共查询到20条相似文献，搜索用时 234 毫秒
1.
McClintock WE Izenberg NR Holsclaw GM Blewett DT Domingue DL Head JW Helbert J McCoy TJ Murchie SL Robinson MS Solomon SC Sprague AL Vilas F 《Science (New York, N.Y.)》2008,321(5885):62-65
During MESSENGER's first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercury's surface. 相似文献
2.
In a first electrophysiological study of worm vision, electroretinograms were measured in two alciopid worms: Torrea, taken at the surface, and deep-sea Vanadis. Both forms possess a primary retina in the focal plane of the lens, and accessory retinas lying beside the lens. Such accessory retinas occur also in deep sea fishes and cephalopods. In Torrea the primary retina peaks in sensitivity at 400 nanometers, the secondary retina at 560 nanometers. Both together could serve as a depth guage, since 560 nanometers attenuates much faster in seawater than 400 nanometers. The Vanadis eyes peaked in sensitivity at 460 to 480 nanometers, a property shared with deep-sea forms of other phyla; and appropriate, since these wavelengths penetrate seawater most deeply, and also are the wavelengths of maximum bioluminescence. 相似文献
3.
Lightly etched single-walled carbon nanotubes are chemically reacted to form rings. The rings appear to be fully closed as opposed to open coils, as ring-opening reactions did not change the structure of the observed rings. The average diameter of the rings was 540 nanometers with a narrow size distribution. The nanotubes in solution were modeled as wormlike polymer chains, yielding a persistence length of 800 nanometers. Nanotubes shorter than this length behave stiffly and stay nearly straight in solution. However, nanotubes longer than the Kuhn segment length of 1600 nanometers undergo considerable thermal fluctuation, suggesting a greater flexibility of these materials than is generally assumed. 相似文献
4.
G Medeiros-Ribeiro AM Bratkovski TI Kamins DAA Ohlberg RS Williams 《Science (New York, N.Y.)》1998,279(5349):353-355
Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been shown to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change. 相似文献
5.
Tensile experiments of fully dense nanocrystalline structures with a mean grain size of less than 100 nanometers demonstrate a considerable increase in hardness but a remarkable drop in elongation-to-failure, indicating brittle behavior. However, dimple structures are often observed at the fracture surface, indicating some type of ductile fracture mechanism. Guided by large-scale atomistic simulations, we propose that these dimple structures result from local shear planes formed around clustered grains that, because of their particular misorientation, cannot participate in the grain boundary accommodation processes necessary to sustain plastic deformation. This raises the expectation that general high-angle grain boundaries are necessary for good ductility. 相似文献
6.
Patterson EM 《Science (New York, N.Y.)》1981,211(4484):836-838
The absorption properties, expressed as a wavelength-dependent imaginary index of refraction, of the Mount St. Helens ash from the 18 May 1980 eruption were measured between 300 and 700 nanometers by diffuse reflectance techniques. The measurements were made for both surface and stratospheric samples. The stratospheric samples show imaginary index values that decrease from approximately 0.01 to 0.02 at 300 nanometers to about 0.0015 at 700 nanometers. The surface samples show less wavelength variation in imaginary refractive index over this spectral range. 相似文献
7.
We analyzed the glassy-state structural relaxation of polymers near surfaces and interfaces by monitoring fluorescence in multilayer films. Relative to that of bulk, the rate of structural relaxation of poly(methyl methacrylate) is reduced by a factor of 2 at a free surface and by a factor of 15 at a silica substrate interface; the latter exhibits a nearly complete arresting of relaxation. The distribution in relaxation rates extends more than 100 nanometers into the film interior, a distance greater than that over which surfaces and interfaces affect the glass transition temperature. 相似文献
8.
Veverka J Robinson M Thomas P Murchie S Bell JF Izenberg N Chapman C Harch A Bell M Carcich B Cheng A Clark B Domingue D Dunham D Farquhar R Gaffey MJ Hawkins E Joseph J Kirk R Li H Lucey P Malin M Martin P McFadden L Merline WJ Miller JK 《Science (New York, N.Y.)》2000,289(5487):2088-2097
Eros is a very elongated (34 kilometers by 11 kilometers by 11 kilometers) asteroid, most of the surface of which is saturated with craters smaller than 1 kilometer in diameter. The largest crater is 5.5 kilometers across, but there is a 10-kilometer saddle-like depression with attributes of a large degraded crater. Surface lineations, both grooves and ridges, are prominent on Eros; some probably exploit planes of weakness produced by collisions on Eros and/or its parent body. Ejecta blocks (30 to 100 meters across) are abundant but not uniformly distributed over the surface. Albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith. On scales of 200 meters to 1 kilometer, Eros is more bland in terms of color variations than Gaspra or Ida. Spectra (800 to 2500 nanometers) are consistent with an ordinary chondrite composition for which the measured mean density of 2.67 +/- 0.1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent. 相似文献
9.
