Giant Impact Explains Mars Dichotomy

PASADENA, Calif.--The surface landscape of Mars, divided into lowlands in the north and highlands in the south, has long perplexed planetary scientists. Was it sculpted by several small impacts, via mantle convection in the planet's interior, or by one giant impact? Now scientists at the California Institute of Technology have shown through computer modeling that the Mars dichotomy, as the divided terrain has been termed, can indeed be explained by one giant impact early in the planet's history.

"The dichotomy is arguably the oldest feature on Mars," notes Oded Aharonson, associate professor of planetary science at Caltech and an author of the study. The feature arose more than four billion years ago, before the rest of the planet's complex geologic history was superimposed.

Scientists had previously discounted the idea that a single, giant impactor could have created the lower elevations and thinner crust of Mars's northern region, says Margarita Marinova, a graduate student in Caltech's Division of Geological and Planetary Sciences (GPS) and lead author of the study, which appears June 26 in the journal Nature. This special issue of the journal features a trio of papers on the Mars dichotomy.

For one thing, Marinova explains, it was thought that a single impact would leave a circular footprint, but the outline of the northern lowlands region is elliptical. There is also a distinct lack of a crater rim: topography increases smoothly from the lowlands to the highlands without a lip of concentrated material in between, as is the case in small craters. Finally, it was believed that a giant impactor would obliterate the record of its own occurrence by melting a large fraction of the planet and forming a magma ocean.

"We set out to show that it's possible to make a big hole without melting the majority of the surface of Mars," Aharonson says. The team modeled a range of projectile parameters that could yield a cavity the size and ellipticity of the Mars lowlands without melting the whole planet or making a crater rim.

After cranking 500 simulations combining various energies, velocities, and impact angles through the GPS division's Beowulf-class computer cluster CITerra, the researchers narrowed in on a "sweet spot"--a range of single-impact parameters that would make exactly the type of crater found on Mars. Although a large impact had been suggested (and discounted) in the past, Aharonson says, computers weren't fast enough to run the models. "The ability to search for parameters that allow an impact compatible with observations is enabled by the dedicated machine at Caltech," he adds.

The favored simulation conditions outlined by the sweet spot suggest an impact energy of around 10 to the 29 joules, which is equivalent to 100 billion gigatons of TNT. The impactor would have hit Mars at an angle between 30 and 60 degrees while traveling at 6 to 10 kilometers per second. By combining these factors, Marinova calculated that the projectile was roughly 1,600 to 2,700 kilometers across.

Estimates of the energy of the Mars impact place it squarely between the impact that is thought to have led to the extinction of dinosaurs on Earth 65 million years ago and the one believed to have extruded our planet's moon four billion years ago.

Indeed, the timing of formation of our moon and the Mars dichotomy is not coincidental, Marinova notes. "This size range of impacts only occurred early in solar system history," she says. The results of this study are also applicable to understanding large impact events on other heavenly bodies, like the Aitken Basin on the moon and the Caloris Basin on Mercury.

The Caltech study comes at a time of renewed interest in the ancient crustal feature on Mars, Aharonson notes. Also in this issue of Nature, Jeffrey Andrews-Hanna and Maria Zuber of MIT and Bruce Banerdt of JPL examine the gravitational and topographic signature of the dichotomy with information from the Mars orbiters. Another accompanying report, from a group at UC Santa Cruz led by Francis Nimmo, explores the expected consequences of mega-impacts.

The other author on this study is Erik Asphaug, a professor of earth and planetary sciences at UC Santa Cruz. 

Writer: 
Elisabeth Nadin
Writer: 

Stress Buildup Precedes Large Sumatra Quakes

PASADENA, Calif.--The island of Sumatra, Indonesia, has shaken many times with powerful earthquakes since the one that wrought the infamous 2004 Indian Ocean tsunami. Now, scientists from the California Institute of Technology and the Indonesian Institute of Sciences are harnessing information from these and earlier quakes to determine where the next ones will likely occur, and how big they will be.

Mohamed Chlieh, the lead author of a new report, looked at the region during his postdoctoral studies at Caltech with Jean-Philippe Avouac, professor of geology and director of Caltech's Tectonics Observatory (TO) and Kerry Sieh, Sharp Professor of Geology. They found that in the time between great earthquakes, some portions of the fault zone locked up while others crept along steadily, and the portions that were locked in the past few decades coincided with portions that rupture to produce large-magnitude quakes. The correlation was especially strong for two temblors of magnitude 8.7 that struck the region in 1861 and again in 2005.

The study also reveals which part of the Sumatra megathrust is storing strain that will be released during future large earthquakes.

Earthquakes in Sumatra are the manifestation of a sudden release of strain that constantly builds as the plates beneath the Indian Ocean creeps steadily toward southeast Asia and dive into the subduction zone under the island. If the total tectonic plate motion in the region is not taken up by fault slip during earthquakes, then a deficit builds until the next earthquake rupture. The patch of the fault where slip is greatest during an earthquake and releases the most pent-up strain, known as an asperity, also gets stuck between quakes. The scientists were interested in what was happening at the land surface, above these asperities, between big earthquakes.

