emissia.offline ART 156
Physics News Update #259
(Новости Физики - бюллетень Американского Института Физики)
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 259 February 21, 1996 by Phillip F. Schewe and Ben Stein
NEW STUDIES CONFIRM THAT CFCs CAUSE OZONE DEPLETION. Because of the "political sensitivity of the ozone- depletion issue," a team of NASA (Langley) and UC Irvine scientists undertook satellite measurements of stratospheric hydrogen chloride and hydrogen fluoride. Their four-year study, the Halogen Occultation Experiment (HALOE), shows that chlorofluorocarbon emissions, and not natural emissions arising, say, from oceans and biomass burning (sources which can now be accounted for), are chiefly to blame for the recent global buildup of stratospheric chlorine and the consequential depletion of ozone. International agreements limiting CFC emissions have already begun to lessen the growth of chlorine in the stratosphere. (James Russell et al., Nature, 8 February 1996.)
DARK MATTER, LIKE LUMINOUS MATTER, IS HIERARCHICAL. That is, it congregates at the galactic level and at the level of galaxy clusters. This view is based on new observations made with the orbiting Japanese x-ray telescope ASCA, which recorded x-ray emissions from gas in the Fornax galaxy cluster (Y. Ikebe et al., Nature, 1 February 1996). The density of dark matter at any location is deduced from the density of the hot (up to 10**8 K) gas, which is probably held in equilibrium by the gravitational influence of the unseen dark matter thereabouts. The ASCA scientists suggest that one explanation of their measurements is the presence of two different kinds of dark matter. This is in keeping with some hybrid cosmological models which propose that cold dark matter (e.g., axions) influences affairs at the galactic level and hot dark matter (e.g., massive neutrinos) at the cluster level. (Science News, 10 Feb. 1996.)
ONE OF THE GREAT MYSTERIES OF WATER, its tendency to shrink when warmed, has been successfully modeled for the first time. As cold water is heated, it reaches a minimum volume--and therefore a maximum density--at around 4 degrees Celsius. No theoretical model has been able to explain this "density anomaly." This shortcoming compromises the accuracy of molecular-scale models of proteins and other systems involving water. Now, researchers at Texas Tech University (G. Wilse Robinson, 806- 742-3099) have proposed an explanation for the density anomaly by looking beyond neighboring molecules in the liquid and focusing on more distant "second neighbors." In all ten known forms of ice, an H20 molecule is surrounded by its closest neighbors in the same way. However, in low-density ice, second-neighbor molecules are relatively distant from the central molecule, while in some of the higher-pressure, dense forms of ice, oxygens belonging to second- neighbor molecules bend around to form a closer approach to the central molecule. The researchers propose that in the liquid these "bent" second neighbors would have somewhat less stability than those in the low-density arrangement and therefore the denser bent neighbors would form only at warmer temperatures. Using a simple model in which an oxygen atom and its second neighbors arrange themselves on a one-dimensional array, the Texas Tech researchers obtained density curves with a temperature and pressure behavior similar to that of water. (C.H. Cho et al., upcoming article in Physical Review Letters.)