Recently in Earth sciences Category

On 30 August 2000, as the sun beat down on Texas’s largest city, ozone concentrations soared to unhealthy levels. Usually in summer, as the city heats up, sea breezes blowing in from nearby Galveston Bay and the Gulf of Mexico refresh the air. But the prevailing winds over Houston, although mild, tend to counteract the sea breeze. Thus, if the breeze collides with the prevailing winds, stagnation sets in over the city and pollutants can build up. Now a numerical study led by Fei Chen of the National Center for Atmospheric Research suggests that the materials of the urban environment are partly to blame for ozone pollution. Chen and colleagues validated their computer model by comparing their simulation of the August 2000 pollution event against extensive data collected in the Texas Air Quality Study 2000. Then, to understand how various environmental features affect the development of the sea breeze, they simulated conditions that were wetter or dryer than normal, and in one simulation they replaced the urban landscape with cropland. The substitution of crops for concrete had the greatest impact on boosting the sea breeze and reducing periods of stagnation. (It also increased the efficacy of the nighttime land breeze that blows pollutants out to sea.) Compared with green space, the researchers found, the urban environment is hotter. That effect actually tends to enhance the sea breeze, but the enhancement is more than offset by the frictional damping from Houston’s buildings. (Fei Chen et al., J. Geophys. Res. [Atmospheres], in press, doi:10.1029/2010JD015533; image from http://www.utexas.edu/research/ceer/texaqs/visitors/photos.html.)—Steven K. Blau

When a transition-metal compound is subject to high pressure, its electronic spin state can change, which in turn can change the compound's material properties. That spin-state crossover is of geophysical relevance because of the iron-bearing minerals in Earth's lower mantle. But the most abundant mantle mineral—Fe-bearing magnesium silicate perovskite (Pv)—is a challenge to study, since it contains three nonequivalent types of Fe atom: Not only can Fe replace either Mg or Si in the crystal lattice, but Fe replacing Mg can be either ferrous (Fe²⁺) or ferric (Fe³⁺). Experiments on spin states under pressure probe the electron configuration indirectly, via its effect on nuclear energy levels, so computational studies are necessary to connect experimental measurements with the correct interpretations. Last year, an experimental study of ferric Fe in Pv yielded results that were at odds with the computational studies to date. Now, Renata Wentzcovitch and colleagues at the University of Minnesota have verified the experimental results computationally and predicted their geophysical consequences. The researchers found that ferric Fe that replaces Si undergoes a spin-state crossover at a pressure somewhere between 40 and 70 GPa, equivalent to a depth between 1100 and 2000 km and consistent with the 50–60 GPa crossover pressure measured experimentally. Since that transition causes the unit cell to shrink in volume by about 1%, it has a significant effect on the mineral’s bulk modulus and thus on the speeds of seismic waves and on mantle convection. (H. Hsu et al., Phys. Rev. Lett. 106, 118501, 2011.)—Johanna Miller

Plutons are mountain-sized formations of igneous rock that poke through Earth's surface. Their origin as solidified eruptions of magma is straightforward to explain. What's puzzling is why plutons are so homogeneous on large scales, despite their immense size and despite being inhomogeneous in their mineral composition on small scales. Alain Burgisser of the Institute of Earth Sciences in Orléans, France, and George Bergantz of the University of Washington in Seattle have proposed an answer. In their model, a mass of viscous, semisolid magma—"mush" is the technical term—lies beneath the surface, hemmed in by walls of more solid rock. Pluton formation begins when the slow churning of the mantle below happens to bring a body of hot magma into contact with the bottom surface of the cooler mush. Over the ensuing decades, the heat rises slowly via conduction, making the mush less viscous and—crucially—less dense. In a process that Burgisser and Bergantz call unzipping, the gradually thickening layer of hot, light mush abruptly undergoes a Rayleigh–Taylor instability, which sends convective plumes of hot mush upward through the nascent pluton. Within a few months, the first plumes reach the top of the nascent pluton, cool, then sink. Only a few successive cycles of overturning suffice to homogenize the mush on plutonic scales. Burgisser and Bergantz's model can plausibly account for the speed with which three real plutons formed, including the one left by the 1991 eruption of Mount Pinatubo in the Philippines. (A. Burgisser, G. W. Bergantz, Nature 471, 212, 2011.)—Charles Day

Global climate change is progressively reducing the Arctic Ocean’s summer ice cover. That retreat harbors an obvious positive-feedback mechanism: Because ice is more reflective than open water, the shrinking cover means more absorption of solar radiation, leading, in turn, to more loss of ice. That raises the prospect of a possible tipping point at which the thus-far relatively gradual retreat of summer ice “runs away,” leaving the Arctic Ocean perennially free of summer ice long before the date—sometime late in this century—generally deduced from climate models. But might not those models be made to reveal such threshold behavior by subjecting them to strong perturbations? Steffen Tietsche and coworkers at the Max Planck Institute for Meteorology in Hamburg, Germany, tried that with a widely used climate model. What would happen, they asked, if by a random fluctuation in some year, the Arctic Ocean became completely free of ice on 1 July? The figure, plotting ice cover in September, when it’s typically least, shows the result (in blue) when that initial perturbation is imposed in a particular year. In every case, the September cover reverts to its gradually falling unperturbed level (black curve) within about two years. Tietsche and company attribute such prompt recovery primarily to a negative-feedback mechanism that damps the albedo reinforcement: During the long, dark winter, the lack of insulating ice produces an anomalously warm arctic atmosphere, whose top radiates heat away faster and whose sides receive less wind-driven heat from temperate latitudes. So, they conclude, a tipping point at which the loss of summer sea ice becomes sudden and irreversible is unlikely. (S. Tietsche et al., Geophys. Res. Lett. 38, L02707, 2011.)—Bertram Schwarzschild