Investigations by Caltech scientists in the region began when Sieh and his students started documenting the history of subsidence and emergence of the islands offshore Sumatra using the record provided by coral heads. Later on, a network of geodetic stations was deployed by the TO. To measure how strain built up in the calm interseismic period between earthquakes, Chlieh and his colleagues analyzed GPS measurements collected since 1991 and annual banding in corals from the past 50 years. Coral growth bands indicate vertical land motion because as the seafloor on which corals live shifts down or up, the creatures either grow to chase sunlight from below water or die back when elevated above water. Both the bands and the GPS data record small land-position shifts in interseismic periods. In contrast, they show drastic shifts during an earthquake, as the corals typically die when they are thrust high enough above or sunk too deep below sea level to survive.

The data provide a record of unevenly distributed deformation of the land surface directly above the subduction zone during the interseismic period. Modeling further indicates that this results from the asperities along the plate interface, while other parts remain smoothly slipping. These interseismic asperities are 10 times as wide--up to 175 kilometers--in the region where great earthquakes have occurred in the past.

"Our model shows asperities exactly at the same places that the 2005 Nias and the 1797 and 1833 earthquakes in the Mentawai islands occurred, indicating that aperities seem to be persistent features from one seismic cycle to another," Chlieh remarks. Avouac adds, "This is clear indication that the characteristics of large earthquakes are somewhat determined by properties of the plate interface that can be gauged in advance from measuring interseismic deformation.

"A priori, large earthquakes should not be expected where the plate interface is creeping, but are inescapable where it is locked. So it seems that we can, with interseismic observations, see these asperities before the earthquake occurs," he says. "The question now is, 'How well are we able to estimate the characteristics of the earthquakes that these asperities could produce?'"

The implications of the study are major, according to Chlieh. "Using the asperity locations, we may be able to construct some more realistic earthquake and tsunami models following different scenarios. Then we will have a good idea of the risk induced by these locked fault zones."

The study appears in the May issue of the Journal of Geophysical Research. Other authors on the paper are Danny Natawidjaja, a former Caltech grad student who is now at the Indonesian Institute of Sciences, and John Galetzka, staff geodesist with the TO.

Abstract: http://www.agu.org/pubs/crossref/2008/2007JB004981.shtml 

Writer: 
Elisabeth Nadin
Writer: 

Partnerships of Deep-Sea Methane Scavengers Revealed

PASADENA, Calif.--The sea floor off the coast of Eureka, California, is home to a diverse assemblage of microbes that scavenge methane from cold deep-sea vents. Researchers at the California Institute of Technology have developed a technique to directly capture these cells, lending insight into the diverse symbiotic partnerships that evolved among different species in an extreme environment.

The community's interconnected metabolism sheds light on how the anaerobic microbes, which consume nearly 80 percent of the methane leaked from marine sediments, limit oceanic emissions of this potent greenhouse gas.

"Ninety-nine percent of what's out there we can't grow in the lab, including these methane-oxidizing organisms," says Victoria Orphan, an assistant professor of geobiology at Caltech in whose lab the cell sampling technique was developed.

"We know from ribosomal RNA studies that there is a lot of microbial diversity in nature, but we don't know what the vast majority of microbes are doing," Orphan adds. "We needed a method for separating specific organisms out of complex environments."

Metagenomic analysis, in which the genetic material of all microorganisms swept from their homes in a sample is sequenced wholesale, yields a plenitude of general information. Annelie Pernthaler, a former Caltech postdoc who is now a research scientist at the Centre for Environmental Research in Leipzig, Germany, and Orphan devised a technique to tease out individuals from the diverse microbial community of the deep-sea sediment. Their aim: to simplify the genomic sequencing to target only the organisms they were interested in.

They began with descents in the manned submersible Alvin, collecting cores of sea-floor sediment from areas where methane migrates from below. Back in the lab, the team used enzyme-tagged short DNA probes to specifically bind the ribosomal RNA in the methane-consuming microbes of the sediment. A second reaction used the enzyme to deposit fluorescent molecules within and around the cell, a method known as CARD-FISH, for "catalyzed reporter deposition fluorescence in situ hybridization."

The fluorescing cells and attached microorganisms were captured using microbeads that are both paramagnetic--a form of magnetism occurring only in the presence of an externally applied magnetic field--and coated with an antibody to the fluorescent molecule. This Caltech-patented technique, called "magneto-FISH," bypasses the need to grow the microorganisms in culture because it targets the fluorescing molecules around the cell instead of a specific molecule within the cell.

