September 2009 Archives

In the 10 years since researchers at the Joint Institute for Nuclear Research in Dubna, Russia, first reported producing the superheavy element 114 (see Physics Today, April 1999, page 21), some tens of other sightings of the element (as well as elements 115, 116, and 118) have been documented—but all by the same group. Now a team at Lawrence Berkeley National Laboratory, led by Heino Nitsche and Ken Gregorich, has confirmed the results. Such independent verification is important, particularly given evidence of fabricated results for other superheavy elements (see Physics Today, September 2002, page 15), but it is complicated by experimental challenges, including picobarn (10⁻⁴⁰ m²) reaction cross sections and radioactive targets. Working with the lab's 88-inch cyclotron, the Berkeley team followed a process similar to that used at Dubna: They aimed an accelerated beam of calcium-48 ions at a target containing plutonium-242. The reaction products passed through a gas-filled mass spectrometer, which separated out the nuclei of interest, to a detector that yielded energy and timing information not only for the products but also for any alpha particles or fission fragments they emitted. Amid the data the researchers collected over their eight days of running the experiment, they found two correlated chains of decays that they identified as starting with ²⁸⁶114 and ²⁸⁷114. Although the lifetimes (tenths of seconds), decay modes, and decay energies agree with the Dubna results, the cross sections measured by the Berkeley team are lower. That discrepancy, say the researchers, could be due to statistical fluctuations or to some of the element-114 nuclei overshooting the detector. (L. Stavsetra et al., Phys. Rev. Lett. 103, 132502, 2009.)—Richard J. Fitzgerald

In a single cell, thousands of simultaneously occurring biochemical reactions carry out such functions as converting and storing energy and regulating nutrient levels; together, those processes make up the cell’s metabolic network. Computational biology involves, among other things, the linking of metabolic pathways to form a metabolic network model, a promising tool for preclinical drug studies and other medical research. However, such computational models do not traditionally include the function-determining structural details of a network’s macromolecules; for example, an enzyme’s ability to catalyze reactions and regulate the cell’s response to external stimuli is determined by its three-dimensional configuration. Now, an international team led by Adam Godzik at the Burnham Institute for Medical Research in California has taken a rare step and introduced atomic-level protein structural data to the metabolic network model of an ancient thermophilic bacterium, Thermotoga maritima, shown in this optical microscope image. The image also shows schematics of proteins in their 3D configurations, which, when they were expressed in the reconstructed metabolic network, helped the research team solve the puzzle of how proteins evolve when their cell networks grow larger.

It turns out that only 37% of T. maritima’s proteins are essential to the formation of its metabolic network; those “core-essential” proteins adopt the bulk—61%—of the bacterium’s relatively few unique 3D configurations. The finding suggests that the core-essential proteins evolved their structure to perform additional functions in distinct pathways. (Y. Zhang et al., Science 325, 1544, 2009.)—Jermey N.A. Matthews

Of the more than 350 planets that have been observed to date orbiting alien suns, only a handful have known densities. (For more on exoplanets, see Physics Today, May 2009, page 46.) Now an international team led by Didier Queloz (Geneva Observatory, Switzerland) has introduced the newest member of that club, Corot-7b, the only exoplanet to have a measured density comparable to that of Earth. The result suggests that Corot-7b, like Earth, has a rocky composition. The planet and its host star Corot-7 were named after the Convection, Rotation, and Planetary Transits satellite, which in 2008 observed a star whose intensity periodically dimmed once every 20.4 hours—the result of a planet partially eclipsing the star. The small degree of dimming and the known size of the star established that Corot-7b has a radius about twice that of Earth’s. To obtain the mass of the planet, Queloz and colleagues turned to the High Accuracy Radial Velocity Planet Searcher, a spectrograph that could precisely measure the sinusoidally modulating Doppler shifts in the light emitted by Corot-7 as it is gravitationally tugged to and fro by the small planet orbiting it. The greater the amplitude of oscillation, the greater the planet’s mass. The result required that the Queloz team filter out obscuring noise due to sunspot-like activity on the star. As a bonus, the residual signal included a component with a 3.69-day period. Queloz and colleagues attribute that to a previously unknown second planet, about eight times as massive as Earth. (D. Queloz et al., Astron. Astrophys., in press, doi:10.1051/0004-6361/200913096.) —Steven K. Blau

