共查询到20条相似文献，搜索用时 15 毫秒
1.
Earth-based telescopic observations indicate that Saturn's rings are about 1 kilometer thick, while spacecraft measurements and theoretical considerations give an upper bound of about 100 meters. Analysis of a shielding effect present in radio occultation provides a sensitive new measure of the ring thickness. On the basis of this effect, Voyager 1 microwave measurements of near-forward scatter imply a thickness ranging from less than 10 meters in ring C to about 20 and 50 meters in the Cassini division and ring A, respectively. Monolayer models do not fit the observations in the latter two regions. The discrepancy between the Earth-based and spacecraft measurements may be due to warps in the ring plane or effects of tenuous material outside the primary ring system. 相似文献
2.
Hinteregger HF Shapiro II Robertson DS Knight CA Ergas RA Whitney AR Rogers AE Moran JM Clark TA Burke BF 《Science (New York, N.Y.)》1972,178(4059):396-398
Very-long-baseline interferometry experiments, involving observations of extragalactic radio sources, were performed in 1969 to determine the vector separations between antenna sites in Massachusetts and West Virginia. The 845.130-kilometer baseline was estimated from two separate experiments. The results agreed with each other to within 2 meters in all three components and with a special geodetic survey to within 2 meters in length; the differences in baseline direction as determined by the survey and by interferometry corresponded to discrepancies of about 5 meters. The experiments also yielded positions for nine extragalactic radio sources, most to within 1 arc second, and allowed the hydrogen maser clocks at the two sites to be synchronized a posteriori with an uncertainty of only a few nanoseconds. 相似文献
3.
Coseismic and Postseismic Fault Slip for the 17 August 1999, M = 7.5, Izmit, Turkey Earthquake 总被引：2，自引：0，他引：2
Reilinger RE Ergintav S Bürgmann R McClusky S Lenk O Barka A Gurkan O Hearn L Feigl KL Cakmak R Aktug B Ozener H Töksoz MN 《Science (New York, N.Y.)》2000,289(5484):1519-1524
We use Global Positioning System (GPS) observations and elastic half-space models to estimate the distribution of coseismic and postseismic slip along the Izmit earthquake rupture. Our results indicate that large coseismic slip (reaching 5.7 meters) is confined to the upper 10 kilometers of the crust, correlates with structurally distinct fault segments, and is relatively low near the hypocenter. Continued surface deformation during the first 75 days after the earthquake indicates an aseismic fault slip of as much as 0.43 meters on and below the coseismic rupture. These observations are consistent with a transition from unstable (episodic large earthquakes) to stable (fault creep) sliding at the base of the seismogenic zone. 相似文献
4.
Cheng AF Barnouin-Jha O Zuber MT Veverka J Smith DE Neumann GA Robinson M Thomas P Garvin JB Murchie S Chapman C Prockter L 《Science (New York, N.Y.)》2001,292(5516):488-491
During the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker's low-altitude flyover of asteroid 433 Eros, observations by the NEAR Laser Rangefinder (NLR) have helped to characterize small-scale surface features. On scales from meters to hundreds of meters, the surface has a fractal structure with roughness dominated by blocks, structural features, and walls of small craters. This fractal structure suggests that a single process, possibly impacts, dominates surface morphology on these scales. 相似文献
5.
Jones JH 《Science (New York, N.Y.)》1969,163(3874):1449-1450
Measurements of current speed and direction from the sea surface to below 400 meters made on the equator in the eastern Pacific in April 1968 indicate that the Equatorial Undercurrent extends from 300 meters to the sea surface. These measurements, when compared with previous observations, indicate that eastward motion at the surface is a result of surfacing of the undercurrent caused by a release of the surface wind stress. 相似文献
6.
Sea level measurements from tide gauges at Miami, Florida, and Cat Cay, Bahamas, and bottom pressure measurements from a water depth of 50 meters off Jupiter, Florida, and a water depth of 10 meters off Memory Rock, Bahamas, were correlated with 81 concurrent direct volume transport observations in the Straits of Florida. Daily-averaged sea level from either gauge on the Bahamian side of the Straits was poorly correlated with transport. Bottom pressure off Jupiter had a linear coefficient of determination ofr(2) = 0.93, and Miami sea level, when adjusted for weather effects, had r(2) = 0.74; the standard errors of estimating transports were +/- 1.2 x 10(6) and +/- 1.9 x 10(6) cubic meters per second, respectively. A linear multivariate regression, which combined bottom pressure, weather, and the submarine cable observations between Jupiter and the Bahamas, had r(2) = 0.94 with a standard error of estimating transport of +/- 1.1 x 10(6) cubic meters per second. These results suggest that a combination of easily obtained observations is sufficient to adequatelv monitor the daily volume transport fluctuations of the Florida Current. 相似文献
7.
