Caltech Faculty Elected to the American Academy of Arts and Sciences

The American Academy of Arts and Sciences has elected three Caltech faculty members as academy fellows. They are John F. Brady, Chevron Professor of Chemical Engineering and Mechanical Engineering and executive officer for chemical engineering; Kenneth A. Farley, W. M. Keck Foundation Professor of Geochemistry and chair of the Division of Geological and Planetary Sciences; and Fiona A. Harrison, Benjamin M. Rosen Professor of Physics.

"It is a privilege to honor these men and women for their extraordinary individual accomplishments," said Don Randel, chair of the academy's board of directors, of the 204 newly elected fellows and 16 foreign honorary members. "The knowledge and expertise of our members gives the academy a unique capacity—and responsibility—to provide practical policy solutions to the pressing challenges of the day. We look forward to engaging our new members in this work."

Brady works in the area of complex fluids and active matter that includes microstructural elements such as suspensions, colloidal dispersions, and self-propelling particles. Understanding these materials led Brady to develop a novel computational method called Stokesian dynamics. He won the 2012 Fluid Dynamics Prize from the American Physical Society and was elected to the National Academy of Engineering in 1999.

Most of Farley's research has focused on terrestrial geochemistry, but he is now increasingly interested in planetary science and especially exploration of the geochemistry, geology, and geomorphology of Mars. In his laboratory on the Caltech campus, Farley and his group measure noble gases such as helium and neon in rock and mineral samples. One major objective of this work is determining the ages and surface exposure history of Earth's geological features. Farley was recently involved in the first-ever experiments of this type carried out on the surface of Mars, via an instrument on board the Mars Science Laboratory's Curiosity rover. He has received the Day Medal of the Geological Society of America and the Macelwane Award of the American Geophysical Union, and was elected to the National Academy of Sciences in 2013.

Harrison specializes in observational and experimental high-energy astrophysics. She is the principal investigator for NASA's NuSTAR Explorer Mission and uses this satellite, along with other satellites and ground-based telescopes, to understand black holes, neutron stars, and supernova remnants. In her labs at Caltech, Harrison's group develops high-energy X-ray detectors and instrumentation for future space missions. She was elected to the American Physical Society in 2012 and won a NASA Outstanding Public Leadership Medal in 2013.

Also named to the academy this year is Katherine T. Faber, the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, who will be joining the Caltech faculty on July 1 as the Simon Ramo Professor of Materials Science. Faber's research focuses on understanding fracture and toughening of brittle materials such as those used for high-temperature coatings for power generation applications. She also works on the fabrication of ceramic materials with controlled porosity. She is cofounder and codirector of the Northwestern University-Art Institute of Chicago Center for Scientific Studies in the Arts (NU-ACCESS), which employs advanced materials science techniques for conservation science. Faber is a Distinguished Life Member of the American Ceramic Society (2013), and became a National Science Foundation American Competitiveness and Innovation Fellow in 2010.

The total number of Caltech faculty named to the academy is now 97.

The academy was founded in 1780 by John Adams, James Bowdoin, John Hancock, and other scholar-patriots "to cultivate every art and science which may tend to advance the interest, honor, dignity, and happiness of a free, independent, and virtuous people." The academy has elected as fellows and foreign honorary members the finest minds and most influential leaders from each generation, including George Washington and Ben Franklin in the 18th century, Daniel Webster and Ralph Waldo Emerson in the 19th, and Albert Einstein and Winston Churchill in the 20th. The current membership includes more than 250 Nobel laureates and 60 Pulitzer Prize winners.

A full list of new members is available on the academy website at https://www.amacad.org/content/members/members.aspx.

The academy will welcome this year's new fellows and foreign honorary members at its annual induction ceremony at the academy's headquarters in Cambridge, Massachusetts, on October 11, 2014.

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On the Front Lines of Sustainability

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On the Front Lines of Sustainability
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The chemical processes used to make products ranging from pharmaceuticals to perfumes can have a harmful impact on the environment. However, Caltech chemist and Nobel laureate Robert Grubbs has spent several decades developing catalysts—compounds that speed up a chemical reaction—that can make the synthesis of these products more efficient and ecologically friendly, ultimately reducing their environmental footprint. Similarly, chemist Brian Stoltz is developing new strategies for the synthesis of compounds needed in the chemical, polymer, and pharmaceutical industries. His new processes rely upon oxygen and organometallic catalysts—greener alternatives to the toxic metals that are normally used to drive such reactions.

