Radar imaging of Asteroid 7 Iris     

S.J. Ostro  C. Magri  L.A.M. Benner  M.C. Nolan  M.W. Busch 《Icarus》2010,207(1):285-294

Arecibo radar images of Iris obtained in November 2006 reveal a topographically complex object whose gross shape is approximately ellipsoidal with equatorial dimensions within 15% of 253 × 228 km. The radar view of Iris was restricted to high southern latitudes, precluding reliable estimation of Iris’ entire 3D shape, but permitting accurate reconstruction of southern hemisphere topography. The most prominent features, three roughly 50-km-diameter concavities almost equally spaced in longitude around the south pole, are probably impact craters. In terms of shape regularity and fractional relief, Iris represents a plausible transition between ∼50-km-diameter asteroids with extremely irregular overall shapes and very large concavities, and very much larger asteroids (Ceres and Vesta) with very regular, nearly convex shapes and generally lacking monumental concavities.  相似文献   

Earth-based 12.6-cm wavelength radar mapping of the Moon: New views of impact melt distribution and mare physical properties     

Bruce A. Campbell  Lynn M. Carter  Michael Nolan  Rebecca R. Ghent  Ross F. Anderson 《Icarus》2010,208(2):565-181

We present results of a campaign to map much of the Moon’s near side using the 12.6-cm radar transmitter at Arecibo Observatory and receivers at the Green Bank Telescope. These data have a single-look spatial resolution of about 40 m, with final maps averaged to an 80-m, four-look product to reduce image speckle. Focused processing is used to obtain this high spatial resolution over the entire region illuminated by the Arecibo beam. The transmitted signal is circularly polarized, and we receive reflections in both senses of circular polarization; measurements of receiver thermal noise during periods with no lunar echoes allow well-calibrated estimates of the circular polarization ratio (CPR) and the four-element Stokes vector. Radiometric calibration to values of the backscatter coefficient is ongoing. Radar backscatter data for the Moon provide information on regolith dielectric and physical properties, with particular sensitivity to ilmenite content and surface or buried rocks with diameter of about one-tenth the radar wavelength and larger.Average 12.6-cm circular polarization ratio (CPR) values for low- to moderate-TiO₂ mare basalt deposits are similar to those of rough terrestrial lava flows. We attribute these high values to abundant few-centimeter diameter rocks from small impacts and a significant component of subsurface volume scattering. An outflow deposit, inferred to be impact melt, from Glushko crater has CPR values near unity at 12.6-cm and 70-cm wavelengths and thus a very rugged near-surface structure at the decimeter to meter scale. This deposit does not show radar-brightness variations consistent with levees or channels, and appears to nearly overtop a massif, suggesting very rapid emplacement. Deposits of similar morphology and/or radar brightness are noted for craters such as Pythagoras, Rutherfurd, Theophilus, and Aristillus. Images of the north pole show that, despite recording the deposition of Orientale material, Byrd and Peary craters do not have dense patterns of radar-bright ejecta from small craters on their floors. Such patterns in Amundsen crater, near the south pole, were interpreted as diagnostic of abundant impact melt, so the fraction of Orientale-derived melt in the north polar smooth plains, 1000 km farther from the basin center, is inferred to be much lower.  相似文献   

Towards an understanding of the Of?p star HD 191612: optical spectroscopy   总被引：1，自引：0，他引：1  

Ian D. Howarth  Nolan R. Walborn  Danny J. Lennon  Joachim Puls  Yaël Nazé  K. Annuk  I. Antokhin  D. Bohlender  H. Bond  J.-F. Donati  L. Georgiev  D. Gies  D. Harmer  A. Herrero  I. Kolka  D. McDavid  T. Morel  I. Negueruela  G. Rauw  P. Reig 《Monthly notices of the Royal Astronomical Society》2007,381(2):433-446

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P _orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.  相似文献   

Young stellar populations in early-type galaxies in the Sloan Digital Sky Survey     

Louisa A. Nolan  Somak Raychaudhury  Ata Kabán 《Monthly notices of the Royal Astronomical Society》2007,375(1):381-387

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Lithium tracer-diffusion in an alkali-basaltic melt — An ion-microprobe determination     

R.K. Lowry  S.J.B. Reed  J. Nolan  P. Henderson  J.V.P. Long 《Earth and Planetary Science Letters》1981,53(1):36-40

An ion-microprobe-based technique has been used to measure lithium tracer-diffusion coefficients (D_Li) in an alkali-basaltic melt at 1300, 1350 and 1400°C. The results can be expressed in the form:D_Li=7.5 ×10^?2exp(?27,600/RT)cm²S^?1The results show significantly faster diffusion rates than those previously recorded for other monovalent, divalent and trivalent cations in a tholeiitic melt. Consequently, diffusive transport of ions acting over a given time in a basaltic melt can produce a wider range of transport distance values than hitherto supposed. Hence, it is concluded that great care should be exercised when applying diffusion data to petrological problems.  相似文献   