When range and Doppler observations of space probes near or on Moon are reduced by use of a lunar ephemeris calculated from the Brown lunar theory, residuals as large as 440 meters in position and 1.5 millimeters per second in velocity are observed. When the calculations are repeated with use of LE 5, the integrated lunar ephemeris described (1), the residuals are greatly reduced. 相似文献
8.
DH Hathaway PA Gilman JW Harvey F Hill RF Howard HP Jones JC Kasher JW Leibacher JA Pintar GW Simon 《Science (New York, N.Y.)》1996,272(5266):1306-1309
Doppler velocity observations obtained by the Global Oscillation Network Group (GONG) instruments directly measure the nearly steady flows in the solar photosphere. The sun's differential rotation is accurately determined from single observations. The rotation profile with respect to latitude agrees well with previous measures, but it also shows a slight north-south asymmetry. Rotation profiles averaged over 27-day rotations of the sun reveal the torsional oscillation signal-weak, jetlike features, with amplitudes of 5 meters per second, that are associated with the sunspot latitude activity belts. A meridional circulation with a poleward flow of about 20 meters per second is also evident. Several characteristics of the surface flows suggest the presence of large convection cells. 相似文献
9.
Zenk W 《Science (New York, N.Y.)》1981,213(4512):1113-1114
During an almost yearlong period of observations made with a current meter in the fracture zone between the Falkland Islands (Islas Malvinas) and South Georgia, several overflow events were recorded at a depth of 3000 meters carrying cold bottom water from the Scotia Sea into the Argentine Basin. The outflow bursts of Scotia Sea bottom water, a mixing product of Weddell Sea and eastern Pacific bottom water, were associated with typical speeds of more than 28 centimeters per second toward the northwest and characteristic temperatures below 0.6 degrees C. The maximum 24-hour average speed of 65 centimeters per second, together with a temperature of 0.29 degrees C, was encountered on 14 November 1980 at a water depth of 2973 meters, 35 meters above the sea floor. 相似文献
10.
Apollo Field Geology Investigation Team 《Science (New York, N.Y.)》1973,179(4068):62-69
The Cayley Plains at the Apollo 16 landing site consist of crudely stratified breccias to a depth of at least 200 meters, overlain by a regolith 10 to 15 meters thick. Samples, photographs, and observations by the astronauts indicate that most of the rocks are impact breccias derived from an anorthositegabbro complex. The least brecciated members of the suite include coarse-grained anorthosite and finer-grained, more mafic rocks, some with igneous and some with metamorphic textures. Much of the traverse area is covered by ejecta from North Ray and South Ray craters, but the abundance of rock fragments increases to the south toward the younger South Ray crater. The Descartes highlands, a distinct morphologic entity, differ from the adjacent Cayley formation more in physiographic expression than in lithologic character. 相似文献
11.
Oceanic plankton have been hypothesized to occur in micropatches (<10 meters) that can have a large impact on marine ecosystem dynamics. Towed video microscopy was used to unobtrusively determine distributions of oceanic plankton over a continuum of scales from microns to hundreds of meters. Distinct, taxa-specific aggregations measuring less than 20 centimeters were found for copepods but not for nonmotile (cyanobacterial colonies) or asexual (doliolid phorozooids) forms, which suggests that these small patches are related to mating. Significant patchiness was also found on larger scales and was correlated among taxa, indicating physical control. These video observations provide new insights into basic plankton ecology by allowing quantitative assessment of individual plankton in their natural, undisturbed state. 相似文献
12.
As a result of increasing frequency and intensity of tropical cyclones, an accurate forecasting of cyclone evolution and ocean response is becoming even more important to reduce threats to lives and property in coastal regions. To improve predictions, accurate evaluation of the air-sea momentum exchange is required. Using current observations recorded during a major tropical cyclone, we have estimated this momentum transfer from the ocean side of the air-sea interface, and we discuss it in terms of the drag coefficient. For winds between 20 and 48 meters per second, this coefficient initially increases and peaks at winds of about 32 meters per second before decreasing. 相似文献
13.
Detrick RS Harding AJ Kent GM Orcutt JA Mutter JC Buhl P 《Science (New York, N.Y.)》1993,259(5094):499-503
Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges. 相似文献
14.
