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51.
Cassini images of Saturn's small inner satellites (radii of less than approximately 100 kilometers) have yielded their sizes, shapes, and in some cases, topographies and mean densities. This information and numerical N-body simulations of accretionary growth have provided clues to their internal structures and origins. The innermost ring-region satellites have likely grown to the maximum sizes possible by accreting material around a dense core about one-third to one-half the present size of the moon. The other small satellites outside the ring region either may be close to monolithic collisional shards, modified to varying degrees by accretion, or may have grown by accretion without the aid of a core. We derived viscosity values of 87 and 20 square centimeters per second, respectively, for the ring material surrounding ring-embedded Pan and Daphnis. These moons almost certainly opened their respective gaps and then grew to their present size early on, when the local ring environment was thicker than it is today. 相似文献
52.
Selective sparing of a class of striatal neurons in Huntington's disease 总被引:29,自引:0,他引:29
R J Ferrante N W Kowall M F Beal E P Richardson E D Bird J B Martin 《Science (New York, N.Y.)》1985,230(4725):561-563
A distinct subpopulation of striatal aspiny neurons, containing the enzyme nicotinamide adenine dinucleotide phosphate diaphorase, is preserved in the caudate nucleus in Huntington's disease. Biochemical assays confirmed a significant increase in the activity of this enzyme in both the caudate nucleus and putamen in postmortem brain tissue from patients with this disease. The resistance of these neurons suggests that the gene defect in Huntington's disease may be modifiable by the local biochemical environment. This finding may provide insight into the nature of the genetically programmed cell death that is a characteristic of the disease. 相似文献
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Recent current velocity measurements across the lower continental rise of Nova Scotia show a deep equatorwardflow with speeds (maximum, 73 centimeters per second) among the highest recorded for the deep sea. Silicate measurements indicate that this flow usually consists of southern-source (Antarctic) bottom water. These measurements confirm the existence of a second and deeper western boundary flow that was earlier inferredfrom geological observations. 相似文献
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Photooxidation of cumene (isopropylbenzene) and pyridine in dilute solution in natural waters gives products characteristic of reactions with alkylperoxy (RO(2).) and hydroxyl (HO.) radicals. On the basis of the rates of formation of the products, the average concentrations of RO(2). and HO. are estimated to be about 10(-9) and 10(-17) mole per liter, respectively. The concentration of RO(2). is large enough that, for some classes of reactive chemicals, oxidation can be an important process in natural waters. 相似文献
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Richardson JR 《Science (New York, N.Y.)》1981,211(4487):1161-1163
Assumptions made from studies of sparse living faunas of brachiopods, namely, that they are intolerant of mud, that the free-lying habit is confined to species without pedicles, and that the pedicle of articulate brachiopods is uniform in structure and function, do not withstand critical examination. Studies in New Zealand show that some species in the same area occur in both attached and free-lying populations. Individuals cannot always be differentiated morphologically, but the structure of populations from hard and soft substrates is distinctive. Attachment to a substrate appears to be a larval rather than an adult requirement in most species. 相似文献
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