Natural Quasicrystals May Be the Result of Collisions Between Objects in the Asteroid Belt

Naturally formed quasicrystals—solids with orderly atomic arrangements with symmetries impossible for conventional crystals—are among the rarest structures on Earth. Only two have ever been found.

A team led by Paul Asimow (MS '93, PhD '97), professor of geology and geochemistry at Caltech, may have uncovered one of the reasons for that scarcity, demonstrating in laboratory experiments that quasicrystals could arise from collisions between rocky bodies in the asteroid belt with unusual chemical compositions.

A paper on their findings was published on June 13 in the advance online edition of the Proceedings of the National Academy of Sciences.

At an atomic level, crystals are both ordered and periodic, meaning that they have a well-defined geometric structure composed of atomic clusters that repeat like building blocks with equal spacings along the repeat directions. Over one hundred years ago, it was shown the crystal can only exhibit one of four types of rotational symmetry: two-fold, three-fold, four-fold, or six-fold.

The number refers to how many times an object will look exactly the same within a full 360-degree rotation about an axis. For example, an object with two-fold symmetry appears the same twice, or every 180 degrees; an object with three-fold symmetry appears the same three times, or every 120 degrees; and an object with four-fold symmetry appears the same four times, or every 90 degrees.

Prior to 1984, it was believed that it would be impossible for a solid to grow with any other type of symmetry, and no examples of materials with other symmetries had ever been discovered in nature or grown in a lab. In that year, however, Princeton physicist Paul Steinhardt (BS '74) and his student Dov Levine (now at the Technion – Israel Institute of Technology) theorized a set of conditions under which other types of symmetry could potentially exist and Dan Shechtman of the Israel Institute of Technology and collaborators published a paper announcing the synthesis in the laboratory of a material with a five-fold rotational symmetry.

The atomic arrangements of these materials were ordered so that, like crystals, X-rays and electrons passing through them form a pattern of sharp spots. However, whereas the spots obtained from a crystal are equally spaced and only form patterns with symmetries from the restricted list, the spots obtained from a quasicrystal form a fractal snowflake pattern that include forbidden symmetries, such as five-fold. Steinhardt and Levine dubbed them "quasiperiodic crystals" or "quasicrystals" for short.

Over the next few decades, researchers figured out how to manufacture more than 100 different varieties of quasicrystals by melting and homogenizing certain elements and then cooling them at very specific rates in the lab. Still, though, no naturally existing quasicrystals were known. Indeed, researchers suspected their formation would be impossible. That is because most lab-grown quasicrystals were metastable, meaning that the same combination of elements could arrange themselves into a crystalline structure using less energy.

Everything changed in the late 2000s, when Steinhardt and colleague Luca Bindi from the Museum of Natural History at the University of Florence (currently in the Faculty of the Department of Earth Sciences of the same University) found a tiny grain of an aluminum, copper, and iron mineral that exhibited five-fold symmetry. The grain came from a small sample of the Khatyrka meteorite, an extraterrestrial object known only from a few pieces found in Russia's Koryak Mountains. Steinhardt and his collaborators found a second natural quasicrystal from the same meteorite in 2015, confirming that the natural existence of quasicrystals was possible, just very rare.

A microscopic analysis of the meteorite indicated that it had undergone a major shock at some point in its lifetime before crashing to Earth – likely from a collision with another rocky body in space. Such collisions are common in the asteroid belt and release high amounts of energy.

Asimow and colleagues hypothesized that the energy released by the shock could have caused the quasicrystal's formation by triggering a rapid cycle of compression, heating, decompression, and cooling.

To test the hypothesis, Asimow simulated the collision between two asteroids in his lab. He took thin slices of minerals found in the Khatyrka meteorite and sandwiched them together in a sample case that resembles a steel hockey puck. He then screwed the "puck" to the muzzle of a four-meter-long, 20-mm-bore single-stage propellant gun, and blasted it with a projectile at nearly one kilometer per second, about equal to the speed of the fastest rifle-fired bullets.

It is important to note that those minerals included a sample of a metallic copper-aluminum alloy, which has only been found in nature in the Khatyrka meteorite.

After the sample was shocked with the propellant gun, it was sawed open, polished, and examined. The impact smashed the sandwiched elements together and, in several spots, created microscopic quasicrystals, according to X-ray and electron diffraction studies at Caltech, Princeton, and Florence.

