Geophysicist David G. Harkrider Dies

David G. Harkrider, professor of geophysics, emeritus, at Caltech and an expert in seismological wave propagation, passed away on Tuesday, February 16, 2016. He was 84.

Born on September 25, 1931 in Houston, Texas, Harkrider received his bachelor's and master's degrees from Rice University in 1953 and 1957, respectively. He earned a doctorate in geophysics from Caltech in 1963 and remained as a research fellow until 1965, when he joined the Department of Geology at Brown University as an assistant professor. Harkrider returned to Caltech as an associate professor in 1970, becoming a professor in 1979 and a professor emeritus in 1995. From 1977–1979 he was the associate director of Caltech's Seismological Laboratory.

He was elected a Fellow of the American Geophysical Union in 1979. From 1982–1988 he was on the board of the Seismological Society of America, serving as vice president in 1987 and as president in 1988. He was elected a Fellow of the Royal Astronomical Society in 2009.

Harkrider investigated diverse topics within the field of geophysics. Early in his career he studied the theory of air-wave trains—the oscillations of the atmosphere in regions experiencing strong shocks, such as a meteor or a nuclear explosion. At Caltech, he collaborated with Professor of Geophysics Donald Helmberger and then-Professor of Geophysics Charles Archambeau (now a retired professor of physics at the University of Colorado) to analyze and interpret the propagation of seismic waves in the earth. Harkrider's work was focused on the analysis of the propagation of surface waves—a type of seismic wave that travels through the crust—and their coupling with air waves and tsunami waves. He led the development of a digital computing system to recognize the seismic signals from earthquakes, rapidly determine their locations, and distinguish the signatures of earthquakes from those of nuclear explosions. Harkrider's modeling efforts played a key role in ensuring the compliance of the Nuclear Test Ban Treaty with the Soviet Union.

Together with Don Helmberger, Harkrider taught a year-long course in seismology. Numerous graduates of the program went on to pursue PhDs in the field and are now professors.

"David took surface-wave theory from the rather primitive state in which it existed in the late 1960's to the point where a generalized seismic source could be embedded at any depth in an arbitrarily layered media and the response, including synthetic seismograms, calculated," says Professor of Geophysics Robert W. Clayton. "He was pioneer in the application of computer techniques to seismological problems."

"Although Harkrider's most widely known published works are on the excitation and propagation of surface waves in multi-layered media, his handwritten class notes on propagation of acoustic-gravity waves were very useful for seismologists who ventured into the field of acoustic-gravity waves from seismic waves," says Hiroo Kanamori, the John E. and Hazel S. Smits Professor of Geophysics, emeritus, and Harkrider's longtime colleague. "When I became interested in acoustic-gravity waves after the large eruption of Mount Pinatubo in 1991, I studied his class notes in great detail. These notes are so unique that I am sure that many students must have benefitted a great deal from them. I wish they were published."

Harkrider, his family notes, was a "kind and generous man with a sharply irreverent wit who loved his family, his friends, his cats and dogs, and his research," and enjoyed football, golf, old musicals, Mexican food, martinis, and "Tabasco on everything." He is survived by his wife, Sara Brydges; daughter, Claire Harkrider Topp; son, John D. Harkrider; and by four grandchildren.

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David G. Harkrider, professor of geophysics, emeritus, passed away on Thursday, February 18, 2016.

A New Twist on the History of Life

The idea that the wholesale relocation of Earth's continents 520 million years ago, also known as "true polar wander," coincided with a burst of animal speciation in the fossil record dates back almost 20 years to an original hypothesis by Joseph Kirschvink (BS, MS '75), Caltech's Nico and Marilyn Van Wingen Professor of Geobiology, and his colleagues. For more than a century, paleontologists including Charles Darwin have debated whether the so-called Cambrian explosion—a rapid period of species diversification that began around 542 million years ago—was the equivalent of an evolutionary "big bang" of biological innovation, or just an artifact of the incomplete fossil record.

