Radar observations of Itokawa in 2004 and improved shape estimation     

Steven J. Ostro  Lance A. M. Benner  Christopher Magri  Jon D. Giorgini  Randy Rose  Raymond F. Jurgens  Donald K. Yeomans  Alice A. Hine  Michael C. Nolan  Daniel J. Scheeres  Stephen B. Broschart  Mikko Kaasalainen  Jean‐Luc Margot 《Meteoritics & planetary science》2005,40(11):1563-1574

Abstract— We present June 2004 radar images of asteroid 25143 Itokawa (1998 SF36) that improve upon the longitude‐latitude coverage of images obtained in 2001 by Ostro et al. (2004) and use the 2001–2004 data to refine that paper's constraints on Itokawa's shape. The 2004 images, the first of the asteroid's southern side, look distinctly different from the 2001 images, revealing leading edges that are much more curved and rugged than the nearly convex leading edges seen at northern latitudes in 2001. Itokawa is shaped like a slightly asymmetrical, bent, lumpy ellipsoid with dimensions along the principal axes within 10% of 594 times 320 times 288 m. To illustrate the uncertainty space associated with shape reconstruction from images with suboptimal orientational coverage, we present two alternative three‐dimensional models of the object.  相似文献   

The Gulf of Maine boundary: Dropping anchor or setting a course?     

Ted L. McDorman  Phillip M. Saunders  David L. VanderZwaag 《Marine Policy》1985,9(2):90-107

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Palaeomagnetism of the Tudor gabbro, Ontario: evidence for divergence between Grenvillia and interior Laurentia     

David J. Dunlop  Hironobu Hyodo  Todd Knight  Alan G. Steele 《Geophysical Journal International》1985,83(3):699-720

Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α₉₅= 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α₉₅= 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent ⁴⁰Ar/³⁹Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed.  相似文献   

The Kaidun meteorite: Mineralogy of an unusual CM1 lithology     

Michael E. ZOLENSKY  Andrei V. IVANOV  S. Vincent YANG  David W. MITTLEFEHLDT  Kazumasa OHSUMI 《Meteoritics & planetary science》1996,31(4):484-493

Abstract Kaidun is a breccia of disparate enstatite and carbonaceous chondrite clasts that continues to provide real surprises. Many Kaidun clasts have been intensely altered by aqueous fluids, as evidenced by the widespread occurrence of ferromagnesian phyllosilicates and by the presence of carbonate- and phyllo-silicate-filled veins. In this report, we describe an unusual CM lithology containing many mineralogical features not previously reported from any meteorite, including pyrrhotite, with exclusive needlelike morphologies and thick mantles of phyllosilicate, and complex aggregates of phyllosilicate, melanite garnet, crosscut by pentlandite veins. The latter features appear to be due in large part to extensive hydrothermal alteration at temperatures on the order of 450 °C, which is significantly higher than that attained during secondary processing from other known CM material.  相似文献   

巴颜喀拉山东段及其邻区大地电磁测深成果地质解释     

庞存廉  方胜  夏元祁 《青海国土经略》1996,(1)

本文主要依据青海省囊歉至大河坝大地电磁测深成果,并结合重力资料对巴颜喀拉及南北两侧地区的壳幔结构进行了地质解释。指出了该区壳幔存在4个明显的低阻层,并简述了它们的形态特征,推断了其地质含意。认为昆中断裂带与昆南断裂带间是一个巨大的构造混杂岩带,影响深度亦很大。推断该区可能存在太古代地层。划出了8条壳幔断裂带,认为昆中断裂应是塔里木—华北板块与华南板块的分界线。  相似文献   

Microwave observations of Saturn's rings: anisotropy in directly transmitted and scattered saturnian thermal emission     

David E. Dunn  Lawrence A. Molnar  Imke de Pater 《Icarus》2004,171(1):183-198

We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae.  相似文献   

A concordance model of the Lyman α forest at z= 1.95     

Tridivesh Jena  Michael L. Norman  David Tytler  David Kirkman  Nao Suzuki  Angela Chapman  Carl Melis  Pascal Paschos  Brian O'Shea  Geoffrey So  Dan Lubin  Wen-Ching Lin  Dieter Reimers  Eckardt Janknecht  Cora Fechner 《Monthly notices of the Royal Astronomical Society》2005,361(1):70-96

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A stepwise method for determining the number of component distributions in a mixture     

Yu-Sheng Hsu  Joseph J. Walker  David E. Ogren 《Mathematical Geology》1986,18(2):153-160

This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given.  相似文献   

STUDY ON GIS FOR YIELD ESTIMATION BY REMOTE SENSING IN JILIN MAIZE BELT     

Liu Xiangnan  Huang Fang  Zhou Zhanao 《中国地理科学(英文版)》1996,6(4):351-358

ＳＴＵＤＹ　ＯＮ　ＧＩＳ　ＦＯＲ　ＹＩＥＬＤ　ＥＳＴＩＭＡＴＩＯＮ　ＢＹ　ＲＥＭＯＴＥ　ＳＥＮＳＩＮＧ　ＩＮ　ＪＩＬＩＮ　ＭＡＩＺＥ　ＢＥＬＴＳＴＵＤＹＯＮＧＩＳＦＯＲＹＩＥＬＤＥＳＴＩＭＡＴＩＯＮＢＹＲＥＭＯＴＥＳＥＮＳＩＮＧＩＮＪＩＬＩＮＭＡＩＺ...  相似文献   

Low-cost volcano surveillance from space: case studies from Etna, Krafla, Cerro Negro, Fogo, Lascar and Erebus     

Andrew J. L. Harris  Anna L. Butterworth  Richard W. Carlton  Ian Downey  Peter Miller  Pedro Navarro  David A. Rothery 《Bulletin of Volcanology》1997,59(1):49-64

 Satellite data offer a means of supplementing ground-based monitoring during volcanic eruptions, especially at times or locations where ground-based monitoring is difficult. Being directly and freely available several times a day, data from the advanced very high resolution radiometer (AVHRR) offers great potential for near real-time monitoring of all volcanoes across large (3000×3000 km) areas. Herein we describe techniques to detect and locate activity; estimate lava area, thermal flux, effusion rates and cumulative volume; and distinguish types of activity. Application is demonstrated using data for active lavas at Krafla, Etna, Fogo, Cerro Negro and Erebus; a pyroclastic flow at Lascar; and open vent systems at Etna and Stromboli. Automated near real-time analysis of AVHRR data could be achieved at existing, or cheap to install, receiving stations, offering a supplement to conventional monitoring methods. Received: 21 January 1997 / Accepted: 3 April 1997  相似文献