Armed with this experimental evidence, Asimow says he is confident that shocks are the source of naturally formed quasicrystals. "We know that the Khatyrka meteorite was shocked. And now we know that when you shock the starting materials that were available in that meteorite, you get a quasicrystal."

Sarah Stewart (PhD '02)—a planetary collision expert from the University of California, Davis, and reviewer of the PNAS paper—admits she was surprised by the findings. "If you had called me before the study and asked if this would work I would have said 'no way.' The astounding thing is that they did it so easily," she says. "Nature is crazy."

Asimow acknowledges that the experiments leave many questions unanswered. For example, it is unclear at what point the quasicrystal formed during the shock's pressure and temperature cycle. A bigger mystery, Asimow says, is the origin of the copper-aluminum alloy in the meteorite, which has never been seen elsewhere in nature.

Next, Asimow plans to shock various combinations of minerals to see what key ingredients are necessary for natural quasicrystal formation.

These results are published in a paper titled "Shock synthesis of quasicrystals with implications for their origin in asteroid collisions." In addition to Asimow, Steinhardt, and Bindi, other coauthors on the paper are Chi Ma, director of analytical facilities in the Geological and Planetary Sciences division at Caltech; Lincoln Hollister (PhD '66) and Chaney Lin from Princeton University; and Oliver Tschauner from the University of Nevada, Las Vegas. Their work was supported by the National Science Foundation (NSF), the University of Florence, and the NSF-Materials Research Science & Engineering Centers Program through New York University and the Princeton Center for Complex Materials.

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Prebiotic Molecule Detected in Interstellar Cloud

Chiral molecules—compounds that come in otherwise identical mirror image variations, like a pair of human hands—are crucial to life as we know it. Living things are selective about which "handedness" of a molecule they use or produce. For example, all living things exclusively use the right-handed form of the sugar ribose (the backbone of DNA), and grapes exclusively synthesize the left-handed form of the molecule tartaric acid. While homochirality—the use of only one handedness of any given molecule—is evolutionarily advantageous, it is unknown how life chose the molecular handedness seen across the biosphere.

Now, Caltech researchers have detected, for the first time, a chiral molecule outside of our solar system, bringing them one step closer to understanding one of the most puzzling mysteries of the early origins of life.

A paper about the work appears in the June 17 issue of the journal Science.

The different forms, or enantiomers, of a chiral molecule have the same physical properties, such as the temperatures at which they boil and melt. Chemical interactions with other chiral species, however, can vary greatly between enantiomers. For instance, many chiral pharmaceutical chemicals are only effective in one handedness; in the other, they can be toxic.

"Homochirality is one of the most interesting properties of life as we know it," says Geoffrey Blake (PhD '86), professor of cosmochemistry and planetary sciences and professor of chemistry. "How did it come to be that all living things use one enantiomer of a particular amino acid, for example, over another? If we could run the tape of life again, would the same enantiomers be selected through a deterministic process, or is a random choice made that depends on a tiny imbalance of one handedness over the other? If there is life elsewhere in the universe, based on the biochemistry we know, will it use the same enantiomers?"

To help answer these questions, Blake and his colleagues at the National Radio Astronomy Observatory (NRAO) searched one particular molecular cloud, called Sagittarius B2(N), for chiral molecules. The team used the Green Bank Telescope Prebiotic Interstellar Molecular Survey (PRIMOS) of Sagittarius B2(N). The PRIMOS project, led by co-senior author Anthony Remijan of the NRAO, examines the spectrum of Sagittarius B2(N) across a broad range of radio frequencies. Every gas-phase molecule can only tumble in specific ways depending on its size and shape, giving it a unique rotational spectrum —like a fingerprint—that makes it readily identifiable in the PRIMOS survey.

The PRIMOS data revealed the signature of a chiral molecule called propylene oxide (CH3CHOCH2); follow-up studies with the Parkes radio telescope in Australia confirmed the findings. "It's the first molecule detected in space that has the property of chirality, making it a pioneering leap forward in our understanding of how prebiotic molecules are made in space and the effects they may have on the origins of life," says Brandon Carroll, co-first author on the paper and a graduate student in Blake's group. "While the technique we used does not tell us about the abundance of each enantiomer, we expect this work to enable future observations that will let us understand a great deal more about chiral molecules, the origins of homochirality, and the origins of life in general."