The cells separated by magneto-FISHing reveal who's partnered up with whom, and provides a fresh look at microbial symbiosis in nature, Orphan says. The main player near the methane vents is a methane-metabolizing member of the Archaea, a prokaryotic domain of life distinct from both bacteria and eukaryotes. Piggybacked on the archaeal cells are some members from among four different species of bacteria--three more than were previously known to be associated with these particular archaea--whose exact roles in the system can now be addressed.

The methane-vent partnership between archaea that consume methane and bacteria that reduce sulfate is believed to be a form of cometabolism or syntrophy, meaning "feeding together," where one species lives off the metabolic products of others. Using the information obtained from the metagenome of these partnerships, says Orphan, biologists can develop specific experiments to directly test the physiological and nutritional relationships between these organisms, as well as the ecological strategies used to successfully colonize deep-sea environments.

One example of such an experiment is highlighted in the group's study, published May 8 in the early online edition of the journal PNAS. The researchers discovered that the organisms possess genes for nitrogen fixation, a process that converts nitrogen gas into nourishing compounds like ammonia. "We were surprised to see these genes in the captured cells," says Anne Dekas, a geobiology graduate student at Caltech, "because we thought these organisms were relatively energy-starved, and nitrogen fixation takes a lot of energy."

Orphan and Dekas were able to show that the organisms are not just equipped for the task, they are actually carrying it out. "Showing nitrogen fixation is a great finding in itself," Dekas comments, "but it is also just one example of the hypothesis testing that can follow magneto-FISH coupled to metagenomic analysis."

Other authors on the study are Caltech's C. Titus Brown, a postdoc in biology; Shana Goffredi, a senior research fellow in environmental science and engineering; and Tsegereda Embaye, a technician in the division of geological and planetary sciences.

Writer: 
Elisabeth Nadin
Writer: 

A Grand Canyon as Old as the Dinosaurs?

PASADENA, Calif.--How the Grand Canyon was carved has been a topic of scientific controversy for nearly 140 years. Now, with new geochronologic data from the canyon and surrounding plateaus, geologists from the California Institute of Technology present significant evidence that the canyon formed nearly 50 million years earlier than previously thought.

The results will be published in the May issue of the Geological Society of America Bulletin in a paper by Rebecca Flowers, a former Caltech postdoctoral scholar now on the faculty of the University of Colorado; Chandler Family Professor of Geology Brian Wernicke; and Keck Foundation Professor of Geochemistry Kenneth Farley.

The team studied the sedimentary rock layers, or strata, of both the canyon and a large area of the surrounding plateaus. These strata were deposited near sea level sometime in the Paleozoic era (540-250 million years ago) and were subsequently uplifted and eroded to form the canyon. But questions like when and why the canyon itself formed have remained open.

The long-held interpretation sets canyon incision at about six million years ago, when the plateau that hosts it began to rise from near sea level to a current elevation of almost 7,000 feet. This view highlights the erosive power of the Colorado River, which cut into the plateau surface like a giant buzzsaw and progressively deepened the canyon at the same time the entire region was rising.

Now, using a radiometric dating method called uranium-thorium-helium [(U-Th)/He] dating, developed in Farley's lab, the researchers paint a different scenario. Uplift and carving of a deep canyon took place more than 55 million years ago, above the present position of the Grand Canyon's Upper Granite Gorge, within strata much younger than the Paleozoic rocks currently exposed in the canyon walls.

"When this canyon was formed, it looked like a much deeper version of present-day Zion Canyon, which cuts through strata of the Mesozoic era," Wernicke says. Then from 28 to 15 million years ago, a pulse of erosion deepened the already-formed canyon and also scoured the surrounding plateaus, stripping off the Mesozoic strata to reveal the Paleozoic rocks that we see today.

The key to the discovery lay in the ancient sandstones of the canyon walls, which contain scant grains of the phosphate mineral apatite that in turn host trace amounts of the radioactive elements uranium and thorium. These elements decay, spitting out helium atoms at well-constrained rates via alpha-particle emission. Although some of those atoms are lost through diffusion early in the grain's history, by measuring the abundances of all three elements, (U-Th)/He dating ultimately yields the time that an apatite crystal cooled below 70 degrees Celsius. Paired with information from boreholes about how Earth's temperature increases with depth, dates from apatite grains in rocks that are now at the surface communicate the last time those rocks were buried a mile deep.

A key finding of the Caltech team is that samples collected from the bottom of the Upper Granite Gorge region yield the same (U-Th)/He apatite dates as samples collected on the plateau surface nearby. "Because both canyon and plateau samples have resided near the same depth since 55 million years ago, a canyon of about the same dimensions as today must have existed at least that far back, and possibly as far back as the time of the last dinosaurs at the end of the Cretaceous period 65 million years ago," Wernicke states.

Wernicke says that the most surprising aspect of their new findings is that, since the Grand Canyon was originally cut, the adjacent plateaus have also eroded downward by about a mile, on average, every bit as fast as the bottom of the canyon. "And so the small, ephemeral streams that cover the arid plateau seem to be just as effective as the mighty Colorado at eroding away rock," he notes.