In a new study published in the December issue of the American Journal of Science, a team of researchers including Kirschvink and Ross Mitchell, a postdoctoral scholar in geology at Caltech, describes a new model showing that during the proposed Cambrian true polar wander event, most continents would have moved toward the equator instead of toward the poles.

"It's long been observed that biological diversity is highest in the tropics, where nutrients and energy tend to be abundant," says Kirschvink. "One of the side effects of true polar wander is that sea level rises near the equator but falls near the poles, so the equatorial migration of most Cambrian land masses would have enhanced diversification into previously lower-diversity environments."

Using a model they developed, the team simulated the pattern of continental migration during the Cambrian and found that their results can explain the distribution of Cambrian fossils.

"Our model provides an explanation for why the fossil record looks the way it does, with many Cambrian fossil groups on some continents but few on others," says study coauthor Tim Raub (BS, MS '02), a lecturer at the University of St. Andrews in Scotland.

"The same sea-level rise which flooded those continents that shifted to the tropics and opened new ecological niches for faster speciation also led to more fossil preservation," Mitchell says. "In contrast, the few areas that shifted to the poles became less biologically diverse and also lost rock volume to erosion following sea-level drops due to true polar wander."

The scientists say their new findings could help resolve the debate started so long ago by Darwin. If their theory is correct, the Cambrian explosion is both a true and dramatic pulse of biological innovation and an expression of preferentially preserved shells on selectively submerged continental margins capable of containing fossils.

Funding for the study was provided by the National Science Foundation.

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Monday, February 29, 2016

Modeling molecules at the microscale

Geobiologist Honored by National Academy of Sciences

Dianne Newman, professor of biology and geobiology at Caltech and an investigator with the Howard Hughes Medical Institute, has been awarded the National Academy of Sciences (NAS) Award in Molecular Biology for her "discovery of microbial mechanisms underlying geologic processes." The award citation recognizes her for "launching the field of molecular geomicrobiology" and fostering greater awareness of the important roles microorganisms have played and continue to play in how Earth evolved.

"Trust me, no one was more shocked than I was by this news," says Newman. "It really honors the many the exceptional people who have come through my lab over the years, as well as the geobiology field more broadly. Geobiology is a venerable old field, which offers many fascinating and important problems that would benefit from the attention of individuals trained in mechanistic research. Hopefully this award will encourage more young people from molecular and cellular biology to enter the field."

Newman's research focuses on the relationship between microorganisms and geologic processes. She has demonstrated that some bacteria in iron-rich environments, such soils and sediments, can utilize extracellular iron as a dump site for excess electrons by generating extracellular electron shuttles, including a class of metabolites formerly considered to be redox-active antibiotics. Newman has also made contributions to our understanding of other microbial metabolic processes of geological significance, including how microbes respire using arsenate instead of oxygen, and how they perform photosynthesis using iron rather than water. In addition, she and her coworkers have studied the mechanisms by which certain microbes make stromatolites and magnetosomes, two types of structures that leave biosignatures in ancient rocks. Perhaps most importantly, her team has demonstrated the power of applying genetic analysis to diverse organisms from iron-rich environments, paving the way for others to do the same.

Newman is now hoping to bring tools commonly used in geochemistry to facilitate environmentally-informed studies of pathogens in chronic infections. For example, in collaboration with Caltech professor of geobiology Alex Sessions and researchers at Children's Hospital Los Angeles, Newman's group has characterized the composition and growth rate of pathogens in mucus collecting in the lungs of individuals with cystic fibrosis. Using this information, her lab is designing new experiments to reveal the survival mechanisms utilized by microorganisms—such as Pseudomonas aeruginosa, an opportunistic bacterium that colonizes the lungs of these patients—in this environment.

The NAS Award in Molecular Biology was first given in 1962. It is presented with a medal and a $25,000 prize. Newman will receive the award on May 1, 2016, during the National Academy of Sciences' annual meeting in Washington, D.C.

Previous recipients of the award include David Baltimore, Caltech President Emeritus and the Robert Andrews Millikan Professor of Biology.

