Cold Case: IceCube is on the verge of solving one of the oldest mysteries in physics

My new book is about IceCube, a unique telescope at the South Pole that is designed to detect not light, but a strange elementary particle called the neutrino. The way the production process for books works, I had my last chance to change anything editorially about three months ago, in late July. Amazingly (and this is the way science works), the IceCube scientists have already taken a dramatic step forward.

This telescope is made out of ice: a cubic kilometer of diamond clear ice, a mile and more beneath the polar surface, outfitted with a grid of more than 5,000 light sensors. The ice itself is the basic detector: when a neutrino collides with an atomic nucleus in the ice in or near the sensor grid, or even in the bedrock below it, the neutrino will die and give birth to a charged particle that travels in the same direction as the neutrino, dragging a cone of a pale blue light along with it. By tracking this streaking particle as it passes through the grid, the physicists can determine the direction of the parent neutrino. In 2013, the IceCube collaboration announced that they had detected the very first high-energy neutrinos coming from outer space and thus gave birth to the field of neutrino astronomy.

They can also do particle physics with this detector. In fact, IceCube is the largest particle physics detector ever built—by a long shot. It weighs about a billion tons.

In the wide, wide world that is physics, the most comfortable home for this experiment is cosmic ray physics, a wonderful combination of astrophysics and particle physics that has an allusive, artistic flavor to it, because cosmic rays are so hard to pin down. A large and venerable field, predating particle physics, nuclear physics, and even Einstein’s theory of general relativity, cosmic ray physics was born in 1912, when the Austrian physicist Victor Hess made the first measurements, at 17,400 feet in a hydrogen balloon, that gave proof to a pervasive “ionizing radiation,” constantly streaming into the atmosphere—and through it, into our bodies and our planet—from space. Hess’s “rays” are now known to consist mostly of protons and larger atomic nuclei—and now that we know how to detect them, neutrinos as well. When they reach our planet, they usually collide with the atmosphere to generate showers of secondary particles—which makes understanding primary cosmic rays all that more difficult.

It is not possible to do astronomy with a charged cosmic ray particle, since interstellar magnetic fields will bend its trajectory as it flies through space, so that its incoming direction gives no clue as to the place it was born. Since the neutrino is uncharged, on the other hand, it travels in a straight line, like light, and can be used for astronomy.

The astrophysical objects that produce cosmic rays are the scenes of the most violent events in the universe: supernovae, active galactic nuclei, supernova remnants, gamma ray bursters, colliding galaxies, and other strange beasts, some not yet imagined. And all of these creatures are basically huge particle accelerators, operating by the same basic principles as the manmade variety here on Earth. The incredibly high electromagnetic fields generated by these violent events accelerate the charged particles in their neighborhoods to extremely high energies and hurl them off into interstellar and intergalactic space, where some will eventually reach our planet. Cosmic ray physics in fact gave birth to particle physics, for until the 1950s when manmade accelerators came on line, most new particles were discovered in cosmic ray air showers.

In the 1960s, physicists began to realize that cosmic rays can reach astounding energies. The record-holder thus far is the so-called Oh-My-God particle, which was observed by an instrument called the Fly’s Eye in a Utah desert in 1991. This single subnuclear spec packed as much punch as a baseball going about sixty miles an hour—about three hundred thousand times the capability of the most powerful accelerator mankind has ever built, the Large Hadron Collider at CERN. When the Oh-My-God particle hit the atmosphere it gave birth to a shower of about two hundred billion secondary particles and decay products. No one is sure what kind of particle it was.

Since most cosmic ray particles are charged and we can’t know where they come from, exactly which type of cosmic object emits these astoundingly energetic particles has been a mystery now for about sixty years. This is probably the most important question in cosmic ray physics. It has always been one of the main items on the menu for neutrino astronomy, and indeed, a little more than a month ago, IceCube, the world’s first working neutrino telescope, may have provided the answer.

On September 22nd, the blue streak from a muon, born from a neutrino, zoomed through the grid of ice-bound light sensors at the South Pole. IceCube detects about three hundred neutrinos a day, so this alone was not particularly special. What was special was the muon’s energy. It was almost ten times as energetic as any particle that could possibly be produced at the Large Hadron Collider. This meant there was a high probability that the neutrino that created this muon came from outer space.