The erosional history proposed by the Caltech team jibes with other recent studies that also involve innovative radiometric dating techniques and speak to the early history of the canyon, Wernicke says. The first, undertaken by researchers led by Karl Karlstrom at the University of New Mexico and published last November in the same journal as the new Caltech study, demonstrated that the amount of downcutting of the Colorado in the Upper Granite Gorge was about 350 feet over the last 700,000 years. Extrapolated back in time, this rate is too slow to have carved the entire canyon in only six million years. Another University of New Mexico study, led by Carol Hill and Yemane Asmerom and published this March in the journal Science, demonstrated by dating cave deposits throughout the canyon that a water table, and therefore an erosion surface, lay somewhere near the canyon rim 17 million years ago, very close to the end of the pulse of erosion suggested by Caltech's (U-Th)/He dating.

The new work also echoes even earlier ideas of Richard Young of the State University of New York at Geneseo, Wernicke notes. In the 1980s, Young led a team that discovered that a group of ancient tributary canyons just south of the western Grand Canyon (Lower Granite Gorge region) were in fact originally formed between 63 and 50 million years ago, about the time the (U-Th)/He data suggest for initial cutting above the Upper Granite Gorge area. "The current wave of research thus strengthens the link between the formation of the tributary canyons and the evolution of the Grand Canyon proper, including the Upper Granite Gorge region," Wernicke says.

Wernicke credits much of the recent discoveries to cutting-edge dating techniques. "Although vigorous debate is sure to continue," he notes, "conventional wisdom about the history of the Grand Canyon in particular, and geology in general, is being challenged by these new, high-tech avenues of research."

Writer: 
Elisabeth Nadin
Writer: 

Water Vapor Detected in Protoplanetary Disks

PASADENA, Calif.--Water is an essential ingredient for forming planets, yet has remained hidden from scientists searching for it in protoplanetary systems, the spinning disks of particles surrounding newly formed stars where planets are born. Now the detection of water vapor in the inner part of two extrasolar protoplanetary disks brings scientists one step closer to understanding water's role during terrestrial planet formation.

By maximizing the spectroscopic capabilities of NASA's Spitzer Space Telescope and high-resolution measurements from the Keck II Telescope in Hawaii, researchers from the California Institute of Technology and other institutes found water molecules in disks of dust and gas around two young stars. DR Tau and AS 205A, respectively around 457 and 391 light-years away from Earth, are each at the center of a spinning disk of particles that may eventually coalesce to form planets.

"This is one of the very few times that water vapor has been detected in the inner part of a protoplanetary disk--the most likely place for terrestrial planets to form," says Colette Salyk, a graduate student in geological and planetary sciences at Caltech. She is the lead author of a group of scientists reporting their findings in the March 20 issue of the Astrophysical Journal Letters.

Salyk and her colleagues first harnessed light-emission data captured by Spitzer to inspect dozens of young stars with protoplanetary disks. They honed in on DR Tau and AS 205A because these presented a large number of water emission lines--spikes of brightness at certain wavelengths that are a unique fingerprint for water vapor. "Only Spitzer is capable of observing these particular lines in a large number of disks because it operates above Earth's obscuring water-vapor–rich atmosphere," says Salyk.

To determine in what part of the disk the vapor resides, the team made high-resolution measurements at shorter wavelengths with NIRSPEC, the Near-InfraRed cross-dispersed echelle grating Spectrometer for the Keck II Telescope. Unlike Spitzer, which observed water lines blended together into clumps, NIRSPEC can resolve individual water lines in selected regions where the atmospheric transmission is good. The shape of each line relays information on the velocity of the molecules emitting the light. "They were moving at fast speeds," says Salyk, "indicating that they came from close to the stars, which is where Earthlike planets might be forming."

"While we don't detect nearly as much water as exists in the oceans on Earth, we see only a very small part of the disk--essentially only its surface--so the implication is that the water is quite abundant," remarks coauthor Geoffrey Blake, professor of cosmochemistry and planetary sciences and professor of chemistry at Caltech.

The presence of water in the inner disk may indicate its stage on the road to planet formation. A planet like Jupiter in our solar system grew as its gravitational field trapped icy solids spinning in the outer part of the sun's planetary disk. However, before Jupiter gained much mass, these same icy solids could have traveled towards the star and evaporated to produce water vapor such as that seen around DR Tau and AS 205A.

Although they have not detected icy solids in the extrasolar disks, says Salyk, "our observations are possible evidence for the migration of solids in the disk. This is an important prediction of planet-forming models."

These initial observations portend more to come, says coauthor Klaus Pontoppidan, a Caltech Hubble Postdoctoral Scholar in Planetary Science. "We were surprised at how easy it is to find water in planet-forming disks once we had learned where to look. It will take years of work to understand the details of what we see."