Propylene oxide is a useful molecule to study because it is relatively small compared to biomolecules such as amino acids; larger molecules are more difficult to detect with radio astronomy, but have been seen in meteorites and comets formed at the birth of the solar system. Though propylene oxide is not utilized in living organisms, its presence in space is a signpost for the existence of other chiral molecules.

"The next step is to detect an excess of one enantiomer over the other," says Brett McGuire (PhD '15), an NRAO Jansky Fellow and former member of the Blake lab, who shares first authorship on the work with Carroll. "By discovering a chiral molecule in space, we finally have a way to study where and how these molecules form before they find their way into meteorites and comets, and to understand the role they play in the origins of homochirality and life."

"The past few years of exoplanetary science have told us there are millions of solar system-like environments in our galaxy alone, and thousands of nearby young stars around which planets are being born," says Blake. "The detection of propylene oxide, and the future projects it enables, lets us begin to ask the question—does interstellar prebiotic chemistry plant the primordial cosmic seeds that determine the handedness of life?"

Additional coauthors on the paper, titled "Discovery of the interstellar chiral molecule propylene oxide (CH3CHOCH2)," include Caltech graduate student Ian Finneran, a member of the Blake group. The work is supported by the National Science Foundation Astronomy and Astrophysics and Graduate Fellowship grant programs and the NASA Astrobiology Institute through the Goddard Team and the Early Career Collaboration award program.

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The Next Big Thing

To get a glimpse into the future, what better place is there to look than the minds of those about to become Caltech's newest alumni? After all, our 2016 graduates have been at the forefront of research in vastly different fields for the past few years. Their unique perspectives have informed their ideas of the future, and their work will reach far beyond the confines of a lab.

With that in mind, in the Summer 2016 issue of E&S magazine, we talked to a handful of undergraduate and graduate students prior to commencement to find out what they think will be the next big thing in science and engineering and how their plans after graduation reflect those ideas.

 

I believe that the future of science, technology, engineering, and mathematics (STEM) will place a greater emphasis on implementation and impact of research. While rapid economic growth and globalization have introduced numerous difficult challenges, society has acquired powerful new tools and technology to develop and implement solutions for these issues.

I will be working as a management consultant after graduating to expose myself to business and strategy. That way, I can perhaps one day help new discoveries and ideas produce a tangible impact on people's lives."

Aditya Bhagavathi
BS in Computer Science

 

I believe the future of planetary and space exploration will follow two paths—one, the search for life beyond Earth within the solar system, and two, the characterization of exoplanets.

For the solar system, the initial survey of its major worlds was just completed with the New Horizons flyby of Pluto, and therefore a new focus will likely emerge. That initial survey has revealed several worlds to be potentially habitable, including Mars, Europa, and Enceladus, with the former two already targets for future missions. These new missions will not only reveal more about these worlds but also force us to reevaluate what life is, how it arises, and how it endures.

For exoplanets, the diversity of worlds is immense. From giant planets that orbit their host stars in less than a day to habitable planets with permanent daysides and nightsides, exoplanets offer a tremendous opportunity to understand the planets in our own solar system. With the rapid development of technologies, instruments, and observing techniques, the flood of data regarding exoplanets will only continue. I plan to be among the scientists who will analyze this data and combine their results with theoretical models to investigate what these distant worlds are like. By doing this, we will be exploring our place in the universe and whether we are alone within it."

Peter Gao
PhD in Planetary Science

 

When asked what he would do with his degree in philosophy during a routine dentist appointment, David Silbersweig, MD at Brigham and Women's Hospital and Academic Dean at Harvard Medical School, responded with a single word that spoke volumes: 'Think.' Simply put, I too want to think.

I want to learn how to think at a complex level such that my ability to think and subsequently solve problems allows me to change lives. The history and philosophy of science degree at Caltech has given me exactly this. According to Silbersweig, 'If you can get through a one-sentence paragraph of Kant, holding all of its ideas and clauses in juxtaposition in your mind, you can think through most anything.' In my first History and Philosophy of Science class, I read Kant. I also find immense happiness in working with and helping other individuals, a sense of euphoria matched by little else in life. I learned this lesson through tutoring students and coaching younger athletes. And finally, as a collegiate athlete myself, I have undergone multiple orthopedic surgeries that ignited an interest in the musculoskeletal system and its ability to suffer injury yet recover remarkably. Together, these three aspects of life are central to my vision of the future. Becoming an orthopedic surgeon is the perfect combination—the career that will give me these components and a lot more.

