, our measurements suggest that a deep canyon existed more than 70 million years ago," he says.

Farley and Rebecca Flowers—a former postdoctoral scholar at Caltech who is now an assistant professor at the University of Colorado, Boulder—outlined their findings in a paper published in the November 29 issue of Science Express.

Building upon previous research by Farley's lab that showed that parts of the eastern canyon are likely to be at least 55 million years old, the team used a new method to test ancient rocks found at the bottom of the canyon's western section. Past experiments used the amount of helium produced by radioactive decay in apatite—a mineral found in the canyon's walls—to date the samples. This time around, Farley and Flowers took a closer look at the apatite grains by analyzing not only the amount but also the spatial distribution of helium atoms that were trapped within the crystals of the mineral as they moved closer to the surface of the earth during the massive erosion that caused the Grand Canyon to form.

Rocks buried in the earth are hot—with temperatures increasing by about 25 degrees Celsius for every kilometer of depth—but as a river canyon erodes the surface downwards towards a buried rock, that rock cools. The thermal history—shown by the helium distribution in the apatite grains—gives important clues about how much time has passed since there was significant erosion in the canyon.   

"If you can document cooling through temperatures only a few degrees warmer than the earth's surface, you can learn about canyon formation," says Farley, who is also chair of the Division of Geological and Planetary Sciences at Caltech.

The analysis of the spatial distribution of helium allowed for detection of variations in the thermal structure at shallow levels of Earth's crust, says Flowers. That gave the team dates that enabled them to fine-tune the timeframe when the Grand Canyon was incised, or cut.

"Our research implies that the Grand Canyon was directly carved to within a few hundred meters of its modern depth by about 70 million years ago," she says.

Now that they have narrowed down the "when" of the Grand Canyon's formation, the geologists plan to continue investigations into how it took shape. The genesis of the canyon has important implications for understanding the evolution of many geological features in the western United States, including their tectonics and topography, according to the team.

"Our major scientific objective is to understand the history of the Colorado Plateau—why does this large and unusual geographic feature exist, and when was it formed," says Farley. "A canyon cannot form without high elevation—you don't cut canyons in rocks below sea level. Also, the details of the canyon's incision seem to suggest large-scale changes in surface topography, possibly including large-scale tilting of the plateau."

"Apatite ⁴He/³He and (U-Th)/He evidence for an ancient Grand Canyon" appears in the November 29 issue of the journal Science Express. Funding for the research was provided by the National Science Foundation. 

Think back to the last time you saw the Milky Way—that faint stripe of stars that thickens and brightens as you get farther from city lights. At least 200 billion stars fill the Milky Way, our galaxy. How many planets might orbit those stars? What would those worlds be like? Twenty years ago, it was anybody's guess.  

In the 1990s, astronomers began to discover planets around other stars—so-called exoplanets. Since then, the confirmed count of exoplanets has skyrocketed to more than 850, with thousands of candidates awaiting follow-up. Astronomers now estimate that the stars in our Milky Way have an average of at least one planet each. (The next time you look up into the night sky, think about that.)

The sudden prospect of characterizing so many solar systems in our own galaxy has brought together two once-isolated camps: planetary scientists, who generally focus on the inside of our solar system, and astronomers, who mostly look beyond it. Planetary scientists see an opportunity to learn about our solar system and its origins by putting it into the context of a huge ensemble of other solar systems, and astronomers have a keen interest in what planetary scientists might help them discover about planet formation on a galactic or even larger scale.

To those ends, nine Caltech astronomers and planetary scientists are forming a Center for Planetary Astronomy. Joining together in a single research center will help them maintain fruitful collaborations, collectively attract research funding and fellowships for young scholars, and recruit top students and postdoctoral scholars.

The nascent center's members bring complementary perspectives to the characterization of our newly discovered neighbors. Planetary science professor Geoff Blake, astronomy professor Lynn Hillenbrand, and senior research associate John Carpenter study planet-forming disks of gas and dust around young stars. Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, and Caltech's infamous Pluto-killer, studies fossil rubble from just such a disk—a fantastic array of thousands of planetesimals and chunks of rock and ice on the fringes of our solar system, known as the Kuiper belt, that yields clues to the primordial solar system. The remaining scientists are focused more on the planets themselves. John Johnson, an assistant professor of planetary astronomy, focuses on the detection and characterization of exoplanets, searches for worlds like Earth, and investigates how stars' masses affect planet formation by studying the relationships between exoplanets and the very different types of stars that they orbit. Heather Knutson, an assistant professor of planetary science, characterizes exoplanets' compositions, temperatures, atmospheres, and even their weather. Yuk Yung, the Smits Family Professor of Planetary Science, studies the atmospheres of planets, and Dave Stevenson, the Marvin L. Goldberger Professor of Planetary Science, studies planetary interiors and how they evolve. Gregg Hallinan, an assistant professor of astronomy, is trying to detect radio signals from exoplanets, which would indicate the presence of magnetic fields that could be a signature of habitability.