Geological exploration in an East coast submarine canyon from a research submersible 总被引：1，自引：0，他引：1
Large talus blocks litter the flat floor of Oceanographer Canyon at a depth of 1460 meters; they indicate down-axis mass transport of floor sediment at an unknown time and rate. From 1460 to 1310 meters the sidewall is covered by unconsolidated sediment lying at 35 degrees to 40 degrees from the horizontal. An outcrop of Pleistocene or younger sediment at 1460 meters is probably a remnant of a former fill. 相似文献
15.
JK Harmon SJ Ostro LAM Benner KD Rosema RF Jurgens R Winkler DK Yeomans D Choate R Cormier JD Giorgini DL Mitchell PW Chodas R Rose D Kelley MA Slade ML Thomas 《Science (New York, N.Y.)》1997,278(5345):1921-1924
Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet. 相似文献
16.
Radar observations of Mars at centimeter wavelengths in May, June, and July 1976 provided estimates of surface roughness and reflectivity in three potential landing areas for Viking 1. Surface roughness is characterized by the distribution of surface landing slopes or tilts on lateral scales of the order of 1 to 10 meters; measurements of surface reflectivity are indicators of bulk surface density in the uppermost few centimeters. By these measures, the Viking 1 landing site at 47.5 degrees W, 22.4 degrees N is rougher than the martian average, although it may be near the martian average for elevations accessible to Viking, and is estimated to be near the Mars average in reflectivity. The AINW site at the center of Chryse Planitia, 43.5 degrees W, 23.4 degrees N, may be an area of anomalous radar characteristics, indicative of extreme, small-scale roughness, very low surface density, or a combination of these two characteristics, Low signal-to-noise ratio observations of the original Chryse site at 34 degrees W, 19.5 degrees N indicate that that area is at least twice as rough as the Mars average. 相似文献
17.
Bice DM Newton CR McCauley S Reiners PW McRoberts CA 《Science (New York, N.Y.)》1992,255(5043):443-446
Quartz grains that appear to have been shock-metamorphosed occur within three closely spaced shale beds from the uppermost Triassic ("Rhaetian") Calcare a Rhaetavicula in the Northern Apennines of Italy. The upper shale coincides with the abrupt termination of the distinctive, uppermost Triassic Rhaetavicula fauna and is overlain by the Hettangian (Lower Jurassic) Calcare Massiccio; no extinctions appear to be associated with the two lower layers, which occur 1.2 and 2.4 meters below the boundary shale. Approximately 5 to 10% of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes (basal, omega, and pi) most commonly observed in shocked quartz. Textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the Triassic. 相似文献
18.
Hay AE 《Science (New York, N.Y.)》1984,225(4667):1154-1156
Acoustic backscatter observations at 200 kilohertz were made of the buoyant plume from a submarine spring at a depth of 47 meters in Cambridge Fiord, Baffin Island. Vertical velocities of up to 37 centimeters per second are inferred from the ascent rates of discrete scattering structures in the plume. 相似文献
19.
During the 1975 El Ni?o expedition an array of conventional and electric field type near-bottom current recorders was deployed at the equator 300 kilometers west of the Galápagos Islands. While hydrographic observations were indicating El Ni?o activity off the South American coast, the current meters recorded an oscillation with a 25-day period, a wavelength of about 1000 kilometers, and an amplitude of 0.04 meter per second propagating westward at approximately 0.5 meter per second. These characteristics agree with theoretical models of a first-mode baroclinic Rossby wave trapped at the equator. 相似文献
20.
MT McCulloch AW Tudhope TM Esat GE Mortimer J Chappell B Pillans AR Chivas A Omura 《Science (New York, N.Y.)》1999,283(5399):202-204
Uplifted coral terraces at Huon Peninsula, Papua New Guinea, preserve a record of sea level, sea-surface temperature, and salinity from the penultimate deglaciation. Remnants have been found of a shallow-water reef that formed during a pause, similar to the Younger Dryas, in the penultimate deglaciation at 130,000 +/- 2000 years ago, when sea level was 60 to 80 meters lower than it is today. Porites coral, which grew during this period, has oxygen isotopic values and strontium/calcium ratios that indicate that sea-surface temperatures were much cooler (22 degrees +/- 2 degreesC) than either Last Interglacial or present-day tropical temperatures (29 degrees +/- 1 degreesC). These observations provide further evidence for a major cooling of the equatorial western Pacific followed by an extremely rapid rise in sea level during the latter stages of Termination II. 相似文献