Flasar FM Achterberg RK Conrath BJ Pearl JC Bjoraker GL Jennings DE Romani PN Simon-Miller AA Kunde VG Nixon CA Bézard B Orton GS Spilker LJ Spencer JR Irwin PG Teanby NA Owen TC Brasunas J Segura ME Carlson RC Mamoutkine A Gierasch PJ Schinder PJ Showalter MR Ferrari C Barucci A Courtin R Coustenis A Fouchet T Gautier D Lellouch E Marten A Prangé R Strobel DF Calcutt SB Read PL Taylor FW Bowles N Samuelson RE Abbas MM Raulin F Ade P Edgington S Pilorz S Wallis B Wishnow EH 《Science (New York, N.Y.)》2005,307(5713):1247-1251
Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites. 相似文献
10.
The magnetization directions of individual monodomain nanoparticles as small as 5 nanometers in diameter are determined using the Foucault method of Lorentz microscopy. A model is developed to explain the images and diffraction patterns of samarium cobalt nanoparticles as a function of the aperture shift direction. Thermally induced changes in the magnetization direction of superparamagnetic magnetite nanoparticles were observed but with a much slower rate than expected, due to surface anisotropy. When the time scale for magnetization reversal is much shorter than the data acquisition time, as in carbon-coated iron cobalt alloy nanoparticles, the images show an average of such thermally induced changes. 相似文献
11.
The amount of carbon dioxide that can be dissolved in surface seawater depends at least partially on the homogeneous buffer factor, which is a mathematical function of the chemical equilibrium conditions among the various dissolved inorganic species. Because these equilibria are well known, the homogeneous buffer factor is well known. Natural spatial variations depend very systematically on sea surface temperatures, and do not contribute significantly to uncertainties in the present or future carbon dioxide budget. 相似文献
12.
Sandroff CJ Harbison JP Ramesh R Andrejco MJ Hegde MS Chang CC Vogel EM Hwang DM 《Science (New York, N.Y.)》1989,245(4916):391-393
Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga(2)O(3) and As(2)O(3)), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime. 相似文献
13.
Nickel metal catalysts composed of nanometer by micrometer strips have been produced with solid-state microfabrication techniques. The strips are actually the edges of nickelcatalyst thin films, which are sandwiched between separating support layers, which are also nanometers thick. These linear nanostructures constitute well-defined and well-controlled catalytic entities that reproduce the size of traditional supported metal clusters in one dimension, thus separating size from total number of atoms in the catalyst. Examination of their catalytic activity showed that they duplicate the behavior of conventional supported clusters. A specific rate maximum was observed for the hydrogenolysis of ethane at a nanoscale dimension similar to the cluster size at which the rate is maximum in the case of the supported cluster studies, whereas the hydrogenation of ethylene shows no such size dependency. The results suggest that the surface-to-volume ratio or the number of atoms in the catalytic entity cannot be the source of these size effects and that either support effects or nonequilibrium surface structures are the determining factors. 相似文献
14.
Lateral variations in membrane composition are postulated to play a central role in many cellular events, but it has been difficult to probe membrane composition and organization on length scales of tens to hundreds of nanometers. We present a high-resolution imaging secondary ion mass spectrometry technique to reveal the lipid distribution within a phase-separated membrane with a lateral resolution of approximately 100 nanometers. Quantitative information about the chemical composition within small lipid domains was obtained with the use of isotopic labels to identify each molecular species. Composition variations were detected within some domains. 相似文献
15.