IceCube sent out an automated, public “alert.” About four hours later, the two IceCube scientists who kept track of such things, Claudio Kopper at the University of Alberta and Erik Blaufuss at the University of Maryland, issued a second alert, encouraging “ground and space-based instruments” to look in the direction the neutrino came from, in order “to help identify a possible astrophysical source for the candidate neutrino.”

For the first time ever, optical telescopes found the likely source of a cosmic neutrino. They identified a blazar, some billions of light years away, within 250 arc-seconds—six-one hundredths of a degree—of the spot where the neutrino was pointing. This is a bullseye.

Blazars are among the brightest, most violent, and least understood creatures in the astronomical zoo. They’re thought to be giant elliptical galaxies with rapidly spinning, supermassive black holes at their cores, which scavenge up the material around them in a sort of cosmic earthquake. What distinguishes them from quasars and other forms of radio galaxy is that they send out high-energy “jets” of light and elementary particles, kind of like laser beams, from their “north and south poles”: up and down along their axes of rotation. The reason they appear so bright is that these jets sometimes aim straight at Earth. And blazars are unruly beasts. For reasons that are not understood, they occasionally flare: they become brighter by factors or ten or more, for period of hours, days, or months. The blazar that was identified with the September 22nd neutrino happened to be flaring.

The first thing to be said is that this was the second indication in the space of about one month that we have now entered a new era in astronomy: multi-messenger astronomy. On August 22nd, three huge gravitational wave instruments had detected a wave from the merger of two neutron stars, they had told optical astronomers where to look, and dozens of telescopes had observed the resulting “gamma ray burst.” Physicists had long suspected that some gamma ray bursts were generated by merging neutron stars, but this was the first proof, and it could only have been provided by this new gravitational messenger. One physicist observed that it was the “first time we have a 3D IMAX view of an astronomical event.” IceCube has now added neutrinos to the palette. Now that we can study the incredible variety of astrophysical phenomena with all wavelengths of light, gravity waves, and neutrinos, it will easier to figure out what makes them tick.

As in the case of the gravity wave, IceCube can do some science with the September 22nd neutrino as well.

It is reasonable to suspect that the objects that emit high-energy cosmic rays, like this neutrino, might also emit high-energy photons, particles of light. So it is especially intriguing that two of the optical telescopes that found the September blazar were gamma ray telescopes: NASA’s Fermi satellite telescope, and MAGIC, the Major Atmospheric Gamma Imaging Cherenkov telescope in the Canary Islands. Gamma rays are the most energetic form of photon.

Fermi has been in low Earth orbit for almost ten years and has made several major discoveries. One is that most of the high-energy gamma rays reaching us from outer space—more than 80 percent—seem to be coming from blazars. So, in recent years, quite a few cosmic ray physicists have been rooting for blazars as the mysterious source of ultrahigh-energy cosmic rays. The simultaneous detection of a high-energy neutrino and gamma rays from a flaring blazar is the best evidence so far that this may be the case.

But one neutrino isn’t all that convincing, is it?

In fact, this same sort of thing, though never quite this convincing, has happened three times before.

About fifteen months ago, on July 31, 2016, IceCube sent out a similar alert. A year later, this past July, the AGILE collaboration (Astro‐Rivelatore Gamma a Immagini Leggero) which operates an orbiting X-ray and gamma ray telescope launched by the Italian Space Agency, announced that they had found an object that looked awfully like a blazar in the direction the neutrino was pointing, and that that blazar had flared for about a day, just one day before the neutrino was detected. Less convincing, as I say, but still there.

In April 2016, the Fermi satellite collaboration announced a similar coincidence between a neutrino that IceCube had detected in 2012, before the days of automatic alerts, and a flaring blazar about 10 billion light years away (which means the blazar flared and the neutrino was born about 10 billion years ago). “Big Bird,” as this neutrino is known, is among the most energetic neutrinos ever detected. (In the early days, the students in IceCube named the most interesting neutrinos they found after Sesame Street characters.) For esoteric reasons that it is not necessary to go into here, it was harder to tell exactly where Big Bird was pointing, so again, this wasn’t entirely convincing.

Perhaps the most intriguing of the four blazar/neutrino connections is the earliest, which occurred fifteen years ago. At that point, IceCube hadn’t been built yet. Its predecessor or prototype, the smaller and much less sensitive Antarctic Muon and Neutrino Detector Array (AMANDA) detected three neutrinos coming from the direction of a flaring blazar in 2002.