Switching from paper files to cloud-based data storage might seem like an obvious choice for sustainability, but can we further reduce the environmental impact of storing data? The theoretical work of engineer and computer scientist Adam Wierman suggests that with the right algorithms, we can. Today, data centers—the physical storage facilities Wierman calls the "SUVs of the Internet"—account for more than 1.5 percent of U.S. electricity usage. And as more data goes online, that number is expected to grow. Wierman's work helps engineers design algorithms that will reroute data, with preference to centers that use renewable energy sources like wind and solar.

Energy from the sun—although free and abundant—cannot easily be stored for use on dreary days or transported to cloudy regions. Caltech engineer and materials scientist Sossina Haile hopes to remove that barrier with a specific type of solar reactor she has developed. The reactor is lined with ceramic cerium oxide; when this lining is heated with concentrated sunlight it releases oxygen, priming it to remove oxygen from water molecules or carbon dioxide on cooling, thus creating hydrogen fuel or "syngas"—a precursor to liquid hydrocarbon fuels. This conversion of the sun's light into storable fuel could allow solar-derived power to be available day and night.

Caltech student participants in the Department of Energy's biennial Solar Decathlon competition set out to prove that keeping a house lit up, cooled down, and comfortable for living is possible—even while off the grid. The Techers teamed up with students at the Southern California Institute of Architecture to create CHIP and DALE, their entries in the 2011 and 2013 competitions, respectively. These functional and stylish homes, powered solely by the sun, were engineered with innovative components including a rainwater collection system and moving room modules that optimize heating and cooling efficiency. 

Although many of us take the nearest bathroom for granted, working toilets require resources and infrastructure that may not be available in many parts of the world. Inspired by the "Reinventing the Toilet Challenge" issued by the Bill and Melinda Gates Foundation, environmental scientist and engineer Michael Hoffmann and his team applied his research in hydrogen evolution and water treatment to reengineer the toilet. The Caltech team's design—which won the challenge in 2012—can serve hundreds of people each day, treat its own wastewater, and generate electricity, providing a sustainable and low-cost solution to sanitation and hygiene challenges in the developing world. Prototypes are being tested in India and China for use in urban and remote environments in the developing world.  

Geophysicist Mark Simons studies the mechanics of the Earth—furthering our understanding of what causes our planet to deform over time. His research often involves using satellite data to observe the movement associated with seismic and volcanic activity, but Simons is also interested in changes going on in the icy parts of Earth's surface, especially the dynamics of glaciers. By flying high above Iceland's ice caps, Simons and his colleagues can track the glaciers' melt-and-freeze response in relation to seasonal and long-term variations in temperature—and their potential response to climate change.

The production of industrial nitrogen fertilizer results in 130 million tons of ammonia annually—while also requiring high heat, high pressure, and lots of energy. However, in a process called nitrogen fixation, soil microorganisms that live near the roots of certain plants can produce a similar amount of ammonia each year. The bugs use catalysts called nitrogenases to convert nitrogen from the air into ammonia at room temperature and atmospheric pressure. By mimicking the behavior of these microorganisms, Jonas Peters and his colleagues synthesized an iron-based catalyst that allows for nitrogen fixation under much milder conditions. The catalyst could one day lead to more environmentally friendly methods of ammonia production.

Traditionally, the photovoltaic cells in solar panels have been expensive and have had limited efficiency—making them a hard sell in the consumer market. Engineer and applied physicist Harry Atwater's work suggests that there is a thinner and more efficient alternative. Atwater, who is also the director of the Resnick Sustainability Institute, uses thin layers of semiconductors to create photovoltaics that absorb sunlight as efficiently as thick solar cells but can be produced with higher efficiency than conventional cells.

The generation of chemical fuels from sunlight could completely change the way we power the planet. Researchers in the laboratory of Caltech chemist Nate Lewis are working to develop different components of a fuel-producing device that could do just that called a photoelectrochemical cell. The cell would consist of an upper layer that could absorb sunlight, carbon dioxide, and water vapor, a middle layer consisting of light absorbers and catalysts that can produce fuels, which are then released through the device's bottom layer. When such a device is created, the Joint Center for Artificial Photosynthesis, of which Lewis is the scientific director, aims to ease the transfer of these technologies to the private sector. 

