San Andreas Earthquakes Have Almost Always Been Big Ones, Paleoseismologists Discover

PASADENA, Calif.—A common-sense notion among many Californians is that frequent small earthquakes allow a fault to slowly relieve accumulating strain, thereby making large earthquakes less likely. New research suggests that this is not the case for a long stretch of the San Andreas fault in Southern California.

In a study appearing in the current issue of the journal Geology, researchers report that about 95 percent of the slippage at a site on the San Andreas fault northwest of Los Angeles occurs in big earthquakes. By literally digging into the fault to look for information about earthquakes of the past couple of millennia, the researchers have found that most of the motion along this stretch of the San Andreas fault occurs during rare but large earthquakes.

"So much for any notion that the section of the San Andreas nearest Los Angeles might relieve its stored strains by a flurry of hundreds of small earthquakes!" said Kerry Sieh, a geology professor at the California Institute of Technology and one of the authors of the paper.

Sieh pioneered the field of paleoseismology years ago as a means of understanding past large earthquakes. His former student, Jing Liu, now a postdoctoral fellow in Paris, is the senior author of the paper.

In this particular study, Liu, Sieh, and their colleagues cut trenches parallel and perpendicular to the San Andreas fault at a site 200 kilometers (120 miles) northwest of Los Angeles, between Bakersfield and the coast. The trenches allowed them to follow the subsurface paths of small gullies buried by sediment over the past many hundreds of years. They found that the fault had offset the youngest channel by nearly 8 meters, and related this to the great (M 7.9) earthquake of 1857. Older gullies were offset progressively more by the fault, up to 36 meters. By subtracting each younger offset from the next older one, the geologists were able to recover the amount of slip in each of the past 6 earthquakes.

Of the six offsets discovered in the excavations, three and perhaps four were offsets of 7.5 to 8 meters, similar in size to the offset during the great earthquake of 1857. The third and fourth events, however, were slips of just 1.4 and 5.2 meters. Offsets of several meters are common when the rupture length is very long and the earthquake is very large. For example, the earthquake of 1857 had a rupture length of about 360 kilometers (225 miles), extending from near Parkfield to Cajon Pass. So, the five events that created offsets measuring between 5.2 and 8 meters likely represent earthquakes that had very long ruptures and magnitudes ranging from 7.5 to 8. Taken together, these five major ruptures of this portion of the San Andreas fault account for 95 percent of all the slippage that occurred there over the past thousand years or so.

The practical significance of the study is that earthquakes along the San Andreas, though infrequent, tend to be very large. Years ago, paleoseismic research showed that along the section of the fault nearest Los Angeles the average period between large earthquakes is just 130 years. Ominously, 147 years have already passed since the latest large rupture, in 1857.

The other authors of the paper are Charles Rubin, of the department of geological sciences at Central Washington University in Ellensburg, and Yann Klinger, of the Institut de Physique du Globe de Paris, France. Additional information about the site, including a virtual field trip, can be found at http://www.scec.org/wallacecreek/.

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Robert Phillip Sharp Dies

PASADENA, Calif.—Robert Phillip Sharp, a leading authority on the surfaces of Earth and Mars and longtime head of the geological sciences division at the California Institute of Technology, died May 25 at his home in Santa Barbara. He was 92.

Though Sharp was a renowned geologist in his own right, his most significant role was arguably his modernization of the earth sciences at Caltech during a time of unparalleled progress in the furtherance of knowledge of both our own world and of others. Sharp brought to the job a remarkable talent for hiring top people, as well as a strong interest in creating new interdisciplinary approaches to take advantage of the dawning age of manned and unmanned planetary exploration.

Particularly noteworthy in Sharp's revamping of the Caltech earth sciences was his support of planetary science as a vehicle for extending geological research to the other planets, notably Mars, and for his contributions to the creation of the field of geochemistry. The latter discipline was especially important in the interpretation of lunar samples that began at Caltech in 1970. Sharp was also closely involved with NASA during the 1960s as an interpreter of the Mariner imagery from Mars.

In an anecdote reported by the Pasadena Star-News during Caltech's centennial celebration in 1991, Sharp recalled how one of the Mariner technicians had told him the imagery just returned from Mars had revealed the presence of a lake. Telling the technician that a lake on Mars was absurd, he looked at the imagery and saw that the rippling features the technician had seen were actually sand dunes. "That was the beauty of it for me," Sharp said. "Astrophysicists, engineers, and computer guys, and they need this dumb ol', dirty fingernail geologist like me!"

