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The Scientist has asked a group of experts to comment periodically upon recent articles that they have found noteworthy. Their selections, presented here in every issue, are neither endorsements of content nor the result of systematic searching. Rather, the list represents personal choices of articles the columnists believe the scientific community as a whole may also find interesting. Reprints of any articles cited here may be ordered through The Genuine Article, 3501 Market St., Philadelphia, Pa. 19104, or by telephoning (215) 386-4399. Author: PETER J. SMITH Department of Earth Sciences Open University Milton Keynes, U.K. Molecular orbit calculations suggest that potassium is not enhanced in sulfur, iron, or iron sulfides at the high pressures of the earth's outer core. Thus, most of the earth's potassium cannot be in the core, and the earth therefore cannot be chondritic in potassium. Nor can the geomagnetic core dynamo be powered by the radioactive decay of 40K. D.M. Sherman, "Chemical bonding and the incorporation of potassium into the Earth's core," Geophysical Research Letters, 17, 693-6, May 1990. (U.S. Geological Survey, Denver) Tephra layers occurring as partings in the Tertiary coal beds of the Kenai lowland, Alaska, enable the frequency of ancient volcanic events to be determined. The layers show, for example, that just over 10.5 million years ago there was a volcanic pulse with activity occurring every 125 to 500 years; whereas from 10.5 to 7.5 million years ago, volcanic events occurred only once every 9,000 years. Such coal records are less complete than those from deep-sea cores but, where they exist, they are more detailed. L.M. Reinink-Smith, "Relative frequency of Neogene volcanic events as recorded in coal partings from the Kenai lowland, Alaska: a comparison with deep-sea core data," Geological Society of America Bulletin, 102, 830-40, June 1990. (University of Alaska, Fairbanks) The frequency of ancient volcanic events in southern Italy may be estimated from tephra in deep-sediment cores from the Mediterranean Sea. During the past 190,000 years, volcanic pulses have occurred roughly every 23,000 years. In addition, each episode apparently correlates with the state of evolution of magma in the region's magma chamber. M. Paterne, J. Labeyrie, F. Guichard, A. Mazaud, F. Maitre, "Fluctuations of the Campanian explosive volcanic activity (South Italy) during the past 190,000 years, as determined by marine tephrochronology," Earth and Planetary Science Letters, 98, 166-74, May 1990. (Domaine du Centre National de la Recherche Scientifique [CNRS], Gif sur Yvette, France) Examination of all known earthquakes in central Greece over the period 1890 to 1988 shows that some tectonic structures known to be active have been aseismic for the past 100 years. A century of earthquake data is thus insufficient for either an assessment of seismic risk or a confident estimation of maximum magnitude. N.N. Ambraseys, J.A. Jackson, "Seismicity and associated strain of central Greece between 1890 and 1988," Geophysical Journal International, 101, 663-708, June 1990. (Imperial College of Science and Technology, London; et al.) A multichannel seismic reflection survey in the western North Atlantic has revealed a remarkable array of features in the Mesozoic oceanic crust. These include banded layering in the lower crust (and, occasionally, the mantle), subhorizontal reflectors in the upper crust, dipping reflectors in the lower crust, and dipping reflectors cutting through the entire crust. R.S. White, R.S. Detrick, J.C. Mutter, P. Buhl, et al., "New seismic images of oceanic crustal structure," Geology, 18, 462-5, May 1990. (University of Cambridge, England; University of Rhode Island, Kingston; et al.) A modeling study of oceanic circulation suggests that a doubling of atmospheric carbon dioxide would result in a mean sea level rise of 19 centimeters owing to thermal expansion alone. Meanwhile, ground observations reveal that between 1976 and 1987 the Arctic ice lost 15 percent of its volume over an area of 300,000 square kilometers. However, because the annual variations in the thickness of Arctic ice are unknown, the loss cannot be attributed to global warming with