Measurements of the geomagnetic field at the smallest scales are used to locate sunken ships and mineral-rich geological formations. Large-scale measurements probe properties of Earth’s core. At length scales of tens to hundreds of kilometers, geomagnetic maps yield clues about the chemical dynamics in Earth’s outer mantle and the effects of ionic currents on ocean circulation. To avoid ground-based electromagnetic interference, geomagnetometers are typically placed aboard orbiting satellites, which are deployed sporadically and at a relatively high cost. Now, an international team of scientists led by James Higbie (Bucknell University), Domenico Bonaccini Calia (European Southern Observatory), and Dmitry Budker (University of California, Berkeley) has proposed a lower-cost ground-to-space system that exploits the interaction of beams from ground-based lasers with sodium atoms in the mesosphere, about 90 km above Earth's surface. The team’s system would harness the existing and expanding infrastructure of high-powered lasers that generate artificial stars for optical telescopes by exciting mesospheric sodium. As shown in the image, an optically pumped laser would spin polarize the sodium atoms, and then a ground-based telescope would measure the fluorescence intensity, which is dependent on the atoms’ precession frequency in the magnetic field. The researchers calculate that such a system would achieve subnanotesla sensitivity and could lead to the formation of a global network for continuous mapping and monitoring of mesospheric magnetic fields. (J. M. Higbie et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1013641108.)—Jermey N. A. Matthews

Superplasticity is the ability of some crystalline materials to stretch up to several times their own length when heated. Although the minerals in Earth's mantle don't endure such large strains, circumstantial evidence suggests that superplasticity helps them respond to the subduction of continental plates and other tectonic processes. Now, a team led by Takehiko Hiraga of Tokyo University and Hidehiro Yoshida of Japan's National Institute for Materials Science has found direct evidence that mantle minerals are indeed superplastic. Like other superplastic materials—real or presumed—those in the mantle are polycrystalline aggregates. For their study, Hiraga, Yoshida, and their team sintered nanoscale powders to make two analogues of mantle minerals, both of which consisted mostly of forsterite (Mg₂SiO₄). In the absence of strain, a superplastic material is made up of nanoscale grains of the majority component interspersed with smaller grains of the minority component. When heated under strain, the majority and minority grains both grow by merging with their neighbors. That response ensures that grains continue to abut each other, forestalling failure of the bulk material. As the accompanying figure shows, samples that consisted of 90% forsterite and 10% periclase (MgO) could withstand strains of more than 500%. Moreover, two electronic diagnostics, electron back-scattered diffraction and transmission electron microscopy, revealed that grains in the mantle analogues grew like grains in materials whose superplasticity is established. Having measured the temperatures and strain rates under which mantle analogues become superplastic, the team estimated that superplasticity could help Earth's mantle accommodate a 200-km slab that takes 60 million years to penetrate 3000 km. (T. Hiraga, T. Miyazaki, M. Tasaka, H. Yoshida, Nature 468, 1091, 2010.)—Charles Day

Global news coverage of the blown-out BP-operated Macondo well documented several efforts to stem the roughly 8 million liters per day of crude oil that gushed into the Gulf of Mexico from April to July 2010. One notable failure was the “top-kill” method to plug the well by pumping a dense slurry of drilling mud into it—reportedly, much of the mud was swept out by the spewing oil. A recent study by scientists at Lawrence Livermore National Laboratory and Washington University in St. Louis points to fluid shearing as the chief culprit in top-kill’s failure. Turbulent eddies, caused by the velocity differences between the counterstreaming mud and the oil, likely sheared the mud, breaking it up into packets of fluid whose settling velocities were an order of magnitude smaller than the upward velocity. In laboratory experiments, the researchers confirmed their theory and demonstrated a possible solution, adding a viscoelastic polymer to an aqueous cornstarch mixture to represent the drilling mud. As the images show, the control fluid (left, in green) suffered turbulent breakup, but the polymer-laced fluid (center) descended as a coherent slug or, at lower flow rates (right), as stringy, connected globules. The researchers calculate that a polymer-doped slug of drilling mud at the Macondo well would have descended with a terminal velocity of roughly 7 m/s, nearly double the estimated 3.7 m/s ascent of escaping oil. (P. Beiersdorfer et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

Terrestrial gamma-ray flashes (TGFs) are the source of the highest-energy nonanthropogenic photons produced on Earth. Associated with thunderstorms—and, in fact, with individual lightning discharges—they are presumed to be the bremsstrahlung produced when relativistic electrons, accelerated by the storms’ strong electric fields, collide with air molecules some 10–20 km above sea level. According to the prevailing theoretical model of that process, and to some previous observations, the TGF energy spectrum should follow a power law at low energies but decay exponentially at energies above about 7 MeV, as shown by the red line in the figure. Now, researchers working with data from the Italian Space Agency’s AGILE satellite find a rather different and theoretically challenging spectrum. Based on data from 130 TGFs collected over a 20-month period, their observed spectrum (black dots) is well fitted at the high-energy end by a second power law (blue line) that extends to at least 100 MeV with no sign of an exponential cutoff. According to the theory, the highest-energy electrons must have had a long history of flying through the electric field, colliding with air molecules, and releasing electrons with relatively low energies. But so many low-energy electrons would produce many more low-energy photons than the AGILE team observed. (M. Tavani et al., Phys. Rev. Lett., in press.)—Johanna Miller