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Dianne Newman has been awarded the National Academy of Sciences Award in Molecular Biology.

White House Puts Spotlight on Earthquake Early-Warning System

Since the late 1970s, Caltech seismologist Tom Heaton, professor of engineering seismology, has been working to develop earthquake early-warning (EEW) systems—networks of ground-based sensors that can send data to users when the earth begins to tremble nearby, giving them seconds to potentially minutes to prepare before the shaking reaches them. In fact, Heaton wrote the first paper published on the concept in 1985. EEW systems have been implemented in countries like Japan, Mexico, and Turkey. However, the Unites States has been slow to regard EEW systems as a priority for the West Coast.

But on February 2, 2016, the White House held the Earthquake Resilience Summit, signaling a new focus on earthquake safety and EEW systems. There, stakeholders—including Caltech's Heaton and Egill Hauksson, research professor in geophysics; and U.S. Geological Survey (USGS) seismologist Lucy Jones, a visiting associate in geophysics at Caltech and seismic risk advisor to the mayor of Los Angeles—discussed the need for earthquake early warning and explored steps that can be taken to make such systems a reality. 

At the summit, the Gordon and Betty Moore Foundation announced $3.6 million in grants to advance a West Coast EEW system called ShakeAlert, which received an initial $6 million in funding from foundation in 2011. The new grants will go to researchers working on the system at Caltech, the USGS, UC Berkeley, and the University of Washington.

"We have been successfully operating a demonstration system for several years, and we know that it works for the events that have happened in the test period," says Heaton. "However, there is still significant development that is required to ensure that the system will work reliably in very large earthquakes similar to the great 1906 San Francisco earthquake. This new funding allows us to accelerate the rate at which we develop this critical system."

In addition, the Obama Administration outlined new federal commitments to support greater earthquake safety including an executive order to ensure that new construction of federal buildings is up to code and that federal assets are available for recovery efforts after a large earthquake.

The commitments follow a December announcement from Congressman Adam Schiff (D-Burbank) that Congress has included $8.2 million in the fiscal year 2016 funding bill specifically designated for a West Coast earthquake early warning system.

"By increasing the funding for the West Coast earthquake early-warning system, Congress is sending a message to the Western states that it supports this life-saving system. But the federal government cannot do it alone and will need local stakeholders, both public and private, to get behind the effort with their own resources," said Schiff, in a press release. "The early warning system will give us critical time for trains to be slowed and surgeries to be stopped before shaking hits—saving lives and protecting infrastructure. This early warning system is an investment we need to make now, not after the 'big one' hits."

ShakeAlert utilizes a network of seismometers—instruments that measure ground motion—widely scattered across the Western states. In California, that network of sensors is called the California Integrated Seismic Network (CISN) and is made up of computerized seismometers that send ground-motion data back to research centers like the Seismological Laboratory at Caltech.

Here's how the current ShakeAlert works: a user display opens in a pop-up window on a recipient's computer as soon as a significant earthquake occurs in California. The screen lists the quake's estimated location and magnitude based on the sensor data received to that point, along with an estimate of how much time will pass before the shaking reaches the user's location. The program also gives an approximation of how intense that shaking will be. Since ShakeAlert uses information from a seismic event in progress, people living near the epicenter do not get much—if any—warning, but those farther away could have seconds or even tens of seconds' notice.

The goal is an improved version of ShakeAlert that will eventually give schools, utilities, industries, and the general public a heads-up in the event of a major temblor.

Read more about how ShakeAlert works and about Caltech's development of EEW systems in a feature that ran in the Summer 2013 issue of E&S magazine called Can We Predict Earthquakes?

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On February 2, 2016, the White House held the Earthquake Resilience Summit, signaling a new focus on earthquake safety and EEW systems.