Indeed, adds Blake, "This is a much larger story than just one or two disks. With upcoming observations of tens of young stars and disks with both Spitzer and NIRSPEC, along with our data in hand, we can construct a story for how water concentrations evolve in disks, and hopefully answer questions like how Earth acquired its oceans."

Other authors on the paper are Fred Lahuis of Leiden Observatory in the Netherlands and SRON, the Netherlands Institute for Space Research; Ewine van Dishoeck, also of Leiden Observatory; and Neal Evans of the University of Texas at Austin. 

Writer: 
Elisabeth Nadin
Writer: 

Tracking Earth Changes with Satellite Images

SAN FRANCISCO, Calif.--For the past two decades, radar images from satellites have dominated the field of geophysical monitoring for natural hazards like earthquakes, volcanoes, or landslides. These images reveal small perturbations precisely, but large changes from events like big earthquake ruptures or fast-moving glaciers remained difficult to assess from afar, until now.

Sebastien Leprince, a graduate student in electrical engineering at the California Institute of Technology, working under the supervision of geology professor and director of Caltech's Tectonics Observatory (TO), Jean-Philippe Avouac, wrote software that correlates any two optical images taken by satellite. It has proved extremely reliable in tracking large-scale changes on Earth's surface, like earthquake ruptures, the mechanics of "slow" landslides, or defining the fastest-moving sections of glaciers that, due to global warming, have recently increased their pace.

Leprince will describe his software and results of many of its applications on December 14 at the annual meeting of the American Geophysical Union (AGU) in San Francisco. His research will also be featured in the January 1 issue of Eos, AGU's weekly newspaper.

When the technique called InSAR, which uses radar images to reveal details about ground displacement, was introduced, it was quickly embraced. No longer did geoscientists have to rely solely on measurements made by troupes of field geologists or by ground-based devices that might not have been optimally placed. But, says Leprince, "InSAR is physically limited: it's good for small displacements but not for large ones. The radar resolution isn't enough to look at deformation with a large gradient."

Using optical images to complement the radar-based InSAR technique seemed like a natural step. When Leprince began grappling with the idea in 2003, he found several baby steps had been taken. "Satellite image correlation was not a science yet, it was more like an art," he says. The first attempts, reported in 1991, were inconclusive but promising. Since then, several teams of scientists had worked on the problem independently. Some had even developed it well enough to monitor glacier flow.

The major obstacle Leprince faced in developing optical image correlation software was that there were several steps involved but no one knew in which order to take them. "Errors came from everywhere, but where exactly?" he noted. "And we found at least one major flaw in each step."

Three of the four main steps involve correcting geometric distortions innate to taking pictures from space and projecting them onto a surface. The first step matches coordinates of the satellite image with coordinates on the ground. "This is not new, but the approximations being made were not okay," says Leprince. The second step describes the satellite's position in its orbit at the time it took the photo. This is just like in everyday life--you need to know how your camera was oriented when you show off a photo you snapped. In the next step, which Leprince says people never knew they were doing wrong, the image is correctly wrapped onto topography. Finally, the images are precisely combined-or coregistered-in order to measure surface displacements accurately.

"What is important is that we identified the steps and took each one independently and did an error analysis for each step to see how errors propagated," says Leprince. His program, which he calls COSI-Corr and which was packaged by the TO's software engineer Francois Ayoub for official release this year, takes all of these steps automatically in just a few hours of processing time. "You start the program, you go home, you have a nice weekend on the beach, and it's done."

The paper describing the software Leprince developed appeared in the June 2007 issue of the journal IEEE Transactions on Geoscience and Remote Sensing. COSI-Corr can now combine any images taken by different satellite imagers from different incidence views. For example, to analyze displacement from the 1999 Hector Mine earthquake near Twenty-Nine Palms in California, Leprince correlated a SPOT 4 image with an ASTER image. This had never been done before. It takes only a few hours to process.

Using his technique, Leprince has precisely measured offset from several notable recent earthquakes, including 2005 Kashmir, Pakistan; 2002 Denali, Alaska; 1999 Hector Mine and Chi Chi, Taiwan; and 1992 Landers, California. In the case of earthquakes, the image correlation technique can be used to map in detail all fault ruptures and to measure displacements both along and across the fault. Uncertainties, typically within centimeters for 10-15-meter-resolution images, are extremely low.

The day after Leprince released his software through the TO website, he was contacted by a geologist in Canada asking how the technique could be used to study glacier flow. Radar images cannot analyze glaciers because they move too fast and ice melting poses a problem. "The tectonic application was pretty well set up and we'd tested it thoroughly," says Leprince. "So we extended it to glaciology." And then to other studies as well.

What's tricky about studying glacier flow is that not only has their pace picked up in recent years due to climate change, but glaciers have a natural yearly cycle of ice gain and loss. The two signals can be discerned with cross-correlation of optical imagery. Leprince's method was used to study Mer de Glace glacier in the Alps, which flows at around 90 meters per year. The optical images provide a full view of the ice flow field, pinpointing exactly where the glacier is moving fastest. The same approach was taken with a landslide above the Alpine town of Barcelonnette in eastern France. Benchmarks had been planted to monitor the landslide's flow, and Leprince's correlation methods showed that all 38 of them missed the fastest-moving region. While the landslide is moving slow now, the town will be threatened when the landslide detaches and descends rapidly.