In 1989, Sharp's own research won him America's highest scientific honor, the National Medal of Science, for having "illuminated the nature and origin of the forms and formation processes of planetary surfaces, and for teaching two generations of scientists and laymen to appreciate them." Sharp was also cited by the White House for having built the multidisciplinary Division of Geological and Planetary Sciences at Caltech.

His many research activities included investigations of basin range structure, continental basin deposits, mountain glaciation, continental glaciation, glacial-lake shorelines, frozen ground, erosion surfaces, desert sand dunes, glaciers, oxygen and hydrogen isotopes in snow and glacier ice, and surface forms and processes on Mars.

His many awards and honors included his being named in 1950 by Life magazine as one of 10 outstanding U.S. college teachers; receiving the Kirk Bryan Award from the Geological Society of America in 1964; and receiving the NASA Exceptional Scientific Achievement Award Medal in 1971; election to the National Academy of Sciences in 1973; and receiving the Penrose Award from the Geological Society of America in 1977 (the society's top honor) and the Charles P. Daly Medal from the American Geographical Society in 1991. In addition, he was honored by the Boy Scouts of America in 1978 with the Distinguished Eagle Scout Award, and by Caltech by the institution of the Robert P. Sharp Professorship in Geology that same year.

A native of Oxnard, California, Sharp first came to Caltech as an undergraduate in 1930, where he was a star quarterback in the days when the institute was still a competitive force in Southern California football. Sharp was one of 25 former gridiron stars who had gone on to significant careers, who were profiled in the Christmas 1958 issue of the magazine Sports Illustrated. At the time "one of the ablest, most popular teachers at Caltech," the 165-pound Sharp lamented having been sacked so many times toward the end of his college days due to temporary 1933 changes in college rules that weighted the game "in favor of brute force." He said of football that it served to show a scientist he needed "to be determined as hell and that there is a certain poise and aggressiveness that is desirable."

After earning his BS and MS at Caltech, in 1934 and 1935, respectively, Sharp went on to Harvard University for a doctorate in geology in 1938. He spent the years from 1938 to 1943 in the geology department at the University of Illinois, and then served in the U.S. Army Air Forces from 1943 to 1945, working in the Arctic, Desert, and Tropic Information Center and reaching the rank of captain. After returning from the war, he served from 1945 to 1947 on the University of Minnesota faculty, and then returned to Caltech as a professor. He was division chairman from 1952 to 1968, and retired in 1979.

Though officially designated emeritus after his retirement, he nevertheless remained quite active, and was especially renowned in the Caltech community for his frequent field trips to numerous remote locales of geological interest. He not only continued to be involved with educational field trips for students in the geological sciences, but also participated in field trips for Caltech alumni, Associates, Caltech staff members, and others.

He was preceded in death by his wife, Jean Todd Sharp. His survivors include two adopted children, Kristin and Bruce.

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White House Names Three from Caltech Faculty as Presidential Early Career Award Winners

PASADENA, Calif.—Three members of the faculty at the California Institute of Technology have been named among the most recent winners of the prestigious Presidential Early Career Award for Scientists and Engineers (PECASE). The honor was announced today by the White House.

The three are Babak Hassibi, an electrical engineer who studies data transmission and wireless communications system; Mark Simons, a geophysicist who specializes in understanding the mechanical behavior of Earth using radar and other satellite observations of the motions of Earth's surface; and Brian Stoltz, an organic chemist who specializes in the synthesis of structurally complex, biologically active molecules.

Hassibi was cited by the White House for his "fundamental contributions to the theory and design of data transmission and reception schemes that will have a major impact on new generations of high-performance wireless communications systems. He has nurtured creativity in his undergraduate and graduate students by involving them in research and inspiring them to apply new approaches to communications problems."

An associate professor of electrical engineering at Caltech and a faculty member since 2001, Hassibi earned his bachelor's degree from the University of Tehran in 1989, and his master's and doctorate degrees from Stanford in 1993 and 1996, respctively. He is the holder or coholder of four patents for communications technology, and is the winner of several awards, including the 2002 National Science Foundation Career Award, the 1999 American Automatic Control Council O. Hugo Schuck Best Paper Award, the 2003 David and Lucille Packard Fellowship for Science and Engineering, and the 2002 Okawa Foundation Grant for Telecommunications and Information Sciences.

Simons, an associate professor of geophysics, combines satellite data with continuum mechanical models of Earth to study ongoing regional crustal dynamics, including volcanic and tectonic deformation in Iceland, crustal deformation and the seismic cycle in California, Chile, and Japan, and volcanic and tectonic deformation in and around Long Valley, California. He also uses the gravity fields of the terrestrial planets to study the large-scale geodynamics of mantle convection and its relationship to tectonics.

