Interaction of S- and L-bursts in Jupiter's decametric radio spectra     

Jorma J. Riihimaa  Thomas D. Carr 《Earth, Moon, and Planets》1981,25(4):373-387

Dynamic spectra of Jupiter's decametric emission are observed with a high-resolution radio spectrograph. Certain events observed in region Io-B display interaction effects between the S- and L-emissions. The effect appears as a gap in the L-emission after a passage of an S-burst. In a typical case the L-emission has a relatively narrow bandwidth, the S-burst appears as a sloping line crossing it, and the duration of the emission gap is a substantial fraction of a second. The gap has sharp, sloped edges; its leading edge has anf-t slope which is the same as that of the S-burst while the slope of the trailing edge is lower. A kind of tilted V-pattern is thus formed. It is suggested that some more complicated spectral patterns may also be produced this way. It seems that an emission model consisting of S-bursts from bunches of gyrofrequency-emitting electrons ascending a flux tube and interacting with L-emitting bunches of electrons having more stationary guiding centers is inadequate to explain the complex S-L interactions.Contribution of No. 36 of the Department of Astronomy, University of Florida, U.S.A.  相似文献   

Earth-orbit-approaching comets and their theoretical meteor radiants     

Jack D. Drummond 《Icarus》1981,47(3):500-517

Sixteen comets produce recognizable meteor showers that are found in A. F. Cook's (1973, In Evolutionary and Physical Properties of Meteoroids (C. L. Hemenway, P. M. Millman, and A. F. Cook, Eds.), pp. 183–191, U.S. Govt. Printing Office, Washington, D.C.), working list of meteor streams. Of these, five are long period, including one in a parabolic and one in a hyperbolic orbit. The largest Earth-comet orbit miss distance is 0.20 AU for P/Encke and the Northern and Southern Taurids. Using this is an upper limit for meteor showers from comets, all comets which approach the Earth's orbit to within 0.20 AU were extracted from the Catalogue of Cometary Orbits (B. G. Marsden, 1979. 3rd ed., Central Bureau of Astronomical Telegrams, IAU SAO, Cambridge, Mass.). A compilation of such comets is presented by date minimum approach, along with the distance of closest approach and the theoretical geocentric radiants and velocities of possible associated meteor showers. Both pre- and postpperihelion encounters with the Earth's orbit are considered. There are 240 entries for 178 long-period comets, and 36 for 28 short-period comets. It is noted that all short-period comets that have approached the Earth's orbit to within 0.08 AU have produced meteors, except P/Lexell, P/Finlay, P/Denning-Fujikawa, and P/Grigg-Skjellerup. Attention is called to the favorable observing conditions for detecting meteors from P/Grigg-Skjellerup in April 1982, and for the possibility of another great Draconid storm from P/Giacobini-Zinner in October 1985. A comparison is made between observed sporadic meteor rates and the distribution of theoretical radiants throughout the year, from which it is concluded that the currently known comets can account for sporadic meteors. A criterion is developed to test whether or not an observed meteor shower can be associated with a given theoretical radiant. Based on known examples, a qualitative model for comet/meteor relationships is also presented.  相似文献   

On periodic flybys of the moon   总被引：1，自引：1，他引：0  

A. D. Bruno 《Celestial Mechanics and Dynamical Astronomy》1981,24(3):255-268

This paper considers the plane circular restricted three-body problem for small . Symmetric periodic solutions of the second species (passing near the body of mass ) and their distance from the center of the body of mass are studied by constructing perturbations of arc-solutions (solutions with consecutive collisions) existing for =0. Orbits which also pass near the body of mass 1- are studied in detail. The results are applied to finding periodic orbits in the Earth-Moon system and in the Sun-Jupiter system.Russian version: Preprint No. 91 (1978) of Inst. Appl. Math.; present English translation was made by L. M. Perko and W. C. Schulz (February 1979).  相似文献   

Hydrocarbons in surficial sediments from the Scotian Shelf     

Paul D Keizer  J Dale  D.C Gordon 《Geochimica et cosmochimica acta》1978,42(2):165-172

Surface sediments from 20 stations on the Scotian Shelf, collected on a transect from Halifax to Emerald Bank and around Sable Island, have been analyzed for hydrocarbon content and composition by gas chromatography and fluorescence spectrophotometry. Some samples were taken near abandoned exploratory drilling sites in the Sable Island area. Hydrocarbons appear to be mainly derived from biogenic terrestrial sources as evidenced by an inverse correlation of concentration with increasing distance from the mainland and a strong odd carbon preference in the n-alkanes. Contribution from petroleum sources, while minor, was most noticeable between Halifax and Emerald Bank. There is also evidence that the hydrocarbon composition at abandoned exploratory drilling sites has been slightly altered.  相似文献   