A New Power Source for Earth's Dynamo

The earth's global magnetic field plays a vital role in our everyday lives, shielding us from harmful solar radiation. The magnetic field, which has existed for billions of years, is caused by a dynamo—or generator—within the mostly molten iron in the earth's interior; this liquid iron churns in a process called convection. But convection does not happen on its own. It needs a driving force—a power source. Now, graduate student Joseph O'Rourke and David Stevenson, Caltech's Marvin L. Goldberger Professor of Planetary Science, have proposed a new mechanism that can power this convection in the earth's interior for all of the earth's history.

A paper detailing the findings appears in the January 21 issue of Nature.

Convection can be seen in such everyday phenomena as a pot of boiling water. Heat at the bottom of the pot causes pockets of fluid to become less dense than the surrounding fluid, and thus to rise. When they reach the surface, the pockets of fluid cool and sink again. This same process occurs in the 1,400-mile-thick layer of molten metal that makes up the outer core.

The earth consists mostly of the mantle (solid material made of oxides and silicate in which magnesium is prominent) and the core (mainly iron). These two regions are usually thought of as completely separated; that is, the mantle materials do not dissolve in the core materials. They do not mix at the atomic level, much as water does not usually mix with oil. The core has a solid inner part that has been slowly growing throughout the earth's history, as liquid iron in the planet's interior solidifies. The outer, liquid part of the core is a layer of molten iron mixed with other elements, including silicon, oxygen, nickel, and a small amount of magnesium. Stevenson and O'Rourke propose that the transfer of the element magnesium in the form of mantle minerals from the outer core to the base of the mantle is the mechanism that powers convection.

Magneisum is a major element in the mantle, but it has low solubility in the iron core except at very high temperatures—above 7,200 degrees Fahrenheit. As the earth's core cools, magnesium oxides and magnesium silicates crystallize from the metallic, liquid outer core, much as sugar that has been dissolved in hot water will precipitate as sugar crystals when the water cools. Because these crystals are less dense than iron, they rise to the base of the mantle. The heavier liquid metal left behind then sinks, and this motion, Stevenson argues, may be the mechanism that has sustained convection for over three billion years—the mechanism that in turn powers the global magnetic field.

"Precipitation of magnesium-bearing minerals from the outer core is 10 times more effective at driving convection than growth of the inner core," O'Rourke says. "Such minerals are very buoyant and the resulting fluid motions can transport heat effectively. The core only needs to precipitate upwards a layer of magnesium minerals 10 kilometers thick—which seems like a lot, but it's not much on the scale of the inner and outer cores—in order to drive the outer core's convection."

Previous models assumed that the steady cooling of iron in the inner core would release heat that could power convection. But most measurements and theory in the past few years for the thermal conductivity of iron—the property that determines how efficiently heat can flow through a metal—indicates that the metal can easily transfer heat without undergoing motion. "Heating up iron at the bottom of the outer core will not cause it to rise up buoyantly—it's just going to dissipate the heat to its surroundings," O'Rourke says.

"Dave had the idea of a magnesium-powered dynamo for a while, but there was supposed to be no magnesium in Earth's core," O'Rourke says. "Now, models of planetary formation in the early solar system are showing that Earth underwent frequent impacts with giant planetary bodies. If these violent, energetic events occurred, Earth would have been experiencing much higher temperatures during its formation than previously thought—temperatures that would have been high enough to allow some magnesium to mix into liquid metallic iron."

These models made it possible to pursue the idea that the dynamo may be powered by the precipitation of magnesium-bearing minerals. O'Rourke calculated that the amounts of magnesium that would have dissolved in the core during Earth's hot early stages would have caused other changes in the composition of the mantle that are consistent with other models and measurements. He also calculated that the precipitation of these magnesium minerals would have enough energy to power the dynamo for four billion years.

Experimental verification of the amount of magnesium that can go into the core is still sparse, O'Rourke and Stevenson say. "Further applications of our proposed mechanism include Venus—where there is no magnetic field—and the abundant exoplanets that are more massive than the Earth but may have similar chemical compositions," Stevenson says.

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Friday, January 29, 2016
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Caltech Researchers Find Evidence of a Real Ninth Planet

Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.