Clean energy from the wind is a promising alternative to fossil fuels, but giant pinwheel-like wind turbines that are common on many wind farms can create dangerous obstacles for birds as well as being an unpleasant addition to a landscape's aesthetic. To combat this problem, Caltech engineer and fluid-mechanics expert John Dabiri is testing a new design for wind turbines, which looks a bit like a spinning eggbeater emerging from the ground. By placing these columnar vertical wind turbines in a careful arrangement—an arrangement inspired by the vortex of water created behind a swimming fish—his smaller vertical turbines create just as much energy as the "pinwheels" and on a much smaller land footprint.

In the early 1990s, Caltech bioengineer Frances Arnold pioneered "directed evolution"—a new method of engineering custom-built enzymes, or activity-boosting proteins. The technique allows mutations to develop in the enzyme's genetic code; these mutations can give the enzyme properties that don't occur in nature but are beneficial for human applications. The selectively enhanced enzymes help microbes turn plant waste and fast-growing grasses into fuels like isobutanol, which could sustainably replace more than half of U.S. oil imports, Arnold says. She's also exploring ways the technique could help factories to make pharmaceuticals and other products in much cleaner and safer ways.

The combined research efforts of Richard Flagan, John Seinfeld, Mitchio Okumura, and Paul Wennberg aim to improve our understanding of various aspects of climate change. Chemical engineer Flagan is pioneering ways to measure the number and sizes of particles in the air down to that of large molecules. Seinfeld studies where particles in the air come from, how they are produced by airborne chemical reactions, and the effect they have on the world's climate. Chemical physicist Okumura studies the chemical reactions that occur when sunlight encounters air pollution and results in smog. Wennberg, an atmospheric chemist, studies the natural and human processes that affect smog formation, the health of the ozone layer, as well as the lifetime of greenhouse gases. Wennberg and his colleagues join a legacy of Caltech researchers who have improved air quality through key discoveries about pollution.

In the past, researchers have discovered materials that can act as reaction catalysts, driving sunlight to split water into hydrogen fuel and an oxygen byproduct. However, these wonder materials are often expensive and in short supply. The research of chemist Harry Gray, who leads the National Science Foundation-funded Center for Chemical Innovation in Solar Fuels program, tests combinations of Earth-abundant metals to search for an inexpensive catalyst that boosts the water-splitting reaction with the sun. Gray also coleads an outreach project in which students in the classroom can participate in the race for solar fuels by testing thousands of materials and reporting their results to Caltech researchers.

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Although Earth Week has officially come to a close, Caltech's commitment to sustainability continues. In this feature, you will meet some of the researchers at Caltech whose work is contributing to a greener planet and to the long-term improvement of our global environment.

Research for a Greener Future

Today's Earth Week feature highlights three cross-disciplinary research centers where Caltech scientists and engineers collaborate on projects that will have a positive impact on energy, the environment, and Earth's sustainable future.

The Ronald and Maxine Linde Center for Global Environmental Science

The Ronald and Maxine Linde Center for Global Environmental Science brings together researchers from chemistry, engineering, geology, environmental science, and other disciplines, with the goal of understanding the global environment and developing solutions to complex environmental problems. Linde Center scientists investigate how Earth's climate and its atmosphere, oceans, and biosphere have varied in the past and how they may change in the future. They are working on solutions to vexing challenges in climate change prognosis and mitigation, and to improve air and water quality.

The Linde Center was established thanks to support from Caltech alumnus and trustee Ronald Linde and his wife, Maxine. Led by acting director Paul Wennberg, Caltech's R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, the center is housed in the Linde + Robinson Laboratory, which was constructed in 1932 as an astronomy lab. The building recently underwent extensive renovations, to become one of the nation's most energy-efficient laboratories and the first existing historic building to earn the LEED Platinum rating. In 2012, Linde + Robinson was honored with a 2012 Los Angeles Conservancy Preservation Award for the "exceptionally creative and sensitive approach" of the renovation. The project, the conservancy noted, "not only preserved the building's unique historic features, it found brilliant new uses for them—particularly the solar telescope, built as the centerpiece of the original building but functionally obsolete. Now it tracks the sun and uses the light it captures for both illumination and exploration."