Simons earned his bachelor's degree at UCLA in 1989, and his doctorate from MIT in 1995. He was a postdoctoral scholar at Caltech for two years before joining the faculty in 1997.

Stoltz has been an assistant professor of chemistry at Caltech since 2000. He earned his bachelor's degree at Indiana University of Pennsylvania in 1993, his master's and doctorate degrees at Yale University in 1996 and 1997, respectively. Before joining the Caltech faculty he spent two years at Harvard University as a National Institutes of Health (NIH) Postdoctoral Fellow. His work is aimed at developing new strategies for creating complex molecules with interesting structural, biological, and physical properties. The goal is to use these complex molecules to guide the development of new reaction methodology to extend fundamental knowledge and to potentially lead to useful biological and medical applications.

Stoltz, an Alfred P. Sloan Fellow, is the recipient of a Research Corporation Cottrell Scholars Award, the Camille and Henry Dreyfus New Faculty Award, and the Pfizer Research Laboratories Creativity in Synthesis Award. Additionally, he was named as an Eli Lilly Grantee in 2003 and has won a number of young faculty awards from pharmaceutical companies such as Merck Research Laboratories, Abbott Laboratories, GlaxoSmithKline, Johnson & Johnson, Amgen, Boehringer Ingelheim, and Roche. At Caltech he won the 2001 Graduate Student Council Teaching Award and Graduate Student Council Mentoring Award.

The PECASE awards were created in 1996 by the Clinton Administration "to recognize some of the nation's most promising junior scientists and engineers and to maintain U.S. leadership across the frontiers of scientific research." The awards are made to those whose innovative work is expected to lead to future breakthroughs.

 

 

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Caltech geoscientist named Guggenheim Fellow

PASADENA—Joann Stock, an authority on plate tectonics and a professor of geology and geophysics at the California Institute of Technology, has been awarded a Guggenheim Fellowship, the John Simon Guggenheim Memorial Foundation has announced. Stock joins 184 other artists, scholars, and scientists this year for the prestigious honor, which is now in its 80th year. A member of the Caltech faculty since 1992, Stock came from Harvard University where she was an associate professor. She also holds an adjunct appointment at the Centro de Investigación Científica y de Educación Superior de Ensenada (the Center of Scientific Investigation and Higher Education of Ensenada) in Baja California, Mexico, and is a former employee of the U.S. Geological Survey office at Menlo Park, California. Her research interests include plate tectonics, structural geology, evolution of plate boundaries, physical volcanology, remote sensing, ground-penetrating radar studies of active faults, and stress and deformation in the lithosphere. The Guggenheim Fellowship has been awarded to Stock for a project on the comparative tectonic history of two rift basins: the Sea of Japan and the Gulf of California. The grant will help to support Stock's sabbatical research on this topic, which she plans to conduct next academic year in Japan and in Mexico. The goal of the project is to better understand the ways in which ocean basins can result from stretching of continental lithosperic plates. Stock will be working in collaboration with scientists from the University of Tokyo and from the University of Sonora for the studies in Japan and in Mexico, respectively.

The Guggenheim Fellows are appointed on the basis of distinguished achievement in the past and exceptional promise for future accomplishment. Each year, the new recipients are appointed on the basis of recommendations from expert advisors and are approved by the foundation's board of trustees. This year's total award funding for the 185 new recipients is $6,912,000. The 2004 Guggenheim Fellowship Awards were announced April 8 in New York by foundation president Edward Hirsch.

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From Cosmos to Climate, Six Caltech Professors Awarded Sloan Research Fellowships

PASADENA, Calif.— Six members of the Caltech faculty have received Alfred P. Sloan Research Fellowships for 2004.

The Caltech recipients in the field of mathematics are Nathan Dunfield and Vadim Kaloshin, both associate professors of mathematics. In physics, Sloan Fellowships were awarded to Andrew Blain, assistant professor of astronomy, Sunil Golwala, assistant professor of physics, Re'em Sari, associate professor of astrophysics and planetary science, and Tapio Schneider, assistant professor of environmental science and engineering.

Each Sloan Fellow receives a grant of $40,000 for a two-year period. The grants of unrestricted funds are awarded to young researchers in the fields of physics, chemistry, computer science, mathematics, neuroscience, computational and evolutionary molecular biology, and economics. The grants are given to pursue diverse fields of inquiry and research, and to allow young scientists the freedom to establish their own independent research projects at a pivotal stage in their careers. The Sloan Fellows are selected on the basis of "their exceptional promise to contribute to the advancement of knowledge."