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

Concrete is the most prevalent synthetic material on Earth, yet the detailed nature of its primary binding constituent, hydrated cement, is only poorly understood. When cement, a dry powder that consists mostly of calcium oxide and silicate, is mixed with water, the material hardens through the formation of a complex hydrated oxide called calcium-silicate-hydrate. But the microscopic structure of C-S-H is largely unknown—even its stoichiometry, as suggested by convention with hyphens. C-S-H's structure had been thought to be related to that of two naturally occurring calcium silicate minerals, but those minerals can't explain C-S-H's observed properties. Armed with recent measurements of C-S-H's density and its ratio of calcium to silicon atoms, a team of researchers at MIT has proposed a new molecular model for C-S-H based on atom-scale simulations: Layers of calcium ions (gray in the figure) are surrounded by silicon (yellow) and oxygen (red) arranged as short silica chains one, two, and five units long; between those layers are water (oxygen in blue, hydrogen in white) and interlayer calcium ions (green) that ensure overall neutrality. The model's chemical composition, (CaO)_1.65(SiO₂)(H₂O)_1.75, agrees well with results from neutron scattering experiments. In addition to reproducing the known structural properties of the material, the model also suggests that at short length scales C-S-H should be viewed as a glassy phase. With an atom-level model of the C-S-H structure now in hand, the researchers hope to be able to manipulate the macroscopic properties of concrete, such as its strength and temperature resistance. (R. J.-M. Pellenq et al., Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0902180106, in press.)—Richard J. Fitzgerald

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

Turbulent flows of a liquid along a surface experience frictional drag, a macroscopic phenomenon that affects the speed and efficiency of marine vessels, the cost of pumping oil through a pipeline, and countless other engineering parameters. The drag arises from shear stress, the rate per unit area of momentum transfer from the flow to the surface. To reduce the flux, engineers could add polymers to the flow, inject bubbles against the surface, or combine the two methods, but those approaches bear their own cost. Jonathan Rothstein and colleagues at the University of Massachusetts Amherst now offer a proof-of-principle demonstration of a new, passive option for reducing drag in turbulent flow. They tailored the microscale structure of a hydrophobic material—polydimethylsiloxane, similar to the rubbery polymer used to caulk bathtubs—to create air pockets at the surface, as shown in the figure, that allow the flow to “slip” (shear free) at the liquid–air interface. The greater the area covered by air pockets, the greater the reduction in shear stress—up to 50%, they estimate, judging from particle-image velocimetry and pressure-drop experiments over a wide range of Reynolds numbers. The researchers found that the critical Reynolds number at which the onset of drag reduction occurs is related to the ratio of two length scales—one associated with the geometry of the hydrophobic surface corrugations, the other with the thickness of the viscous boundary layer there. (R. J. Daniello, N. E. Waterhouse, J. P. Rothstein, Phys. Fluids 21, 085103, 2009.) —R. Mark Wilson

Rocky planets like our own are thought to form by accretion. Small dust grains agglomerate to make bigger grains, then pebbles, rocks, boulders—all the way up to objects the size of Mars. At that threshold—about 1/10 of Earth's mass—a rocky protoplanet has consumed all the locally available material. To grow further, Mars-sized protoplanets must collide and merge. There's ample evidence throughout the solar system for impacts of the scale needed to complete Earth's formation. Now, a team led by Carey Lisse of the Johns Hopkins University in Baltimore has found evidence of a giant impact outside the solar system, in a dusty disk around a nearby star. Lisse and his colleagues did not detect the impact itself, shown here in an artist's impression. Rather, they found IR spectral features attributable to the impact's blasted and ground-up debris: amorphous silica dust. They also detected the product of vaporized planetary crust: silicon monoxide gas. From those spectral features, Lisse could deduce that the impact spalled a mass of dust at least as big as Pluto, 10²² kg. If the impactor had the same mass and enough kinetic energy to create the observed SiO, then its impact speed must have been 10 km/s. Such high relative velocities are not uncommon in the solar system. Indeed, the Mars-sized object that struck Earth to form the Moon had a relative velocity of that magnitude. (C. M. Lisse et al., Astrophys. J. 701, 2019, 2009.)—Charles Day