Resnick Sustainability Institute

Caltech's Resnick Sustainability Institute was created to fund and foster innovative Caltech-based sustainability and energy-science research collaborations with the potential to develop renewable-energy technologies that may one day help solve our global energy and climate challenges. The mission of the institute, which was founded with a generous gift from Stewart and Lynda Resnick, spans research, education, and communications. Current projects include research into energy generation, such as advanced photovoltaics, photoelectrochemical solar fuels, cellulosic biofuels, and wind-energy system design; energy conversion work on batteries and fuel cells; and research into technologies for energy efficiency and management, such as fuel-efficient vehicles, green chemical synthesis, and thermoelectric materials, as well as advanced research on electrical grid control and distribution.

This year the Resnick Sustainability Institute debuted two new initiatives: the Resonate Awards, which honor breakthrough achievements in energy science and sustainability, and a prize postdoctoral fellowship program. The Resonate Award winners will be announced at the Fortune Brainstorm GREEN conference in May 2014, and the inaugural class of postdoctoral fellows will be announced this fall.

Led by Harry Atwater, Caltech's Howard Hughes Professor and professor of applied physics and materials science, the institute is collocated with the Joint Center for Artificial Photosynthesis (JCAP) in the recently renovated Jorgensen Laboratory, which has been awarded LEED Platinum certification. In the renovation, Caltech and its partners were able to reuse or recycle over 90 percent of the materials removed from the original facility, a computer science building. Jorgensen has high-efficiency lighting and HVAC systems, a "living roof" composed of evergreen and drought-tolerant grasses, and water-saving plumbing and landscaping, among other green features.

The Joint Center for Artificial Photosynthesis (JCAP)

JCAP, established in 2010 as a U.S. Department of Energy (DOE) Energy Innovation Hub, is the nation's largest research effort focused on artificial photosynthesis. Led by researchers from Caltech (JCAP South, housed at the Jorgensen Laboratory) and partner Lawrence Berkeley National Laboratory (JCAP North), the center aims to create a low-cost artificial generator that uses sunlight, carbon dioxide, and water to make fuel from the sun 10 times more efficiently than current living crops. Once a prototype generator is developed, it will be handed off to private-sector companies to launch a new solar-fuels industry. Such a transformative breakthrough would reduce our country's dependence on oil and enhance energy security.

JCAP researchers include Scientific Director Nathan S. Lewis, Caltech's George L. Argyros Professor and professor of chemistry; Jonas Peters, the Bren Professor of Chemistry; William A. Goddard, Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics; and Harry Atwater.

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Spring Break in the Galápagos

 

As the final element of Evolution, Caltech's new Bi/Ge 105 course, a dozen students spent their spring break snorkeling with penguins and sharks, hiking a volcano, and otherwise taking in the natural laboratory for evolution that is the Galápagos Islands. The second-term course was created and is taught by Rob Phillips, the Fred and Nancy Morris Professor of Biophysics and Biology, and Victoria Orphan, professor of geobiology, and is designed to give students both a broad picture of evolution and a chance to make their own up-close-and-personal observations.

"Rob and I both feel very strongly that lab and field experiences are essential for the growth of the students as scientists," Orphan says. "Being at a place like Caltech that's small and where you have a lot of talented and enthusiastic students is the perfect environment to create those kinds of opportunities."

So with their trusty mascot—a bobblehead Darwin—in tow, the undergraduate students, their teaching assistant, and the two professors flew to Ecuador and then to the archipelago off the coast to spend a week living as field researchers and learning from Ecuadorian naturalist Ernesto Vaca and from their natural surroundings.

"The Galápagos are completely iconic," says Phillips. "Right before your eyes you can see the products of evolution, if you like. You can swim in the water with the flightless cormorants. The famed Darwin's finches are there. You can wonder what penguins are doing at the equator. What especially impresses me about seeing species such as the cormorants is the way they teach us about some of the most important evolutionary features seen on islands, such as dwarfism, gigantism, and flightlessness."

During the trip, each student made a presentation to the group, discussing a species or topic specific to the islands. One spoke about the Galápagos fur seal; another presented about the opuntia, a variety of cactus; another about marine iguanas. Senior bioengineering major Laura Santoso spoke about invasive species on the island. She says that although she had researched the subject extensively ahead of time, she saw things differently once she was actually in the Galápagos. For example, she had read that a particular invasive insect had been essentially eradicated from the islands, but while there she actually saw a number of the bugs. "It drove home how challenging it is to get rid of these invasive species," she says. "I find that observing the complexity of the issue in person and developing my own inferences makes it more meaningful."