Even in the absence of the Moon and stars, the night sky would not be black. Chemical reactions in the upper atmosphere emit faint light called airglow, whose spectrum has been studied for more than a century. Much of the emission comes from atomic and molecular oxygen, hydroxyl radicals, sodium, and, at higher latitudes, nitrogen dioxide. But after subtracting those species' contributions from the airglow spectrum at lower latitudes, observers have noted a broad band of unaccounted-for emission in the orange part of the visible spectrum, between roughly 550 and 650 nm. Using measurements from the Optical Spectrograph and Infrared Imager System (OSIRIS) on Sweden's Odin spacecraft, Dick Gattinger and Ted Llewellyn (University of Saskatchewan) and colleagues have now identified the source of that band: iron monoxide. Several factors supported the team's conclusions. Nitrogen dioxide was ruled out since sunlight destroys it at lower latitudes and, moreover, it has a different spectral signature. Laboratory measurements of FeO, on the other hand, yield a similar emission spectrum. Iron's atmospheric abundance is comparable to that of sodium, a known airglow contributor. And an orange emission band observed a decade ago originating from iron-containing meteors was also matched to the laboratory FeO spectrum. Subsequent ground-based measurements show that the FeO emission exhibits dramatic temporal fluctuations. The team postulates that FeO is generated through reactions between atmospheric iron and ozone. (W. F. J. Evans et al., Geophys. Res. Lett. 37, L22105, 2010, doi:10.1029/2010GL045310.)—Richard J. Fitzgerald

Despite global warming, the rate of water evaporation over land surfaces has steadily declined in the past few decades. That unexpected trend, observed by farmers and climate-change scientists alike, has been linked to a decline in surface wind speeds over the same period. The challenge of quantifying the stilling-winds phenomenon on a global scale was recently taken up by Robert Vautard and his colleagues at the Climate Science and Environment Laboratory in France and the European Centre for Medium-Range Weather Forecasts in the UK. By analyzing data from more than 800 weather stations, the researchers found that 73% reported that wind speeds measured 10 meters above the ground had declined by up to 15% from 1979 to 2008. As the image shows, some regions in Europe experienced declines of as much as 5 m/s per decade. After studying climate-model simulations, the researchers attribute much of the slowdown to an increase in topographical surface roughness from a surge in vegetation growth induced by excess atmospheric carbon and also anthropogenic activities such as urbanization. Less of a slowdown, or even an increase, was seen in regions that did not experience significant vegetation growth. The analysis assigned a lesser role to reduced atmospheric circulation caused by global warming. Wind energy enthusiasts should not necessarily be worried, say the researchers, since most wind turbines operate at 50–100 m, where the analysis did not detect any noticeable trend. (R. Vautard et al., Nat. Geo., in press, doi:10.1038/ngeo979.)—Jermey N. A. Matthews

A team from Japan has measured the crystal structure of iron under conditions that prevail in Earth's solid inner core—that is, at temperatures and pressures higher than 5000 K and 300 GPa. To reach those extreme values, Shigehiko Tateno and Kei Hirose of the Tokyo Institute of Technology and their collaborators placed Fe powder inside the 20-μm-wide cell of a diamond anvil. Tightening the anvil's screw squeezed the sample to pressures up to 377 GPa, while two 100-W ytterbium fiber lasers raised the sample's temperature as high as 5700 K. Placing the cell in a beamline at the SPring-8 synchrotron in Sayo, Japan, yielded the structural information and enabled the team to fill in the uncharted top corner of Fe's phase diagram. Under ambient conditions, Fe adopts a body-centered cubic (bcc) structure (the red region in the bottom left corner of the phase diagram). As the temperature increases, the pressure needed to forestall melting increases too. Previous measurements (solid diamonds) had shown that Fe switches from a bcc to a hexagonal close-packed (hcp) structure (blue region) at modest temperatures and pressures. That the hcp structure survives at inner core conditions has now been established by the SPring-8 measurements (open symbols). If Fe in Earth's inner core really is hcp, then the lengthening of the crystal's c-axis parallel to Earth's rotation would naturally account for a certain anomaly in seismic signals that pass through the core. (S. Tateno, K. Hirose, Y. Ohishi, Y. Tatsumi, Science 330, 350, 2010. )—Charles Day

Oxygen in the air and dissolved oxygen in the ocean are crucial for sustaining animal life on Earth. But that oxygen wasn’t always there. Researchers can gain some information about the history of ocean oxygenation from the compositions of sedimentary rocks, because dissolved O₂ affects the solubility and precipitation of certain elements. Now, an international team of researchers led by Tais W. Dahl (University of Southern Denmark) has studied the problem using a new technique based on molybdenum isotopes. In oxygenated water, Mo forms the soluble molybdate ion, MoO₄²⁻, and precipitates in small quantities by a mechanism that favors lighter isotopes. But when hydrogen sulfide is present instead of O₂, all Mo isotopes precipitate quickly. By analyzing the Mo in 180 rock samples with ages spanning almost 2 billion years, the researchers concluded that O₂ levels increased twice: 550 million years ago and 400 million years ago, as shown in the figure. The first increase coincides with the emergence of the first motile animals, and the second coincides with the evolution of much larger animals including large predatory fish. The method does not provide a quantitative measure of O₂ levels over time. Still, based on the physiological requirements of fish, the researchers suggest that prior to 400 million years ago the ocean and atmosphere contained O₂ at just 15–50% of their present levels, which means that the earliest animals survived with much less oxygen than we breathe today. (T. W. Dahl et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1011287107.)—Johanna Miller