There are many more applications for correlating optical images to monitor Earth surface changes. Caltech geologists and their collaborators began to apply it to studying dunes, which radars cannot image, after they were contacted by labs in Egypt who need information on dune migration for urban planning.

"Radar interferometry is a huge technique, but you can only measure half of the world with it. Now we can measure the other half with this technique," comments Leprince. "The biggest thing is what's to come."

COSI-Corr and many of its applications will be presented by Leprince on Friday morning, December 14, in Moscone South Exhibit Hall B. To learn more about the technique, visit http://www.tectonics.caltech.edu/slip_history/spot_coseis/

Writer: 
Elisabeth Nadin
Writer: 

Earthquake Season in the Himalayan Front

SAN FRANCISCO, Calif.--Scientists have long searched for what triggers earthquakes, even suggesting that tides or weather play a role. Recent research spearheaded by Jean-Philippe Avouac, professor of geology and director of the Tectonics Observatory at the California Institute of Technology, shows that in the Himalayan mountains, at least, there is indeed an earthquake season. It's winter.

For decades, geologists studying earthquakes in the Himalayan range of Nepal had noted that there were far more quakes in the winter than in the summer, but it was difficult to assign a cause. "The seasonal variation in seismicity had been noticed years ago," says Avouac. Now, over a decade of data from GPS receivers and satellite measurements of land-water storage make it possible to connect the monsoon season with the frequency of earthquakes along the Himalaya front. The analysis also provides key insight into the timescale of earthquake nucleation in the region.

Avouac will present the results of the study on December 12 at the annual meeting of the American Geophysical Union (AGU) in San Francisco. They are also available online through the journal Earth and Planetary Science Letters, and will appear in print early next year.

The world's tallest mountain range, the Himalaya continues to rise as plate tectonic activity drives India into Eurasia. The compression from this collision results in intense seismic activity along the front of the range. Stress builds continually along faults in the region, until it is released through earthquakes.

Avouac and two collaborators from France and Nepal--Laurent Bollinger and Sudhir Rajaure--began their earthquake seasonality investigation by analyzing a catalog of around 10,000 earthquakes in the Himalaya. They saw that, at all magnitudes above this detection limit, there were twice as many earthquakes during the winter months--December through February--as during the summer. That is, in winter there are up to 150 earthquakes of magnitude three per month, and in summer, around 75. For magnitude four, the winter average is 16 per month, while in summer the rate falls to eight per month. They ran the numbers through a statistical calculation and ruled out the possibility that the seasonal signal was due merely to chance.

"The signal in the seismicity is real; there is no discussion," Avouac says. "We see this seasonal cycle," he adds. "We didn't know where it came from but it is really strong. We're looking at something that is changing on a yearly basis-the timescale over which stress changes in this region is one year."

Earlier studies suggested that seasonal variations in atmospheric pressure set off earthquakes, and this had been proposed for seasonal seismicity following the 1992 Landers, California, quake.

The scientists turned to satellite measurements of water levels in the region. Using altimetry data from TOPEX/Poseidon, a satellite launched in 1992 by NASA and the French space agency CNES (Centre National d'Etudes Spatiales), they evaluated the water level in major rivers of the Ganges basin to within a few tens of centimeters. They found that the water level over the whole basin begins its four-meter rise at the onset of the monsoon season in mid-May, reaching a maximum in September, followed by a slow decrease until the next monsoon season.

They combined river level measurements with data from NASA's GRACE--Gravity Recovery and Climate Experiment--mission, which studies, among other things, groundwater storage on landmasses. The data revealed a strong signal of seasonal variation of water in the basin. Paired with the altimetry data, these measurements paint a complete picture of the hydrologic cycle in the region.

In the Himalaya, monsoon rains swell the rivers of the Ganges basin, increasing the pressure bearing down on the region. As the rains stop, the river water soaks through the ground and the built-up load eases outward, toward the front of the range. This outward redistribution of stress after the rains end leads to horizontal compression in the mountain range later in the year, triggering the wintertime earthquakes.

The final piece connecting winter earthquake frequency to season, and lending insight into the process by which earthquakes nucleate, lay in GPS data. Installation of GPS instruments across the Himalayan front began in 1994, and now they provide a decade's worth of measurements showing land movement across the region. Instead of looking at vertical motions, which are widely believed to be sensitive to weather and the same forces that cause tides on Earth, the scientists concentrated on horizontal displacements. The lengthy records, analyzed by Pierre Bettinelli during his graduate work at Caltech, show that horizontal motion is continuous in the range front. Stress constantly builds in the region. But just as water levels near their lowest in the adjacent Ganges basin and earthquakes begin their doubletime, horizontal motion reaches its maximum speed.