Junior bioengineering major Aleena Patel agrees, adding that the trip suggested new ways to ask questions, to study, and to explore. "Being there in person piques curiosity in ways that other facets of learning don't," she says. "At times, there was so much to see it was almost overwhelming. But as scientists, we need that inspiration to ask questions and to be emotionally motivated."

That is just the kind of motivation Phillips and Orphan hoped to impart. "My view is that the most important point is to get students to plug into the idea of looking at nature and wondering, 'Why is that like that? How could science attack that question?' It's not so much a course about learning what is," says Phillips. "It's a course about saying, 'I wonder . . .'"

In addition to the Galápagos trip, the class took smaller day trips closer to campus during the winter term. On a special behind-the-scenes tour of the Page Museum at the La Brea Tar Pits, they were able to collect samples from one of the current excavations in order to study the microbes that make a living in such a unique environment. They also visited the Moore Lab of Zoology at Occidental College, where they used calipers to measure beaks in one of the world's largest collections of Mexican birds. The goal of the exercise was to get a feel for the kinds of measurements that biologists have conducted on finches on Daphne Major, one of the islands of the Galápagos, to study evolution in action.

The new class was supported by Caltech's Innovation in Education Fund, the Division of Geological and Planetary Sciences, and the Division of Biology and Biological Engineering through its William K. Bowes Jr. Leadership fund. As for why its focus was evolution, Orphan explains, "Evolution is of course integral to anyone doing biology. But when you start to look around, you find that evolution has its tendrils in a lot of different areas of research beyond biological research—even in computer science. We wanted to give the students that perspective, that even if they weren't going to be evolutionary biologists, per se, that the concepts and the way of perceiving the world in this class were going to help them."

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Spring Break in the Galapagos
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Caltech and JPL Experts Discuss Climate Change

In today's installment of our Earth Week 2014 features, Caltech experts share their knowledge about climate change.

 

Caltech professor Jess Adkins, a geochemist and paleoclimatologist, offers insight into global warming in the video "Is There Hope for Planet Earth?" produced by Green Wish, a nonprofit organization that funds green initiatives at the local level. In the video, Adkins talks about the history of climate change, where we are today, and what we can do to mitigate its impact in the future.

 

On April 22—Earth Day—the Caltech Center for Teaching, Learning, & Outreach sponsored an "Ask A Scientist" panel on the Caltech campus to answer questions about climate change and earth sciences. Panel members included David Crisp, JPL senior research scientist and principal investigator of the NASA Earth System Science Pathfinder Orbiting Carbon Observatory mission; Joshua Fisher, JPL research scientist, Water and Carbon Cycles Group; science communicator and education specialist Laura Tenenbaum; and Caltech alumnus Julius Su (BS '98 and '99, PhD '07), cofounder of Su-Kam Intelligent Education Systems (SKIES). Su used the SKIES iPad app, an online learning and information crowdsourcing tool, to relay online and audience questions to the panelists. The video is a recording of the live event.

 

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Experiences from two years of MOOCs at Caltech: A WEST Public Seminar

Spring Break in the Galápagos

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Spring Break in the Galápagos
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Credit: Kevin Yu

As the culminating event of the new Evolution course at Caltech, a dozen Techers, their TA, and two professors—Rob Phillips and Victoria Orphan—spent a week of spring break living as field researchers on the Galápagos Islands.

Credit: Kevin Yu

Ecuadorian naturalist Ernesto Vaca led the group in their studies of the natural world on the Galápagos. Here, at Playa Las Bachas on Santa Cruz Island, he is describing the molting of the Sally Lightfoot crab. The students kept scientific journals during the trip, writing down questions and observations along the way.

Credit: Laura Santoso

Marine iguanas are endemic to the Galápagos and are the only modern lizards that swim. They offer an excellent example of the way isolation on islands can lead to unique speciation.

Credit: Jeff Marlow

The group's home base for the trip was the research vessel Daphne, shown here anchored in James Bay.

Credit: Jeff Marlow

The group walks over solidified volcanic ash on Santiago Island.