Kulmala M Riipinen I Sipilä M Manninen HE Petäjä T Junninen H Maso MD Mordas G Mirme A Vana M Hirsikko A Laakso L Harrison RM Hanson I Leung C Lehtinen KE Kerminen VM 《Science (New York, N.Y.)》2007,318(5847):89-92
Atmospheric aerosol formation is known to occur almost all over the world, and the importance of these particles to climate and air quality has been recognized. Although almost all of the processes driving aerosol formation take place below a particle diameter of 3 nanometers, observations cover only larger particles. We introduce an instrumental setup to measure atmospheric concentrations of both neutral and charged nanometer-sized clusters. By applying the instruments in the field, we come to three important conclusions: (i) A pool of numerous neutral clusters in the sub-3 nanometer size range is continuously present; (ii) the processes initiating atmospheric aerosol formation start from particle sizes of approximately 1.5 nanometers; and (iii) neutral nucleation dominates over the ion-induced mechanism, at least in boreal forest conditions. 相似文献
16.
Switzer JA Raffaelle RP Phillips RJ Hung CJ Golden TD 《Science (New York, N.Y.)》1992,258(5090):1918-1921
Cleaved cross sections of nanometer-scale ceramic superlattices fabricated from materials of the lead-thallium-oxygen system were imaged in the scanning tunneling microscope (STM). The apparent height differences between the layers were attributed to composition-dependent variations in local electrical properties. For a typical superlattice, the measured modulation wavelength was 10.6 nanometers by STM and 10.8 nanometers by x-ray diffraction. The apparent height profile for potentiostatically deposited superlattices was more square than that for galvanostatically deposited samples. These results suggest that the composition follows the applied potential more closely than it follows the applied current. The x-ray diffraction pattern of a superlattice produced under potential control had satellites out to the fourth order around the (420) Bragg reflection. 相似文献
17.
Toxic potential of materials at the nanolevel 总被引：12，自引：0，他引：12
Nanomaterials are engineered structures with at least one dimension of 100 nanometers or less. These materials are increasingly being used for commercial purposes such as fillers, opacifiers, catalysts, semiconductors, cosmetics, microelectronics, and drug carriers. Materials in this size range may approach the length scale at which some specific physical or chemical interactions with their environment can occur. As a result, their properties differ substantially from those bulk materials of the same composition, allowing them to perform exceptional feats of conductivity, reactivity, and optical sensitivity. Possible undesirable results of these capabilities are harmful interactions with biological systems and the environment, with the potential to generate toxicity. The establishment of principles and test procedures to ensure safe manufacture and use of nanomaterials in the marketplace is urgently required and achievable. 相似文献
18.
Spectral tuning of pigments underlying red-green color vision 总被引：18，自引：0，他引：18
Variations in the absorption spectra of cone photopigments over the spectral range of about 530 to 562 nanometers are a principal cause of individual differences in human color vision and of differences in color vision within and across other primates. To study the molecular basis of these variations, nucleotide sequences were determined for eight primate photopigment genes. The spectral peaks of the pigments specified by these genes spanned the range from 530 to 562 nanometers. Comparisons of the deduced amino acid sequences of these eight pigments suggest that three amino acid substitutions produce the approximately 30-nanometer difference in spectral peaks of the pigments underlying human red-green color vision, and red shifts of specific magnitudes are produced by replacement of nonpolar with hydroxyl-bearing amino acids at each of the three critical positions. 相似文献
19.
Prolonged exposure of rhesus monkeys to intense blue light produces long-term changes which are consistent with loss of response of those cones that contain a photopigment with peak absorption at 445 nanometers. The 90 percent reduction of spectral sensitivity in the blue region has lasted more than 5 months. Reduced sensitivity to long wavelengths is produced by adaptation to light of 520 nanometers. This reduced sensitivity, which lasts no more than 30 days, is attributed to a temporary loss of response of the cones containing a photopigment with peak absorption at 535 nanometers. 相似文献
20.
Ion microprobe analyses show that solar wind nitrogen associated with solar wind hydrogen implanted in the first tens of nanometers of lunar regolith grains is depleted in 15N by at least 24% relative to terrestrial atmosphere, whereas a nonsolar component associated with deuterium-rich hydrogen, detected in silicon-bearing coatings at the surface of some ilmenite grains, is enriched in 15N. Systematic enrichment of 15N in terrestrial planets and bulk meteorites relative to the protosolar gas cannot be explained by isotopic fractionation in nebular or planetary environments but requires the contribution of 15N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin and never equilibrated with the 15N-depleted protosolar nebula. 相似文献