The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling and computer simulations but have not yet observed the object directly.

"This would be a real ninth planet," says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting."

Brown notes that the putative ninth planet—at 5,000 times the mass of Pluto—is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighborhood of the solar system. In fact, it dominates a region larger than any of the other known planets—a fact that Brown says makes it "the most planet-y of the planets in the whole solar system."

Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.

"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," says Batygin, an assistant professor of planetary science. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."

The road to the theoretical discovery was not straightforward. In 2014, a former postdoc of Brown's, Chad Trujillo, and his colleague Scott Sheppard published a paper noting that 13 of the most distant objects in the Kuiper Belt are similar with respect to an obscure orbital feature. To explain that similarity, they suggested the possible presence of a small planet. Brown thought the planet solution was unlikely, but his interest was piqued.

He took the problem down the hall to Batygin, and the two started what became a year-and-a-half-long collaboration to investigate the distant objects. As an observer and a theorist, respectively, the researchers approached the work from very different perspectives—Brown as someone who looks at the sky and tries to anchor everything in the context of what can be seen, and Batygin as someone who puts himself within the context of dynamics, considering how things might work from a physics standpoint. Those differences allowed the researchers to challenge each other's ideas and to consider new possibilities. "I would bring in some of these observational aspects; he would come back with arguments from theory, and we would push each other. I don't think the discovery would have happened without that back and forth," says Brown. " It was perhaps the most fun year of working on a problem in the solar system that I've ever had."

Fairly quickly Batygin and Brown realized that the six most distant objects from Trujillo and Sheppard's original collection all follow elliptical orbits that point in the same direction in physical space. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates.

"It's almost like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place," says Brown. The odds of having that happen are something like 1 in 100, he says. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent. "Basically it shouldn't happen randomly," Brown says. "So we thought something else must be shaping these orbits."

The first possibility they investigated was that perhaps there are enough distant Kuiper Belt objects—some of which have not yet been discovered—to exert the gravity needed to keep that subpopulation clustered together. The researchers quickly ruled this out when it turned out that such a scenario would require the Kuiper Belt to have about 100 times the mass it has today.

That left them with the idea of a planet. Their first instinct was to run simulations involving a planet in a distant orbit that encircled the orbits of the six Kuiper Belt objects, acting like a giant lasso to wrangle them into their alignment. Batygin says that almost works but does not provide the observed eccentricities precisely. "Close, but no cigar," he says.

Then, effectively by accident, Batygin and Brown noticed that if they ran their simulations with a massive planet in an anti-aligned orbit—an orbit in which the planet's closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets—the distant Kuiper Belt objects in the simulation assumed the alignment that is actually observed.

"Your natural response is 'This orbital geometry can't be right. This can't be stable over the long term because, after all, this would cause the planet and these objects to meet and eventually collide,'" says Batygin. But through a mechanism known as mean-motion resonance, the anti-aligned orbit of the ninth planet actually prevents the Kuiper Belt objects from colliding with it and keeps them aligned. As orbiting objects approach each other they exchange energy. So, for example, for every four orbits Planet Nine makes, a distant Kuiper Belt object might complete nine orbits. They never collide. Instead, like a parent maintaining the arc of a child on a swing with periodic pushes, Planet Nine nudges the orbits of distant Kuiper Belt objects such that their configuration with relation to the planet is preserved.

"Still, I was very skeptical," says Batygin. "I had never seen anything like this in celestial mechanics."

But little by little, as the researchers investigated additional features and consequences of the model, they became persuaded. "A good theory should not only explain things that you set out to explain. It should hopefully explain things that you didn't set out to explain and make predictions that are testable," says Batygin.

And indeed Planet Nine's existence helps explain more than just the alignment of the distant Kuiper Belt objects. It also provides an explanation for the mysterious orbits that two of them trace. The first of those objects, dubbed Sedna, was discovered by Brown in 2003. Unlike standard-variety Kuiper Belt objects, which get gravitationally "kicked out" by Neptune and then return back to it, Sedna never gets very close to Neptune. A second object like Sedna, known as 2012 VP113, was announced by Trujillo and Sheppard in 2014. Batygin and Brown found that the presence of Planet Nine in its proposed orbit naturally produces Sedna-like objects by taking a standard Kuiper Belt object and slowly pulling it away into an orbit less connected to Neptune.


A predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (blue) fit this prediction precisely.
Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]

But the real kicker for the researchers was the fact that their simulations also predicted that there would be objects in the Kuiper Belt on orbits inclined perpendicularly to the plane of the planets. Batygin kept finding evidence for these in his simulations and took them to Brown. "Suddenly I realized there are objects like that," recalls Brown. In the last three years, observers have identified four objects tracing orbits roughly along one perpendicular line from Neptune and one object along another. "We plotted up the positions of those objects and their orbits, and they matched the simulations exactly," says Brown. "When we found that, my jaw sort of hit the floor."

"When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome—you kill two birds with one stone," says Batygin. "But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn't realize was sitting in a nearby tree."

Where did Planet Nine come from and how did it end up in the outer solar system? Scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets—Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. "But there is no reason that there could not have been five cores, rather than four," says Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit.

Batygin and Brown continue to refine their simulations and learn more about the planet's orbit and its influence on the distant solar system. Meanwhile, Brown and other colleagues have begun searching the skies for Planet Nine. Only the planet's rough orbit is known, not the precise location of the planet on that elliptical path. If the planet happens to be close to its perihelion, Brown says, astronomers should be able to spot it in images captured by previous surveys. If it is in the most distant part of its orbit, the world's largest telescopes—such as the twin 10-meter telescopes at the W. M. Keck Observatory and the Subaru Telescope, all on Mauna Kea in Hawaii—will be needed to see it. If, however, Planet Nine is now located anywhere in between, many telescopes have a shot at finding it.

"I would love to find it," says Brown. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."

In terms of understanding more about the solar system's context in the rest of the universe, Batygin says that in a couple of ways, this ninth planet that seems like such an oddball to us would actually make our solar system more similar to the other planetary systems that astronomers are finding around other stars. First, most of the planets around other sunlike stars have no single orbital range—that is, some orbit extremely close to their host stars while others follow exceptionally distant orbits. Second, the most common planets around other stars range between 1 and 10 Earth-masses.

"One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune," says Batygin. "Until now, we've thought that the solar system was lacking in this most common type of planet. Maybe we're more normal after all."

Brown, well known for the significant role he played in the demotion of Pluto from a planet to a dwarf planet adds, "All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found," he says. "Now we can go and find this planet and make the solar system have nine planets once again."

The paper is titled "Evidence for a Distant Giant Planet in the Solar System."

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The giant planet, nicknamed Planet Nine, traces a bizarre, highly elongated orbit in the outer solar system.

15 for 2015: The Year in Research News at Caltech

The year 2015 proved to be another groundbreaking year for research at Caltech. From seeing quantum motion, to reconfiguring jellyfish limbs, to measuring stellar magnetic fields, researchers continued to ask and answer the deepest scientific questions.

In case you missed any of them, here are 15 stories highlighting a few of the discoveries, methods, and technologies that came to life at Caltech in 2015.

 

 

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Here are 15 stories highlighting a few of the discoveries, methods, and technologies that came to life at Caltech in 2015.

Developing a Picture of the Earth's Mantle

Deep inside the earth, seismic observations reveal that three distinct structures make up the boundary between the earth's metallic core and overlying silicate mantle at a depth of about 2,900 kilometers—an area whose composition is key to understanding the evolution and dynamics of our planet. These structures include remnants of subducted plates that originated near the earth's surface, ultralow-velocity zones believed to be enriched in iron, and large dense provinces of unknown composition and mineralogy. A team led by Caltech's Jennifer Jackson, professor of mineral physics has new evidence for the origin of these features that occur at the core-mantle boundary.