Now, none these events alone is enough to claim a discovery. Together, however, I think I can go out on a limb and say that they probably add up to one.

To say that the thousand or so physicists who work on IceCube, Fermi, and MAGIC are excited would be an understatement. They seem to be very close to answering the longest-standing and arguably most fundamental question in cosmic ray physics. They’re doing what physicists do, arguing strenuously among themselves about the statistical significance of what they have found and what sort of public claims they might eventually make. And they’re combing feverishly through their historical data to see what they can see about blazars and cosmic neutrinos … .

That is quite a treasure trove. There is almost certainly more to come.

The Telescope in the Ice excerpted in Nautilus

Ice Fishing for Neutrinos
Ignorant and lucky at the bottom of the Earth

This excerpt is from chapter 8, “Enter Bruce,” in which the incomparable master driller, Bruce Koci, joins the project. The editors at Nautilus magazine have done a fine job editing it for online publication. I’m somewhat relieved that they’ve left out some of the juicier parts, but I think you’ll find them entertaining in the book itself.

Recent and Upcoming Appearances by Mark Bowen

Sunday, September 3rd, 2017, 5:30 pm
Book reading: THE TELESCOPE IN THE ICE
BigTown Gallery, Rochester, VT

Thursday, July 13, 2017, 10 am
Interview on Knowledge@Wharton (@Bizradio111), the online business analysis journal of the  School at Subjects: the new iceberg in Antarctica, EPA Administrator Scott Pruitt’s idea of having a “red team-blue team” debate about global warming, and my forthcoming book, The Telescope in the Ice

Calling all Don Quixotes

Here, three years after my last post, I am moved to blog again by the superb short story that one of our local treasures, novelist Howard Frank Mosher, has published in another of our local treasures, Northern Woodlands Magazine. It comes out seasonally, so it doesn’t grow on one’s shelf like moss. I find that I read every issue cover-to-cover.

Mosher’s piece, Where is Don Quixote?, was published in the Autumn 2015 issue. Curiously, the only letter that appeared in the subsequent issue was critical of Mosher’s message, so I wrote the following letter, which has just been published in the current, Spring 2016 issue. It summarizes my present thinking on alternative energy in Vermont pretty well, with the caution that “this is,” as Guy Stewart Callendar wrote about the greenhouse effect back in the 1930s, “a difficult subject: by long tradition the happy hunting ground for robust speculation, it suffers much because so few can separate fact from fancy.”

 


 

Watching the Wind

To the Editors,

How surprising that the only letter evoked by Howard Frank Mosher’s fine fictional piece in your Autumn 2015 issue was critical of his message. I have written two books about global warming, no one can call me a denier, yet I agree 100 percent with Ezekial Kinnneson, the story’s protagonist, that the wind towers looming in increasing number over New England’s mountain ridgelines bear very close watching. There will always be trade-offs in the development of alternatives to fossil fuel burning, but it is now clear that those associated with industrial-scale “wind factories” make them essentially unsupportable in this region. They do make sense on the vast flat plains of the Midwest, for example, where they can be “two-dimensional,” but on our one-dimensional ridgelines, they make no sense economically and have a negligible effect on carbon emissions, while destroying some of the loveliest scenery and most delicate habitat in our region.

Five and ten years ago, the notion was that we shouldn’t pick a winner, we should let the different alternative energy technologies mature and sort themselves out. Well, New England has now tried wind, and it is failing. Solar is already competitive in price, if not cheaper, and it is nowhere near as destructive to landscapes and ecosystems. It would take wind towers on half the iconic mountain ridgelines of my home state to supply our own modest electrical needs, while it would only take one percent of our existing farmland to do the job with solar—and this is leaving out rooftops and small yards and fields.

As a non-fiction writer, I found myself envious of Mosher’s ability to encapsulate the issue so succinctly and to hit all the right notes. One he hits very well is the fact that industrial wind is almost always located in the most distant and pristine—and usually poorest—areas, and is gradually transforming them into “energy ghettos” in order to supply power to distant cities. The next thing coming down the pike will be thousands of miles of transmission lines. Rooftop solar, which is intrinsically distributed and won’t need such infrastructure, is clearly the better alternative.