certainty. U. Mikolajewicz, B.D. Santer, E. Maier-Reimer, "Ocean response to greenhouse warming," Nature, 345, 589-93, 14 June 1990. (Max-Planck Institut für Meteorologie, Hamburg, W.Ger.) P. Wadhams, "Evidence for thinning of the Arctic ice cover north of Greenland," Nature, 345, 795-7, 28 June 1990. (University of Cambridge, England) Author: MARYE ANNE FOX Department of Chemistry University of Texas Austin Progress in understanding enzyme-catalyzed concerted reactions requires access to an enzyme amenable to physical study; that is, one that retains the activity of the native complex catalyst. The use of recombinant DNA techniques to prepare a low molecular weight chorismate mutase, containing only 109 amino acids, is described. J. Stewart, D.B. Wilson, B. Ganem, "A genetically engineered monofunctional chorismate mutase," Journal of the American Chemical Society, 112, 4582-4, 23 May 1990. (Cornell University, Ithaca, N.Y.) Micelle structures are often ambiguous. Yet they typically exhibit useful properties with respect to solubilization of apolar substances by complexation in water. A description is presented of the formation of a fully aromatic water-soluble hyper- branched polymer that is capable of generating an environment that resembles a micelle. Y.H. Kim, O.W. Webster, "Water-soluble hyperbranched polyphenylene: `A unimolecular micelle'?" JACS, 112, 4592-3, 23 May 1990. (E.I. Du Pont de Nemours & Co. Inc., Wilmington, Del.) Lacking both ã and n electrons, silicon atoms provide unique stabilization of carbocations from the à, á, and ç positions. An overview of evidence documenting the effect of silicon is presented, and a rationale for the stabilization of silicon-containing cations is offered. J.B. Lambert, "The interaction of silicon with positively charged carbon," Tetrahedron, 46 (8), 2677-89, 1990. (Northwestern University, Evanston, Ill.) Chiral organometallic compounds can precisely distinguish atoms, groups, and faces in chiral or prochiral organic molecules, and can catalyze the formation of enantiomerically pure substances. Examples of such compounds covered are the addition of dialkyl zinc to aldehydes and asymmetric hydrogenations using chiral Rhodium(I) and Ruthenium(II) complexes. R. Noyori, "Chiral metal complexes as discriminating molecular catalysts," Science, 248, 1194-9, 8 June 1990. (Nagoya University, Japan) Semiconductor clusters represent aggregates with a structural complexity between that of simple monomeric constituent molecules and bulk semiconductor solids. An overview of their synthesis and physical characterization by theory and experimentation is provided. M.L. Steigerwald, L.E. Brus, "Semiconductor crystallites: a class of large molecules," Accounts of Chemical Research, 23, 183-8, June 1990. (AT&T Bell Laboratories, Murray Hill, N.J.) Antibody-mediated ç-imaging or á-therapy of tumors occurs via conjugation with bifunctional chelation reagents containing a radiometal. The synthesis and properties of an aqueous, bridged commobis(dicarbollide) cluster, which may serve as a biologically inviolable radio-carrier, are described. M.F. Hawthorne, A. Varadarajan, C.B. Knobler, S. Chakrabarti, et al., "Radiometallacarboranes as tumor imaging reagents," JACS, 112, 5365-6, 20 June 1990. (University of California, Los Angeles; et al.) Author: FRANK A. WILCZEK School of Natural Sciences Institute for Advanced Study Princeton, N.J. Delicate and ingenious methods of keeping ultra-accurate time, and some of their applications, are discussed in the 1989 Nobel Prize in Physics acceptance lecture. N. Ramsey, "Experiments with separated oscillatory fields and hydrogen masers," Science, 248, 1612-18, 29 June 1990. (Harvard University, Cambridge, Mass.) Currently, the most accurate clocks are probably not actually man-made ones at all. Rather, they are rotating neutron stars (pulsars). Very few known physical effects are capable of interfering with their periodicity. Among those that can are some extremely interesting ones, such as the emission of gravity waves, or the distortion of space between us and the neutron star owing to gravity waves from the early universe. Cosmological phase transitions of various sorts (associated with symmetry breaking in quantum chromodynamics [QCD], the weak interactions, or even grand