Like a batch of cookie dough, Earth is constantly being stirred: Volcanism, tectonic-plate movements, and thermal convection can all transport material between and within the mantle and crust. Even the lower mantle is not immune to mixing: Studies of seismic waves’ speeds (see Physics Today, August 1997, page 17), which measure the temperature profile across Earth’s interior, show slabs of cool, dense oceanic crust sinking nearly to the core–mantle boundary. But now, Boston University’s Matthew Jackson and collaborators have found that rock samples from Baffin Island in northeastern Canada, the products of an enormous volcanic eruption some 60 million years ago, show all the signs of having come from an ancient mantle reservoir, unstirred since not long after Earth’s formation 4.5 billion years earlier. The researchers based their conclusion on isotopic measurements of several chemical elements, each with some isotopes produced by radioactivity and others that are purely primordial. If any mixing had occurred, it would have changed the relative amounts of a radioactive parent element and its daughter—and thus the daughter’s present-day isotopic composition, which is easier to measure than the parent–daughter ratio. If the Baffin samples did indeed come from a primitive reservoir, geophysical modelers will be challenged to explain how any part of the mantle could have remained so isolated for so long; one possible solution involves convective eddies that trap and preserve the pristine mantle material. (M. G. Jackson et al., Nature 466, 853, 2010.)—Johanna Miller

Under the hot summer sun, the ocean’s surface waters become warmer than the atmosphere above them. As the heat is transferred to the atmosphere, it can strengthen low-pressure disturbances and drive the characteristic weather phenomena known in the Atlantic region as hurricanes and in the Pacific as typhoons or tropical cyclones (see the Quick Study on hurricane formation by Kerry Emanuel in Physics Today, August 2006, page 74). A new model from a research collaboration led by Anand Gnanadesikan at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in New Jersey shows how strongly correlated the sea surface temperature (SST) is to the ocean’s color. The image depicts average concentrations (in mg/m³) of chlorophyll—the green pigment in phytoplankton—from 1997–2000 in the Pacific Ocean, where more than half of the reported typhoon-force winds (greater than 32 m/s) occur. Considering an extreme scenario, the researchers set the concentration of chlorophyll to zero and then studied the evolution of cyclones in the North Pacific Ocean. Without chlorophyll to absorb much of the solar radiation, SSTs drop. Air over the colder water sinks, drying the atmosphere and increasing wind shear, which quenches typhoon formation. Although typhoon frequency increased by 20% along the equator, the simulation predicted an overall drop in the region—up to 70% for areas beyond 15 degrees north of the equator—and a decrease in frequency of the most intense typhoons. (A. Gnanadesikan et al., Geophys. Res. Lett., in press.)—Jermey N. A. Matthews

Since industrialization, the amount of mercury circulating in the air, water, and living organisms has roughly tripled. But the physical and chemical processes involved in that Hg cycle are poorly understood. In 2007 Bridget Bergquist and Joel Blum of the University of Michigan in Ann Arbor discovered that Hg’s two stable magnetic isotopes, ¹⁹⁹Hg and ²⁰¹Hg, sometimes exhibit slightly different behavior from that of the five stable nonmagnetic isotopes. The difference is attributed mostly to coupling between nuclear and electronic spins, which facilitates electron spin flips in certain photoinitiated reactions. Consequently, the varying isotopic compositions of natural samples can provide information about the Hg cycle and sources of Hg pollution. (See Physics Today, December 2008, page 25.) Now Blum, Laura Sherman, and colleagues have studied Hg isotope fractionation in Arctic snow. The Hg is deposited seasonally: Shortly after the annual polar sunrise, sunlight induces precipitation of almost all the Hg in the polar atmosphere. How and to what extent Hg leaves the snow before the snow melts is uncertain, but the researchers found that of the snow samples they collected, those that had been exposed to more sunlight contained less ¹⁹⁹Hg and ²⁰¹Hg. They conclude that Hg is emitted from the snow via a photochemical reaction that favors the odd-numbered isotopes. And they suggest that any Hg that enters Arctic ecosystems from the melted snow could be identified by its isotopic composition. (L. S. Sherman et al., Nat. Geosci. 3, 173, 2010.) —Johanna Miller

When Jean-Dominique Cassini discovered Iapetus in 1671, he was surprised to find it visible on just one side of its orbit around Saturn. The moon’s orbit had to be synchronous, he correctly inferred, with its leading hemisphere far darker than its trailing one. More than 300 years later, Voyager 2 revealed that the charcoal dark and frosty bright surfaces interleave, like two halves of a tennis ball. But the pattern and sharpness of the dark–bright boundaries remained mysterious. Using data collected by Cassini–Huygens since 2004, John Spencer (Southwest Research Institute), Tilmann Denk (Free University of Berlin), and colleagues have now confirmed a hypothesis first proposed in 1974. Micrometeorites swept up on the leading hemisphere during the moon’s orbit, they argue, darken it enough to trigger the thermal migration of ice: sublimation from dark, warmer patches centered around the equator and subsequent recondensation at bright, colder areas near the poles and trailing side. Visible-spectrum images such as the ones shown here indicate that the dust coating Iapetus’s leading side is redder than the dirt presumed intrinsic to the moon. Judging from IR data, the dust reaches temperatures up to 129 K during Iapetus’s 79-day rotation. Sublimed ice molecules can travel ballistically hundreds of kilometers before recondensing at cold (113 K) traps. An enormous gossamer ring around Saturn detected last year by the Spitzer Space Telescope is the putative source of reddish dust. (J. R. Spencer, T. Denk, Science, in press, doi:10.1126/science.1177132; T. Denk et al., Science, in press, doi:10.1126/science.1177088.)—R. Mark Wilson