Comparison of crude oils and their alteration products     

John D. Saxby 《Geochimica et cosmochimica acta》1978,42(2):215-217

A simple method is proposed for using gas Chromatographic data on the high molecular weight saturated fractions of crude oils to make comparisons of source materials, migration mechanisms and, in particular, alteration processes within reservoirs. The preferential loss of normal alkanes by bacterial attack on a light crude is one of several processes readily followed using this scheme.  相似文献   

Formation of the Bencubbin polymict meteoritic breccia     

Gregory W. Kallemeyn  William V. Boynton  John Willis  John T. Wasson 《Geochimica et cosmochimica acta》1978,42(5):507-515

The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.  相似文献   

Primary fractionation of elements among iron meteorites     

Edward R.D. Scott 《Geochimica et cosmochimica acta》1978,42(9):1447-1458

The primary fractionation process in iron meteorites is that responsible for the distribution of elements between the groups, most notably Ga and Ge, which show concentration ranges of 10³ and 10⁴ respectively. To investigate the cause of the primary fractionation, concentrations of 16 elements were converted to relative abundances by dividing the element/Ni ratio by the CI chondrite ratio. These abundances were plotted on logarithmic graphs with data for each group (except IB and IIICD) and each cluster of closely related anomalous irons averaged.Co, P, Au, As, Cu, Sb, Ge and Zn are positively correlated with Ga. For most groups (except IA, IC and IIAB) relative abundances of these elements tend to decrease from about 1 in approximately the order listed above. This is the expected order in which these elements will condense into Fe, Ni during equilibrium nebular condensation. Mean relative abundances of refractory elements in groups generally lie within a narrow range of 0.5–2, and are uncorrelated with Ga. Although the equilibrium model may be only a gross approximation, it suggests that most primary fractionation did occur during nebular condensation.The anomalous irons are essential for defining many of the primary fractionation trends. On several element-Ga graphs the displacements of the anomalous irons from the primary curves indicate that these irons experienced the same secondary fractionation process (probably fractional crystallization) that produced the trends within most groups. The anomalous irons appear to be samples from over 50 minor groups, which have similar histories to the 12 major groups.  相似文献   

Chemical exchange during hydrothermal alteration of basalt by seawater—I. Experimental results for major and minor components of seawater     

Michael J. Mottl  Heinrich D. Holland 《Geochimica et cosmochimica acta》1978,42(8):1103-1115

Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO₂, SO₄), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO₂, Ca, K, Ba, B and CO₂ were leached from basalt and enriched in solution. SiO₂ concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.  相似文献   

Determination of the sources and circulation paths of thermal fluids: the Abano region,northern Italy     

D. Norton  C. Panichi 《Geochimica et cosmochimica acta》1978,42(8):1283-1294

Water samples from natural springs and artesian wells in the Abano region of northern Italy are characterized by anomalous temperatures and compositions. Concentrations of major components and oxygen isotopes in samples of these fluids have been interpreted in the context of the regional geologic environment, circulation of groundwaters, and reactions between rocks and circulating groundwaters. These considerations define sourceregions and pathlines of the groundwaters.Circulation of fluids in the Abano region is interpreted to be the result of meteoric water infiltration into outcrops of Permian and Mesozoic aquifers in the pre-Alps, which lie north of the Abano region. These aquifers have southerly dips and, therefore, extend under the Abano region at depths between 0.5 and 2.5 km. This aquifer geometry is conducive to forced convection both of aqueous ions derived from the evaporate, limestone and dolomite bearing formations, and of thermal energy along flow paths which extend from the outcrops in the sourceregions to depths of 2 km below the Abano region and upward along high angle faults to thermal springs. Local variation in compositions of water samples is consistent with the mixing of local meteoric waters and formation fluids that were ultimately derived from Alpine sources.  相似文献   

Fair weather and storm sand transport on the Long Island, New York, inner shelf     

J. W. LAVELLE  D. J. P. SWIFT  P. E. GADD  W. L. STUBBLEFIELD  F. N. CASE†  H. R. BRASHEAR†  K. W. HAFF† 《Sedimentology》1978,25(6):823-842

Both spring-summer and fall-winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20-22 m using a radio-isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall-winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near-bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least-squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10^?3 gm/cm/sec and 1.7 × 10^?1 gm/cm/sec for ‘typical’ and extreme winter storm activity.  相似文献