The center's members are excited about its potential contribution to the major discoveries that are sure to come in this field. "The unique combination of Caltech's top-ranked astronomical facilities, astronomy program, and planetary science program will allow us to access the deep and broad knowledge about planets and planetary systems that only comes from such a joint endeavor," says Brown.

Says Knutson, "I was trained as an astronomer, but what I do is planetary science. Caltech is one of the few places where we have great conversations between the two groups. And Caltech's resources, in terms of telescopes, give us the opportunity to move quickly and think big." 

Caltech geology graduate student Katie Stack says her Caltech experience has provided her with the best of both worlds. Literally.

As one of five Caltech graduate students currently staffing the Mars Science Laboratory mission, Stack is simultaneously exploring the geologic pasts of both Mars and Earth. She and her student colleagues apply their knowledge of Earth's history and environment—gleaned from Caltech classes and field sites across the globe—to the analysis of Curiosity's discoveries as well as the hunt for evidence of past life on the Red Planet.

"Mars exploration is that perfect combination of understanding what is close to home and far afield," says Stack, who studies sedimentology and stratigraphy in the lab of John Grotzinger, the mission's project scientist and Caltech's Fletcher Jones Professor of Geology.

"The mission is providing a different perspective for seeing the world . . . as well as for seeing myself," she adds. "As a graduate student, you often struggle with your place in your academic community, and taking part in the mission is one of the ways that we are just thrown into the mix. We are working on the same level as a bunch of senior scientists, who have a lot of experience, and yet they are asking us questions—seeking our expertise. That's an experience you don't often get to have."

Caltech's graduate student participants on the MSL—who include Stack, Kirsten Siebach, Lauren Edgar, Jeff Marlow, and Hayden Miller, all from the Division of Geological and Planetary Sciences—represent the largest contingent of students from any one institution in a mission that has more than 400 participating scientists. Caltech's strong student presence is aided in large part by the leadership role that faculty are playing in the mission as well as the Institute's close proximity to mission control at the Jet Propulsion Laboratory (JPL), which Caltech manages for NASA.

Caltech's graduate students are among the mission personnel responsible for sequencing the scientific plan and programming the rover each day, as well as for documenting the scientific discussion and decisions at each step of the mission. As the mission's blogger, Marlow also helps share the science team's work with the public.

"The graduate students are the heart of the mission," Grotzinger says. "They are the keepers of the plan and are able to efficiently operate the technology to run the rover every day, especially when senior scientists are unable to do so."

Making the science plans for the rover, says graduate student Kirsten Siebach, is "as close as I get to driving the rover. I can help program it to take pictures, analyze samples, and shoot the ChemCam laser."

"It's always fun when something that I helped command the rover to do, like take a picture, ends up making the news," she adds. "I helped command it to take one such picture of the Hottah outcrop that showed evidence of an ancient streambed."

In addition to staffing operations for the mission, Caltech's students are also key contributors to the scientific analysis of the data and help make decisions about where Curiosity goes.

Before Curiosity landed, for instance, Stack and Lauren Edgar, helped compile a geologic map of the Gale Crater landing ellipse, using orbital images to identify the geologic diversity and relationships among rocks. Their work has continued to serve as a "road map" for the rover's research. Meanwhile, Siebach has been exploring the history of water on Mars, looking at the geomorphology of channel structures and fractures on the planet.

"We really have grown up in the golden age of Mars exploration," Edgar says, noting that while at Caltech, she's had the opportunity to contribute to three Mars rover missions—Spirit, Opportunity, and now Curiosity. "They just keep getting better and better."

In addition to the graduate students, several undergraduate students have taken part in the mission, participating through Caltech's Summer Undergraduate Research Fellowships (SURF) program. This past summer, a student working with Bethany Ehlmann, an assistant professor of planetary science at Caltech and an MSL science team member, helped to characterize and classify hundreds of Earth rock samples for potential comparison with Mars specimens that will be analyzed by the ChemCam instrument. Meanwhile, over the past two summers, Solomon Chang, a Caltech sophomore studying computer science, worked with JPL engineers to model Curiosity's mobility to ensure that it would actually move on Mars as it had been programmed to do.

