Articles Alert

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. Department of Earth Sciences Open University Milton Keynes, U.K. A combined paleomagnetic, sedimentological, and stratigraphic study of a Late Devonian iridium anomaly in Western Australia suggests that the iridium was concentrated over a long period by purely biological processes. In short, meteorite impact is not the only way of generating iridium anomalies in the earth's crust. N.F. Hurley, R. Van der Voo, "Magnetostratigraphy, Late Devonian iridium anomaly, and impact hypotheses," Geology, 18, 291-4, April 1990. (University of Michigan, Ann Arbor) The breakup of Pangea may have been a two-stage process involving two distinct types of faulting. Global continental angular momentum in the hotspot frame of reference would have been high during the period 180 million to 150 million years ago, leading to passive rifting arising from motion-induced stresses. By contrast, angular momentum over the period 140 million to 90 million years ago would have been low, leading to active rifting resulting from the buildup of heat beneath a near-stationary land mass (the Anderson model). A. Hynes, "Two-stage rifting of Pangea by two different mechanisms," Geology, 18, 323-6, April 1990. (McGill University, Montreal, Quebec) A new global model of current plate motions (NUVEL-1), which incorporates new and better data obtained since the PO71 and RM2 models were presented in 1978, has been devised. The new model, based on spreading rates of the past 3 million years, comprises 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. It differs significantly from the previous models but fits the data much better. (For example, NUVEL-1's systematic misfitting of spreading rates nowhere exceeds 3 millimeters per year.) C. DeMets, R.G. Gordon, D.F. Argus, S. Stein, "Current plate motions," Geophysical Journal International, 101, 425-78, May 1990. (Naval Research Laboratory, Washington, D.C.; Northwestern University, Evanston, Ill.) Concentration on the study of Jurassic specimens has obscured ichthyosaurian affinities, with the result that ichthyosaurs are usually placed in their own subclass of reptiles. However, studies of the skull roofs and palatal features of the more primitive Triassic specimens now suggest that ichthyosaurs should be included in the subclass Diopsida. Triassic and Jurassic ichthyosaurs differed not only in morphology but also in ecological behavior. The former probably tended to be ambush predators, whereas the latter were mainly pursuit predators. J.A. Massare, J.M. Callaway, "The affinities and ecology of Triassic ichthyosaurs," Geological Society of America Bulletin, 102, 409-16, April 1990. (University of Rochester, N.Y.) Department of Chemistry University of Texas Austin Quantitative knowledge of the thermodynamic factors influencing molecular recognition promises to be increasingly important as systems containing multiple recognition sites are designed. Variable temperature binding has been used to reveal significant heat capacity effects and a predominantly enthalpic origin for stabilization through cation-p complexation. D.A. Stauffer, R.E. Barrans, Jr., D.A. Dougherty, "Concerning the thermodynamics of molecular recognition in aqueous and organic media. Evidence for significant heat capacity effects," Journal of Organic Chemistry, 55, 2762-7, 27 April 1990. (California Institute of Technology, Pasadena) Although aggregates in which tetrahedra share faces are rare, a recent paper describes the electronic structures of several classes of chains of this aesthetically pleasing type. C. Zheng, R. Hoffmann, D.R. Nelson, "A helical face-sharing tetrahedron chain with irrational twist, stella quadrangula, and related matters," Journal of the American Chemical Society, 112, 3784-91, 9 May 1990. (Cornell University, Ithaca, N.Y.; Harvard University, Cambridge, Mass.) The synthesis and characterization of a purely organic ferromagnet with the highest spin multiplicity (10 parallel spins) known to date are reported. I. Fujita, Y. Teki, T. Takui, T. Kinoshita, et al., "Design, preparation, and electron spin resonance detection of a ground-state undecet (S=5) hydrocarbon," JACS, 112, 4074-5, 9 May 1990. (Osaka City University, Japan; Nagoya University, Japan; Institute for Molecular Science, Okazaki, Japan) Molecular dynamics simulations, even in such complex environments as the active site of RNase A, have now become sophisticated enough to supplement structural and chemical data. This allows testing of the feasibility of repositioning important residues as they fulfill their catalytic roles. K. Haydock, C. Lim, A.T. Brunger, M. Karplus, "Simulation analysis of structures on the reaction pathway of RNAse A," JACS, 112, 3826-31, 9 May 1990. (Harvard University, Cambridge, Mass.) The Cambridge Structural Database provides a source for surveying conformational preference in classes of molecules. Here, such a tabulation shows that preferential conformations of Lewis acid-carbonyl complexes and pligated organometallic complexes can be understood on the basis of the effects of p-bonding hybridization and frontier molecular orbital theory. This information thus provides an analysis relevant to mechanisms and transition state structures of synthetically important transformations. S. Shambayati, W.E. Crowe, S.L. Schreiber, "On the conformation and structure of organometal complexes in the solid state: two studies relevant to chemical synthesis," Angewandte Chemie, International Edition in English, 29, 256-72, March 1990. (Harvard University, Cambridge, Mass.) Department of Computer Science University of Pittsburgh Pittsburgh, Pa. As we rely more on software to keep the wheels of our technological society turning, we must pay even more attention to the consequences of program failures. A recent article examines some of the many issues involved in testing software. Separate measures of reliability and trustworthiness are proposed, with