The temperature of interstellar iron grains     

Ronald G. Tabak 《Astrophysics and Space Science》1977,49(1):41-46

In order for catalytic reactions to occur in interstellar dust clouds, it is necessary for the temperature of the grains to be about an order of magnitude hotter than usually calculated for grains of dielectric materials. However, transition metal (e.g., iron) grains should be fairly abundant, and because they absorb strongly in the visible and ultraviolet regions of the interstellar radiation field, they have equilibrium temperatures 133 K. This was determined by a new method which utilizes Semi-Empirical free electron theory parameters in the infrared, coupled with a reiteration scheme which takes into consideration the change of conductivity of the iron with temperature.  相似文献   

Sunspot geometry and pressure balance     

Ronald G. Giovanelli 《Solar physics》1982,80(1):21-31

Measurements on magnetic canopies extending from sunspots show that, at the outer penumbral edge, heights of the bases are independent of sunspot diameter and average 180 km. This places a lower limit on the outer penumbral base; with an assumed thickness of 250 km, the top is 430 km above z = 0 ( _c = 1) in the photosphere.Chistyakov's (1962) observations require the penumbral surface to be convex in radial section. The Wilson depression, able thus to be found only from limb-side penumbras, is 1360 km from his selected measurements. Averaged over all regular sunspots without special selection, this drops to 1040 km. Thus ^* = 1 in umbras lies around z = -610 km.Magnetic field-strength measurements relate probably to ^* 0.02, some 160 km higher, where z -450 km. The magnetic pressure of the typical 3250 G sunspot field would support the external-axial gas-pressure difference at z = -330 km, the difference of 120 km lying well within the uncertainties. Tension forces, commonly invoked to achieve pressure balance, do not exceed the uncertainties of measurement.Beyond the sunspot, the base of the sunspot field rises only slowly over at least 16 000 km horizontally, whereas Beckers (1963) found the inclination of H superpenumbral fibrils to be some 13°. These results are nicely compatible since the field angle is typically of this magnitude at the minimum heights where H fibrils will be observed, say 1400 km.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.  相似文献   

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E     

Anton P le Roex  Henry J B Dick  Ronald T Watkins 《Contributions to Mineralogy and Petrology》1992,110(2-3):253-268

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K₂O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)_n ratios are particularly high (3.4–7.8). ⁸⁷Sr/⁸⁶Sr (0.70290–0.70368), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51284) and ²⁰⁶Pb/²⁰⁴Pb (18.708–19.564) isotopic ratios further indicate the geochemically enriched nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.  相似文献   

Book reviews     

Tom L. McKnight  J. M. Powell  Stephen Codrington  John Connell  Stewart F. Richards  Ronald J. Horvath 《The Australian geographer》1991,22(1):86-96

REGIONAL GEOGRAPHY: Current Developments and Future Prospects edited by R. J. Johnston, J Hauer and G. A. Hoekveld. 14 × 22 cm, xiii and 216 pages. Routledge: London 1990 (ISBN 0 415 05247 5) $A89.95 (hard).

THE MAKING OF THE AMERICAN LANDSCAPE edited by M. P. Conzen. 19 × 25 cm, xvi and 433 pages. Unwin Hyman: Boston 1990 (ISBN 0 04 917010 4) $A59.95 (soft).

EASTERN EUROPE: An Historical Geography 1815–1945 by D. Turnock. 14 × 22 cm, ix and 357 pages. Routledge: London 1989 (ISBN 0 415 01269 4) $A115.00 (hard), distributed by the Law Book Co.

THE HUMAN GEOGRAPHY OF EASTERN EUROPE by D. Turnock. 14 × 22 cm, xii and 345 pages. Routledge: London 1989 (ISBN 0 415 00469 1) $A105.00 (hard), distributed by the Law Book Co.

A HISTORY OF THE PACIFIC ISLANDS by I. C. Campbell. 14 × 21 cm, 239 pages. University of Queensland Press: St Lucia 1990 (ISBN 0 7022 2291 7) $A34.95 (soft).

THE CORPORATE FIRM IN A CHANGING WORLD ECONOMY: Case Studies in the Geography of Enterprise edited by M. de Smidt and E. Wev‐er. 14 × 23 cm, xiv and 247 pages. Routledge: London 1990 (ISBN 0 415 03497 3) $A105.00 (hard).

THE CAPITALIST CITY: Global Restructuring and Community Politics by M. P. Smith and J. R. Feagin. 15 × 23 cm, 393 pages. Basil Black‐well: Oxford 1988 (ISBN 0 631 15182 6) £35 (hard).

CONFLICT AND CHANGE IN THE COUNTRYSIDE: Rural Society, Economy and Planning in the Developed World by G. M. Robinson. 15 × 23 cm, xxvi and 482 pages. Belhaven Press: London 1990 (ISBN 1 85293 043 8) £40.00 (hard); (ISBN 1 85293 044 6) £14.95 (soft).

ENVIRONMENTAL SCIENCE (Collins Reference Dictionary) by G. Jones, A. Robertson, J. Forbes and G. Hollier. 13 × 20 cm, vi and 473 pages. Collins: London 1990 (ISBN 0 00 434348 4) £5.95 (soft).

THE HUMAN IMPACT ON THE NATURAL ENVIRONMENT (Third Edition) by A. Goudie. 17 × 24 cm, xi and 388 pages. Blackwell: Oxford 1990 (ISBN 0 631 16164 3) $A39.95 (soft).