Those summer projects have ended, but for the Caltech grad students on the MSL team, the work continues. Indeed, says Grotzinger, because many of the mission's scientists will be leaving Pasadena to return to their home institutions during the coming months, the grad students will be called upon to fill additional roles in the rover's daily operation and science.

"One of the great things about working on a mission as a student is that science is a fairly merit-driven process," says Ehlmann, who participated in Mars exploration missions as both an undergraduate and graduate student. "So if you have a good idea and you are there, you can contribute to deciding what measurements to make, can develop hypothesis about what's going on. It's a very inspiring and empowering experience."

The Jet Propulsion Lab's Cassini spacecraft roared toward Saturn in a spectacular nighttime launch on October 15, 1997. After two gravity assists from Venus and one each from Earth and Jupiter, the school-bus-sized orbiter arrived at Saturn on June 30, 2004 Pasadena time, putting itself on a wildly looping path designed to give its Earthbound handlers repeated close-up looks at Saturn's rings, all of Saturn's big moons, a good few of the small ones—and, of course, Saturn itself. Saturn and its retinue have not disappointed; new wonders have been revealed with every orbit.

Cassini carried a second spacecraft along for the ride—a probe named Huygens, built by the European Space Agency and designed to land on Titan, Saturn's largest moon. Titan could have been a planet itself, if it hadn't grown up in the wrong neighborhood. Titan is bigger than Mercury (but smaller than Mars) and has an atmosphere that's four times denser at the surface than Earth's is at sea level. The surface, however, had never been seen  before—or at least not clearly. There's so much methane in Titan's atmosphere that the entire moon is wrapped in an impenetrable, orange pall of photochemical smog.

Cassini dropped off its hitchhiker six months after arriving, and on January 14, 2005, Huygens took a two-hour, 28-minute parachute ride through Titan's atmosphere, smelling, tasting, and feeling its winds all the way down. At an altitude of about 40 kilometers, the hydrocarbon haze became transparent, revealing a vista much like the mountain ranges and dry lakes of the Mojave Desert. But because of Titan's average surface temperature of 95 kelvins (nearly –290° F) the gullies on these mountainsides had been carved by methane rain; the mountains themselves proved to be made of water ice frozen as hard as any rock on Earth.

Hyugens made a soft landing on material with the consistency of wet sand and was still broadcasting 72 minutes later when Cassini dropped below Titan's horizon and radio contact was lost. The pictures Huygens beamed back showed that it had come to rest on a sepia-tinted plain strewn with cantaloupe-sized cobblestones of ice that had clearly been rounded by a flowing liquid. It didn't take long to identify this fluid as methane, since Huygens's mass spectrometer detected a puff of the stuff vaporized from the greasy mudflat by the lander's body heat.

Cassini's radar mapper has since confirmed what earlier observations had suggested—that lakes of liquid methane and ethane exist, primarily in Titan's northern hemisphere. One, near the north pole, is the size of Lake Superior. Titan has taken this low-temperature parody of Earth, where methane is water and ice is rock, even further. Cassini's radar has spotted areas where there may be "cryovolcanoes" that ooze slush instead of lava.

Titan, big as it is, is far too small to have retained the warmth that volcanoes—even of ice—demand. However, Cassini found that it flexes like a stress ball in the grip of Saturn's gravity, which induces a 10-meter-tall "solid tide" in Titan's crust. This periodic deformation not only generates heat, but implies the presence of liquid water under the ice. A Titan made of solid rock could only bulge by about a meter, but a liquid would be more susceptible to Saturn's pull. "We have long suspected that Titan, like Jupiter's moons Europa, Ganymede, and Callisto, has a subsurface ocean," says David Stevenson, Caltech's Goldberger Professor of Planetary Science. "These results show that this is very likely the case."

Titan isn't the only satellite of Saturn to join the subsurface saltwater society. The biggest surprise of the Cassini mission has come from Enceladus, a small, undistinguished moon that turns out to be spewing curtains of icy mist from the vicinity of its south pole. These geysers issue from fissures called tiger stripes—dark parallel gashes on Enceladus's otherwise bright surface. These intriguing stripes were discovered in May 2005, and the trajectory of Cassini's next flyby in July was altered to take a closer look. Cassini spotted the geysers on its way in, shortly before whizzing through them at an altitude of about 170 kilometers. A thermal map of the surface below linked the plumes to hot spots with temperatures of some 145 K—on a moon whose average surface temperature does not exceed 70 K. (See "A Nice Place to Visit?" in E&S No. 1, 2006.)