Mosher’s piece was so inspiring that it prompted me to give a subscription to some friends, so they could read it and all the other wonderful articles you have in your magazine.

Thanks.

Earth Day Revelation: Wind Power Doesn’t Make Sense in Vermont

Sometimes it’s a good idea to go back to school.

On Earth Day, which was a week ago Monday, I spoke to a seminar class in the natural sciences at our local college, Lyndon State. It was fun talking to the students, and I was reassured to see how committed they are to doing something about global warming, which was the main subject of my talk. I learned the most, however, from a professor who sat in on the class, who changed my mind about wind power.

I had met physicist Ben Luce by chance at a lecture in St. Johnsbury over the winter. We enjoyed speaking to each other and decided to get together again sometime. I was surprised during that first conversation when this obviously intelligent person, an alternative energy and even wind activist who seems to have all the numbers and regulations at his fingertips, came out strongly against wind power, not only in our home state but pretty much anywhere on land anywhere in the Northeast. (He believes it makes sense in the Midwest, where the resource is “two dimensional,” that is, it can be spread out over a plain, rather than along a one-dimensional mountain ridge. Ben’s advocacy of wind in New Mexico helped lead to the installation of several hundred megawatts of wind generation there.)  He fears that industrial wind development of a magnitude that could make any significant contribution to the Northeast’s energy needs would set us back years if not decades by giving alternative energy a bad name, both politically and economically.

That short conversation had me intrigued but not convinced. I’ve been aware of the resistance to the ridge line wind projects near my home in northern Vermont, Lowell and Sheffield in particular, but I assumed it was motivated by NIMBY-ism or knee-jerk environmentalism in people who aren’t aware how important it is to cut  greenhouse emissions.  But last week when Ben sat me down and walked me through the numbers, he convinced me that economic and scientific logic comes down strongly on the side of resistance as well.

In a nutshell, wind power is not only highly destructive to Vermont’s lovely and iconic mountain ridge lines, it also loses hands down to solar, technically and economically, while solar is nowhere near as destructive to the landscape. This would appear to be a no-brainer. It would take wind development on half the state’s ridge lines to supply our small state’s relatively modest electrical needs, while it would only take 1 percent of our already existing farmland to do the job with solar. Furthermore, Ben writes in an e-mail, “Focusing on just Vermont’s energy demand is irrelevant to the climate issue (its a red herring so to speak): VT’s consumption is only a few percent of the overall region’s demand. … What really matters is how the overall region is going to significantly reduce emissions, and this is where wind falls down badly.” If ALL of Vermont’s ridge lines were dedicated to wind, they would supply only about 1 percent of the Northeast’s energy demand. This is down at the noise level.

“Blasting and bulldozing ridge lines for wind power in Vermont is therefore analogous to cutting down a forest for a garden when there is already a large field prepared adjacent,” Ben writes elsewhere. “It just doesn’t make sense on scientific grounds, even from the standpoint of combating climate change.”

Wind advocates tend to emphasize the potential for Vermont alone, conveniently neglecting to point out that solar could do the job much less destructively and also be capable of supplying a large fraction of the Northeast’s needs and therefore having a real impact on the entire region’s greenhouse emissions. The advocates also “hide the fact that we are not supplying wind energy to Vermont for the most part anyways,” writes Ben, “because we are selling most of the credits to utilities in other states (while we double-count the same power towards our own ‘renewable energy targets’). This latter part is basically just fraud. … The real reason there is a wind rush on is simply that people haven’t studied it much, it became somewhat cheaper more quickly than solar, and because utilities despise distributed generation.”

This brings up another of wind’s drawbacks: transmission lines. Owing both to the small capacity of the local electrical grid and the fact that it’s not a “smart grid,” the Sheffield wind project (I’m tempted to call them “wind factories,” as the performers at Bread and Puppets did last summer) actually has to discard power when its output exceeds local demand. A wind project is a centralized power station–in this respect it’s like a fossil fuel-burning power plant–so significant wind development leads to the need for more transmission lines. Conservative goals for wind power in the Northeast over the next fifteen years would “carry transmission costs of between $7 billion and $12 billion” and require four thousand miles of new lines! These costs are not often mentioned by advocates. Even ignoring them, while the cost of wind dropped until about 2000, it has been increasing ever since, mostly due to the commodity costs associated with the enormous metal infrastructure involved. And in Vermont there is the added cost of massive road building in inhospitable terrain. Meanwhile, solar is by its nature distributed and less capital intensive, and its cost continues to drop. It’s already competitive with wind on its face, while the future costs of wind from transmission corridors and so on remain hidden.

