Cassini Turns 15

Some highlights from JPL's mission to Saturn

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. 

Writer: 
Douglas Smith
Writer: 
Exclude from News Hub: 
No

Planetary Weatherman: An Interview with Andrew Ingersoll

Andrew Ingersoll, Earle C. Anthony Professor of Planetary Sciences, has been a leader in the investigation of planetary weather and climate for nearly five decades. His research has included studies of the so-called runaway greenhouse effect that is thought to have boiled away Venus's oceans, the presence of liquid water on Mars, the supersonic winds on Jupiter's moon Io, and the atmospheric dynamics of Jupiter, Saturn, Uranus, and Neptune. He has been a key player on the instrument teams for many NASA/JPL missions, including Pioneer Venus, Pioneer Saturn, Voyager, Mars Global Surveyor, Galileo, and Cassini.

Ingersoll recently spoke more about his research and why he has dedicated his career to exploring atmospheres in outer space. In particular, he spoke, about the Cassini mission—a robotic spacecraft that was launched in 1997 and has been orbiting Saturn, its rings, and its moons since 2004.

 

What do you study?

In a single word, it is atmospheres. I study the weather and climate of Earth and other planets.

 

Why?

It may be in my genes to like that kind of phenomenon. I like things that I can stick my hands into and feel and imagine. Other branches of physics were too abstract for me. The objects of interest were either too large or too small. I like the human scale.

I also really like the unknowns. At the end of my doctorate in 1966, I could have gone into the fields of meteorology, oceanography, or planetary science. Meteorology seemed too routine because it involved forecasting the weather every day. That's unfair, because there are big challenges, but still, it wasn't as appealing to me as the planets. We knew next to nothing about them in the 1960s and Caltech offered me a nice job as an assistant professor, studying the weather and climate of other planets.

 

What was the field like when you first started out?

In those days, I felt I was doing what early meteorologists and oceanographers were doing in the 1930s, during the war and right after, which was just kind of getting the basic understanding of what was going on.

In the 1960s, 70s, 80s—and even today—we were and are still building that fundamental understanding of the planets. What are the basic gears? What's making Venus so hot and Mars so cold and Jupiter a striped Christmas tree ball? All of these things are fascinating.

 

What do we learn from studying the atmospheres of other planets?

By studying the planets—with their wide array of personalities and behaviors—we learn about how atmospheres and how weather and climate work in general and we get a much broader picture and perspective than we could ever get from studying Earth alone. Earth has ice ages—and that's pretty interesting—but we can't look at the ice ages today except by proxies. But we can look at the planets right now.

 

What have we learned from studying other planets that you are most excited about?

In the broadest sense, we have learned humility—how difficult it is to extend our knowledge of Earth's weather and climate to other planets. The planets continue to surprise us with things we never imagined. For me, the planets themselves are the heroes of the space program. They keep coming through with exciting stuff that was waiting to be discovered.

 

What have we discovered by studying Saturn through the Cassini mission?

Saturn has everything: rings, magnetosphere, icy moons, a moon—Titan—with a dense atmosphere, and the giant planet itself. In the realm of atmospheres, three things stand out. One is the way Saturn erupts every 20 to 30 years with a giant thunderstorm that wraps around the planet and then dies. Another is the way Titan has developed a methane-based hydrologic cycle complete with lakes, rivers, clouds, and rain. The third is the way Enceladus, a little icy moon, manages to emit plumes of ice particles, water vapor, and other gases despite its low temperatures. The underground plumbing is a big mystery, but it may involve liquid water close to the surface.

 

What big questions/discoveries are you still interested in pursuing?

Cassini has answered many old questions and raised many new ones, and that's good. It means there is stuff out there waiting to be discovered. I hope to answer a few of the new questions. Also, there is Earth. My career as a planetary scientist has given me a willingness to speculate about processes here on Earth and maybe to see things that other people have missed. Even if it doesn't work out, I am going to take a look at some of the outstanding questions of Earth's weather and climate. For instance, there's a 60-day cycle of tropical rainfall known as the Madden Julian oscillation. That's a long lifetime for a weather pattern, and the models don't reproduce it very well. Also, there are 1000-year cycles recorded in the Greenland ice known as Dansgaard-Oeschger events. That's much shorter than the orbital cycles and may represent sudden turnover of the oceans. Students and I are working on both projects right now.

