ScienceWeek

SCIENCEWEEK

March 23, 2007

Vol. 11 - Number 12

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To the first flaker of flints who forgot his dinner.

-- W.H. Auden (1907-1973)

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Contents (full text below):

1. Geophysics: The Next Great Earthquake

2. Physics: How Does the Proton Spin?

3. Toxicology: A Sluggish Response to Humanity's Biggest Mass Poisoning

4. Behavioural Neuroscience: Hare-Brained Flies

5. Asteroids: Spun in the Sun

6. Mathematics Professor at New York U. Wins $975,000 Abel Prize

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1. Science 23 March 2007: Vol. 315. no. 5819, pp. 1675 - 1676 DOI: 10.1126/science.1140173

Geophysics: The Next Great Earthquake

Robert McCaffrey

Earthquakes with a magnitude 9 (M9) or larger occur very infrequently but can cause widespread damage and loss of life, as we saw with the Sumatra-Andaman earthquake in December 2004. Most of these earthquakes occur at the type of tectonic boundaries where one plate slides at a gentle angle beneath another (a process known as subduction). Because they happen mostly beneath the ocean, they often generate destructive tsunami waves.

There are more than 40,000 km of subduction boundaries (see the figure). The rupture of any one contiguous segment ~800 km or more in length can produce an M9 earthquake. Seismologists have long tried to determine which segments are more likely than others to break. Yet, the M9 earthquake of 2004 ruptured a segment that was thought to be among the least likely to go. What governs the frequency of these massive quakes, and are all subduction segments capable of producing one?

Earthquake frequency can be estimated on the basis of plate tectonics. An M9 earthquake accounts for about 20 meters of slip on the boundary between two plates, which converge at 0.02 to 0.10 meters per year; thus, an average time between them is 200 to 1000 years, assuming all the slip is by M9 quakes. If some slip occurs through smaller quakes or creep, the interval will be longer.

From an observational standpoint, this long interval is problematic, because in most places, reliable records of earthquakes date back only a century. Historic accounts and geologic observations can be used to extend the record, but they lack detail.

In places where long histories are available, the times between great earthquakes appear to be highly irregular. In Cascadia, for example, disturbances of the soft sediments by shaking and deposits of sand by tsunamis, both suggestive of past great earthquakes, show an average time between events of 600 years (1). However, the actual times range from 200 to 1500 years, revealing a very large randomness to when the margin breaks.

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2. Science 23 March 2007: Vol. 315. no. 5819, pp. 1672 - 1673 DOI: 10.1126/science.1140165

Physics: How Does the Proton Spin?

Steven D. Bass

Many particles, such as electrons, protons, and neutrons, behave like spinning tops. Unlike classical tops, however, the spin of these particles is an intrinsic quantum mechanical phenomenon. This spin is responsible for many fundamental properties of matter, including the proton's magnetic moment, the different phases of matter in low-temperature physics, the properties of neutron stars, and the stability of the known universe. In recent experiments, a number of research groups have been seeking to shed some light on the puzzling origin of spin and how this might resolve some large discrepancies between theory and experiment.

Particles such as the proton are actually combinations of more basic entities called quarks and gluons (which bind the quarks together). One of the challenges to physicists over the past 20 years has been to understand how the proton's spin is built up from its quark and gluon constituents. Models of the proton generally predict that about 60% of the proton's spin should be carried by the intrinsic spin of its three quarks, with the rest carried by orbital angular momentum (that is, the quarks flying around inside the proton). However, experiments at CERN (European Organization for Nuclear Research), DESY (Deutsches Elektronen-Synchrotron), and SLAC (Stanford Linear Accelerator Center) have taught us that the contribution from the spin of the quarks inside is small, only about 30% (1-4). This shortfall offers a substantial challenge to our understanding about the structure of the proton. To sort this out, a vigorous global program has produced about 1000 theoretical papers, and dedicated spin experiments are under way at CERN, DESY, Jefferson Laboratory, and RHIC (Relativistic Heavy Ion Collider) to map individual quark and gluon angular momentum contributions to the proton's spin. These experiments are now yielding exciting results (5).

