Where Is Solar Energy Headed?

In a new paper in ScienceNate Lewis, the George L. Argyros Professor of Chemistry at Caltech, reviews recent developments in solar-energy utilization and looks at some of the challenges and opportunities that lie ahead in the research and development of solar-electricity, solar-thermal, and solar-fuels technologies. Read the full paper.

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

Toward Liquid Fuels from Carbon Dioxide

In the quest for sustainable alternative energy and fuel sources, one viable solution may be the conversion of the greenhouse gas carbon dioxide (CO2) into liquid fuels.

Through photosynthesis, plants convert sunlight, water, and CO2 into sugars, multicarbon molecules that fuel cellular processes. CO2 is thus both the precursor to the fossil fuels that are central to modern life as well as the by-product of burning those fuels. The ability to generate synthetic liquid fuels from stable, oxygenated carbon precursors such as CO2 and carbon monoxide (CO) is reminiscent of photosynthesis in nature and is a transformation that is desirable in artificial systems. For about a century, a chemical method known as the Fischer-Tropsch process has been utilized to convert hydrogen gas (H2) and CO to liquid fuels. However, its mechanism is not well understood and, in contrast to photosynthesis, the process requires high pressures (from 1 to 100 times atmospheric pressure) and temperatures (100–300 degrees Celsius).

More recently, alternative conversion chemistries for the generation of liquid fuels from oxygenated carbon precursors have been reported. Using copper electrocatalysts, CO and CO2 can be converted to multicarbon products. The process proceeds under mild conditions, but how it takes place remains a mystery.

Now, Caltech chemistry professor Theo Agapie and his graduate student Joshua Buss have developed a model system to demonstrate what the initial steps of a process for the conversion of CO to hydrocarbons might look like.

The findings, published as an advanced online publication for the journal Nature on December 21, 2015 (and appearing in print on January 7, 2016), provide a foundation for the development of technologies that may one day help neutralize the negative effects of atmospheric accumulation of the greenhouse gas CO2 by converting it back into fuel. Although methods exist to transform CO2 into CO, a crucial next step, the deoxygenation of CO molecules and their coupling to form C–C bonds, is more difficult.

In their study, Agapie and Buss synthesized a new transition metal complex—a metal atom, in this case molybdenum, bound by one or more supporting molecules known as ligands—that can facilitate the activation and cleavage of a CO molecule. Incremental reduction of the molecule leads to substantial weakening of the C–O bonds of CO. Once weakened, the bond is broken entirely by introducing silyl electrophiles, a class of silicon-containing reagents that can be used as surrogates for protons.

This cleavage results in the formation of a terminal carbide—a single carbon atom bound to a metal center—that subsequently makes a bond with the second CO molecule coordinated to the metal. Although a carbide is commonly proposed as an intermediate in CO reductive coupling, this is the first direct demonstration of its role in this type of chemistry, the researchers say. Upon C–C bond formation, the metal center releases the C2 product. Overall, this process converts the two CO units to an ethynol derivative and proceeds easily even at temperatures lower than room temperature.

"To our knowledge, this is the first example of a well-defined reaction that can take two carbon monoxide molecules and convert them into a metal-free ethynol derivative, a molecule related to ethanol; the fact that we can release the C2 product from the metal is important," Agapie says.

While the generated ethynol derivative is not useful as a fuel, it represents a step toward being able to generate synthetic multicarbon fuels from carbon dioxide. The researchers are now applying the knowledge gained in this initial study to improve the process. "Ideally, our insight will facilitate the development of practical catalytic systems," Buss says.

The scientists are also working on a way to cleave the C–O bond using protons instead of silyl electrophiles. "Ultimately, we'd like to use protons from water and electron equivalents derived from sunlight," Agapie says. "But protons are very reactive, and right now we can't control that chemistry."

The research in the paper, "Four-electron deoxygenative reductive coupling of carbon monoxide at a single metal site," was funded by Caltech and the National Science Foundation.