"We had been staring at [the seasonal signal] for years, and then the satellite data came in and we deployed the GPS network and suddenly it became crystal clear," says Avouac. "It's like something you dream of."

While many scientists have suggested that changing water levels can influence the earthquake cycle, a definitive mechanism had yet to be pinpointed. "There are two main avenues by which people have tried to understand the physics of earthquakes: Earth tides and aftershocks," says Avouac. With the water level data, he could show that the rate at which stress builds along the rangefront, rather than the absolute level of stress, triggers earthquakes.

Although Earth tides induce stress levels similar to what builds up during seasonal water storage, they only vary over a 12-hour period. The Himalayan signal shows that it is more likely that earthquakes are triggered after stress builds for weeks to months, which matches the timescale of seasonal stress variation in that region.

About other earthquake-prone regions Avouac says, "seasonal variation has been reported in other places, but I don't know any other place where it is so strong or where the cause of the signal is so obvious."

Other authors on the paper are Pierre Bettinelli, Mireille Flouzat, and Laurent Bollinger of the Commissariat a l'Énergie Atomique, France; Guillaume Ramillien of the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, France; and Sudhir Rajaure and Som Sapkota of the National Seismological Centre in Nepal.

Avouac will present details of the group's findings at AGU on Wednesday, December 12, at 2 p.m., Moscone West room 3018, in session T33F: Earthquake geology, active tectonics, and mountain building in south and east Asia.

Writer: 
Elisabeth Nadin
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Tracing the Roots of the California Condor

Pasadena, Calif.--At the end of the Pleistocene epoch some 10,000 years ago, two species of condors in California competed for resources amidst the retreating ice of Earth's last major glacial age. The modern California condor triumphed, while its kin expired.

In the past century, paleontologists have been unsure whether the modern California condor is different enough from a larger, extinct condor that lived during Pleistocene time to classify the two as distinct species. Now, after the most extensive study of condor fossils and skeletal remains to date, Caltech senior undergraduate Valerie Syverson has documented evidence that confirms the two are different enough for the distinction.

Her findings will be presented on October 28 at the annual meeting of the Geological Society of America in Denver.

To solve the puzzle, Syverson teamed up with Donald Prothero, a paleontologist at Occidental College and a guest lecturer in geobiology at Caltech. They studied bones from recently dead condors and compared them with those found in the extensive bone pile of Los Angeles's Pleistocene-aged La Brea tar pits. What they found, Syverson says, is that "there's definitely one species distinction, and possibly two."

Syverson began her study by examining bones from condor skeletons housed at the Los Angeles Museum of Natural History, the Museum of Vertebrate Zoology at UC Berkeley, and the Santa Barbara Museum of Natural History. One interesting finding was that among these modern birds, Gymnogyps californianus, there was no distinction in bone size between males and females.

After looking at modern condors, Syverson turned to La Brea. She examined Pleistocene specimens from various tar pits, the oldest 35,000 and the youngest 9,000 years old. The record thus provides a glimpse into a long time variation within a species restricted to one location. Over the entire 26,000-year record, Syverson found no change in condor morphology. Although this had been previously discovered in a similar study of golden eagles from La Brea, Syverson says it's remarkable to see that the drastic climate change accompanying the end of the last ice age had no impact on the size of the species that lived through it.

When Syverson plotted her measurements of modern and Pleistocene condor bones, she found there was a definite size distinction between the two. "The ancients are decidedly bigger," she says, and the difference is especially notable in the femur, or thigh bone. These birds were heavier, with a longer, narrower skull and beak than the modern California condor. At first blush, they seem to belong to the species Gymnogyps amplus, first described in 1911 based on a broken tarsometatarsus, a bone found in the lower leg of birds.

In fact, that type specimen suggests that the Pleistocene condors at La Brea may be a third distinct condor species. The broken tarsometatarsus--housed in the Berkeley collection--is larger than any other condor bone Syverson studied. "It would've been an outlier from either species," she says. "Based on the fact that the type specimen is outside the range for both of the groups, I wonder if we need to define a third species for the extinct La Brea condor."

This study also documents evidence that ancient and modern condors coexisted for some time, and that the Pleistocene species may have lived at the same time as humans in western North America. Several tarsometatarsi of the older, bigger species were found in the youngest pit at La Brea. This pit also contains the remains of the La Brea woman, the only prehistoric human discovered in the tar pits. Another piece of evidence pointing to the same conclusion comes from the Berkeley museum collection. It is a bone from a Native American midden--a garbage heap--in Oregon, and it falls into the size range of the ancient group. Although its age is unknown, it must have lived at the same time as the people who disposed of it.

Syverson hopes to use radiocarbon dating to determine the age of the Oregon specimen. She'd also like to apply the technique to date the G. amplus type specimen, to see if its age does indeed distinguish a third condor species.