“If you add in all the impact and cost issues, then wind looks really crazy,” Ben concludes.

Makes sense to me, and it is also refreshing to see such clear thinking and explaining. There is no easy answer to the daunting challenge of global warming, and dogmatism simply won’t work. We need all the clarity and level-headedness we can summon.

Selected previous appearances by Mark Bowen

Monday, April 22, 2013 (Earth Day), 3 pm
Natural Sciences Seminar
Lyndon State College, Lyndonville, Vermont

Tuesday, April 16, 2013, 8:45 am/12:30 pm
Short talk to introduce Lonnie Thompson, with Book Signing
American Polar Society 75th Anniversary Meeting and Symposium:
The Polar Regions in the 21st Century: Globalization, Climate Change and Geopolitics
Marine Biological Laboratory, Woods Hole, Massachusetts

Friday, March 25, 2011, 7 pm
Trekking in Nepal
St. Johnsbury Shambhala Center
17 Eastern Avenue
St. Johnsbury, Vermont
For more information call Caroline DeMaio at (802)748-4240
Suggested donation: $10
All Proceeds to benefit Community Action Nepal,
a charity founded by the British Mountaineer Doug Scott

Wednesday, June 3, 2009, 2:00 PM – 3:30 PM
Panel discussion on Censorship of Science
at the
Global Forum on Freedom of Expression
Oslo, Norway

Monday, March 2, 2009, 4-5 PM
Interview on Stand UP! with Pete Dominick
P.O.T.U.S. SIRIUS 110 / XM 130

Saturday, October 4, 2008, 8:00 PM
Censoring Science on Book TV (C-SPAN2)
This show streams on the Web, here.
It was edited from talks given by Mark Bowen and Jim Hansen
in Lexington, Massachusetts on June 1, 2008 in an event sponsored by the Lexington Global Warming Action Coalition

Friday, July 18, 2008
Panel discussion on Restructuring U.S. Science Policy
at the annual meeting of the progressive blogging community:
Netroots Nation (formerly YearlyKos), Austin, Texas

June 1, 2008
Talks by and discussion with Jim Hansen and Mark Bowen
Sponsored by the Lexington Global Warming Action Coalition
Cary Hall, Lexington, Massachusetts
(Daily Kos features a wonderful diary about this event by a “Kossack” who attended. It was also covered by New England Cable News, LGWAC has produced a video of the event, and it has been broadcast on C-SPAN’s Book TV.)

May 22, 2008
Interview on Chicago Public Radio’s Worldview (podcast)

April 29, 2008
“Climate Change from the Mountains to the Sea”
Talk in connection with Earth Day
Edinboro University, Edinboro, Pennsylvania

March 22, 2008
Talk: “Climate Change from the Mountains to the Sea”
American Alpine Club, New England Section, Annual Dinner, Weston, MA

March 21, 2008
Interview with Jim Hansen and Mark Bowen
Democracy Now! with Amy Goodman and Juan Gonzalez

March 11, 2008
Book reading: Censoring Science
Brookline Booksmith, Brookline, MA

February 17, 2008
Censoring Science, talk and book signing
Annual meeting of the American Assoc. for the Advancement of Science, Boston, MA

February 15, 2008
Interviewed along with Jim Hansen on Nature magazine’s Climate Podcast

February 2, 2008
Interviewed by Robert F. Kennedy Jr. on Ring of Fire (audio or video)

 January 27, 2008
Interview on The John Batchelor Show, KFI-AM, Los Angeles

 January 26, 2008
Interview on Left Jab, XM Satellite Radio, Channel 167 (Air America)

January 14, 2008
Interview on the Michelangelo Signorile Show
Sirius Satellite Radio, Channel 109 (OutQ)

January 13, 2008
Interview on The John Batchelor Show
WABC Radio, New York City

January 10, 2008
Talk: “Responsible Climate Science Reporting”
19th Annual Weather Summit: Understanding & Communicating Climate Change More Effectively
Steamboat Springs, CO

January 8, 2008
Interviewed along with Jim Hansen
Fresh Air with Terry Gross, National Public Radio

Continue reading

Censoring Science Reprise

Hmmh … . Even though it’s more than four years old, Censoring Science was reviewed just the other day on the Daily Kos. Might have something to do with the fact that this is an election year. And, come to think of it, it might be worthwhile to refresh our memories about the brutal tactics of the last Republican administration, since they will almost certainly return if a Republican wins the election this year. After all, as I show in the book, every Republican administration since 1988, when Jim Hansen turned global warming into a public issue, has censored government scientists who work in this area.