Our historical record of seismic activity is very short, by geological time scales. So extrapolating that record to predict future earthquakes can lead to nasty surprises, such as 2008's devastating earthquake in Sichuan, China, which occurred on a fault that had seen little recent activity. Large earthquakes are typically followed by aftershocks whose frequency decays to some background level of seismicity, following an empirical relation known as Omori's law. But determining the time scale of the decay and the baseline activity can be difficult. A new model by Seth Stein of Northwestern University and Mian Liu of the University of Missouri–Columbia posits an inverse relationship between the aftershock-sequence durations and the slip rates along faults. Large earthquakes are most common along the boundaries of tectonic plates, and the occurrences of aftershocks tend to decay quickly—within a decade or so—to a relatively high background. The relative plate motion at such boundaries can be rapid, faster than 10 mm/yr. Continental interiors, far away from plate boundaries, deform much more slowly, typically less than 1 mm/yr. And thanks to that slower rate of fault loading, aftershocks can last hundreds of years or longer, as shown in the figure. Thus, warn the researchers, interpreting continental earthquakes as steady-state seismicity can overestimate the hazard in presently active areas and underestimate it elsewhere. (S. Stein, M. Liu, Nature 462, 87, 2009.)—Richard J. Fitzgerald

With political sentiment growing in favor of greenhouse gas (GHG) restrictions, biofuels from plant cellulose are being considered among the alternatives to fossil fuels: Plants are renewable and biodegradable, and they sequester carbon. Yet a new report validates concerns that a global biofuels program could put intense pressure on land supply and distribution. To predict the impact of a biofuels-based economy on climate change, an international team of researchers from the US, Brazil, and China linked an economic model of land use with a terrestrial biogeochemical model of global GHG levels. The team considered two cases for cellulosic biofuel crop growth: The primary focus of case1 is on converting unfarmed areas such as forests, as shown in the image; of case 2, on exploiting existing farmland to the extent possible. In both cases, biofuel feedstock becomes a dominant global crop by year 2100, but in the process, total forest area is cut—by 56% in case 1 and by 24% in case 2. The loss of carbon-sequestering trees in case 1 results in a net release of carbon. In case 2, the gains from biofuel production ultimately lead to increased carbon sequestration in the farmed soil from the addition of nitrogen fertilizer, which paradoxically releases N₂O, another potent GHG. The research suggests that stabilizing GHG levels will require a limited and more efficient use of forests and fertilizers for biofuel crop production. (J. M. Melillo et al., Science Express, 22 October 2009, doi:10.1126/science.1180251. Image courtesy of Chris Neill, Marine Biological Laboratory, Woods Hole, MA.)—Jermey N. A. Matthews

Combining the powerful notion that energy is conserved with observational data on surface temperature, ocean heat content, and radiative fluxes, researchers have determined our planet's energy budget for the past half century—without recourse to any climate models. Energy in the form of heat is added to Earth by the Sun's direct radiation and by the well-characterized radiative contributions from greenhouse gases. Those positive contributions (so-called forcings) are balanced, as shown in the figure, by stratospheric aerosols of volcanic origins that reflect incoming sunlight, increased outgoing IR radiation from a warming Earth, long-term heating of Earth—almost entirely of its oceans, which have far higher heat capacities than the atmosphere, land, or ice—and a residual term that mainly represents direct and indirect cooling effects of anthropogenic aerosols throughout the atmosphere. The analysis shows that the anthropogenic aerosols contribute a net cooling of 1.1 Wm⁻², in agreement with the 2007 assessment by the Intergovernmental Panel on Climate Change but with tighter error bars. The researchers, led by Daniel Murphy of the National Oceanic and Atmospheric Administration in Boulder, Colorado, say that the agreement gives confidence both in their technique and in the global circulation models used by the IPCC. They also found that the aerosols’ effects over time reflected the stabilized amount of atmospheric sulfates that resulted from the adoption of emission controls in North America in the 1970s. (D. M. Murphy et al., J. Geophys. Res. 114, D17107, 2009, doi: 10.1029/2009JD012105.) —Stephen G. Benka