One of the major developments in medicine will be 3-D printing, primarily in order to provide individuals with replacement bones and organs. Combining new progress in computer science will facilitate immense progress in 3-D printing, which also aligns well with the use of robotics in surgery. As an athlete who has torn my ACL and had bone spurs in the past year, I'm excited to be a part of this field in the future and hopefully help other athletes succeed in pursuing their passions."

Harinee Maiyuran
BS in History and Philosophy of Science

 

My personal hunch, and perhaps a somewhat common one, is that all disciplines—and not just STEM ones—are moving toward being increasingly data driven, a phenomenon rooted in freer dissemination and greater influx of research data. Correspondingly, computers and programming drive data processing in all disciplines; a common joke is that every scientist is automatically a software engineer. Statistical and machine learning techniques that are designed to tackle vast quantities of data are increasingly common in academic papers and will probably continue to climb in popularity.

I am planning to go into computational astrophysics research because I believe that the recent influx of data from new detectors will drive a huge surge of research questions to be investigated. And as a physics/computerscience double major, I'm uniquely equipped to analyze big data and extract scientific meaning from it."

Yubo Su
BS in Physics and Computer Science

 

Many aspects about future climate are unclear, such as how cloudiness, precipitation, and extreme events will change under global warming. But recent progress in observational and computational technology has provided great potential for clarifying these uncertainties. I plan to continue my research and utilize new data and models to develop theoretical understanding of these problems. I hope that such new insight will be helpful for assessing climate change impacts and designing effective adaptation and mitigation strategies."

Zhihong Tan
PhD in Environmental Science and Engineering

 

The future of science and engineering depends on closing the huge gap between the general public and scientists and engineers. I think this stems from a good deal of ignorance about what it is we do and hope to achieve, which leads to misconceptions about our work and community, and the separation between 'us' and 'them.' But if we're trying to understand and solve problems that affect everyone, shouldn't everyone be more involved?

When I graduate, I'm going to take a year off to try and bridge this gap in my own life. I don't know what I'll do yet, but it will be decidedly nonacademic. I want to travel, work odd jobs, and pursue hobbies I've set aside to finish my education. If I want to help people understand why I do what I do, I need to be certain that I understand first. After only four years surrounded almost exclusively by scientists and engineers, I want to get away a little. That way, when I inevitably return, I'll have a bit more perspective."

Valerie Pietrasz
BS in Mechanical Engineering and Planetary Science

 

Driven by the goal of reducing fossil fuel use and pollution, clean energy research plays and will play a pivotal role in America's energy future. Clean energy research spans disciplines such as biological and environmental sciences, advanced materials, nuclear sciences, and chemistry. Therefore, multidisciplinary efforts are not only necessary but also crucial to develop and deploy real-world solutions for energy security and protecting the environment.

As a graduate student, I have focused on understanding nanoscale energy transport in novel energy-efficient materials. In the future, I plan to further advance and apply my expertise to solve real-world problems in an integrated and multidisciplinary approach. I hope this effort will eventually lead to developing advanced clean energy technologies that could not only ease today's energy crisis but also improve our quality of life."

Chengyun Hua
PhD in Mechanical Engineering

 

I believe that in the next decade, the behavioral and computational subfields of neuroscience will work together seamlessly. I think this change will be primarily fueled by the development of new tools that allow us to measure the activity of large populations of neurons more precisely.

A prominent behavioral method of research, in mice at least, is to activate large structures in the brain and observe the aggregate behavioral effect. However, it is unlikely that all of these neurons are responsible for the same signal, so this approach may be too crude. I think new measurement techniques will enable behavioralists to collect large-scale population activity that computationalists can use in order to find subtle differences of function within these structures. Hopefully this collaboration will lead to generating and validating fundamental theories underlying how the brain works.

Currently, I am in the process of developing a method to measure the activity from over 10,000 neurons simultaneously. I hope to validate this technique before I graduate and then apply it to studying large-scale population activity during various behaviors. My future aim is to work closely with computationalists with the hope of discovering fundamental theories of brain function."