Credit: Victoria Orphan

Flightlessness is one of the key evolutionary adaptations seen on islands. Here, a flightless cormorant is seen diving to gather food.

Credit: Kevin Yu

The landscape of Cerro Dragón (Dragon Hill) on Santa Cruz Island. This was one of many sites where the students were able to see the impact of invasive species such as goats.

Credit: Laura Santoso

The group's mascot—a Darwin bobblehead doll—posing in front of the third largest oceanic caldera in the world at the Sierra Negra volcano.

Credit: Pushpa Neppala

The Sierra Negra volcano on Santa Cruz Island.

Credit: Jeff Marlow

A young sea lion serves as an unexpected roadblock upon the group's arrival at North Seymour Island.

Credit: Ketaki Panse

A blue-footed booby perched atop a volcanic rock on North Seymour Island.

Credit: Laura Santoso

A land iguana with the island Daphne Minor in the background. One of the central questions about the iguanas on the Galápagos is how they arrived on the islands in the first place.

Credit: Aleena Patel

Part of the group explores a mangrove lagoon in Elizabeth Bay on Isla Isabela. According to Orphan, the mangroves are a nursery for many animals, and she encouraged the students to examine the mangrove roots closely. "Really looking closely, you start to see little transparent shrimp running up and down. There's a lot of richness that you can see even by just sitting and observing," she says.

Credit: Ketaki Panse

A beautiful sunset seen from the top of Bartholomew Island.

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As the final element of Evolution, Caltech's new Bi/Ge 105 course, a dozen students spent their spring break snorkeling with penguins and sharks, hiking a volcano, and otherwise taking in the natural laboratory for evolution that is the Galápagos Islands. The second-term course was created and is taught by Rob Phillips, the Fred and Nancy Morris Professor of Biophysics and Biology, and Victoria Orphan, professor of geobiology, and is designed to give students both a broad picture of evolution and a chance to make their own up-close-and-personal observations.

 

A New Tool for Unscrambling the Rock Record

Caltech-developed technique shows sulfur reducers were at work on the early Earth

A lot can happen to a rock over the course of two and a half billion years. It can get buried and heated; fluids remove some of its minerals and precipitate others; its chemistry changes. So if you want to use that rock to learn about the conditions on the early Earth, you have to do some geologic sleuthing: You have to figure out which parts of the rock are original and which came later. That is a tricky task, but now a team of Caltech researchers has developed and applied a unique technique that removes much of the guesswork.

"We want to know what Earth looked like when these ancient rocks were deposited. That's a giant challenge because a number of processes have scrambled and erased the original history," says Woodward Fischer, an assistant professor of geobiology at Caltech. "This is a first big effort to try to wrestle with that."

Fischer is the lead author on a paper that describes the new technique and findings in the current issue of the Proceedings of the National Academy of Sciences.

Using the new method, Fischer and his colleagues have examined ancient rocks dating to an age before the rise of oxygen. Today, water feeds the biosphere, providing the electrons needed to support life. But before the evolution of photosynthesis and the accumulation of oxygen in the atmosphere, elements such as iron and sulfur were the source of electrons. Researchers interested in the early Earth would like to determine how and when life figured out how to use these elements. The Caltech team has identified clear evidence that 2.5 billion years ago, sulfate-reducing microbes were already at work.

The researchers studied drill core samples collected in South Africa from sedimentary rocks that are slightly older than 2.5 billion years old. They focused on small features within the rocks, called nodules, made of the mineral pyrite. Also known as fool's gold, pyrite can be made in a number of ways, including as a product of respiratory metabolism: sulfate-reducing microbes reduce sulfate, which is present in seawater, yielding hydrogen sulfide, and when that hydrogen sulfide mingles with iron, pyrite is produced.

Today, sulfate-reducing microbes are often found in anoxic environments such as marine sediments where the oxygen has been consumed by aerobes but where there is still plenty of organic matter. It is logical, then, to suspect that these microbes would have been important players on the early Earth, when oxygen was scarce. Comparative genomics studies of sulfate reducers that are living today also suggest that these microbes should have been present 2.5 billion years ago. But this has been difficult to confirm in the rock record.