The proton is described by quantum chromodynamics (QCD, the theory of quarks and gluons) as a bound state of three confined "valence" quarks (6). The quarks have spin 1/2 and interact through the exchange of gluons, which have a spin of 1 (where spin is quoted in units of Planck's constant divided by pi). When we probe deep inside the proton, the strength of quark-gluon and gluon-gluon interactions is small because of "asymptotic freedom." This unusual idea means that, unlike some interactions, such as electrostatic forces, the force between quarks and gluons weakens as they get closer together. If a quark tries to escape, though, the force becomes stronger--so strong, in fact, that the quarks and gluons are always bound inside nuclear particles such as the proton; they are never observed by themselves as free particles.

In low-energy experiments, the proton behaves like a system of three massive "constituent" quarks carrying about 1/3 each of the mass of the proton. When we look deeper inside in high-energy experiments, these constituent quarks dissolve into near massless "current" quarks and a sea of quark-antiquark pairs and gluons.

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3. Science 23 March 2007: Vol. 315. no. 5819, pp. 1659 - 1661 DOI: 10.1126/science.315.5819.1659

Toxicology: A Sluggish Response to Humanity's Biggest Mass Poisoning

Yudhijit Bhattacharjee

Arsenic-laced water has sickened thousands in South Asia. After delays and false starts, India is addressing the problem with a $500 million safe-water initiative

CHANDALATHI, INDIA--Until the mid-1990s, the biggest foe of Gouchan and Renubala Ari and their extended family was poverty. Then a more insidious menace began to stalk the Ari home in Chandalathi, a cluster of mud huts on the edge of a yellow mustard field some 60 kilometers north of Kolkata. The first signs of trouble were brown spots on their hands and feet that, as the months passed, developed into thick calluses and lesions. It was several years later that doctors visiting the area recognized the hallmark symptoms of arsenic poisoning.

Tests confirmed that water from the well the Aris were using was laden with arsenic. Their oldest son and his wife were diagnosed with skin cancer, a disease linked with chronic low-level arsenic exposure. Gouchan sold his cow, goats, and ducks to pay for their treatment. The couple died anyway. Afraid of suffering the same fate, two younger sons moved to other parts of India. "Arsenic destroyed our home," says Gouchan, a frail 76-year-old who walks with a limp because of arsenic lesions. "I'm tired of showing my calluses to strangers," adds Renubala. "Who can understand our misery?"

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4. Nature 446, 385 (22 March 2007) | doi:10.1038/446385a; Published online 21 March 2007

Behavioural Neuroscience: Hare-Brained Flies

Sadaf Shadan

Using the fruitfly (pictured) as a model to investigate human traits such as attention span might seem odd. But the power of Drosophila genetics, together with previous studies pointing to sophisticated behavioural responses in this organism, in fact makes it an ideal choice for studying how our minds wander. Behavioural neuroscienceHare-brained flies

Bruno van Swinderen suspended flies in a cylindrical arena with rotating walls on which one of two simple visual stimuli was displayed (B. van Swinderen Science 315, 1590–1593; 2007). He found that, each time the stimuli were switched, the fly's local field potential (LFP) activity — a measure of the total electrical activity at the junctions between neurons — increased. When the same object was displayed on both sides of the rotating cylinder there was no increased LFP response when it appeared anew. This ruled out the possibility that the elevated LFP response to a second object was simply due to a startle reflex.

When alternating the two visual stimuli, van Swinderen found that an interval of at least 50 seconds was required since the flies last saw the object for that stimulus to regain its novelty value, as measured by increased LFP activity. This response lasted an average of 9 seconds before the object lost its salience once more.