unification) will produce gravity waves of this sort. Accurate timing of the famous binary pulsar over the past seven years has begun to put some constraints on these possibilities. D.R. Stinebring, M.F. Ryba, J.H. Taylor, "Cosmic gravitational-wave background: limits from millisecond pulsar timing," Physical Review Letters, 65, 285-8, 16 July 1990. (Princeton University, N.J.; Institute for Advanced Study, Princeton, N.J.) A very entertaining and informative review of competing theories and relevant observations in the field of formation of galaxies and other large-scale structures is presented in the form of a table of odds. P.J.E. Peebles, J. Silk, "A cosmic book of phenomena," Nature, 346, 233-9, 19 July 1990. (Princeton University, N.J.; University of California, Berkeley) The idea of compensating for the fluctuating atmosphere in real time in order to obtain sharper astronomical images using ground-based telescopes is an old one. Recent developments, some involving ideas of considerable ingenuity, promise to soon bring this long-standing dream to reality. Planned telescopes using these principles are designed to reach resolution quite close to Hubble's (planned) resolution. H.W. Babcock, "Adaptive optics revisited," Science, 249, 253-7, 20 July 1990. (The Observatories of the Carnegie Institution of Washington, Pasadena, Calif.) Author: BRUCE G. BUCHANAN Department of Computer Science University of Pittsburgh Pittsburgh, Pa. Stable matching problems are those in which each person in a specified group of agents is to be matched with another according to the preferences of both. Examples include assigning roommates and assigning medical interns to hospitals. Results show that the intern and the roommate assignment problems with ties are NP-complete. The author also shows that allowing ties makes this class of problems more difficult. E. Ronn, "NP-complete stable matching problems," Journal of Algorithms, 11, 285-304, June 1990. (Technion-Israel Institute of Technology, Haifa) The n-dimensional Boolean cube, with 2ü vertices whenever their binary labels differ in only one coordinate, is the basis of several multiprocessor architectures. Many computational advantages of hypercubes derive from the ability to embed other graph structures in a hypercube. Substantial past work on subgraph isomorphism has established that deciding if a general graph is a partial subgraph of a hypercube is NP-complete. Similar results are given for trees. A. Wagner, D.G. Corneil, "Embedding trees in a hypercube is NP-complete," SIAM Journal on Computing, 19, 570-90, June 1990. (University of British Columbia, Vancouver; University of Toronto, Ontario) Computing with power series (for example, multiplying two series) is simplified in this paper by translating the series into streams of coefficients with the exponents being given implicitly by ordinal position. Communicating sequential processes over data streams reduces the bookkeeping that normally accompanies the recursive computations. This suggests a different style of programming, which the author predicts will become increasingly important. M.D. McIlroy, "Squinting at power series," Software--Practice & Experience, 20, 661-83, July 1990. (AT&T Bell Laboratories, Murray Hill, N.J.) Local area networks (LANs) are becoming more essential. A recent article, written for network managers, discusses implications of an emerging network standard, the IEEE 10 Base T standard. Advantages of telephone twisted pair Ethernets include fault tolerance and standardization for multiple LANs. D. Zwicker, P. Nikolich, P. Andres, "Ethernet over twisted pair: not as easy as it looks," Telecommunications, 24, 23-8, June 1990. (Racal Interlab, Bosborough, Mass., et al.) "Synthetic aperture radar (SAR) techniques provide images from which surface topography can be calculated. Because the physics of SAR imaging is different, however, existing shape-from-shading algorithms are not directly applicable. A new algorithm is proposed and has been tested with good results. R.T. Frankot, R. Chellappa, "Estimation of surface topography from SAR imagery using shape from shading techniques," Artificial Intelligence, 43, 271-310, June 1990. (Hughes Aircraft Co., El Segundo, Calif., et al.)

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