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Caltech researchers gain insight into carbon monoxide coupling, one carbon atom at a time

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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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.

15 for 2015: The Year in Research News at Caltech

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Credit: K.Batygin/Caltech

New Research Suggests Solar System May Have Once Harbored Super-Earths

Thanks to recent surveys of exoplanets—planets in solar systems other than our own—we know that most planetary systems typically have one or more super-Earths (planets that are substantially more massive than Earth but less massive than Neptune) orbiting closer to their suns than Mercury does. In March, researchers showed that our own solar system may have once had these super-Earths, but they were destroyed by Jupiter's inward and outward migration through the solar system. This migration would have gravitationally flung small planetesimals through the solar system, setting off chains of collisions that would push any interior planets into the sun.
Credit: Lance Hayashida/Caltech and the Hoelz Laboratory/Caltech

Caltech Biochemists Shed Light on Cellular Mystery

The nuclear pore complex (NPC) is an intricate portal linking the cytoplasm of a cell to its nucleus. It is made up of many copies of about 34 different proteins. Around 2,000 NPCs are embedded in the nuclear envelope of a single human cell and each NPC shuttles hundreds of macromolecules of different shapes and sizes between the cytoplasm and nucleus. In February, Caltech biochemists determined the structure of a significant portion of the NPC called the outer rings; in August, the same group solved the structure of the pore's inner ring. Understanding the structure of the NPC could lead to new classes of cancer drugs as well as antiviral medicines.
Credit: iStockphoto

Research Suggests Brain's Melatonin May Trigger Sleep

For decades, supplemental melatonin has been sold over the counter as a sleep aid despite the absence of scientific evidence proving its effectiveness. Few studies have investigated melatonin produced naturally in the human body. This March, Caltech researchers studying zebrafish—animals that, like humans, are awake during the day and asleep at night—determined that the melatonin hormone does help the body fall asleep and stay asleep. Specifically, they found that zebrafish larvae that could not produce melatonin slept for only half as long as normal larvae.
Credit: Gregg Hallinan/Caltech

Advances in Radio Astronomy

In May, a new radio telescope array called the Owens Valley Long Wavelength Array (OV-LWA) saw its first light. Developed by a consortium led by Caltech, the OV-LWA has the ability to image simultaneously the entire sky at radio wavelengths with unmatched speed, helping astronomers to search for objects and phenomena that pulse, flicker, flare, or explode.

In July, Caltech researchers used both radio and optical telescopes to observe a brown dwarf located 20 light-years away and found that these so-called failed stars host powerful auroras near their magnetic poles.
Credit: Michael Abrams and Ty Basinger

Injured Jellyfish Seek to Regain Symmetry

Some kinds of animals can regrow lost limbs and body parts, but moon jellyfish have a different strategy. In June, Caltech researchers reported that the star-shaped eight-armed moon jellyfish rearranges itself when injured to maintain symmetry. It is hypothesized that the rearrangement helps to preserve the jellyfish's propulsion mechanism.
Credit: NASA/JPL-Caltech

Geologists Characterize Nepal Earthquake

In April, a magnitude 7.8 earthquake rocked Nepal. While the damage was extensive, it was not as severe as many geologists predicted. This year, a Caltech team of geologists used satellite radar imaging data and measurements from seismic instruments in Nepal to create models of fault rupture and ground movement. They found that the quake ruptured only a small fraction of the "locked" tectonic plate and that there is still the potential for the locked portion to produce a large earthquake.
Credit: Caltech/JPL

New Polymer Creates Safer Fuels

Plane crashes cause devastating damage, but this damage is often exacerbated by the highly explosive nature of jet fuel. This October, researchers at Caltech and JPL discovered a polymeric fuel additive that can reduce the intensity of postimpact explosions that occur during accidents and crashes. Preliminary results show that the additive can provide this benefit without adversely affecting fuel performance. The polymer works by inhibiting "misting"—the process that causes fuel to rapidly disperse and easily catch fire—under crash conditions.
Credit: Spencer Kellis/Caltech

Controlling a Robotic Arm with a Patient's Intentions

When you reach for a glass of water, you do not consciously think about moving your arm muscles or grasping with your fingers—you think about the goal of the movement. This May, by implanting neural prosthetic devices into the posterior parietal cortex (PCC)—the region of the brain that governs intentions for movement—rather than the motor cortex, which controls movement, Caltech researchers enabled a paralyzed patient to more smoothly and naturally control a prosthetic limb. In November, the researchers showed that there are individual neurons in the PPC that encode for entire hand shapes, such as those used for grasping or gesturing.