The author next performed these tests on two fly mutants — dunce and rutabaga. The proteins encoded by these genes normally alter levels of the same molecule, cAMP, and the mutants show similar defects in short-term memory. Van Swinderen found that the LFP activity in the brains of these mutants did not fluctuate appropriately in response to novel visual stimuli.

Might it simply be that the general responsiveness to visual cues is defective in these mutants? Surprisingly, the answer seems to be no. Van Swinderen found not only that visual responsiveness was unaffected, but also that, following an initial delay, it was in fact far higher in the dunce mutants than in normal flies. Further experiments confirmed that although the mutant flies responded normally to visual stimuli, they were defective in identifying a new stimulus.

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5. Nature 446, 382-383 (22 March 2007) | doi:10.1038/nature05711; Published online 7 March 2007

Asteroids: Spun in the Sun

William F. Bottke

Two asteroids have been observed gradually spinning faster and faster, and the hot tip is that sunlight is the cause. If so, this could give us a handle on the dynamics and evolution of the asteroid belt in general.

"Captain, something is spinning up these asteroids, and there's no way we can stop it!" What might sound like an excerpt from a long-lost episode of Star Trek is, in fact, a pretty precise description of a genuine scientific mystery. On page 420 of this issue, Kaasalainen et al.1 show that asteroid (1862) Apollo, a 1,400-metre-diameter near-Earth asteroid (one whose orbit intersects that of Earth), has noticeably increased its rate of rotation over 25 years of observation*. And in papers published in Science, Lowry et al.2 and Taylor et al.3 report ground-based optical and radar observations showing that (54509) 2000 PH5, a 114-metre-diameter near-Earth asteroid, is also spinning up.

So what's the cause, if it is not tractor beams or dilithium crystals? The answer, it seems, is sunlight. The measurements might be the first direct detection of a long-hypothesized phenomenon known as the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect. This is a torque produced when sunlight from an asteroid's surface is reflected and re-emitted at thermal, infrared wavelengths4, 5. According to the theory, these thermal torques can cause small asteroids to spin up or down with time, the direction and acceleration of the spin being determined by the shape and orientation of each body (Fig. 1). Given enough time, the YORP effect can even flip a body so it ends up spinning in the opposite direction.

The idea that solar radiation can affect asteroid dynamics goes back to the first of the eponymous YORP scientists, Ivan Osipovich Yarkovsky, a Polish civil engineer who worked for a Russian railway company by day, and bent his mind to scientific problems by night6. Shortly before his death in 1902, Yarkovsky published a pamphlet describing how infrared heat that was re-radiated away from the surface of an asteroid could provide a small thrust. In much the same manner, ices sublimating to their gaseous state off the surface of a comet create a 'rocket effect' that propels the comet strongly enough to change its orbit. Yarkovsky's effect works much more slowly, mainly because the angular momentum of a body changes more easily when comparatively massive gas and dust are thrown off than when infrared photons are radiated away. Nevertheless, it is now recognized that the thermal forces of the Yarkovsky effect can cause small bodies to drift slowly towards or away from the Sun to an extent determined by their size, the direction of their spin axis and several other parameters7.

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6. The Chronicle of Higher Education: Friday, March 23, 2007

Mathematics Professor at New York U. Wins $975,000 Abel Prize

By Jason M. Breslow

S.R. Srinivasa Varadhan, a professor of mathematics at New York University, has won the 2007 Abel Prize for achievement in mathematics, the Norwegian Academy of Science and Letters announced on Thursday. The $975,000 prize is considered an honor on par with the Nobel Prize, which has no mathematics category.

Mr. Varadhan is being recognized for his contributions to probability theory, the mathematical tool for analyzing chance. His work within probability theory has been credited with providing mathematicians with a unified way to calculate the likelihood of very rare events. His research has been applied to disciplines as diverse as biology, economics, statistics, and computer science.

Mr. Varadhan's accomplishments have "become a cornerstone of modern probability, both pure and applied," according to a statement released by the academy. It also commended the "great conceptual strength and ageless beauty" of his work.

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