From current studies, scientists know that sulfate reducers metabolize the various stable isotopes of sulfur in a predictable way: producing light sulfur isotopes first before moving on to produce heavier ones as they run out of substrate. This provides a chemical thumbprint that researchers can look for as they examine pyrite nodules. The nodules crystalize early within the sediments, with the material at their core forming before the material at their edges. Therefore, to check whether sulfur-reducing organisms were active when a particular pyrite-containing rock formed, a geobiologist should be able to measure the ratios of a nodule's sulfur isotopes at different points—both near the core and closer to the edges—to see how those ratios changed as the nodule grew. But the nodules are only about a millimeter in diameter, so researchers have not been able to collect the fine-grained measurements they need in order to identify the isotopic thumbprint. Instead, they often grind up an entire rock sample, measure its isotopic composition, and then compare it to another rock.

Muddying the interpretation even more, these ancient rocks have all been deeply complicated by the wrinkles of time. All of the events and circumstances that have affected them since their deposition have left their chemical marks, by carving away old materials and precipitating new ones. A geologist can use some of the textures—the marks left in the fabric of the rock—to unravel some of a rock's history, but only if those textures clearly crosscut or overlap one another. Some of the visual cues can also be misleading. So it can be difficult just to identify which parts of a rock are original and can therefore provide insight about the early Earth.

Fischer's new technique changes all that. It allows researchers to untangle a rock's history and to then zoom in and measure the isotopic ratios at a number of points within a single pyrite nodule.

He begins as any geologist would—by looking at a sample with light and electron microscopy to identify the different textures within the rock. Doing that, he might identify a number of pyrite nodules that "look good"—that appear to date to the rock's original deposition.

He then uses a technique called scanning SQUID (superconducting quantum interference device) microscopy, which uses a quantum detector to produce a magnetic map of the sample at a very small scale. Pyrite itself is not magnetic, but when it is later altered, it forms a mineral called pyrrhotite, which is magnetic. Using scanning SQUID microscopy, Fischer has been able to rule out a number of nodules that had appeared to be original but that were in fact magnetic, meaning that they included pyrrhotite. In his South African samples, those deceptive features dated to a volcanic event 500 million years after the rocks were deposited, which sent chemistry-altering fluids through all the layers of sediment and rock that were present at the time.

"If you weren't using this technique, you'd miss the later alteration," Fischer says. "Those textures looked good. They would have passed naive tests."

The final step in the process is to measure the isotopic composition of the nodules using an analytical method called secondary ion mass spectrometry (SIMS). This specialized technique is used to measure the chemistry of thin films and solids with very fine spatial resolution. Materials scientists use it to analyze silicon wafers, for example, and planetary scientists have used it to study bits of rock from the moon. Fischer's group is one of the few in the world that uses it to study ancient rocks.

In SIMS, a sample under very strong vacuum is bombarded with a beam of cesium ions, which displaces ions from the surface of the sample. A mass spectrometer can measure those so-called secondary ions, providing a count of the sample's sulfur isotopes. Since the beam can be focused very precisely, the method allows researchers to sample many points within a single nodule, measuring a 13 x 5 grid within a millimeter, for example. The product is essentially a map of the sample's isotopic composition.

"It's one thing to say, 'Wow, rocks are really complicated. There's just going to be information lost.' It's another thing to be able to go back in and say, 'I know how to piece together the history of this rock and learn something about the early Earth that I didn't know previously.'"

Using the new technique, Fischer and his colleagues were able to identify which parts of their drill core samples were truly ancient and to then measure the sulfur isotopic composition of those nodules as they grew. And indeed they found the isotopic signature expected as a result of the activity of sulfur-reducing microbes.

"This work supports the hypothesis that microbial sulfate reduction was an important metabolism in organic-rich environments on the early Earth," Fischer says. "What's more, we now know how we can ask better questions about ancient rocks. That, for me, is incredibly exciting."

The paper is titled "SQUID-SIMS is a useful approach to uncover primary signals in the Archean sulfur cycle." Along with Fischer, additional Caltech coauthors are John Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry; Joseph Kirschvink, the Nico and Marilyn Van Wingen Professor of Geobiology; Jena Johnson, a graduate student in geobiology; and Yunbin Guan, director of the Center for Microanalysis. David Fike of Washington University in St. Louis and Timothy Raub of the University of St. Andrews are also coauthors. Scanning SQUID microscopy is a technique that was developed by researchers at Caltech and Vanderbilt University. The work was supported by the Agouron Institute and by a NASA Exobiology Award.