Gregory Stevens
BS in Biology

 

I think the future of planetary science is to discover and characterize more and more extra-solar planets, including their orbital configurations, atmospheres, and habitability. This is a challenging task because it requires a solid understanding of how chemistry and physics work on a planetary scale. Learning more about the planets closest to us paves a way toward the understanding of exoplanets that are far beyond our reach, since we can send missions to them. So after graduation, I will join the team for Juno—the spacecraft that will arrive at Jupiter in summer 2016—at JPL. New discoveries about Jupiter will also tell us more about what other planets beyond our solar system could look like."

Cheng Li
PhD in Planetary Science

 

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The Next Big Thing
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We hear from a handful of graduating students to learn what they think will be the next big thing in science and engineering and how their plans reflect those ideas.

Gilmartin Named Dean of Undergraduate Students

On July 1, 2016, Kevin Gilmartin, professor of English, will begin serving as Caltech's dean of undergraduate students.

In announcing Gilmartin's appointment, Joseph E. Shepherd, vice president for student affairs and the C. L. Kelly Johnson Professor of Aeronautics and Mechanical Engineering, described him as "an accomplished scholar and author who brings to this position twenty-five years of experience in teaching and mentoring our students, and who has shown a keen interest in the welfare of our undergraduate students in and outside of the classroom."

In his new role as dean of undergraduate students, Gilmartin will work on fostering academic and personal growth through counseling and support for student activities as well as acting as a liaison between students and faculty, says Shepherd.

A recipient the Feynman Prize, Caltech's highest teaching award, Gilmartin says he was attracted to the job of dean because "I have always found our students to be so interesting, and engaging. They are extraordinarily optimistic. They seem to have a positive attitude toward the world—they're curious, and they're open to new things. What more could you ask for?"

He says he sees his role as helping undergraduates develop and thrive. "I'm excited to work with students to help foster their intellectual and academic growth and their development as individuals," he says. "Our students are remarkably diverse and they have diverse interests. The Caltech curriculum is demanding, and focused, no doubt. But within it, and through it, our students do find so many opportunities."

He adds, "The dean's office provides essential support. But we can also encourage our students to do more than they are inclined to do, to challenge themselves, to try new things."

Gilmartin received his undergraduate degree in English from Oberlin College in 1985. He received both his MS ('86) and PhD ('91) in English from the University of Chicago, joining the faculty of Caltech in 1991.

Barbara Green, who has served as the interim dean over the past year will return to her regular position as associate dean in July. In his announcement, Shepherd thanked Green "for her work with our students and service to the Institute [and for] being so willing and committed to the success of our undergraduate student body."

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On July 1, 2016, Kevin Gilmartin, professor of English, will begin serving as Caltech's dean of undergraduate students.

Ditch Day? It’s Today, Frosh!

Today we celebrate Ditch Day, one of Caltech's oldest traditions. During this annual spring rite—the timing of which is kept secret until the last minute—seniors ditch their classes and vanish from campus. Before they go, however, they leave behind complex, carefully planned out puzzles and challenges—known as "stacks"—designed to occupy the underclassmen and prevent them from wreaking havoc on the seniors' unoccupied rooms.

Follow the action on Caltech's Facebook, Twitter, and Instagram pages as the undergraduates tackle the puzzles left for them to solve around campus. Join the conversation by sharing your favorite Ditch Day memories and using #CaltechDitchDay in your tweets and postings.

          

 

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Seven from Caltech Elected to National Academy of Sciences

Three Caltech professors and four Caltech alumni have been elected to the prestigious National Academy of Sciences (NAS). The announcement was made Tuesday, May 3.

Raymond Deshaies is a professor of biology, investigator at the Howard Hughes Medical Institute, and executive officer for molecular biology. Deshaies's work focuses on understanding the basic biology of protein homeostasis, the mechanisms that maintain a normal array of functional proteins within cells and organisms. He is the founder of Caltech's Proteome Exploration Laboratory to study and sequence proteomes, which are all of the proteins encoded by a genome.

John Eiler is the Robert P. Sharp Professor of Geology and professor of geochemistry, as well as the director of the Caltech Microanalysis Center. Eiler uses geochemistry to study the origin and evolution of meteorites and the earth's rocks, atmosphere, and interior. Recently, his team published a paper detailing how dinosaurs' body temperatures can be deduced from isotopic measurements of their eggshells.