 

What's next for you?

I'm involved with Juno, a spacecraft that is going into orbit around Jupiter in 2016. It carries a microwave radiometer that allows us to see down below the clouds at radio wavelengths. We will learn if the weather has roots in the deep atmosphere and whether the roots account for the long-term stability of the cloud features. Juno does many things, but it also gives us our first good look at the poles, where the weather is very unlike that at lower latitudes. Cassini goes on until 2017, and those two spacecraft will keep me busy. And there are always bright students who challenge me to think about new things.

 

Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Caltech Again Named World's Top University in <i>Times Higher Education</i> Global Ranking

PASADENA, Calif.—The California Institute of Technology (Caltech) has been rated the world's number one university in the 2012–2013 Times Higher Education global ranking of the top 200 universities.

Oxford University, Stanford University, Harvard University, and MIT round out the top five.

"We are pleased to be among the best, and we celebrate the achievements of all our peer institutions," says Caltech president Jean-Lou Chameau. "Excellence is achieved over many years and is the result of our focus on extraordinary people. I am proud of our talented faculty, who educate outstanding young people while exploring transformative ideas in an environment that encourages collaboration rather than competition."

Times Higher Education compiled the listing using the same methodology as in last year's survey. Thirteen performance indicators representing research (worth 30 percent of a school's overall ranking score), teaching (30 percent), citations (30 percent), international outlook (which includes the total numbers of international students and faculty and the ratio of scholarly papers with international collaborators, 7.5 percent), and industry income (a measure of innovation, 2.5 percent) make up the data. Included among the measures are a reputation survey of 17,500 academics; institutional, industry, and faculty research income; and an analysis of 50 million scholarly papers to determine the average number of citations per scholarly paper, a measure of research impact.

In addition to placing first overall in this year's survey, Caltech came out on top in the teaching indicator as well as in subject-specific rankings for engineering and technology and for the physical sciences.

"Caltech held on to the world's number one spot with a strong performance across all of our key performance indicators," says Phil Baty, editor of the Times Higher Education World University Rankings. "In a very competitive year, when Caltech's key rivals for the top position reported increased research income, Caltech actually managed to widen the gap with the two universities in second place this year—Stanford University and the University of Oxford. This is an extraordinary performance."

Data for the Times Higher Education's World University Rankings were provided by Thomson Reuters from its Global Institutional Profiles Project, an ongoing, multistage process to collect and validate factual data about academic institutional performance across a variety of aspects and multiple disciplines.

The Times Higher Education site has the full list of the world's top 400 schools and all of the performance indicators.

Writer: 
Kathy Svitil
Frontpage Title: 
Caltech Again Named World's Top University by <i>Times Higher Ed</i>
Writer: 
Exclude from News Hub: 
No
News Type: 
In Our Community

Mars Rover Finds Evidence of Ancient Streambed

An ankle- or hip-deep stream once flowed with force across the surface of Mars in the very spot where NASA's Curiosity rover is currently exploring. The finding, announced by members of the project's science team today at the Jet Propulsion Laboratory (JPL), provides new information about a once wet environment in Gale Crater, the ancient impact crater where the rover touched down in early August.

Using Curiosity's mast camera to analyze two rock outcrops known as Hottah and Link, the team has identified a tilted block of an ancient streambed—a layer of conglomerate rock, which is made up of stones of different sizes and shapes cemented together.

"Curiosity's discovery of an ancient streambed at Gale Crater provides confirmation of the decades-old hypothesis that Mars once had rivers that flowed across its surface," says John Grotzinger, the mission's project scientist and the Fletcher Jones Professor of Geology at Caltech. "This is the starting point for our mission to explore ancient, potentially habitable environments, and to decode the early environmental history of Mars."