From over 500 nominees, a total of 116 young scientists and economists from 51 different colleges and universities in the United States and Canada, including Caltech's six, were selected to receive a Sloan Research Fellowship.

Twenty-eight previous Sloan Fellows have gone on to win Nobel Prizes.

The Alfred P. Sloan Research Fellowship program was established in 1955 by Alfred P. Sloan, Jr., who was the chief executive officer of General Motors for 23 years. Its objective is to encourage research by young scholars at a time in their careers when other support may be difficult to obtain. It is the oldest program of the Alfred P. Sloan Foundation and one of the oldest fellowship programs in the country.

Nathan Dunfield conducts research in topology, the study of how geometric structures in three-dimensional space can be altered. His focus is on the connections to the symmetries of rigid geometric objects, especially certain types of non-Euclidean geometries, and he also uses computer experiments to probe some of the central questions in the study of topology. Dunfield will utilize his Sloan Fellowship to further his research in this area.

Vadim Kaloshin is an expert in chaos theory and "strange attractors." He is especially interested in mathematical equations known as Hamiltonian systems and how they apply to stability. His work could lead to a better understanding of how chaotic systems behave. Kaloshin will use his Sloan Fellowship to continue investigation in these fields.

Andrew Blain probes the origin of galaxies by observing them at great distances in the process of formation. He concentrates on the signatures that can be seen in the short-wavelength radio and long-wavelength infrared spectrum, where the gas and soot-like dust particles between the stars emit energy they absorb from the youngest and most luminous parts of galaxies. Most studies of the process are still carried out using the direct light from stars at shorter optical wavelengths, but the complementary information from longer wavelengths is essential to build up a more complete picture. The Sloan Foundation Fellowship will be used to link together these two techniques by investigating differences between the way distant galaxies found at each wavelength are distributed in space.

Sunil Golwala's research focuses on understanding dark matter and dark energy, components that dominate the universe but whose identity and nature are unknown. Golwala is interested in the development and use of particle detectors for observing the direct scattering of "Weakly Interacting Massive Particles," one of the leading candidates for dark matter. His work also involves the observation of varying aspects of the cosmic microwave background that inform us about the nature of dark energy via its effect on the growth of galaxy clusters and its clustering effects on super-horizon scales. Golwala will utilize his Sloan Fellowship in pursuit of this endeavor to better understand the universe.

Re'em Sari intends to utilize his Sloan Fellowship to examine the origin of planet formation, a first step in a long journey to look for life around other stars. Some of the fundamental questions he will investigate are: How do planets form? What are the necessary initial conditions for planet formation? What factors determined the number of planets in our solar system? How many planets like Earth do we expect to find around other stars? Are there binary giant planets? Sari will apply his fellowship to further understanding the "grand scheme of planetary systems."

Tapio Schneider works on understanding climate and the dynamical processes in the atmosphere that determine basic climatic features such as the pole-to-equator temperature gradient and the distribution of water vapor. Developing mathematical models of the large-scale (1000 km) turbulent transport of heat, mass, and water vapor is one central aspect of this research. The Sloan Fellowship will provide computing equipment and support to expand these studies on climate.

Contact: Deborah Williams-Hedges (626) 395-3227 debwms@caltech.edu

Visit the Caltech Media Relations Web site at: http://pr.caltech.edu/media

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Researchers demonstrate existenceof earthquake supershear phenomenon

PASADENA, Calif.--As if folks living in earthquake country didn't already have enough to worry about, scientists have now identified another rupture phenomenon that can occur during certain types of large earthquakes. The only question now is whether the phenomenon is good, bad, or neutral in terms of human impact.

Reporting in the March 19 issue of the journal Science, California Institute of Technology geophysics graduate student Kaiwen Xia, aeronautics and mechanical engineering professor Ares Rosakis, and geophysics professor Hiroo Kanamori have demonstrated for the first time that a very fast, spontaneously generated rupture known as "supershear" can take place on large strike-slip faults like the San Andreas. They base their claims on a laboratory experiment designed to simulate a fault rupture.

While calculations dating back to the 1970s have predicted that such supershear rupture phenomena may occur in earthquakes, seismologists only recently began assuming that supershear was real. The Caltech experiment is the first time that spontaneous supershear rupture has been conclusively identified in a controlled laboratory environment, demonstrating that super-shear fault rupture is a very real possibility rather than a mere theoretical construct.

In the lab, the researchers forced two plates of a special polymer material together under pressure and then initiated an "earthquake" by inserting a tiny wire into the interface, which is turned into an expanding plasma by the sudden discharge of an electrical pulse. By means of high-speed photography and laser light, the researchers photographed the rupture and the stress waves as they propagated through the material.