Lithium diffusion in silicate melts     

G.J. Cunningham  P. Henderson  R.K. Lowry  J. Nolan  S.J.B. Reed  J.V.P. Long 《Earth and Planetary Science Letters》1983,65(1):203-205

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Tracer diffusion of some alkali,alkaline-earth and transition element ions in a basaltic and an andesitic melt,and the implications concerning melt structure     

Roy K. Lowry  Paul Henderson  John Nolan 《Contributions to Mineralogy and Petrology》1982,80(3):254-261

The diffusion properties of Na, Cs, Sr, Ba, Co, Mn, Fe and Sc ions in a basaltic and an andesitic melt have been determined experimentally using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,300–1,400° C. Diffusion of all cations follows an Arrhenius relationship; the values of the activation energies range from 24 kcal mol^–1 for Na to 67 kcal mol^–1 for Co in the andesitic melt, and from 39 kcal mol^–1 for Na to 65 kcal mol^–1 for Cs in the basaltic melt. Relative diffusivities in the basaltic melt, but not in the andesitic melt, correlate with assumed ionic radii values. Each cation, except Na⁺, diffuses faster in the basaltic melt than in the andesitic melt over the studied temperature range. Sodium shows similar diffusivity in the two melts.Compensation diagrams incorporating new and some previously-published data indicate that Cs probably diffuses by different mechanisms in different silicate glass and melt systems. Iron has a relatively high activation energy which is consistent with its part occupancy of tetrahedral co-ordination polyhedra.  相似文献   

Studies in Military Geography and Geology - Edited by Douglas R Caldwell, Judy Ehlen and Russell S Harmon     

Thomas  Nolan 《The Geographical journal》2006,172(3):266-266

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Structural controls and mechanisms of diffusion in natural silicate melts     

P. Henderson  J. Nolan  G. C. Cunningham  R. K. Lowry 《Contributions to Mineralogy and Petrology》1985,89(2-3):263-272

The diffusion properties of Na, Cs, Ba, Fe and Eu ions have been determined experimentally for a pantellerite melt and of these ions plus Li, Mn and Co in pitchstone melt, using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,200–1,400° C. In addition, Eu diffusion in a basaltic and an andesitic melt was determined. Diffusion of all cations follows an Arrhenius relationship, activation energy values being high for diffusion in the pantellerite melt (e.g. Eu: 100 kcal mol^–1) except in the case of Na (24.3 kcal mol^–1). Activation energies of diffusion in the pitchstone melt are similar to values recorded earlier for andesitic and basaltic melts.The new data are used, along with previously published data for diffusion in other composition melts, to examine the compositional and structural controls on diffusion. The range of diffusivities shows a marked change with melt composition; over two orders of magnitude for a basaltic melt, and nearly four orders for a pantellerite melt (both at 1,300° C). Diffusivity of all cations (except Li and Na) correlates positively with the proportion of network modifying cations. In the case of Li and Na the correlation is negative but the diffusivity of these ions correlates positively with the proportion of Na or of Na + K ions in the bulk melt. Diffusion behaviour in the pantellerite melt departs from the relationships shown by the data for other melt compositions, which could be partly explained by trivalent ions (such as Fe) occupying network forming positions. The diffusivity of alkali metal ions is strongly dependent on ionic radius, but this is not the case with the divalent and trivalent ions; diffusivity of these ions remains relatively constant with change in radius but decreases with increase in ionic charge.A compensation diagram shows four distinct but parallel trends for the majority of the cations in four melt types but the data for Li and Na plot on a separate trend. This and the other relationships are used to elucidate possible mechanisms of diffusion. Exchange mechanisms appear to be common, with the preservation of local charge balance. Li and Na diffuse by a distinct mechanism which involves exchange of similar or identical ions. The diffusion behaviour of the smaller alkali metal ions is sufficiently distinct from all other cations to indicate that diffusion could be an important factor in the geochemical fractionation of the alkali elements.s  相似文献   

Basic hydrology,limnology, and meteorology of modern Lake El’gygytgyn,Siberia     

Matt Nolan  Julie Brigham-Grette 《Journal of Paleolimnology》2007,37(1):17-35

A survey of the modern physical setting of Lake El’gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km², 110 km² of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures (>6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km³. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves (<10 m) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was −10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948–2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below −30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s⁻¹ (30 mph) typically at least once each month and 17.8 m s⁻¹ (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed. This is the second in a series of eleven papers published in this special issue dedicated to initial studies of El'gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.  相似文献