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Kimm Fesenmaier
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Spring Ombudsperson Training

Gravity Measurements Confirm Subsurface Ocean on Enceladus

In 2005, NASA's Cassini spacecraft sent pictures back to Earth depicting an icy Saturnian moon spewing water vapor and ice from fractures, known as "tiger stripes," in its frozen surface. It was big news that tiny Enceladus—a mere 500 kilometers in diameter—was such an active place. Since then, scientists have hypothesized that a large reservoir of water lies beneath that icy surface, possibly fueling the plumes. Now, using gravity measurements collected by Cassini, scientists have confirmed that Enceladus does in fact harbor a large subsurface ocean near its south pole, beneath those tiger stripes.

"For the first time, we have used a geophysical method to determine the internal structure of Enceladus, and the data suggest that indeed there is a large, possibly regional ocean about 50 kilometers below the surface of the south pole," says David Stevenson, the Marvin L. Goldberger Professor of Planetary Science at Caltech and an expert in studies of the interior of planetary bodies. "This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole."

Stevenson is one of the authors on a paper that describes the finding in the current issue of the journal Science. Luciano Iess of Sapienza University of Rome is the paper's lead author.

During three flybys of Enceladus, between April 2010 and May 2012, the scientists collected extremely precise measurements of Cassini's trajectory by tracking the spacecraft's microwave carrier signal with NASA's Deep Space Network. The gravitational tug of a planetary body, such as Enceladus, alters a spacecraft's flight path ever so slightly. By measuring the effect of such deflections on the frequency of Cassini's signal as the orbiter traveled past Enceladus, the scientists were able to learn about the moon's gravitational field. This, in turn, revealed details about the distribution of mass within the moon.

"This is really the only way to learn about internal structure from remote sensing," Stevenson says. In fact, more precise measurements would require the placement of seismometers on Enceladus's surface—something that is certainly not going to happen anytime soon.

The key feature in the gravity data was a so-called negative mass anomaly at Enceladus's south pole. Put simply, such an anomaly exists when there is less mass in a particular location than would be expected in the case of a uniform spherical body. Since there is a known depression in the surface of Enceladus's south pole, the scientists expected to find a negative mass anomaly. However, the anomaly was quite a bit smaller than would be predicted by the depression alone.

"So, you say, 'Aha! This is compensated at depth,'" Stevenson says.

Such compensation for mass is commonly found on planetary bodies, including on Earth. In some cases, the absence of material at the surface is compensated at depth by the presence of denser material. In other cases, the presence of extra material at the surface is compensated by the existence of less dense material at depth. In fact, when the first gravity measurements were made in India, people were struck by the fact that Mount Everest did not seem to produce much of an effect. Today we know that, like most mountains on Earth, Mount Everest is compensated by a low-density root that extends many tens of kilometers below the surface. In other words, the material protruding above the surface is compensated by a reduction of density at depth.

In the case of Enceladus, the opposite is true. The absence of material at the surface is compensated at depth by the presence of material that is denser than ice. "The only sensible candidate for that material is water," Stevenson says. "So if I have this depression at the south pole, and I have beneath the surface 50 kilometers down a layer of water or an ocean, that layer of water at depth is a positive mass anomaly. Together the two anomalies account for our measurements."

Although no one can say for certain whether the subsurface ocean supplies the water that has been seen spraying out of the tiger stripes on Enceladus's surface, the scientists say that it is possible. The suspicion is that the fractures—in some way that is not yet fully understood—connect down to a part of the moon that is being tidally heated by the globe's repeated flexing as it traces its eccentric orbit. "Presumably the tidal heating is also replenishing the ocean," Stevenson says, "so it is possible that some of that water is making its way up through the tiger stripes."

The paper is titled "The Gravity Field and Interior Structure of Enceladus." Additional coauthors are Marzia Parisi, Douglas Hemingway, Robert A. Jacobson, Jonathan I. Lunine, Francis Nimmo, John W. Armstrong, Sami W. Asmar, Maria Ducci, and Paolo Tortora. The work was supported by the Italian Space Agency and by NASA through the Cassini project. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. The Jet Propulsion Laboratory manages the mission for NASA's Science Mission Directorate.

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Kimm Fesenmaier
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