 

Caltech Scientists Develop Cool Process to Make Better Graphene

Graphene is an ultrastrong and conductive material made of a single layer of carbon atoms. While it is a promising material for scientific and engineering advances, manufacturing it on an industrially relevant scale has proven to be impractical, requiring temperatures of around 1,800 degrees Fahrenheit and long periods of time. A new technique invented at Caltech allows the speedy production of graphene—in just a few minutes—at room temperatures. The technique also produces graphene that is stronger, smoother, and more electrically conductive than normally produced synthetic graphene.
Credit: Rafael A. García (SAp CEA), Kyle Augustson (HAO), Jim Fuller (Caltech) & Gabriel Pérez (SMM, IAC), Photograph from AIA/SDO

Astronomers Peer Inside Stars, Finding Giant Magnets

Before this October, astronomers have only been able to study the magnetic fields of stars on the stellar surfaces. Now, using a technique called asteroseismology, scientists were able to probe the fusion-powered hearts of dozens of red giants (stars that are evolved versions of our sun) to calculate the magnetic field strengths inside those stars. They found that the internal magnetic fields of the red giants were as much as 10 million times stronger than Earth's magnetic field. Magnetic fields play a key role in the interior rotation rate of stars, which has a dramatic effect on how the stars evolve.
Credit: Chan Lei and Keith Schwab/Caltech

Seeing Quantum Motion

To the casual observer, an object at rest is just that—at rest, motionless. But on the subatomic scale, the object is most certainly in motion—quantum mechanical motion. Quantum motion, or noise, is ever-present in nature, and in August, Caltech researchers discovered how to observe and manipulate that motion in a small device. By creating what they called a "quantum squeezed state," they were able to periodically reduce the quantum fluctuations of the device. The ability to control quantum noise could one day be used to improve the precision of very sensitive measurements.
Credit: Ali Hajimiri/Caltech

New Camera Chip Provides Superfine 3-D Resolution

3-D printing can produce a wide array of objects in relatively little time, but first the printer needs to have a blueprint of what to print. The blueprints are provided by 3-D cameras, which scan objects and create models for the printer. Caltech researchers have now developed a 3-D camera that produces the highest depth-measurement accuracy of any similar device, allowing it to deliver replicas of an object to be 3-D printed within microns of similarity to the original object. In addition, the camera, known as a nanophotonic coherent imager, is inexpensive and small.
Credit: Image provided courtesy of Joint Center for Artificial Photosynthesis; artwork by Darius Siwek.

One Step Closer to Artificial Photosynthesis and 'Solar Fuels'

Plants are masters of photosynthesis—the process of turning carbon dioxide, sunlight, and water into oxygen and sugar. Inspired by this natural and energy-efficient process, Caltech researchers have created an "artificial leaf" that takes in CO2, sunlight, and water to produce hydrogen fuels. This solar-powered system, one researcher says, shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more.
Credit: Santiago Lombeyda and Robin Betz

Potassium Salt Outperforms Precious Metals As a Catalyst

Rare precious metals have been the standard catalyst for the formation of carbon-silicon bonds, a process crucial to the synthesis of a host of products from new medicines to advanced materials. However, they are expensive, inefficient, and produce toxic waste byproducts. This February, Caltech researchers discovered a much more sustainable catalyst in the form of a simple potassium salt that is one of the most abundant metals on Earth and thousands of times less expensive than other commonly used catalysts. In addition, the potassium salt is much more effective at running challenging chemical reactions than state-of-the-art precious metal complexes.
Credit: Qi Zhao/National University of Singapore