"We have discovered that bridgmanite, the most abundant mineral on our planet, is a reasonable candidate for the material that makes up these dense provinces that occupy about 20 percent of the core-mantle boundary surface, and rise up to a depth of about 1,500 kilometers. Integrated by volume that's about the size of our moon!" says Jackson, coauthor of a study that outlines these findings and appears online in the Journal of Geophysical Research: Solid Earth. "This finding represents a breakthrough because although bridgmanite is the earth's most abundant mineral, we only recently have had the ability to precisely measure samples of it in an environment similar to what we think the materials are experiencing inside the earth."

Previously, says Jackson, it was not clear whether bridgmanite, a perovskite structured form of (Mg,Fe)SiO3, could explain seismic observations and geodynamic modeling efforts of these large dense provinces. She and her team show that indeed they do, but these structures need to be propped up by external forces, such as the pinching action provided by cold and dense subducted slabs at the base of the mantle.

Jackson, along with then Caltech graduate student Aaron Wolf (PhD '13), now a research scientist at the University of Michigan at Ann Arbor, and researchers from Argonne National Laboratory, came to these conclusions by taking precise X-ray measurements of synthetic bridgmanite samples compressed by diamond anvil cells to over 1 million times the earth's atmospheric pressure and heated to thousands of degrees Celsius.

The measurements were done utilizing two different beamlines at the Advanced Photon Source of Argonne National Laboratory in Illinois, where the team used powerful X-rays to measure the state of bridgmanite under the physical conditions of the earth's lower mantle to learn more about its stiffness and density under such conditions. The density controls the buoyancy—whether or not these bridgmanite provinces will lie flat on the core-mantle boundary or rise up. This information allowed the researchers to compare the results to seismic observations of the core-mantle boundary region.

"With these new measurements of bridgmanite at deep-mantle conditions, we show that these provinces are very likely to be dense and iron-rich, helping them to remain stable over geologic time," says Wolf.

Using a technique known as synchrotron Mössbauer spectroscopy, the team also measured the behavior of iron in the crystal structure of bridgmanite, and found that iron-bearing bridgmanite remained stable at extreme temperatures (more than 2,000 degrees Celsius) and pressure (up to 130 gigapascals). There had been some reports that iron-bearing bridgmanite breaks down under extreme conditions, but the team found no evidence for any breakdown or reactions.

"This is the first study to combine high-accuracy density and stiffness measurements with Mössbauer spectroscopy, allowing us to pinpoint iron's behavior within bridgmanite," says Wolf. "Our results also show that these provinces cannot possibly contain a large complement of radiogenic elements, placing strong constraints on their origin. If present, these radiogenic elements would have rapidly heated and destabilized the piles, contradicting many previous simulations that indicate that they are likely hundreds of millions of years old."

In addition, the experiments suggest that the rest of the lower mantle is not 100 percent bridgmanite as had been previously suggested. "We've shown that other phases, or minerals, must be present in the mantle to satisfy average geophysical observations," says Jackson. "Until we made these measurements, the thermal properties were not known with enough precision and accuracy to uniquely constrain the mineralogy."

"There is still a lot of work to be done, such as identifying the dynamics of subducting slabs, which we believe plays a role in providing an external force to shape these large bridgmanite provinces," she says. "We know that the earth did not start out this way. The provinces had to evolve within the global system, and we think these findings may help large-scale geodynamic modeling that involves tectonic plate reconstructions."

The results of the study were published in a paper titled "The thermal equation of state of (Mg,Fe)SiO3bridgmanite (perovskite) and implications for lower mantle structures." In addition to Jackson and Wolf, other authors on the study are Przemeslaw Dera and Vitali B. Prakapenka from the Center for Advanced Radiation Sources at Argonne National Laboratory. Support for this research was provided by the National Science Foundation, the Turner Postdoctoral Fellowship at the University of Michigan, and the California Institute of Technology.

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Developing a Picture of the Earth's Mantle
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A team led by Caltech's Jennifer Jackson, professor of mineral physics has new evidence for the origin of features that occur at the core-mantle boundary.

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