The sizes of the gravels in the conglomerate rock suggest that the stream once flowed at a rate of about a meter per second. The discovery marks the first time scientists have identified gravel that was once transported by water on Mars.

In coming weeks and months, the team plans to use all of Curiosity's analytical instruments to study these types of rocks. And Grotzinger points out, "Finding geological evidence for past water is a prerequisite to beginning geochemical measurements that inform analysis of ancient potentially habitable environments. Curiosity has the most sophisticated and comprehensive suite of geochemical instruments ever flown to Mars."

For more about the finding, read the full JPL release.

Writer: 
Kimm Fesenmaier
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Two Caltech Researchers Receive NIH Director's Awards

Two members of the California Institute of Technology (Caltech) faculty have been given National Institutes of Health (NIH) Director's Awards. The awards are administered through the NIH's Common Fund, which provides support for research deemed to be both innovative and risky.

"The Common Fund High Risk–High Reward program provides opportunities for innovative investigators in any area of health research to take risks when the potential impact in biomedical and behavioral science is high," said NIH Director Francis S. Collins in a press release.

There are three types of NIH Director's Awards: the Pioneer Award, the New Innovator Award, and the Transformative Research Award. This year, biologist Doris Ying Tsao was given one of 10 Pioneer Awards, and geobiologist Dianne K. Newman was among the 20 scientists to receive Transformative Research Awards.

The Pioneer Award, established in 2004, "challenges investigators at all career levels to develop highly innovative approaches that have the potential to produce a high impact on a broad area of biomedical or behavioral research," according to the NIH. Tsao, an assistant professor of biology, will explore the question of how objects that we see are initially processed in the brain. 

"The retina essentially transmits an array of unconnected pixels to the brain. These are first processed locally, through various local filters for color, motion, etc., and the image does not yet contain objects, or bound units," says Tsao. "But after this, there is a mysterious operation that puts all these local pieces together for the first time—and that is what I am studying. I want to know how the brain dynamically links all these pixels over space and time, based on surface contiguity, to form bound units."

For example, as a truck makes a U-turn, the pixels defining the truck can change completely, yet we have no trouble tracking those pixels and seeing they belong to a single invariant object, she explains. 

"I am incredibly excited to have the chance, with the NIH Pioneer Award, to hunt for the circuits implementing these computations within the brain," says Tsao. "We just have to open our eyes to know the circuits exist, but understanding them is going to be an immense challenge that will require huge resources, and to now suddenly have these resources is unbelievable to me." 

The Transformative Research Award program, established in 2009, "promotes cross-cutting, interdisciplinary approaches" for research that "has the potential to create or overturn fundamental paradigms," according to the NIH. Newman, professor of biology and geobiology, will use the award to apply geobiological approaches to understanding the progression of pulmonary infections. 

Due to the challenges of working in situ, or in the body, most studies of infectious disease agents are conventionally performed with representative isolates and imperfect disease models in the laboratory, says Newman. Very few direct measurements of the physiological state of drug-tolerant populations in the host exist, and little is known about which metabolic pathways are actually in play, much less how they change over time in response to coevolving conditions within the lung, she explains. 

"We will tackle this critical knowledge gap using an approach inspired by geobiology," says Newman, who is also an investigator with the Howard Hughes Medical Institute. "Geobiologists are experienced in studying the growth and metabolism of microbial populations in poorly accessible natural habitats by combining molecular biology and stable isotope geochemistry; we will apply these tools to the lung." 

The goal of this project, she says, is to lay a foundation for novel therapeutics to modify and control infectious disease agents. 

"I am honored to have received this award because it will take our research in a meaningful new direction," says Newman. "The opportunity to collaborate with an outstanding group of colleagues at Caltech and at USC and Johns Hopkins hospitals is very exciting. I'm grateful that the NIH took a chance on our idea, and I hope it fulfills its promise."