Probing the Mysterious Perceptual World of Autism

The way in which people with autism spectrum disorder (ASD) perceive the world is unique. It has been a long-standing belief that people with ASD often miss facial cues, contributing to impaired social interaction. In a study published in October, Caltech researchers showed 700 images to 39 subjects and found that people with ASD pay closer attention to simple edges and patterns in images than to the faces of people. The study also found that subjects were strongly attracted to the center of images—regardless of what was placed there—and to differences in color and contrast rather than facial features. These findings may help doctors diagnose and more effectively treat the different forms of autism.
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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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Popping Microbubbles Help Focus Light Inside the Body

A new technique developed at Caltech that uses gas-filled microbubbles for focusing light inside tissue could one day provide doctors with a minimally invasive way of destroying tumors with lasers, and lead to improved diagnostic medical imaging.

The primary challenge with focusing light inside the body is that biological tissue is optically opaque. Unlike transparent glass, the cells and proteins that make up tissue scatter and absorb light. "Our tissues behave very much like dense fog as far as light is concerned," says Changhuei Yang, professor of electrical engineering, bioengineering, and medical engineering. "Just like we cannot focus a car's headlight through fog, scientists have always had difficulty focusing light through tissues."

To get around this problem, Yang and his team turned to microbubbles, commonly used in medicine to enhance contrast in ultrasound imaging.

The gas-filled microbubbles are encapsulated by thin protein shells and have an acoustic refractive index—a property that affects how sound waves propagate through a medium—different from that of living tissue. As a result, they respond differently to sound waves. "You can use ultrasound to make microbubbles rapidly contract and expand, and this vibration helps distinguish them from surrounding tissue because it causes them to reflect sound waves more effectively than biological tissue," says Haowen Ruan, a postdoctoral scholar in Yang's lab.

In addition, the optical refractive index of microbubbles is not the same as that of biological tissue. The optical refractive index is a measure of how much light rays bend when transitioning from one medium (a liquid, for example) to another (a gas).

Yang, Ruan, and graduate student Mooseok Jang developed a novel technique called time-reversed ultrasound microbubble encoded (TRUME) optical focusing that utilizes the mismatch between the acoustic and optical refractive indexes of microbubbles and tissue to focus light inside the body. First, microbubbles injected into tissue are ruptured with ultrasound waves. By measuring the difference in light transmission before and after such an event, the Caltech researchers can modify the wavefront of a laser beam so that it is focuses on the original locations of the microbubbles. The result, Yang explains, "is as if you're searching for someone in a dark field, and suddenly the person lets off a flare. For a brief moment, the person is illuminated and you can home in on their location."

In a new study, published online November 24, 2015, in the journal Nature Communications, the team showed that their TRUME technique could be used as an effective "guidestar" to focus laser beams on specific locations in a biological tissue. A single, well-placed microbubble was enough to successfully focus the laser; multiple popping bubbles located within the general vicinity of a target functioned as a map for the light.

"Each popping event serves as a road map for the twisting light trajectories through the tissue," Yang says. "We can use that road map to shape light in such a way that it will converge where the bubbles burst."

If TRUME is shown to work effectively inside living tissue—without, for example, any negative effects from the bursting microbubbles—it could enable a range of research and medical applications. For example, by combining the microbubbles with an antibody probe engineered to seek out biomarkers associated with cancer, doctors could target and then destroy tumors deep inside the body or detect malignant growths much sooner.

"Ultrasound and X-ray techniques can only detect cancer after it forms a mass," Yang says. "But with optical focusing, you could catch cancerous cells while they are undergoing biochemical changes but before they undergo morphological changes."

The technique could take the place of other of diagnostic screening methods. For instance, it could be used to measure the concentrations of a protein called bilirubin in infants to determine their risk for jaundice. "Currently, this procedure requires a blood draw, but with TRUME, we could shine a light into an infant's body and look for the unique absorption signature of the bilirubin molecule," Ruan says.