Ares Rosakis is the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering in the Division of Engineering and Applied Science. His research interests span a wide spectrum of length and time scales and range from the mechanics of earthquake seismology, to the physical processes involved in the catastrophic failure of aerospace materials, to the reliability of micro-electronic and opto-electronic structures and devices.

Deshaies, Eiler, and Rosakis join 70 current Caltech faculty and three trustees as members of the NAS. Also included in this year's new members are four alumni: Ian Agol (BS '92), Melanie S. Sanford (PhD '01), Frederick J. Sigworth (BS '74), and Arthur B. McDonald (PhD '70).

The National Academy of Sciences is a private, nonprofit organization of scientists and engineers dedicated to the furtherance of science and its use for the general welfare. It was established in 1863 by a congressional act of incorporation signed by Abraham Lincoln that calls on the academy to act as an official adviser to the federal government, upon request, in any matter of science or technology.

A full list of new members is available on the academy website at: http://www.nasonline.org/news-and-multimedia/news/may-3-2016-NAS-Electio...

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Three faculty members and four alumni have been elected to the National Academy of Sciences.
Wednesday, May 11, 2016
Noyes 147 (J. Holmes Sturdivant Lecture Hall) – Arthur Amos Noyes Laboratory of Chemical Physics

Administrative Contact Information Session

Aliso Canyon, Methane, and Global Climate: A Conversation with Paul Wennberg

On October 23, 2015, the Aliso Canyon underground storage facility for natural gas in the San Fernando Valley—the fourth largest of its kind in the United States—had one of its wells blow out, leading to a large release of methane. The leak was not fully under control until February 11, 2016. In the interim, residents of nearby neighborhoods were sickened by the odorants added to the gas, thousands of households were displaced, and California's governor declared a state of emergency for the area. The story made international headlines; the BBC's headline, for example, read, "California methane leak 'largest in US history.'"

The leak was indeed large and undoubtedly difficult for the residents of the area. However, Caltech's Paul Wennberg says there is also a bigger picture to keep in mind: enormous methane and carbon dioxide (CO2) emissions occur all the time, with troubling implications for global climate. Wennberg is Caltech's R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, executive officer for Environmental Science and Engineering, and director of the Ronald and Maxine Linde Center for Global Environmental Science.

We recently sat down with him to talk about methane emissions and how to put the Aliso Canyon event into perspective.

What was your involvement with the Aliso Canyon event?

We have a greenhouse gas remote sensing system here at Caltech that is part of TCCON—the Total Carbon Column Observing Network. The day after the Aliso Canyon leak started, we observed something really weird in the air above Pasadena. There was a large, big plume of methane and ethane gas that came over. We now know that it was from the Aliso Canyon facility. We are providing data for the final analyses of the leak.

In the past you have suggested that the methane emissions from Los Angeles are much larger than was previously included in models.

Right. Thankfully, models are now catching up as we learn more from the data.

What does the Aliso Canyon event suggest about Los Angeles's methane emissions in general?

Aliso Canyon was a very dramatic event. Everyone heard about it worldwide. The leak continued for about 100 days, and yet it only doubled the amount of methane being emitted by LA during that period. This was a tragedy for the people living next to it, who had to deal with horrible nausea and other side effects of the chemicals associated with the natural gas. But from a climate point of view, the methane leak was actually quite trivial.

There are enormous amounts of methane being released into the atmosphere globally as a result of human activity. That is certainly true of LA, but as far as climate goes, it doesn't matter whether it's released in LA or New Zealand. On the timescale that methane sticks around in the atmosphere, it gets well mixed and affects the entire planet.

How much methane is emitted per year?

About three hundred teragrams [Tg; one teragram is equivalent to one billion kilograms] of methane are emitted every year by people and the activities of people, like agriculture and energy. Los Angeles emits about 0.4 Tg. That means that of the human methane emissions, LA as a total is one part in a thousand—not nothing, but a pretty small amount.

For perspective, Aliso Canyon emitted around 0.1 Tg. It was a big event, but what it really illustrates is how big a challenge we truly face. There are many sources emitting methane into the atmosphere and they are very diffuse. Reducing them will require hard work on many, many fronts. So it's not just, "If we solve this one problem, everything will be beautiful in the world."

You could imagine the response to the Aliso Canyon leak might be that we would all of a sudden focus all of our efforts trying to prevent leaks in natural gas storage facilities. That would not be the right answer from a climate perspective.

