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排序方式: 共有1089条查询结果,搜索用时 15 毫秒
81.
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Trombka JI Squyres SW Bruckner J Boynton WV Reedy RC McCoy TJ Gorenstein P Evans LG Arnold JR Starr RD Nittler LR Murphy ME Mikheeva I McNutt RL McClanahan TP McCartney E Goldsten JO Gold RE Floyd SR Clark PE Burbine TH Bhangoo JS Bailey SH Petaev M 《Science (New York, N.Y.)》2000,289(5487):2101-2105
We report major element composition ratios for regions of the asteroid 433 Eros imaged during two solar flares and quiet sun conditions during the period of May to July 2000. Low aluminum abundances for all regions argue against global differentiation of Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive, chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial melting or impact volatilization. 相似文献
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A copper-nitrosyl intermediate forms during the catalytic cycle of nitrite reductase, the enzyme that mediates the committed step in bacterial denitrification. The crystal structure of a type 2 copper-nitrosyl complex of nitrite reductase reveals an unprecedented side-on binding mode in which the nitrogen and oxygen atoms are nearly equidistant from the copper cofactor. Comparison of this structure with a refined nitrite-bound crystal structure explains how coordination can change between copper-oxygen and copper-nitrogen during catalysis. The side-on copper-nitrosyl in nitrite reductase expands the possibilities for nitric oxide interactions in copper proteins such as superoxide dismutase and prions. 相似文献
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Murphy R 《Science (New York, N.Y.)》1939,89(2305):203-204
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Environmental change and Antarctic seabird populations 总被引:1,自引:0,他引:1
Recent changes in Antarctic seabird populations may reflect direct and indirect responses to regional climate change. The best long-term data for high-latitude Antarctic seabirds (Adélie and Emperor penguins and snow petrels) indicate that winter sea-ice has a profound influence. However, some effects are inconsistent between species and areas, some in opposite directions at different stages of breeding and life cycles, and others remain paradoxical. The combination of recent harvest driven changes and those caused by global warming may produce rapid shifts rather than gradual changes. 相似文献
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Ablation, flux, and atmospheric implications of meteors inferred from stratospheric aerosol 总被引:1,自引:0,他引:1
Single-particle analyses of stratospheric aerosol show that about half of the particles contain 0.5 to 1.0 weight percent meteoritic iron by mass, requiring a total extraterrestrial influx of 8 to 38 gigagrams per year. The sodium/iron ratio in these stratospheric particles is higher and the magnesium/iron and calcium/iron ratios are lower than in chondritic meteorites, implying that the fraction of material that is ablated must lie at the low end of previous estimates and that the extraterrestrial component that resides in the mesosphere and stratosphere is not of chondritic composition. 相似文献