In combination with existing techniques that allow scientists to activate individual neurons in lab animals using light, TRUME could help neuroscientists better understand how the brain works. "Currently, neuroscientists are confined to superficial layers of the brain," Yang says. "But our method of optical focusing could allow for a minimally invasive way of probing deeper regions of the brain."

The paper is entitled "Optical focusing inside scattering media with time-reversed ultrasound microbubble encoded (TRUME) light." Support for the research was provided by the National Institutes of Health, the National Institutes of Health BRAIN Initiative, and a GIST-Caltech Collaborative Research Proposal.

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Microbubbles Help Focus Light Inside the Body
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A new technique could one day provide doctors with a minimally invasive way of destroying tumors with lasers, and lead to improved diagnostic medical imaging.

Simulation Shows Key to Building Powerful Magnetic Fields

When certain massive stars use up all of their fuel and collapse onto their cores, explosions 10 to 100 times brighter than the average supernova occur. Exactly how this happens is not well understood. Astrophysicists from Caltech, UC Berkeley, the Albert Einstein Institute, and the Perimeter Institute for Theoretical Physics have used the National Science Foundation's Blue Waters supercomputer to perform three-dimensional computer simulations to fill in an important missing piece of our understanding of what drives these blasts.  

The researchers report their findings online on November 30 in advance of publication in the journal Nature. The lead author on the paper is Philipp Mösta, who started the work while a postdoctoral scholar at Caltech and is now a NASA Einstein Fellow at UC Berkeley.

The extremely bright explosions come in two varieties—some are a type of energetic supernovae called hypernovae, while others are gamma-ray bursts (GRBs). Both are driven by focused jets formed in some collapsed stellar cores. In the case of GRBs, the jets themselves escape the star at close to the speed of light and emit strong beams of extremely energetic light called gamma rays. The necessary ingredients to create such jets are rapid rotation and a magnetic field that is a million billion times stronger than Earth's own magnetic field.

In the past, scientists have simulated the evolution of massive stars from their collapse to the production of these jet-driven explosions by factoring unrealistically large magnetic fields into their models—without explaining how they could be generated in the first place. But how could magnetic fields strong enough to power the explosions exist in nature?

"That's what we were trying to understand with this study," says Luke Roberts, a NASA Einstein Fellow at Caltech and a coauthor on the paper. "How can you start with the magnetic field you might expect in a massive star that is about to collapse—or at least an initial magnetic field that is much weaker than the field required to power these explosions—and build it up to the strength that you need to collimate a jet and drive a jet-driven supernova?"

For more than 20 years, theory has suggested that the magnetic field of the inner- most regions of a massive star that has collapsed, also known as a proto-neutron star, could be amplified by an instability in the flow of its plasma if the core is rapidly rotating, causing its outer edge to rotate faster than its center. However, no previous models could prove this process could strengthen a magnetic field to the extent needed to collimate a jet, largely because these simulations lacked the resolution to resolve where the flow becomes unstable.


Magnetic field amplification in hypernovae
Supercomputer visualization of the toroidal magnetic field in a collapsed, massive star, showing how in a span of 10 milliseconds the rapid differential rotation revs up the stars magnetic field to a million billion times that of our sun (yellow is positive, light blue is negative). Red and blue represent weaker positive and negative magnetic fields, respectively. Credit: Philipp Mösta

Mösta and his colleagues developed a simulation of a rapidly rotating collapsed stellar core and scaled it so that it could run on the Blue Waters supercomputer, a powerful supercomputer funded by the NSF located at the National Center for Supercomputing Applications at the University of Illinois. Blue Waters is known for its ability to provide sustained high-performance computing for problems that produce large amounts of information. The team's highest-resolution simulation took 18 days of around-the-clock computing by about 130,000 computer processors to simulate just 10 milliseconds of the core's evolution.