PATTERNS OF LIFE: Biogeography of a Changing World by H. W. Mielke. 19 × 24 cm, xiv and 370 pages. Unwin Hyman: London 1989 (ISBN 0 04 574032 1).  相似文献   

Aeolian processes on Venus     

Greeley  Ronald  Arvidson  Raymond E. 《Earth, Moon, and Planets》1990,(1):127-157

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about 10^–2 km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils 10 cm deep, except for ~5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO₂ and SO₂ incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO₂ and SO₂. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply, the flux could be as high as 2.5 × 10^–5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <10³ years. In addition, comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surficial processes) is correct, given the global coverage, high spatial resolution, the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.Geology and Tectonics of Venus, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).  相似文献   

Intertidal distribution and feeding habits of the mud crab,Eurytium limosum     

Ronald T. Kneib  Cynthia A. Weeks 《Estuaries and Coasts》1990,13(4):462-468

Average summer densities of the xanthid crab,Eurytium limosum, in an intertidal salt marsh on Sapelo Island, Georgia were in the range of 7.5 to 80.0 individuals m^?2. Crab densities were lowest in wet, lowlying marsh and highest in well-drained creekbank and mussel mound habitats. An analysis of crab stomach contents indicated that feeding occurred mostly around high tide, especially at night. Although the diet included some plant material,Eurytium limosum is primarily predatory and consumed crabs, polychaetes, ostracods, bivalves, and snails. In the laboratory, under simulated low-tide field conditions, both small (11–15 mm carapace width) and large (20–37 mm CW)Eurytium could capture and consume young killifish (Fundulus heteroclitus). Large crabs consumed the entire size range (7–19 mm total length) of larval/juvenile fish offered, but small crabs did not prey upon fish >11.5 mm TL. The potential importance ofE. limosum as a predator on young killifish may not be realized in the field because alternative prey are available and the crabs feed primarily at high tide, when young killifish are dispersed in the water column and are less vulnerable to benthic predators.  相似文献   

Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar   总被引：5，自引：0，他引：5  

David P. Mattey  W. R. Taylor  D. H. Green  C. T. Pillinger 《Contributions to Mineralogy and Petrology》1990,104(4):492-505

The carbon isotopic fractionation between CO₂ vapour and sodamelilite (NaCaAlSi₂O₇) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag₂C₂O₄ was the source of CO₂ and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the ¹³C of coexisting vapour was determined directly by capsule piercing. CO₂ solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO₂ dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO₂ reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO₂ solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO₂ vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring¹³C enrichment in CO₂ vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO₂ vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.  相似文献   

Meridional characteristics of a Pc 4 micropulsation event in the plasmasphere     

C.A. Green 《Planetary and Space Science》1978,26(10):955-967

An analysis was made of a complex large amplitude Pc 4 micropulsation, of four hours duration around local noon, observed at five ground stations in the United Kingdom (2.4? L ?3.8). The final pulsation waveform was shown to be the results of the superposition of wave packets of different periods. The meridional variation of the amplitude of the different period wavepackets was consistent with their being fundamental “toroidal” field line resonances within the plasmasphere, rotated through 90° in their transmission through the ionosphere in accordance with recent theoretical predictions. Other predicted ionospheric effects, such as the loss of the sense-of-polarization reversal across the amplitude maximum, were apparent in the meridional variation of the polarization characteristics.  相似文献   

The formation of saturn's satellites and rings,as influenced by saturn's contraction history     

James B. Pollack  Allen S. Grossman  Ronald Moore  Harold C. Graboske 《Icarus》1976,29(1):35-48

We have used Pollack et al.'s 1976 calculations of the quasi-equilibrium contraction of Saturn to study the influence of the planet's early high luminosity on the formation of its satellites and rings. Assuming that the condensation of ices ceased at the same time within Jupiter's and Saturn's primordial nebulae, and using limits for the time of cessation derived for Jupiter's system by Pollack and Reynolds (1974) and Cameron and Pollack (1975), we arrive at the following tentative conclusions. Titan is the innermost satellite at whose position a methane-containing ice could condense, a result consistent with the presence of methane in this satellite's atmosphere. Water ice may have been able to condense at the position of all the satellites, a result consistent with the occurrence of low-density satellites close to Saturn. The systematic decrease in the mass of Saturn's regular satellites with decreasing distance from Saturn may have been caused partially by the larger time intervals for the closer satellites between the start of contraction and the first condensation of ices at their positions and between the start of contraction and the time at which Saturn's radius became less than a satellite's orbital radius. Ammonia ices, principally NH₄SH, were able to condense at the positions of all but the innermost satellites.Water ice may bave been able to condense in the region of the rings close to the end of the condensation period. We speculate that the rings are unique to Saturn because on the one hand, temperatures within Jupiter's Roche limit never became cool enough for ice particles to form before the end of the condensation period and on the other hand, ice particles formed only very early within Uranus' and Neptune's Roche limits, and were eliminated by gas drag effects that caused them to spiral into the planet before the gas of these planets' nebula was eliminated. Gas drag would also have eliminated any rocky particles initially present inside the Roche limit.We also derive an independent estimate of several million years for the time between the start of the quasi-equilibrium contraction of Saturn and the cessation of condensation. This estimate is based on the density and mass characteristics of Saturn's satellites. Using this value rather than the one found for Jupiter's satellites, we find that the above conclusions about the rings and the condensation of methane-and ammonia-containing ices remain valid.  相似文献   

Immigrants And Boarding Houses: Simple And Multidimensional Propinquity     

Ronald Foresta 《The Professional geographer》1975,27(2):171-178

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