Writer: 
Katie Neith
Frontpage Title: 
Researchers Receive NIH Director's Awards
Writer: 
Exclude from News Hub: 
No
News Type: 
In Our Community

Martian Clay Minerals Might Have Much Hotter Origin

Ancient Mars, like Earth today, was a diverse planet shaped by many different geologic processes. So when scientists, using rovers or orbiting spacecraft, detect a particular mineral there, they must often consider several possible ways it could have been made.

Several such hypotheses have been proposed for the formation of clay minerals, which have been detected from orbit and are sometimes considered indicators that the surface has, in the past, been altered by liquid water. Now, publishing in the journal Nature Geoscience, a team of French and American scientists led by Alain Meunier of the Université de Poitiers and including Caltech's Bethany Ehlmann, has suggested a new, very different possibility.

Previously, planetary scientists considered two hypotheses—both offering the potential for once-habitable environments on Mars—that explain clay mineral formation. One holds that over long enough periods, contact with liquid water can alter igneous rock, such as basalt, producing clays; the other proposes that waters flowing through the martian subsurface can produce clays through a hydrothermal process.

In the new paper, the authors suggest that the clay minerals instead might have precipitated directly from scalding hot magmas.

"This new hypothesis is less exciting for astrobiology because life could not survive in those types of conditions," says Ehlmann, an assistant professor of planetary science at Caltech and a research scientist at the Jet Propulsion Laboratory. "But all three hypotheses need to be on the table as we consider a given clay-bearing deposit. Each hypothesis has a different implication for the history and habitability of ancient Mars."

Ehlmann says that scientists hope to use the Curiosity rover and its suite of instruments to study the clays found in sediments at Gale Crater—the impact crater that the robotic geologist was sent to explore. However, she notes, clays are typically found in even older igneous bedrock on Mars. Future rover missions would need to study clay formation in that ancient crust to rigorously test the various clay formation hypotheses. "There's more exploration that needs to be done before we understand all the mysteries of Mars," she says.

The Los Angeles Times recently spoke to Ehlmann about the new paper and its implications.

Writer: 
Kimm Fesenmaier
Frontpage Title: 
Hotter Origin Possible for Martian Clays
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Geologists in the Field

Field geologists at Caltech come face to face with bears and wolverines, climb steep cliffs and mountains, and endure scorching sunlight and frigid temperatures. Sometimes risking life and limb, they travel to some of the most remote corners of the globe—all in the name of science. A feature-length story in the Summer 2012 issue of E&S magazine describes some of their adventures in the quest to understand our planet. 

Writer: 
Marcus Woo
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Anchors Aweigh

At Caltech, hydrophilic researchers in the Division of Geological and Planetary Sciences take to the salty seas to gather data, explore the deep, and get a firsthand view of the beasts at the bottom. The briny treasures they collect along the way are helping them learn more about past, present, and future environmental conditions and hazards. Read about their ocean adventures in a feature-length story in the Summer 2012 issue of E&S magazine.

Writer: 
Katie Neith
Frontpage Title: 
Ocean-based Researchers Take to the Sea
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Countdown to Mars: Caltech goes to the Red Planet

On Sunday, August 5, the Mars Science Laboratory (MSL) rover, known as Curiosity, will make its dramatic descent onto Mars's surface. Once it lands, the rover will check its instruments to make sure everything's functioning properly—and then it will get right to work.

In fact, on just the second day of its Martian excursion, Curiosity is scheduled to turn on some of its key scientific instruments, taking pictures and making measurements of its immediate environment. About a week later, the rover will spin its six wheels and start exploring. Curiosity will ramp up its science mission over the following weeks, deploying its suite of instruments to scoop, drill, zap, examine, and analyze. And through it all, a couple of Caltech researchers will be among the 300 scientists working here on Earth, taking the information Curiosity sends home and trying to figure out what it all means.

Back in February, Caltech's Ken Farley and Bethany Ehlmann were named as participating scientists on the mission; each will be directing their own Martian science projects. Ehlmann, an assistant professor of planetary science and a JPL research scientist, has been analyzing rocks and minerals on Earth to help MSL's science team choose which Martian rocks to study in detail. Farley, the W. M. Keck Foundation Professor of Geochemistry and chair of the Division of Geological and Planetary Sciences, will measure the presence of noble gases in Martian rocks to determine their ages. Here are some excerpts from Kimm Fesenmaier's story about how these two scientists intend to make their mark on Mars.