In the end, the researchers were able to simulate the so-called magnetorotational instability responsible for the amplification of the magnetic field. They saw—as theory predicted—that the instability creates small patches of an intense magnetic field distributed in a chaotic way throughout the core of the collapsed star.

"Surprisingly, we found that a dynamo process connects these patches to create a larger, ordered structure," explains David Radice, a Walter Burke Fellow at Caltech and a coauthor on the paper. An early type of electrical generator known as a dynamo produced a current by rotating electromagnetic coils within a magnetic field. Similarly, astrophysical dynamos generate currents when hydromagnetic fluids in stellar cores rotate under the influence of their magnetic fields. Those currents can then amplify the magnetic fields.

"We find that this process is able to create large-scale fields—the kind you would need to power jets," says Radice.

The researchers also note that the magnetic fields they created in their simulations are similar in strength to those seen in magnetars—neutron stars (a type of stellar remnant) with extremely strong magnetic fields. "It takes thousands or millions of years for a proto-neutron star to become a neutron star, and we have not yet simulated that. But if you could transport this thing thousands or millions of years forward in time, you would have a strong enough magnetic field to explain magnetar field strengths," says Roberts. "This might explain some fraction of magnetars or a particular class of very bright supernovae that are thought to be powered by a spinning magnetar at their center."

Additional authors on the paper, "A large-scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae," are Christian Ott, professor of theoretical astrophysics; Erik Schnetter of the Perimeter Institute for Theoretical Physics, the University of Guelph, and Louisiana State University; and Roland Haas of the Max Planck Institute for Gravitational Physics in Potsdam-Golm, Germany. The work was partially supported by the Sherman Fairchild Foundation, by grants from the NSF, by NASA Einstein Fellowships, and by an award from the Natural Sciences and Engineering Research Council of Canada.

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Kimm Fesenmaier
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How to Power Jet-Driven Supernovae
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New simulations show how a dynamo in collapsed massive stars can build the strong magnetic fields needed to power extremely energetic blasts.

Tracking Down the "Missing" Carbon From the Martian Atmosphere

Caltech and JPL scientists suggest the fingerprints of early photochemistry provide a solution to the long-standing mystery

Mars is blanketed by a thin, mostly carbon dioxide atmosphere—one that is far too thin to prevent large amounts of water on the surface of the planet from subliming or evaporating. But many researchers have suggested that the planet was once shrouded in an atmosphere many times thicker than Earth's. For decades that left the question, "Where did all the carbon go?"

Now a team of scientists from Caltech and JPL thinks they have a possible answer. The researchers suggest that 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. They have identified a photochemical process that could have helped such an early atmosphere evolve into the current thin one without creating the problem of "missing" carbon and in a way that is consistent with existing carbon isotopic measurements.

The scientists describe their findings in a paper that appears in the November 24 issue of the journal Nature Communications.

"With this new mechanism, everything that we know about the martian atmosphere can now be pieced together into a consistent picture of its evolution," says Renyu Hu, a postdoctoral scholar at JPL, a visitor in planetary science at Caltech, and lead author on the paper.

When considering how the early martian atmosphere might have transitioned to its current state, there are two possible mechanisms for the removal of excess carbon dioxide (CO2). Either the CO2 was incorporated into minerals in rocks called carbonates or it was lost to space.

A separate recent study coauthored by Bethany Ehlmann, assistant professor of planetary science and a research scientist at JPL, used data from several Mars-orbiting satellites to inventory carbonate rocks, showing that there are not enough carbonates in the upper kilometer of crust to contain the missing carbon from a very thick early atmosphere that might have existed about 3.8 billion years ago.

To study the escape-to-space scenario, scientists examine the ratio of carbon-12 and carbon-13, two stable isotopes of the element carbon that have the same number of protons in their nuclei but different numbers of neutrons, and thus different masses. Because various processes can change the relative amounts of those two isotopes in the atmosphere, "we can use these measurements of the ratio at different points in time as a fingerprint to infer exactly what happened to the martian atmosphere in the past," says Hu.