Kudos to Bill Blakemore, too.

While I’m at it, I think I ought to recommend Bill Blakemore, too. His work has defined the cutting edge of accurate, facts-based reporting on global warming since at least 2005. I had the privilege of talking to him a few times as I was writing Censoring Science, and, as I say in the acknowledgments to that book, those interactions were high points of that experience.

Bill not only reports magnificently on the subject, he is also eloquent on the subtleties and responsibilities of reporting on such a complex, ideology-laden issue. Most other journalists could benefit from reading and watching his work.

Here’s an excellent recent piece by Bill on the ABC News Web site.

 

Kudos to Justin Gillis

I’ve been meaning for a while to mention how impressed I am with the new climate science reporter at the New York Times, Justin Gillis. Now, I can’t put it off any longer, since he’s just written a profile of Lonnie Thompson, the protagonist in my first book. Justin did an excellent job, considering how difficult it is to fit as rich a life as Lonnie’s into 3,000 words. He hit just the right tones in his brief encapsulations of the various incidents and scientific matters. I was lucky to have more room when I wrote my book. (Some might argue too much!)

I first noticed a sea-change in the climate science reporting at the Times about two years ago in Justin’s first in-depth piece on climate, a story about the connection between the chaotic weather we’ve been experiencing for the past few years and global warming. The story was wonderfully clear, and reflected current scientific knowledge and uncertainty quite accurately, in my opinion. Justin took a rational approach. He went to the best scientists, used his own intelligence to understand the data and the crux of the matter, and reported his findings simply and straightforwardly. The previous reporter on this beat at the Times (I’m afraid that would be Andy Revkin) had approached the subject as if it were a political issue and he was an investigative reporter. He wasn’t particularly interested in getting to the bottom of scientific issues himself. Instead, he would go out of his way to get the “other side” of the story, which meant that he treated people with little scientific expertise, a history of intellectual dishonesty, or some very obvious bias–propagandists basically–as if they were experts on a par with the world’s leading scientists. As a result, as insightful as his articles could sometimes be, they were generally garbled and delivered mixed and misleading messages. I saw this as a sort of intellectual laziness and shirking of responsibility. Not so with Justin. I know how hard he worked on the Lonnie profile, for example, because he talked with me about it, off and on, for about six months.

I recommend all of Justin’s pieces, and especially his profile of Charles David Keeling. You can find most of them here and here.

And, Justin, please keep up the good work. We need it. It’s helpful.

I knew living in the country was good for you …

There’s been a lot of talk about the ill effects of loosing touch with nature, but most has focused on how it affects the spirit or leads to behavior that damages the natural world. Now, here’s a physical health reason to drop the smartphone and take a walk in the woods. Reminds me of a remark my quite healthy dairy farming neighbor made the other day: “You’re not a real Vermonter unless you’re knee deep in shit.”

From the Editor’s Choice section of the 8 June issue of Science:

MICROBIOLOGY

Hygiene Can Hurt

As human societies urbanize, chronic inflammatory disorders become more apparent. The hygiene hypothesis suggests that individuals exposed to infection in childhood are less likely to develop inflammatory disease because exposure to microorganisms is important for stimulating responses that maintain epithelial cell integrity. Hence, in urban environments, reduced contact with the full diversity of the microbial world may be leading to the increased incidence of inflammatory disorders. Hanski et al. took a random sample of 118 adolescents from towns, villages, and isolated dwellings in eastern Finland, tested their immune function and allergic responses, surveyed their skin microflora, and investigated the biodiversity within their homes. They found several significant correlations, not least that low biodiversity was surprisingly strongly associated with atopy, and concluded that humans need to interact with natural environments for their physical health, not just for their peace of mind.

Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1205624109 (2012).