The data shows that, under the right conditions, the rupture propagates much faster than the shear speed in the plates, producing a shock-wave pattern, something like the Mach cone of a jet fighter breaking the sound barrier.

The split-second photography also shows that such ruptures may travel at about twice the rate that a rupture normally propagates along an earthquake fault. However, the ruptures do not reach supershear speeds until they have propagated a certain distance from the point where they originated. Based on the experiments, a theoretical model was developed by the researchers to predict the length of travel before the transition to supershear.

In the case of a strike-slip fault like the San Andreas, the lab results indicate that the rupture needs to rip along for about 100 kilometers and the magnitude must be about 7.5 or so before the rupture becomes supershear. Large earthquakes along the San Andreas tend to be at least this large if not larger, typically involving rupture lengths of about 300 to 400 kilometers.

"Judging from the experimental result, it would not be surprising if supershear rupture propagation occurs for large earthquakes on the San Andreas fault," said Kanamori.

Similar high-speed ruptures propagating along bimaterial interfaces in engineering composite materials have been experimentally observed in the past (by Rosakis and his group, reporting in an August 1999 issue of Science). These ruptures took place under impact loading; only in the current experiment have they been initiated in an earthquake-like set-up.

According to Rosakis, an expert in crack propagation, the new results show promise in using engineering techniques to better understand the physics of earthquakes and its human impact.

According to Kanamori, the human impact of the finding is still debatable. The most damaging effect of a strike-slip earthquake is believed to be caused by a pulse-like motion normal to the fault caused by the combined effect of the rupture and shear wave. The supershear rupture suppresses this pulse, which is good, but the persistent shock-wave (Mach wave) emitted by the supershear rupture enhances the fault-parallel component of motion (the ground motion that runs in the same direction that the plates slip) and could amplify the destructive power of ground motion, which is bad.

The outstanding question about supershear at this point is which of these two effects dominates. "This is still being debated," says Kanamori. "We're not committed to one view or the other." Only further laboratory-level experimentation can answer this question conclusively.

Several seismologists believe that supershear was exhibited in some large earthquakes, including those that occurred in Tibet in 2001 and in Alaska in 2002. Both earthquakes were located in a remote region and had little, if any, human impact, but analysis of the evidence shows that the fault rupture propagated much faster than would normally be expected, thus implying supershear.

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Most distant object in solar system discovered; could be part of never-before-seen Oort cloud

PASADENA, Calif.--A planetoid more than eight billion miles from Earth has been discovered by researchers led by a scientist at the California Institute of Technology. The new planetoid is more than three times the distance of Pluto, making it by far the most distant body known to orbit the sun.

The planetoid is well beyond the recently discovered Kuiper belt and is likely the first detection of the long-hypothesized Oort cloud. With a size approximately three-quarters that of Pluto, it is very likely the largest object found in the solar system since the discovery of Pluto in 1930.

At this extreme distance from the sun, very little sunlight reaches the planetoid and the temperature never rises above a frigid 400 degrees below zero Farenheit, making it the coldest known location in the solar system. According to Mike Brown, Caltech associate professor of planetary astronomy and leader of the research team, "the sun appears so small from that distance that you could completely block it out with the head of a pin."

As cold as it is now, the planetoid is usually even colder. It approaches the sun this closely only briefly during the 10,500 years it takes to revolve around the sun. At its most distant, it is 84 billion miles from the sun (900 times Earth's distance from the sun), and the temperature plummets to just 20 degrees above absolute zero.

The discoverers---Brown and his colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University--have proposed that the frigid planetoid be named "Sedna," after the Inuit goddess who created the sea creatures of the Arctic. Sedna is thought to live in an icy cave at the bottom of the ocean--an appropriate spot for the namesake of the coldest body known in the solar system.

The researchers found the planetoid on the night of November 14, 2003, using the 48-inch Samuel Oschin Telescope at Caltech's Palomar Observatory east of San Diego. Within days, the new planetoid was being observed on telescopes in Chile, Spain, Arizona, and Hawaii; and soon after, NASA's new Spitzer Space Telescope was trained on the distant object.

The Spitzer images indicate that the planetoid is no more than 1,700 kilometers in diameter, making it smaller than Pluto. But Brown, using a combination of all of the data, estimates that the size is likely about halfway between that of Pluto and that of Quaoar, the planetoid discovered by the same team in 2002 that was previously the largest known body beyond Pluto.

