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排序方式: 共有473条查询结果,搜索用时 15 毫秒
41.
The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181 at the lunar surface. We report tungsten isotope data for lunar metals, which contain no 181Ta-derived cosmogenic 182W. The data reveal differences in indigenous 182W/184W of lunar mantle reservoirs, indicating crystallization of the lunar magma ocean 4.527 +/- 0.010 billion years ago. This age is consistent with the giant impact hypothesis and defines the completion of the major stage of Earth's accretion. 相似文献
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Sex pheromone of the winter moth, a geometrid with unusually low temperature precopulatory responses
Roelofs WL Hill AS Linn CE Meinwald J Jain SC Herbert HJ Smith RF 《Science (New York, N.Y.)》1982,217(4560):657-659
The sex pheromone for the winter moth, Operophtera brumata (L.), has been identified as the novel compound (Z,Z,Z)-1,3,6,9-nonadecatetraene. The male moths respond to the pheromone at low temperatures (4 degrees to 15 degrees C) and exhibit an upper response limit that coincides with the lower response limit for other reported moth sex pheromone systems. The pheromone attracted two other geometrid species, O. bruceata (Bruce spanworm) and O. occidentalis. 相似文献
44.
Atlas R Campbell P Cozzarelli NR Curfman G Enquist L Fink G Flanagin A Fletcher J George E Hammes G Heyman D Inglesby T Kaplan S Kennedy D Krug J Levinson R Marcus E Metzger H Morse SS O'Brien A Onderdonk A Poste G Renault B Rich R Rosengard A Salzburg S Scanlan M Shenk T Tabor H Varmus H Wimmer E Yamamoto K;Journal Editors Authors Group 《Science (New York, N.Y.)》2003,299(5610):1149
45.
Whitney D Goltz HC Thomas CG Gati JS Menon RS Goodale MA 《Science (New York, N.Y.)》2003,302(5646):878-881
Although the visual cortex is organized retinotopically, it is not clear whether the cortical representation of position necessarily reflects perceived position. Using functional magnetic resonance imaging (fMRI), we show that the retinotopic representation of a stationary object in the cortex was systematically shifted when visual motion was present in the scene. Whereas the object could appear shifted in the direction of the visual motion, the representation of the object in the visual cortex was always shifted in the opposite direction. The results show that the representation of position in the primary visual cortex, as revealed by fMRI, can be dissociated from perceived location. 相似文献
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47.
Lichtenegger HC Schöberl T Bartl MH Waite H Stucky GD 《Science (New York, N.Y.)》2002,298(5592):389-392
Biominerals are widely exploited to harden or stiffen tissues in living organisms, with calcium-, silicon-, and iron-based minerals being most common. In notable contrast, the jaws of the marine bloodworm Glycera dibranchiata contain the copper-based biomineral atacamite [Cu2(OH)3Cl]. Polycrystalline fibers are oriented with the outer contour of the jaw. Using nanoindentation, we show that the mineral has a structural role and enhances hardness and stiffness. Despite the low degree of mineralization, bloodworm jaws exhibit an extraordinary resistance to abrasion, significantly exceeding that of vertebrate dentin and approaching that of tooth enamel. 相似文献
48.
A twenty-one-year-old male sandbar shark (Carcharhinus plumbeus) demonstrated signs of chronic weight loss even though its appetite remained unchanged. In addition, the gradual development of a red, circular lesion on its ventral body was noted. After months without any significant changes, a ventral fistula appeared revealing the tip of a wire exposed within its center. The shark was immobilized via intramuscular remote injection through dartgun with etomidate; removed from its exhibit tank; and clinically examined with radiographic imaging, a cell blood count/serum chemistry evaluation, and the use of a metal detector along the body wall. A metallic hook was identified in the coelom about 10 cm cranial to the external fistula. The shark was transferred to an isolation pool for 1 mo. A second immobilization via immersion in eugenol was conducted in order to perform a celiotomy. The hook was located in a liver lobe and was surgically removed. After a prolonged recovery from anesthesia, the shark was released into its primary tank and recovered uneventfully, although some of the skin sutures sloughed prematurely. 相似文献
49.
Gene E. Likens F. Herbert Bormann John S. Eaton Robert S. Pierce Noye M. Johnson 《Water, air, and soil pollution》1976,6(2-4):435-445
Being downwind of eastern and midwestern industrial centers, the Hubbard Brook Experimental Forest offers a prime location to monitor long-term trends in atmospheric chemistry. Continuous measurements of precipitation chemistry during the last 10 yr provide a measure of recent changes in precipitation inputs of H ion. The weighted average pH of precipitation during 1964–65 to 1973–74 was 4.14, with a minimum annual value of 4.03 in 1970–71 and a maximum annual value of 4.21 in 1973–74. The sum of all cations except H ion decreased from 51 μeq 1?1 in 1964–65 to 23 μeq 1?1 in 1973–74 providing a significant drop in neutralizing capacity during this period. Based upon regression analysis, the input in equivalents of H ion and nitrate increased by 1.4-fold and 2.3-fold respectively, from 1964–65 to 1973–74. Input of all other ions either decreased or showed no trend. Based upon a stoichiometric formation process in which a sea-salt, anionic component is subtracted from the total anions in precipitation, SO4 =, contribution to acidity dropped from 83% to 66%, whereas NO3 ? increased from 15% to 30% during 1964–65 to 1973–74. The increased annual input of H ion at Hubbard Brook during the past 10 yr is highly correlated with the increased input of nitrate in precipitation. 相似文献
50.