Examining the capabilities of one of Curiosity's science instruments, Sample Analysis at Mars (SAM), which was designed to look for organic matter, Farley realized that it might be possible to detect the isotope helium-3 with SAM's spectrometer. Since helium-3 is produced by the bombardment of surface materials by incoming cosmic rays, measurements of the isotope can be used to determine how long rocks or other geologic features have been exposed on the surface of a planet. This could help scientists date features on Mars such as impact craters, but it could also come in handy in terms of target selection.

Imagine, for example, that the science team locates an impact site on Mars that seems to have been created within the last few million years. The rocks around that site would be ideal targets for closer study because any organic matter they contain would have been protected from cosmic rays until they were thrown out onto the surface. "The trouble is, it's easy to say that an impact happened a million years ago, but how would you ever really know?" Farley says. "The technique that I have is a way to say, 'Yes, this is, in fact, a very young surface.'"

Farley says his involvement with MSL is a major departure from his previous work, as he has not been heavily involved in the study of the geology of Mars. "Most of what I do is technique development," he says. "So it's interesting for me to see potential application of one of those techniques in another very different setting."

In contrast, space missions are Ehlmann's bread and butter. She got her first taste of rover operations while an undergraduate student working on the Mars Exploration Rovers mission. Then as a graduate student, she was a collaborator on the Mars Reconnaissance Orbiter mission and used data from multiple orbiters to locate hydrated minerals on the surface of the red planet.

For MSL, Ehlmann proposed to help improve the team's ability to select rocks from a distance for more extensive study. Curiosity is equipped with a laser and a telescope known collectively as ChemCam, mounted on its mast. From a distance of about seven meters, the laser can zap a rock to create a plasma, which can be analyzed with the ChemCam spectrometer to get a sense of the chemical elements present. The science team needs to be able to use those results and their knowledge of mineralogy to quickly decide whether to spend more time and energy on a particular target or to keep Curiosity moving.

To give them a leg up, Ehlmann is analyzing rocks from places such as Iceland as well as synthetic mixtures of minerals in the lab that are similar to the materials observed from Mars orbit at the Gale Crater landing site. The rocks can be well characterized in Earth laboratories to understand their mineralogy, the elements they are made of, and how they would appear from orbit. She is studying how they have been changed by different types of interaction with water. She plans to take the samples to Los Alamos National Laboratory, which partnered with the French national space agency to build the ChemCam, in order to test her samples with a ChemCam-like instrument.

"It's very helpful to have this beforehand understanding of what you might see or what kinds of analyses you need to run," Ehlmann says. "If you understand the processes enough that you develop quick-look algorithms for detecting different styles of alteration, it will help in the tactical timescale when you only have a few hours to come up with a plan for the rover for the next day."

Writer: 
Marcus Woo
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

An Earthquake in a Maze

Caltech researchers provide highest-resolution observations yet of the complex 2012 Sumatra earthquake

PASADENA, Calif.—The powerful magnitude-8.6 earthquake that shook Sumatra on April 11, 2012, was a seismic standout for many reasons, not the least of which is that it was larger than scientists thought an earthquake of its type—an intraplate strike-slip quake—could ever be. Now, as Caltech researchers report on their findings from the first high-resolution observations of the underwater temblor, they point out that the earthquake was also unusually complex—rupturing along multiple faults that lie at nearly right angles to one another, as though racing through a maze.

The new details provide fresh insights into the possibility of ruptures involving multiple faults occurring elsewhere—something that could be important for earthquake-hazard assessment along California's San Andreas fault, which itself is made up of many different segments and is intersected by a number of other faults at right angles.