A journey to Earth's center would take us through the crust, the mantle, and then two regions of the core. Innermost of those is an iron crystalline solid; surrounding that is a freely flowing, conducting fluid, capped by a thin boundary layer that abuts the rocky mantle. The presence of the fluid core is deduced from seismic studies, and its dynamics can be extracted—with a few assumptions—from time-dependent observations of Earth's surface magnetic field (see Physics Today, February 2008, page 31). A new study does just that, by using nearly 150 years' worth of surface magnetic measurements to determine the magnetic flux in the liquid core. Motions there are modeled with a set of 20 nested cylinders coaxial with Earth's rotation axis and the key assumption that fluid waves in the core must balance Lorentz, Coriolis, buoyancy, and pressure forces. Jean Dickey (Jet Propulsion Laboratory) and Olivier de Viron (Institute of Earth Physics, Paris, and University Paris Diderot) found four robust modes of angular-momentum oscillations corresponding to waves—with periods of 85, 50, 35, and 28 years and diminishing amplitudes—that propagate inward from the core–mantle boundary. There was some previous observational evidence for two of the modes, and theorists had predicted all four modes having similar periods. Now, the strong concurrence of all the results lends credibility to the new modes' existence. (J. O. Dickey, O. de Viron, Geophys. Res. Lett. 36, L15302, 2009.) —Stephen G. Benka

The Arctic Ocean’s floating sea-ice cover naturally waxes and wanes with the seasons. But the decline in area covered by perennial ice—that which survives the summer melt—has been accelerating in recent years. The nonlinear trend, no doubt, reflects the Arctic’s response to a warming climate: Thanks to the ice albedo–ocean feedback, a drop in ice cover increases the absorption of solar radiation, which warms the ocean, prolongs the melting, and reduces ice cover yet further. For quantitative details on the ice pack’s changing mass and heat capacity, though, researchers need more than areal measurements. Ronald Kwok and colleagues at NASA and the University of Washington have now published what may be the most comprehensive thickness maps of the entire Arctic basin. The lidar system they use aboard NASA’s ICESat can precisely distinguish height differences between the sea surface and ice floes. After measuring that “freeboard,” Kwok and company use Archimedes’ principle to calculate the portion that’s underwater. Because much of the brine drains from sea ice as it ages, making it more reflective to microwaves, the researchers are also able to distinguish between young, seasonal ice and older, perennial ice. The surveys taken by ICESat over five years reveal that the Arctic has thinned by about 0.7 m and lost 1.5 × 10⁶ km² of perennial ice—more than twice the area of Texas. Moreover, the volume of perennial ice shrank by 57%—so much that seasonal ice has become dominant for the first time on record. (R. Kwok et al., J. Geophys. Res. 114, C07005, 2009, doi:10.1029/2009JC005312.)—R. Mark Wilson

Atmospheric aerosols affect climate: The particles scatter, absorb, and emit radiation, and they also induce cloud formation. (See Physics Today, May 2004, page 24.) Much of the aerosol mass is produced by oxidation of organic compounds emitted into the atmosphere through human activity and from the biosphere. But many aspects of aerosol formation are poorly understood. It’s thought that biogenic aerosols might be formed from isoprene, a light hydrocarbon given off in large quantities by certain trees. Isoprene itself is volatile—at sea level it boils at 34 °C—so it must undergo a series of chemical reactions before it can form long-lived aerosol particles. Now Caltech’s Paul Wennberg, graduate student Fabien Paulot, and their colleagues offer some insight into what those reactions are. In laboratory experiments designed to replicate atmospheric conditions far from any human pollution, they found that isoprene reacts several times with OH radicals to ultimately form large amounts of dihydroxyepoxide, a newly identified airborne molecule and a likely aerosol precursor. Its hydroxyl groups make it hydrophilic, so it should be readily taken up by existing aqueous aerosol particles. And epoxides under acidic conditions can form low-volatility polymers; a similar reaction is used in epoxy adhesive. Knowing the reactants and their mechanisms will help researchers improve atmospheric models to better predict the consequences of human activity such as deforestation and pollution. (F. Paulot et al., Science 325, 730, 2009.) —Johanna Miller

Iceland, located on top of the Mid-Atlantic Ridge, is one of the most geologically active places on Earth. Its Katla volcano—buried deep under a huge glacier—is particularly large and active: It tends to explosively erupt every 50 or 60 years, although its most recent eruption was back in 1918. One type of nearby seismic activity is so-called long-period (lp) earthquakes, which have shallow origins and magnitudes of less than 3.3. Swarms of those quakes are sometimes thought to indicate an imminent eruption; after more than 900 lp events took place in October 2002, local authorities developed evacuation plans. But Kristín Jónsdóttir (shown here making field measurements) and her colleagues at Uppsala University in Sweden conclude that the glacier, not the volcano, is the culprit. As the glacier gradually flows down a long valley, part of it reaches a cliff where gargantuan 80-meter-thick ice sheets break off and drop 100 meters, carrying more than enough energy to account for the seismicity. The researchers analyzed data from more than 14 000 lp events in 1991–2007. The events' locations, waveforms, seasonal variations, and other variables all pointed to the calving outlet glacier as the seismic source. The authors note that continued global warming could increase such seismicity. For more on glaciers and earthquakes, see Physics Today, September 2008, page 17. (K. Jónsdóttir et al., Geophys. Res. Lett. 36, L11402, 2009, doi:10.1029/2009GL038234.) —Stephen G. Benka