How should people go about eliminating methane emissions?

There is not "one" fix. Each source requires a different strategy for mitigation.

First, there is fixing leaks in the pipelines and storage facilities.

Then, it turns out that ruminants like cows and sheep produce a lot of methane—probably a third, if not more, of the human emissions. A paper about this, recently in Science, suggests that an important part of the recent increases in methane is coming from agriculture. Depending on what you feed these ruminants, they produce less methane. They eat grass, but they can't metabolize it: they have a fermenter going in their bellies—a whole microbiome that breaks the grass down into smaller things like acetate that they can metabolize. Depending on the microbiome of their guts, the cows and sheep make more or less methane. And it turns out that you can manage this.

Then there are the wetlands used for rice agriculture. Methane is produced anaerobically—in places with no oxygen—by Archaea. If you have a flooded rice paddy, the methane is produced at the roots and is transpired through the rice plants into the atmosphere. Quite a few studies now show that if you can change your rice agricultural practices to allow the fields to dry periodically, the methane emissions drop hugely.

If you were able to fix all of these things what would the impact be in terms of climate change?

If we could really knock the methane emissions back to what they were before people started emitting methane, it would be a large change. It would be a half a watt per meter squared. The total global warming would drop by around 25 percent.

How does the importance of reducing methane emissions compare to the importance of reducing carbon dioxide emissions?

Globally, methane is important. It's maybe a third of the climate forcing of CO2—that is, the increase in methane has contributed about one third of the total change in Earth's climate over the last 100 years. In terms of climate impact, however, the methane emissions from people in Los Angeles are absolutely dwarfed by their CO2 emissions—all of our driving, going on airplanes, and everything else that we do. Still, if we are to reduce our global warming potential and the amount of greenhouse gasses we emit to the atmosphere, methane has to be part of the equation.

We like to think that we can solve these problems by fixing singular events, but climate doesn't work that way. We're talking about the emissions of 7 billion people. If it were that this was produced by 100 events like Aliso Canyon, this would be a simple problem: we solve the 100 problems, and we're done. But it's all of us, and it's all of what we eat, it's all of the energy that we use, it's all of the miles that we drive. It's a much more complex problem.

What work is your group currently doing in terms of methane?

One of the things we've been doing is long-term monitoring. Natural gas is mostly methane (CH4) but there's also ethane (C2H6) in it and this provides a way of separating the signature of methane emitted from agriculture, which has no ethane, and emissions from natural gas, which does.

Over the last five years or so, the production of oil in the United States has increased hugely, and associated with that oil production is natural gas, and therefore methane and ethane. Traditionally, most of the ethane produced at a wellhead was pulled off and sent to the plastic industry. With the changing oil production, the market has become flooded in ethane: there's simply not enough plastic to be made. When the industry can't sell the ethane to the plastic industry, they simply leave it in the natural gas. We see this in the natural gas delivered to Los Angeles. Five years ago natural gas had about 2 percent ethane. Now it's 5 percent—it's more than doubled. What we've seen—and this has nothing to do with Aliso Canyon—is that over the last five years, the amount of ethane in the air over Pasadena has increased.

That's important because it tells us that a significant fraction of the methane that's being released in LA is coming from natural gas brought into Los Angeles. This has been a topic of a lot of debate. Is the big methane emitter the oil production down in the Long Beach area? Is it waste treatment plants? Is it garbage dumps? What we find is that about half of all the methane emitted in this part of LA is gas that originally came in on a pipeline.

How do you know that?

We actually know from the gas company how much ethane is in the natural gas. They report this publically from one of their storage fields and this matches the ethane in samples of the natural gas coming into our buildings.

Are there other projects under way at Caltech to study methane emissions?

Christian Frankenberg [associate professor of environmental science and engineering at Caltech and a JPL research scientist] has been leading an effort to build remote sensing instruments that allow imaging of methane plumes. Using small spectrometers on airplanes, he has flown over areas where you might have a lot of methane emissions and identified individual sources. Last year they were able to find individual pipelines that were leaking in Colorado and in New Mexico. They found several big leaks from pipelines and were able to tell the pipeline operators, who shut them down and fixed them.

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Aliso Canyon, Methane, and Global Climate
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We recently sat down with Paul Wennberg to talk about methane emissions and how to put the Aliso Canyon event into perspective.
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