The extremely elliptical orbit of Sedna is unlike anything previously seen by astronomers, but it resembles in key ways the orbits of objects in a cloud surrounding the sun predicted 54 years ago by Dutch astronomer Jan Oort to explain the existence of certain comets. This hypothetical "Oort cloud" extends halfway to the nearest star and is the repository of small icy bodies that occasionally get pulled in toward the sun and become the comets seen from Earth.

However, Sedna is much closer than expected for the Oort cloud. The Oort cloud has been predicted to begin at a distance 10 times greater even than that of Sedna. Brown believes that this "inner Oort cloud" where Sedna resides was formed by the gravitational pull of a rogue star that came close to the sun early in the history of the solar system. Brown explains that "the star would have been close enough to be brighter than the full moon and it would have been visible in the daytime sky for 20,000 years." Worse, it would have dislodged comets further out in the Oort cloud, leading to an intense comet shower, which would have wiped out any life on Earth that existed at the time.

There is still more to be learned about this newest known member of the solar system. Rabinowitz says that he has indirect evidence that there may be a moon following the planetoid on its distant travels--a possibility that is best checked with the Hubble Space Telescope--and he notes that Sedna is redder than anything known in the solar system with the exception of Mars, but no one can say why. Trujillo admits, "We still don't understand what is on the surface of this body. It is nothing like what we would have predicted or what we can currently explain."

But the astronomers are not yet worried. They can continue their studies as Sedna gets closer and brighter for the next 72 years before it begins its 10,500-year trip out to the far reaches of the solar system and back again. Brown notes, "The last time Sedna was this close to the sun, Earth was just coming out of the last the last ice age; the next time it comes back, the world might again be a completely different place."

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Caltech mineralogy professor George Rossman wins Feynman Prize for teaching excellence

PASADENA, Calif.—Ever wonder what becomes of the type of kid who keeps a rock collection?

For some of the brighter ones, at least, the answer is that they still collect rocks and minerals under the tutelage of award-winning California Institute of Technology professor George Rossman. As the member of the geology faculty most directly involved in mineralogy research, Rossman teaches a popular class at Caltech on the subject, and many of his former students are now prominent mineralogists themselves.

Rossman has been named this year's recipient of Caltech's most prestigious teaching honor, the Feynman Prize. The award, given to an outstanding faculty member each year, recognizes "exceptional ability, creativity, and innovation in both laboratory and classroom instruction." Named in honor of legendary Caltech physics professor and Nobel Laureate Richard Feynman, the prize is made possible by the generosity of an endowment from Ione and Robert E. Paradise, along with additional contributions from Mr. and Mrs. William H. Hurt.

Rossman won the award based on significant input from current and former students. Among the highlights of those comments is that he "is probably the best, clearest, and most exciting teacher I have ever had," that he "is such a great lecturer that he can make the class and each mineral very funny," and that he "is probably the best professor at Caltech."

For Rossman's part, he rather modestly says that minerals are inherently interesting subject matter for the classroom. "Students relate to tangible, visible items," he says, and the specimens sitting on the floor behind his desk easily make his point. One item, for example, is a rather large conglomeration of rock and minerals called pegmatite. Found in San Diego County, the rock contains minerals such as mica, tourmaline, and quartz.

"For me, the minerals are a beautiful entry into the science, because the beautiful colors and shapes are always due to underlying scientific principles," he says. "Nature has the ability to bring together a large number of the elements of the periodic table, and combine them under different pressure and temperature conditions for some really spectacular results."

The practical results are more widespread than one might assume. Synthetic minerals are found in a number of high-tech electronic devices these days, and applications include quartz oscillators, emerald and ruby lasers and such, and the field of mineralogy laps over into a variety of disciplines, including chemistry, solid-state physics, materials science, industrial technology, environmental science, biology, and planetary science.

In fact, the young science student who hopes to study other worlds some day--or perhaps even go to some of them--might do well to study geology and mineralogy. "We presume that the physical principles we learn on Earth are applicable to Mars and other planets," Rossman says. "In fact, they should be applicable in other solar systems."

Rossman's research interests involve the study of how electromagnetic radiation interacts with minerals. His lab's work concentrates on the visible and infrared, but past research has involved pretty much every other region of the electromagnetic spectrum.

"Our goals include understanding at a very basic level the nature of the interaction--in other words, how we can use photons to study minerals," he says. "We've developed a variety of analytical protocols, and I suppose one of our most recent successes has been in learning that the hydrogen content of nominally anhydrous minerals constitutes an important global reservoir that is capable of holding much of the world's water."