The hallmark of a conventional crystal such as table salt is translational symmetry. Quasicrystals do not have that symmetry and so can exhibit a wider structural variety than their more constrained brethren. But quasicrystals, like crystals, do have long-range correlations and display sharp, structure-revealing diffraction patterns. To date, more than 100 quasicrystals have been synthesized in the lab. Now Luca Bindi of the Natural History Museum of Florence has teamed up with Paul Steinhardt and colleagues from Princeton and Harvard universities to present evidence for a natural version of one of those quasicrystals: icosahedral Al₆₃Cu₂₄Fe₁₃. The material, a 100-μm grain, is from a mineral assemblage (left figure) excavated from the Koryak Mountains in Russia and now housed in the Florence museum; the very complexity of the sample argues for its natural formation. In consultation with his US-based colleagues, Bindi identified the sample as possibly hosting a quasicrystal. The US team then probed a small piece of it with transmission electron microscopy. Diffraction patterns such as shown in the right figure identified quasicrystal regions; the 10-fold symmetry cannot be generated by crystals. Subsequent analysis of x rays scattered off pure quasicrystal grains determined the material’s chemical formula. Geologists and physicists have much to learn about the conditions under which quasicrystals form. The study of natural materials can help address that question and may turn up new, never-before contemplated structures. (L. Bindi et al., Science 324, 1306, 2009.) —Steven K. Blau

In an effort to reduce the pervasive smog in Beijing (see photo), Chinese authorities imposed measures to restrict traffic and close factories around the city during the 2008 Olympics. Were those efforts successful in reducing total atmospheric aerosol? Climate scientists Jan Cermak and Reto Knutti at ETH Zürich in Switzerland attempted to find out. They began by comparing absolute values of aerosol optical thickness—transmittance measurements from the Moderate-Resolution Imaging Spectroradiometer aboard NASA's Terra satellite—for the years 2002–08. They found that within a 150-km radius of Beijing, the average 2008 AOT value was more than 14% lower than the previous years. But what would it have been without the mandated emissions reductions? To answer that question, the researchers used a neural network approach: With data from the preceding six summers, they trained a model to predict AOT as a function of relative humidity, wind velocity, and precipitation. The model then predicted that within a 500-km radius of the city, AOTs in 2008 would have been 10%–14% higher than the actual observed values; the model was less accurate when larger regions were analyzed. Although the magnitude of the reductions was lower than expected, the emissions restrictions did have a statistically significant local impact. (J. Cermak, R. Knutti, Geophys. Res. Lett. 36, L10806, 2009, doi:10.1029/2009GL038572. Photo by Michael Silverman, 6 August 2006.)—Jermey N. A. Matthews

The couplings between Earth's solid surface and atmosphere are a rich area for study. For example, it is known that soil "respiration" plays a large role in the global water cycle. Researchers have long assumed that diffusion is the dominant mechanism for transferring gases across the interface between air and soil or rock, enhanced somewhat by wind- and pressure-fluctuation-driven transport. But scientists working in Israel's Negev Desert have uncovered a surprisingly important new mechanism: In regions where Earth's porous surface has cracks, fractures, or other discontinuities, thermal convection can expel, on a daily basis, up to 200 times more gas than diffusion, depending on the surrounding conditions. The team from Ben Gurion and Oregon State Universities installed arrays of sensors in the large--2-cm-wide, 1-m-deep--crack in the foreground of the photograph: Temperature and relative-humidity data were acquired every 10 minutes for more than two years. Every day, the Sun's warmth propagates slowly down through the rock until, as the evening cools, the air in the crack becomes less dense than the overlying atmosphere, and convection sets in. Venting of warm, moist air from the rock surrounding the crack, along with the entrainment of cool dry atmospheric air, then continues until dawn. Convection takes place for up to 19 hours a day in the winter, 12 during the summer. The process, say the researchers, is natural and pervasive and could have a large impact on Earth-atmosphere gas exchange. Also, because water vapor, carbon dioxide, and other gases are involved, there may be implications for climate change studies. (N. Weisbrod et al., Geophys. Res. Lett. 36, L02401, 2009, doi:10.1029/2008GL036096 — Stephen G. Benka

Through its influence on evaporation rates, humidity levels, and other factors, the moisture content of soil has a significant impact on weather. Accurate measurements of that content, though important for meteorological, hydrological, and ecological forecasting, are difficult to make. Extrapolating point measurements to larger areas is inaccurate, and satellite-based remote-sensing methods are hindered by ground cover, surface roughness, and other limitations. A team from the University of Arizona and the Southwest Watershed Research Center in Tucson has shown that just above the ground surface, so-called fast neutrons with energies on the order of an MeV are quantifiably correlated with soil moisture and thus provide a noninvasive means for measuring the average moisture levels over regions several hundred meters wide and tens of centimeters deep. The neutrons are generated by cosmic rays. Upon collision with atmospheric nuclei, cosmic rays create showers of high-energy secondary particles, and those that reach Earth's surface can penetrate it, collide with nuclei there, and produce among their debris fast neutrons, some of which escape back into the atmosphere. Marek Zreda and colleagues discovered that hydrogen, mostly found in water, dominates soil's ability to moderate fast neutrons and that a strong inverse correlation, independent of soil chemistry, exists between moisture content and the intensity of the fast neutrons that escape out of the ground. The team demonstrated that with an independent measurement of the moisture content for calibration, a neutron detector a few meters above the ground can give precise measurements of soil moisture on the time scale of minutes to a few hours. As the figure shows, the hourly soil moisture determined by a cosmic-ray neutron detector (top) agrees with that determined by time-domain reflectometry probes (middle) and with the monitored daily precipitation (bottom). (M. Zreda et al., Geophys. Res. Lett. 35, L21402, 2008, doi:10.1029/2008GL035655.) — Richard J. Fitzgerald