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Elisabeth Nadin
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Caltech's Ingersoll Receives Achievement Award

PASADENA, Calif.-- Andrew P. Ingersoll of the California Institute of Technology has been awarded the 2007 Gerard P. Kuiper Prize by the Division for Planetary Sciences (DPS) of the American Astronomical Society in honor of his outstanding contributions to planetary science. The award was presented this week during the annual DPS meeting in Orlando, Florida.

Ingersoll, the Earle C. Anthony Professor of Planetary Science at Caltech, has been a leader in the investigation of planetary atmospheres for more than four decades. His research has included studies of the runaway greenhouse effect on Venus, the occurrence of liquid water on Mars, the supersonic winds on Jupiter's moon Io, and the atmospheric dynamics of Jupiter, Saturn, Uranus, and Neptune. He participated on the instrument teams for many NASA/JPL missions including Pioneer Venus, Pioneer Saturn, Voyager, Mars Global Surveyor, Galileo, and Cassini.

Ingersoll is a recipient of NASA's Exceptional Scientific Achievement Medal and is a Fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Science, the American Geophysical Union, and the American Astronomical Society.

The Gerard P. Kuiper Prize has been given annually since 1984 to scientists "whose achievements have most advanced our understanding of the planetary system," among them Carl Sagan, James Van Allen and Eugene Shoemaker. The award is named after the pioneering Dutch-born astronomer, who is considered the father of modern planetary science. In 1951, Kuiper proposed the existence of a belt of minor planets at the edge of the solar system; after its discovery, the region was named the Kuiper Belt in his honor. He also discovered the atmosphere of Saturn's moon Titan, the carbon dioxide atmosphere of Mars, Uranus's satellite Miranda, and Neptune's moon Nereid.

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Kathy Svitil
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New Method of Studying Ancient Fossils Points to Carbon Dioxide As a Driver of Global Warming

PASADENA, Calif.—A team of American and Canadian scientists has devised a new way to study Earth's past climate by analyzing the chemical composition of ancient marine fossils. The first published tests with the method further support the view that atmospheric CO2 has contributed to dramatic climate variations in the past, and strengthen projections that human CO2 emissions could cause global warming.

In the current issue of the journal Nature, geologists and environmental scientists from the California Institute of Technology, the University of Ottawa, the Memorial University of Newfoundland, Brock University, and the Waquoit Bay National Estuarine Research Reserve report the results of a new method for determining the growth temperatures of carbonate fossils such as shells and corals. This method looks at the percentage of rare isotopes of oxygen and carbon that bond with each other rather than being randomly distributed through their mineral lattices.

Because these bonds between oxygen-18 and carbon-13 form in greater abundance at low temperatures and lesser abundance at higher temperatures, a precise measurement of their concentration in a carbonate fossil can quantify the temperature of seawater in which the organisms lived. By comparing this record of temperature change with previous estimates of past atmospheric CO2 concentrations, the study demonstrates a strong coupling of atmospheric temperatures and carbon dioxide concentrations across one of Earth's major environmental shifts.

According to Rosemarie Came, a postdoctoral scholar in geochemistry at Caltech and lead author of the article, only about 60 parts per million of the carbonate molecular groups that make up the mineral structures of carbonate fossils are a combination of both oxygen-18 and carbon-13, but the amount varies predictably with temperature. Therefore, knowing the age of the sample and how much of these exotic carbonate groups are present allows one to create a record of the planet's temperature through time.

"This clumped-isotope method has an advantage over previous approaches because we're looking at the distribution of rare isotopes inside a single shell or coral," Came says. "All the information needed to study the surface temperature at the time the animal lived is stored in the fossil itself."

In this way, the method contrasts with previous approaches that require knowledge of the chemistry of seawater in the distant past--something that is poorly known.

The study contrasts the growth temperatures of fossils from two times in the distant geological past. The Silurian period, approximately 400 million years ago, is thought to have been a time of highly elevated atmospheric CO2 (more than 10 times the modern concentration), and was found by the researchers to be a time of exceptionally warm shallow-ocean temperatures—nearly 35 degrees C. In contrast, the Carboniferous period, roughly 300 million years ago, appears to have been characterized by far lower levels of atmospheric carbon dioxide (similar to modern values) and had shallow marine temperatures similar to or slightly cooler than today-about 25 degrees C. Thus, the draw-down of atmospheric CO2 coincided with strong global cooling.

"This is a huge change in temperature," says John Eiler, a professor of geochemistry at Caltech and a coauthor of the study. "It shows that carbon dioxide really has been a powerful driver of climate change in Earth's past."

The title of the Nature paper is "Coupling of surface temperatures and atmospheric CO2 concentrations during the Paleozoic era." The other authors are Jan Veizer of the University of Ottawa, Karem Azmy of Memorial University of Newfoundland, Uwe Brand of Brock University, and Christopher R. Weidman of the Waquoit National Estuarine Research Reserve, Massachusetts.

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Robert Tindol
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