As is typically the case in the Caltech labs, students find more than enough opportunity to immerse themselves in the research. Rossman's lab includes staff scientists and postdoctoral researchers, but graduate students and undergraduates are welcome members of the team. One of Rossman's recent graduate students, Elizabeth Johnson, is now a researcher at the Smithsonian Institution in Washington, D.C. An undergraduate now coming on board is currently a sophomore.

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Planetary scientists find planetoid in Kuiper Belt; could be biggest yet discovered

PASADENA, Calif.—Planetary scientists at the California Institute of Technology and Yale University on Tuesday night discovered a new planetoid in the outer fringes of the solar system.

The planetoid, currently known only as 2004 DW, could be even larger than Quaoar--the current record holder in the area known as the Kuiper Belt--and is some 4.4 billion miles from Earth.

According to the discoverers, Caltech associate professor of planetary astronomy Mike Brown and his colleagues Chad Trujillo (now at the Gemini North observatory in Hawaii), and David Rabinowitz of Yale University, the planetoid was found as part of the same search program that discovered Quaoar in late 2002. The astronomers use the 48-inch Samuel Oschin Telescope at Palomar Observatory and the recently installed QUEST CCD camera built by a consortium including Yale and the University of Indiana, to systematically study different regions of the sky each night.

Unlike Quaoar, the new planetoid hasn't yet been pinpointed on old photographic plates or other images. Because its orbit is therefore not well understood yet, it cannot be given an official name.

"So far we only have a one-day orbit," said Brown, explaining that the data covers only a tiny fraction of the orbit the object follows in its more than 300-year trip around the sun. "From that we know only how far away it is and how its orbit is tilted relative to the planets."

The tilt that Brown has measured is an astonishingly large 20 degrees, larger even than that of Pluto, which has an orbital inclination of 17 degrees and is an anomaly among the otherwise planar planets.

The size of 2004 DW is not yet certain; Brown estimates a size of about 1,400 kilometers, based on a comparison of the planetoid's luminosity with that of Quaoar. Because the distance of the object can already be calculated, its luminosity should be a good indicator of its size relative to Quaoar, provided the two objects have the same albedo, or reflectivity.

Quaoar is known to have an albedo of about 10 percent, which is slightly higher than the reflectivity of our own moon. Thus, if the new object is similar, the 1,400-kilometer estimate should hold. If its albedo is lower, then it could actually be somewhat larger; or if higher, smaller.

According to Brown, scientists know little about the albedos of objects this large this far away, so the true size is quite uncertain. Researchers could best make size measurements with the Hubble Space Telescope or the newer Spitzer Space Telescope. The continued discovery of massive planetoids on the outer fringe of the solar system is further evidence that objects even farther and even larger are lurking out there. "It's now only a matter of time before something is going to be discovered out there that will change our entire view of the outer solar system," Brown says.

The team is working hard to uncover new information about the planetoid, which they will release as it becomes available, Brown adds. Other telescopes will also be used to better characterize the planetoid's features.

Further information is at the following Web site: http://www.gps.caltech.edu/~chad/2004dw

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Caltech geophysicists gain new insights on Earth's core–mantle boundary

Earth's core–mantle boundary is a place none of us will ever go, but researchers using a special high-velocity cannon have produced results showing there may be molten rock at this interface at about 1,800 miles. Further, this molten rock may have rested peacefully at the core-mantle boundary for eons.

In a presentation at the fall meeting of the American Geophysical Union (AGU) today, California Institute of Technology geophysics professor Tom Ahrens reports new measurements of the density and temperature of magnesium silicate--the stuff found in Earth's interior--when it is subjected to the conditions that exist at the planet's core-mantle boundary.

The Caltech team did their work in the institute's shock wave laboratory, where an 80-foot light-gas gun is specially prepared to fire one-ounce tantalum-faced plastic bullets at mineral samples at speeds up to 220 thousand feet per second--about a hundred times faster than a bullet fired from a conventional rifle. The 30-ton apparatus uses compressed hydrogen as a propellant, and the resulting impact replicates the 1.35 million atmospheres of pressure and the 8,500 degrees Fahrenheit temperature that exist at the core–mantle boundary.

The measurements were conducted using natural, transparent, semiprecious gem crystals of enstatite from Sri Lanka, as well as synthetic glass of the same composition. Upon compression, these materials transform to a 30–percent denser structure called perovskite, which also dominates Earth's lower mantle at depths from 415 miles to the core–mantle boundary.

According to Ahrens, the results "have significant implications for understanding the core–mantle boundary region in the Earth's interior, the interface between rocky mantle and metallic core." The report represents the work of Ahrens and assistant professor of geology and geochemistry Paul Asimow, along with graduate students Joseph Akins and Shengnian Luo.