As concern over global warming continues to grow, pressure and funding are increasing to find ways to reduce the growth and, in time, the actual levels of atmospheric carbon dioxide (see PHYSICS TODAY, August 2008, page 26). Peter Kelemen and Jürg Matter of Columbia University's Lamont-Doherty Earth Observatory have proposed a new approach for CO₂ sequestration: accelerating the natural carbonation of exposed mantle rock. In many places around the globe—perhaps most dramatically in Oman—sections of the upper mantle have been raised through subduction or tectonic spreading. The resulting outcrops, termed ophiolites, are rich in peridotite, a rock primarily composed of the minerals olivine and pyroxene. (For more on the Oman ophiolite, see PHYSICS TODAY, January 2005, page 21.) Strongly out of chemical equilibrium with the atmosphere, the mantle rock naturally reacts with water and CO₂ to form silicates, carbonates, and iron oxides. Kelemen and Matter find that atmospheric CO₂ reacts with peridotite surprisingly quickly, at a rate of about 4 × 10⁷ kg/yr for the 500-km-long Oman ophiolite. The researchers suggest several options for boosting that reaction rate even higher, starting with increasing the interaction volume by drilling and fracturing the peridotite. Some fracturing will happen spontaneously as the hydration and carbonation reactions expand the rock volume and give off heat. When the two scientists incorporate into their model the effects of raising the CO₂ concentration near the rock and elevating the peridotite temperature, they estimate a potential increase of 10⁹ in the reaction rate, or 2 × 10⁹ tons of CO₂ captured and sequestered each year per cubic kilometer of ophiolite. The researchers call for further modeling and field testing of what could be a permanent storage solution. (P. B. Kelemen, J. Matter, Proc. Natl. Acad. Sci. USA 105, 17295, 2008.) — Richard J. Fitzgerald

Seismic sensors at the surface of a borehole near the epicenter of a magnitude-6.9 earthquake this year in Japan revealed unpredicted asymmetry in the vertical wave amplitudes at the soil surface: The largest upward acceleration was more than twice that of the largest downward acceleration. The data also showed that the soil surface layer was tossed upward at nearly four times the gravitational acceleration— more than twice the peak horizontal acceleration. These findings run contrary to current structural engineering models, which presume that seismic waves from earthquakes shake the ground horizontally more than vertically. Shin Aoi and colleagues at Japan’s National Research Institute for Earth Science and Disaster Prevention propose what they call a trampoline model to explain the observed nonlinear bouncing behavior. In their model, the soil undergoes compression in the upward direction and behaves as a rigid mass with no intrinsic limit on acceleration, much like an acrobat rebounding from a trampoline (figures 1 and 3). In the downward direction, though, dilatational strains break up the soil and the loose particles fall freely at or below gravitational acceleration (figures 2 and 4). The observed seismographic data were simulated by combining the theoretical waveform from the trampoline model with selected borehole data that resembled elastic deformation of a deformable mass. The researchers say that other events need to be analyzed to learn how material conditions affect vertical ground response during large earthquakes. (S. Aoi et al., Science 322, 727, 2008.) — Jermey N.A. Matthews

Related links:

Kik-Net: Japanese network of strong-motion seismographs
United States Geological Survey ShakeMaps
Stanford University Quake Catcher Network

Different isotopes of the same element don’t always behave identically in chemical reactions. As a result, naturally occurring samples can have measurably different ratios of stable isotopes. In most observed isotope fractionation, deviations in reactivity vary with the mass difference between isotopes, due either to kinetic effects or to differences in the zero-point vibrational energy of chemical bonds. Last year Bridget Bergquist and Joel Blum of the University of Michigan in Ann Arbor found that photochemical reactions of mercury can result in isotope fractionation that does not fit the mass-dependent pattern: Odd-numbered Hg isotopes behave differently from even-numbered ones. Such mass-independent fractionation, observed in only a few elements so far, may be due to spin–spin interactions between nuclei and the unpaired electrons created in light-initiated reactions. Now, Abir Biswas, working with Blum and other Michigan colleagues, has found that Hg stored in coal deposits shows the effects of both mass-dependent and mass-independent fractionation. Moreover, coal samples from different regions—the US, China, and Russia–Kazakhstan—bear different Hg isotopic signatures. The researchers suggest that those signatures could provide some information about how Hg pollution (produced when the coal is burned) circulates in the environment, a process that is poorly understood. (A. Biswas et al., Environ. Sci. Technol., in press.) — Johanna L. Miller

During an effusive volcanic eruption—one that produces flowing lava, as shown here, as opposed to projectile material or clouds of ash—civil authorities need to know which way and how far the lava will flow so that they can decide whether and when to order evacuations. But lava is a difficult fluid to model, because as it cools, it crystallizes and eventually stops flowing. Robert Wright and colleagues at the University of Hawaii in Honolulu have developed a new model for forecasting lava flows. Their model combines two previously published ones: FLOWGO, which simulates lava's heat loss to predict how far it will flow before solidifying, and DOWNFLOW, which takes a stochastic approach to predict the lava's direction. Since DOWNFLOW’s stochastic method is computationally simple—but still accurate—Wright and colleagues’ model yields results much more quickly than other forecasting techniques. Moreover, FLOWGO accounts for the effusion rate—the rate of lava coming out of the ground—which strongly affects the flow length and can change substantially over the course of an eruption. When combined with satellite monitoring of the effusion rate, Wright and colleagues’ model can potentially provide updated forecasts in near real time. (R. Wright, H. Garbeil, A. J. L. Harris, Geophys. Res. Lett., in press.) —Johanna L. Miller