To establish a starting point, the researchers used measurements of the carbon isotope ratio in martian meteorites that contain gases that originated deep in the planet's mantle. Because atmospheres are produced by outgassing of the mantle through volcanic activity, these measurements provide insight into the isotopic ratio of the original martian atmosphere.

The scientists then compared those values to isotopic measurements of the current martian atmosphere recently collected by NASA's Curiosity rover. Those measurements show the atmosphere to be unusually enriched in carbon-13.

Previously, researchers thought the main way that martian carbon would be ejected into space was through a process called sputtering, which involves interactions between the solar wind and the upper atmosphere. Sputtering causes some particles—slightly more of the lighter carbon-12 than the heavier carbon-13—to escape entirely from Mars, but this effect is small. So there had to be some other process at work.

That is where the new mechanism comes in. In the study, the researchers describe a process that begins with a particle of ultraviolet light from the sun striking a molecule of CO2 in the upper atmosphere. That molecule absorbs the photon's energy and divides into carbon monoxide (CO) and oxygen. Then another ultraviolet particle hits the CO, causing it to dissociate into atomic carbon (C) and oxygen. Some carbon atoms produced in this way have enough energy to escape the atmosphere, and the new study shows that carbon-12 is far more likely to escape than carbon-13.

Modeling the long-term effects of this ultraviolet photodissociation mechanism coupled with volcanic gas release, loss via sputtering, and loss to carbonate rock formation, the researchers found that it was very efficient in terms of enriching carbon-13 in the atmosphere. Using the isotopic constraints, they were then able to calculate that the atmosphere 3.8 billion years ago might have had the pressure of Earth's or less under most scenarios.

"The efficiency of this new mechanism shows that there is in fact no discrepancy between Curiosity's measurements of the modern enriched value for carbon in the atmosphere and the amount of carbonate rock found on the surface of Mars," says Ehlmann, also a coauthor on the new study. "With this mechanism, we can describe an evolutionary scenario for Mars that makes sense of the apparent carbon budget, with no missing processes or reservoirs."

The authors conclude their work by pointing out several tests and refinements for the model. For example, future data from the ongoing Mars Atmosphere and Volatile EvolutioN (MAVEN) mission could provide the isotope fractionation of presently ongoing atmospheric loss to space and improve the extrapolation to early Mars.

Hu emphasizes that the work is an excellent example of multidisciplinary effort. On the one hand, he says, the team looked at the atmospheric chemistry—the isotopic signature, the escape processes, and the enrichment mechanism. On the other, they used geological evidence and remote sensing of the martian surface. "By putting these together, we were able to come up with a summary of evolutionary scenarios," says Hu. "I feel that Caltech/JPL is a unique place where we have the multidisciplinary capability and experience to make this happen."

Additional authors on the paper, "Tracing the Fate of Carbon and the Atmospheric Evolution of Mars," are Yuk Yung, the Smits Family Professor of Planetary Science at Caltech and a senior research scientist at JPL, and David Kass, a research scientist at JPL. The work was supported by funding from NASA.

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Kimm Fesenmaier
Home Page Title: 
The Atmospheric Evolution of Mars
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Caltech and JPL researchers identify a process that helps explain how a moderately dense early martian atmosphere could have evolved into the current thin one.

Neurons Encoding Hand Shapes Identified in Human Brain

Neural prosthetic devices, which include small electrode arrays implanted in the brain, can allow paralyzed patients to control the movement of a robotic limb, whether that limb is attached to the individual or not. In May 2015, researchers at Caltech, USC, and Rancho Los Amigos National Rehabilitation Center reported the first successful clinical trial of such an implant in a part of the brain that translates intention—the goal to be accomplished through a movement (for example, "I want to reach to the water bottle for a drink")—into the smooth and fluid motions of a robotic limb. Now, the researchers, led by Richard Andersen, the James G. Boswell Professor of Neuroscience, report that individual neurons in that brain region, known as the posterior parietal cortex (PPC), encode entire hand shapes which can be used for grasping—as when shaking someone's hand—and hand shapes not directly related to grasping, such as the gestures people make when speaking.