The researchers demonstrated by two independent experimental methods that the major mineral of Earth's lower mantle, magnesium silicate in the perovskite structure, melts at the pressure of the core–mantle boundary to produce a liquid whose density is greater than or equal to the mineral itself. This implies that a layer of partially molten mantle would be gravitationally stable over geologic times at the boundary, where seismologists have discovered anomalous features best explained by the presence of partial melt.

Two types of experiments were conducted: pressure-density experiments and shock temperature measurements. In the pressure-density experiments, the velocity of the projectile prior to impact and the velocity of the shock wave passing through the target after impact are measured using high-speed optical and x-ray photography. These measurements allow calculation of the pressure and density of the shocked target material. In shock temperature measurements, thermal emission from the shocked sample at visible and near-infrared wavelengths is monitored with a six-channel pyrometer, and the brightness and spectral shape are converted to temperature.

In both types of experiments, the shock wave takes about one ten-millionth of a second to pass through the dime-sized sample, and the velocity and optical emission measurements must resolve this extremely short duration event.

The pressure-density experiments yielded a surprising result. When the glass starting material is subjected to increasingly strong shocks, densities are at first consistent with the perovskite structure, and then a transition is made to a melt phase at a pressure of 1.1 million atmospheres. As expected for most materials under ordinary conditions, the melt phase is less dense than the solid. Shock compression of the crystal starting material, however, follows a lower temperature path, and the transition from perovskite shock states to molten shock states does not occur until a pressure of 1.7 million atmospheres is reached. At this pressure, the liquid appears to be 3 to 4 percent denser than the mineral. Like water and ice at ordinary pressure and 32 °F, under these high-pressure conditions the perovskite solid would float and the liquid would sink.

Just as the negative volume change on the melting of water ice is associated with a negative slope of the melting curve in pressure-temperature space (which is why ice-skating works-- the pressure of the skate blade transforms ice to water at a temperature below the ordinary freezing point), this result implies that the melting curve of perovskite should display a maximum temperature somewhere between 1.1 and 1.7 million atmospheres, and a negative slope at 1.7 million atmospheres. This implication of the pressure-density results was tested using shock temperature measurements. In a separate series of experiments on the same starting materials, analysis of the emitted light constrained the melting temperature at 1.1 million atmospheres to about 9,900 °F. However, at the higher pressure of 1.7 million atmospheres, the melting point is 8,500o F. This confirms that somewhere above 1.1 million atmospheres, the melting temperature begins to decrease with increasing pressure and the melting curve has a negative slope.

Taking the results of both the pressure-density and shock temperature experiments together confirms that the molten material may be neutrally or slightly negatively buoyant at the pressure of the base of the mantle, which is 1.35 million atmospheres. Molten perovskite would, however, still be much less dense than the molten iron alloy of the core. If the mantle were to melt near the core–mantle boundary, the liquid silicate could be gravitationally stable in place or could drain downwards and pond immediately above the core–mantle boundary. The work has been motivated by the 1995 discovery of ultralow velocity zones at the base of the Earth's mantle by Donald Helmberger, who is the Smits Family Professor of Geophysics and Planetary Science at Caltech, and Edward Garnero, who was then a Caltech graduate student and is now a professor at Arizona State University. These ultralow velocity zones (notably underneath the mid-Pacific region) appear to be 1-to-30-mile-thick layers of very low-seismic-velocity rock just above the interface between Earth's rocky mantle and the liquid core of the Earth, at a depth of 1,800 miles.

Helmberger and Garnero showed that, in this zone, seismic shear waves suffer a 30 percent decrease in velocity, whereas compressional wave speeds decrease by only 10 percent. This behavior is widely attributed to the presence of some molten material. Initially, many researchers assumed that this partially molten zone might represent atypical mantle compositions, such as a concentration of iron-bearing silicates or oxides with a lower melting point than ordinary mantle--about 7,200 oF at this pressure.

The new results, however, indicate that the melting temperature of normal mantle composition is low enough to explain melting in the ultralow velocity zones, and that this melt could coexist with residual magnesium silicate perovskite solids. Thus the new Caltech results indicate that no special composition is required to induce an ultralow velocity zone just above the core–mantle boundary or to allow it to remain there without draining away. The patchiness of the ultralow velocity zones suggests that Earth's lowermost mantle temperatures can be just hotter than, or just cooler than, the temperature that is required to initiate melting of normal mantle at a depth of 1,800 miles.

Writer: 
Robert Tindol
Writer: 

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