"Our results indicate that the earthquake rupture followed an exceptionally tortuous path, breaking multiple segments of a previously unrecognized network of perpendicular faults," says Jean-Paul Ampuero, an assistant professor of seismology at Caltech and one of the authors of the report, which appears online today in Science Express. "This earthquake provided a rare opportunity to investigate the physics of such extreme events and to probe the mechanical properties of Earth's materials deep beneath the oceans."

Most mega-earthquakes occur at the boundaries between tectonic plates, as one plate sinks beneath another. The 2012 Sumatra earthquake is the largest earthquake ever documented that occurred away from such a boundary—a so-called intraplate quake. It is also the largest that has taken place on a strike-slip fault—the type of fault where the land on either side is pushing horizontally past the other.

The earthquake happened far offshore, beneath the Indian Ocean, where there are no geophysical monitoring sensors in place. Therefore, the researchers used ground-motion recordings gathered by networks of sensors in Europe and Japan, and an advanced source-imaging technique developed in Caltech's Seismological Laboratory as well as the Tectonics Observatory to piece together a picture of the earthquake's rupture process. 

Lingsen Meng, the paper's lead author and a graduate student in Ampuero's group, explains that technique by comparing it with how, when standing in a room with your eyes closed, you can often still sense when someone speaking is walking across the room. "That's because your ears measure the delays between arriving sounds," Meng says. "Our technique uses a similar idea. We measure the delays between different seismic sensors that are recording the seismic movements at set locations." Researchers can then use that information to determine the location of a rupture at different times during an earthquake. Recent developments of the method are akin to tracking multiple moving speakers in a cocktail party.

Using this technique, the researchers determined that the three-minute-long Sumatra earthquake involved at least three different fault planes, with a rupture propagating in both directions, jumping to a perpendicular fault plane, and then branching to another.

"Based on our previous understanding, you wouldn't predict that the rupture would take these bends, which were almost right angles," says Victor Tsai, an assistant professor of geophysics at Caltech and a coauthor on the new paper. 

The team also determined that the rupture reached unusual depths for this type of earthquake—diving as deep as 60 kilometers in places and delving beneath the Earth's crust into the upper mantle. This is surprising given that, at such depths, pressure and temperature increase, making the rock more ductile and less apt to fail. It has therefore been thought that if a stress were applied to such rocks, they would not react as abruptly as more brittle materials in the crust would. However, given the maze-like rupture pattern of the earthquake, the researchers believe another mechanism might be in play.

"One possible explanation for the complicated rupture is there might have been reduced friction as a result of interactions between water and the deep oceanic rocks," says Tsai. "And," he says, "if there wasn't much friction on these faults, then it's possible that they would slip this way under certain stress conditions."

Adding to the list of the quake's surprising qualities, the researchers pinpointed the rupture to a region of the seafloor where seismologists had previously considered such large earthquakes unlikely based on the geometry of identified faults. When they compared the location they had determined using source-imaging with high-resolution sonar data of the topography of the seafloor, the team found that the earthquake did not involve what they call "the usual suspect faults."

"This part of the oceanic plate has fracture zones and other structures inherited from when the seafloor formed here, over 50 million years ago," says Joann Stock, professor of geology at Caltech and another coauthor on the paper. "However, surprisingly, this earthquake just ruptured across these features, as if the older structure didn't matter at all."

Meng emphasizes that it is important to learn such details from previous earthquakes in order to improve earthquake-hazard assessment. After all, he says, "If other earthquake ruptures are able to go this deep or to connect as many fault segments as this earthquake did, they might also be very large and cause significant damage."

Along with Meng, Ampuero, Tsai, and Stock, additional Caltech coauthors on the paper, "An earthquake in a maze: compressional rupture branching during the April 11 2012 M8.6 Sumatra earthquake," are postdoctoral scholar Zacharie Duputel and graduate student Yingdi Luo. The work was supported by the National Science Foundation, the Gordon and Betty Moore Foundation, and the Southern California Earthquake Center, which is funded by the National Science Foundation and the United States Geological Survey.

Writer: 
Kimm Fesenmaier
Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

Pages

Subscribe to RSS - GPS