discussion on how to estimate these quantities. Managers and programmers alike will benefit from bringing these considerations into current practice. D.L. Parnas, J.A. van Schouwen, S.P. Kwan, "Evaluation of safety-critical software," Communications of the ACM, 33, 636-48, June 1990. (Queen's University, Kingston, Ontario; private contributor) Before optical disk technology, the lowest-cost mass storage medium available was microfilm. A model has been presented of an optical disk-based document storage and retrieval system for office environments, in which throughput and users' wait time are determined. J.N. Daigle, R.B. Kuehl, J.D. Langford, "Queueing analysis of an optical disk jukebox based office system," IEEE Transactions on Computers, 39, 819-28, June 1990. (Virginia Polytechnic Institute and State University, Blacksburg; Eastman Kodak Co., Rochester, N.Y.; University of Washington, Seattle) Security and privacy considerations have occupied computer scientists since the 1940s and are the focus of a special issue of an IEEE journal. In one article, a practical method is presented for identifying possible channels for storing and retrieving information, some of which may be illicit. The method has been applied to the operating system Secure Xenix, and results are described. C.-R. Tsai, V.D. Gligor, C.S. Chandersekaran, "On the identification of covert storage channels in secure systems," IEEE Transactions on Software Engineering, 16, 569-80, June 1990. (VDG Inc., Chevy Chase, Md.; University of Maryland, College Park; IBM Corp., Gaithersburg, Md.) The theoretically most efficient known algorithm for computing a minimum cost path between two nodes of a graph is Dijkstra's algorithm. Efficient implementations depend on the data structures used. An interesting investigation has been published of the variations on the heap data structure used in the algorithm under the special case that all arc costs are integers of moderate size. R.K. Ahuja, K. Mehlhorn, J.B. Orlin, R. Tarjan, "Faster algorithms for the shortest path problem," Journal of the Association for Computing Machinery, 37, 213-23, April 1990. (Massachusetts Institute of Technology, Cambridge; Universit„t des Saarlandes, Saarbrücken, W.Ger.; Princeton University, N.J.) Reading text from arbitrary books and journals is a challenge in computer vision because of problems with printing quality, such as changing typefaces, ink quality, and paper stock. However, one approach that exploits the semantics of a domain (in this case, chess) is shown to achieve high levels of accuracy. A program implemented in C averages about 10 minutes per page for analysis. H.S. Baird, K. Thompson, "Reading chess," Machine Intelligence, 12, 552-9, June 1990. (Princeton University, N.J.; University of California, Berkeley) Occasionally, a researcher needs a simple hashing function without consideration of the theoretical underpinnings. One has recently been presented, and its performance on 26,662 dictionary entries shows uniform distribution of entries into the 256 hash bias. P.K. Pearson, "Fast hashing of variable-length text strings," Communications of the ACM, 33, 677-80, June 1990. (Lawrence Livermore National Laboratory, Calif.) School of Natural Sciences Institute for Advanced Study Princeton, N.J. A rich variety of queer but fascinating and useful materials-- among them, various liquid crystals that turn electronic signals into visual displays for computer terminals--is presented in a collection of related articles. P. Day, D.C. Bradley, D. Bloor, editors, "Molecular chemistry for electronics," Philosophical Transactions of the Royal Society of London, 330A, 71-215, 23 February 1990. (The University, Hull, England; Universit„t Mainz, W.Ger.; et al.) The superfluid phases of isotopically pure 3 He have proved to be exceptionally fertile sources of intricate and exotic phenomena. Roughly speaking, whereas the superfluidity of 4 He is a simple Bose condensation for spherical atoms, and in most superconductors the Cooper pairs are spinless spherical molecules, in 3 He the Cooper pairs are spinning cigars. These objects can organize themselves into a bewildering variety of textures and flow patterns. Many of the topological dreams of quantum field theory find realizations here. An accessible introduction by one of the leading experimental wizards in the field is presented. O. Lounasmaa, G. Pickett, "The 3 He superfluids," Scientific American, 262, 104-11, June 1990. (Academy of Finland and Helsinki University of Technology; University of Lancaster, England) Magnetic fields penetrate type II superconductors in vortices less than a micron in radius. The properties of vortices are crucial to all high-field applications of superconductivity. New techniques are making it possible to get a good look at the internal structure of the electric fields inside vortices. H. Hess, R. Robinson, J. Waszczak, "Vortex-core structure observed with a scanning tunneling microscope," Physical Review Letters, 64, 2,711-14, 28 May 1990. (AT&T Bell Laboratories, Murray Hill, N.J.) In chemical reactions, much of the action takes place on sub-picosecond time scales. Amazing techniques of laser spectroscopy allow a good look into this realm. M. Grubele, A.H. Zewail, "Ultrafast reaction dynamics," Physics Today, 43, 24-33, May 1990. (California Institute of Technology, Pasadena) Old pulsars, which have been "spun up" by stealing matter from a less dense massive companion, are very lively and interesting objects that have provoked a good deal of investigation lately. The huge gravitational and centrifugal forces governing such objects, combined with their thoughtfulness in providing means whereby we can observe them, make them an exciting testing ground for high-energy astrophysics. D.C. Backer, S.R. Kulkarni, "A new class of pulsars," Physics Today, 43, 26-35, March 1990. (University of California, Berkeley; Caltech, Pasadena)

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