Most neuroprostheses are implanted in the motor cortex, the part of the brain controlling limb motion. But the movement of these robotic arms are jerky, probably due to the complicated mechanics for controlling muscle movement. Having eliminated that problem by implanting the device in the PPC, the brain region that encodes the intent, led Andersen and colleagues to further investigate the role specific neurons play in this part of the brain.

The research appears in the November 18 issue of the Journal of Neuroscience.

"The human hand has the ability to do numerous complex operations beyond just grasping," says Christian Klaes, a postdoctoral fellow at Caltech and first author of the paper. "We gesture when we speak, we manipulate objects, we use sign language to communicate with the hearing impaired. Tetraplegic patients rate hand and arm function to be of the highest importance to have better control over their environment. So our ultimate goal is to improve the range of neuroprostheses using control signals from the PPC.

"The more precisely we can identify individual neurons involved with hand movements, the better the capability these robotic devices will provide. Ultimately, we hope to mimic in a robotic hand the same freedom of movement of the human hand."

In the study, the researchers used the rock-paper-scissors game and a variation, rock-paper-scissors-lizard-Spock. The game, says Andersen, is "perfect" for this kind of research. "The addition of a lizard, depicted as a cartoon image of a lizard, and Spock—a picture of Leonard Nimoy in character—was to increase the repertoire of possible hand shapes available to our tetraplegic participant, Erik G. Sorto, whose limbs are completely paralyzed. We assigned a pinch gesture for the lizard and a spherical shape for Mr. Spock."

The game was played in two phases, first rock-paper-scissors and then the expanded game with the lizard and Spock. In the task, Sorto was briefly shown an object on a screen that corresponded to one of the hand shapes—for example, a picture of a rock or Mr. Spock. The image was followed by a blank screen, and then text appeared instructing Sorto to imagine making the corresponding hand shape with his right hand—a fist for the rock, an open hand for paper, a scissors gesture for scissors, a pinch for the lizard, and a spherical shape (loosely analogous to the Vulcan salute) for Spock—and to say which visual image he had seen, as the neuroprosthetic device recorded the activity of neurons in the PPC.

The researchers were able to identify single neurons in the PPC that fired when Sorto was presented with an image of an object to be grasped—a rock, say—and identified a nearly completely separate class of neurons that responded when Sorto engaged in motor imagery (the mental planning and imagined execution of a movement without the subject actually trying to move the limb).

"We found two mostly separate populations of neurons in the PPC that show either visual responses or motor-imagery responses during the task, the former when Erik identified a cue and the latter when he imagined performing a corresponding hand shape," says Andersen.

The researchers discovered that individual neurons in the PPC also responded to hand shapes that did not directly correspond to a grasp-related visual stimulus. The paper shape can be related to the initial opening of the hand to grasp a paper, and the rock closing the hand to grasp a rock—and in fact, these imagined hand shapes were used by Sorto to imagine opening a robotic hand by imagining paper and closing the robotic hand around an object by imagining rock. However, scissors, lizard, and Spock call for imagining hand gestures that are more abstract and iconic than those needed to grasp the visual objects, and suggests, says Andersen, that this area of the brain may also be involved in more general hand gestures, such as ones we use when talking, or for sign language.

The results of the trial were published in a paper titled, "Hand Shape Representations in the Human Posterior Parietal Cortex." In addition to Andersen and Klaes, other authors on the study are Spencer Kellis, Tyson Aflalo, and Kelsie Pejsa from Caltech; Brian Lee, Christi Heck, and Charles Liu from USC; and Kathleen Shanfield, Stephanie Hayes-Jackson, and Mindy Aisen from Rancho Los Amigos National Rehabilitation Center.

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Hand Shape Neurons Identified in Brain
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The neurons, identified through brain studies using the game rock-paper- scissors-lizard-Spock, may lead to improved prosthetic devices.

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