Life is found wherever liquid water exists on Earth. Could this hold for the outer solar system as well? Perhaps, says Professor of Planetary Science Andrew Ingersoll. "The idea of a 'habitable zone' has existed for a hundred years. All the water is frozen out on Mars, and it's all vaporized on Venus. Earth is just right—we have oceans, and we have life. But now, the habitable zone might also include an archipelago of these isolated moons."

Enceladus probably gets its heat by flexing, like Titan, Ingersoll says. "But this would dissipate orbital energy, and eventually Enceladus wouldn't be where it is any more. And neither would the other nearby moons, because they all influence each other." This could mean that the geysering is a relatively recent phenomenon, says Ingersoll. "Or is it cyclic, and we just got lucky enough to see it?"

Getting lucky may be putting it mildly. "You have to get the energy from somewhere," Stevenson observes. The proximate source, he says, could be a moon named Dione, with which Enceladus is in an orbital resonance. Enceladus makes two trips around Saturn for every one that Dione makes, and each time Enceladus catches up, Dione gives it a gravitational tug. Like a child being pushed ever higher on a swing, these gravity assists could nudge Enceladus into an increasingly eccentric orbit, Stevenson says. Eventually enough orbital energy might accumulate to trigger the geysers, which would then erupt for a while as Enceladus's orbit became more circular again. "This cycle might well take a million years or more," Stevenson says. "But Io, Jupiter's volcanic moon, is also overly active. And now you start to get scared, because this means you're looking at a very special time for both of them." If the odds of this coincidence are 1 in 10, it's not so bad; but if they work out to 1 in 100, it's time to find another theory.

Saturn's rings have also held surprises. Cassini arrived just after the northern hemisphere's winter solstice, when the rings were at their maximum tilt relative to the sun. This allowed the spacecraft to measure the sunlight passing through the rings, revealing them to be incredibly thin—a mere 10 to 20 meters thick, on average. During Saturn's equinox in August 2009, Cassini got a never-before-seen view of the rings aimed edge-on to the sun. As the light swept across them, giant shadows were cast by anything sticking up from the surface. Suddenly, towering waves of ice as much as four kilometers tall were thrown into sharp relief. They proved to be disturbances set off by the passage of the nearby moon Daphnis, whose orbit is slightly tilted relative to the ring plane. (Many more discoveries are described in "Cassini's Ringside Seat" in the Spring 2010 issue of E&S.)

And what of Saturn itself? Unlike Jupiter's psychedelic swirls of candy-cane colors bedecked with semipermanent storms such as the Great Red Spot, the bands in Saturn's atmosphere are muted shades of butterscotch. Not that there's nothing going on down there—Cassini has witnessed 10 thunderstorms in its eight-plus years over Saturn, all in the summer skies of the southern hemisphere. But on December 5, 2010, Cassini caught a storm 500 times the size of the others—in the northern hemisphere. The spacecraft heard the storm before it saw it, says Ingersoll. "So everything is very quiet, and then this giant thunderstorm goes off. We were getting copious noise on the radio receiver, and then we see this." What showed up as a tiny white spot in a picture shot that day grew to the diameter of Earth in three weeks, spawning a trail of white clouds that wrapped all the way around the planet. Such storms have been seen once every 30-odd years since 1876. They usually appear late in the northern hemisphere's summer, but this one arrived 10 years early at the beginning of spring.

"Saturn is normally very bland," Ingersoll says, "which makes this giant storm just weird. It's just crying out to be explained." Like thunderstorms on Earth, these storms are driven by convection, as air masses of different temperatures redistribute their heat. But rather than supporting a bunch of small storms all the time, Saturn stores up these heat differences until it unleashes a doozy. In some ways, Ingersoll says, Saturn behaves less like a terrestrial weather system and more like a volcano, in which the pressure builds up for many years before an eruption. Volcanic pressure is contained by layers of rock, but what keeps the lid on Saturn's atmospheric energy?

"To some extent, that same question applies to both Titan's and Saturn's meteorology," Ingersoll says. "There is more methane in Titan's atmosphere waiting to rain out than there is water vapor in the atmosphere on Earth. There's more latent heat stored, too, but only 1 percent as much sunlight to evaporate it. So there are two ways this could work: tremendously violent but infrequent rainstorms, because there isn't much energy to resupply them; or a steady, continuous drizzle. And the choice seems to be the same on both Saturn and Titan. We've seen the storms on Saturn, and we see erosion on Titan. A drizzle won't carve those channels. So that seems to be how weather works. You turn down the energy, and you still get storms. Who knew? That's why we study this stuff."

There's still time to figure it out. Cassini's mission is now planned to run through the northern hemisphere's summer solstice, in May 2017, allowing us to observe one complete change of Saturn's seasons, or a little less than half of a Saturnian year.