Characterization of the near-Earth Asteroid 2002 NY₄₀     

Lewis C. Roberts Jr.  Doyle T. Hall  John V. Lambert  John L. Africano  Keith T. Knox  Jacob K. Barros  Kris M. Hamada  Dennis Liang  Paul F. Sydney  Paul W. Kervin 《Icarus》2007,192(2):469-474

In August 2002, the near-Earth Asteroid 2002 NY₄₀, made its closest approach to the Earth. This provided an opportunity to study a near-Earth asteroid with a variety of instruments. Several of the telescopes at the Maui Space Surveillance System were trained at the asteroid and collected adaptive optics images, photometry and spectroscopy. Analysis of the imagery reveals the asteroid is triangular shaped with significant self-shadowing. The photometry reveals a 20-h period and the spectroscopy shows that the asteroid is a Q-type.  相似文献   

Episodic intraplate deformation of stable continental margins: evidence from Late Neogene and Quaternary marine terraces,Cape Liptrap,Southeastern Australia     

Thomas Gardner  John Webb  Claudia Pezzia  Terri Amborn  Robert Tunnell  Sarah Flanagan  Dorothy Merritts  Jeffrey Marshall  Derek Fabel  Matthew L. Cupper 《Quaternary Science Reviews》2009,28(1-2):39-53

The Waratah Fault is a northeast trending, high angle, reverse fault in the Late Paleozoic Lachlan Fold Belt at Cape Liptrap on the Southeastern Australian Coast. It is susceptible to reactivation in the modern intraplate stress field in Southeast Australia and exhibits Late Pliocene to Late Pleistocene reactivation. Radiocarbon, optically stimulated luminescence (OSL), and cosmogenic radionuclide (CRN) dating of marine terraces on Cape Liptrap are used to constrain rates of displacement across the reactivated Waratah Fault. Six marine terraces, numbered Qt₆–Tt₁ (youngest to oldest), are well developed at Cape Liptrap with altitudes ranging from ～1.5 m to ～170 m amsl, respectively. On the lowest terrace, Qt₆, barnacles in wave-cut notches ～1.5 m amsl, yielded a radiocarbon age of 6090–5880 Cal BP, and reflect the local mid-Holocene sea level highstand. Qt₅ yielded four OSL ages from scattered locations around the cape ranging from ～80 ka to ～130 ka. It formed during the Last Interglacial sea level highstand (MIS 5e) at ～125 ka. Inner edge elevations (approximate paleo high tide line) for Qt₅ occur at distinctly different elevations on opposite sides of the Waratah Fault. Offsets of the inner edges across the fault range from 1.3 m to 5.1 m with displacement rates ranging from 0.01 mm/a to 0.04 mm/a. The most extensive terrace, Tt₄, yielded four Early Pleistocene cosmogenic radionuclide (CRN) ages: two apparent burial ages of 0.858 Ma ± 0.16 Ma and 1.25 Ma ± 0.265 Ma, and two apparent exposure ages of 1.071 Ma ± 0.071 Ma (¹⁰Be) and 0.798 Ma ± 0.066 Ma (²⁶Al). Allowing for muonic production effects from insufficient burial depths, the depth corrected CRN burial ages are 1.8 Ma ± 0.56 Ma and 2.52 Ma ± 0.88 Ma, or Late Pliocene. A Late Pliocene age is our preferred age. Offsets of Tt₄ across the Waratah Fault range from a minimum of ～20 m for terrace surface treads to a maximum of ～70 m for terrace bedrock straths. Calculated displacement rates for Tt₄ range from 0.01 mm/a to 0.04 mm/a (using a Late Pliocene age, ～2 Ma), identical to the rates calculated for the Last Interglacial terrace, Qt₅. This indicates that deformation at Cape Liptrap has been ongoing at similar time-averaged rates at least since the Late Pliocene. The upper terraces in the sequence, Tt₃ (～110 m amsl), Tt₂ (～140 m) and Tt₁ (～180 m) are undated, but most likely correlate to sea level highstands in the Neogene. Terraces Tt1–Tt4 show an increasing northward tilt with age.The Waratah Fault forms a prominent structural boundary in the Lachlan Fold Belt discernible from airborne magnetic and bouger gravity anomalies. Seismicity and deformation are episodic. Episodic movement on the Waratah Fault may be coincident with sea level highstands since the Late Pliocene, possibly from increased loading and elevated pore pressure within the fault zone. This suggests that intervals between major seismic events could be on the order of 100 ka.  相似文献   

The very long baseline interferometric polarization structure of 3C 345 at λ=2.8 cm     

D. C. Gabuzda  A. J. Mioduszewski †  D. H. Roberts  J. F. C. Wardle 《Monthly notices of the Royal Astronomical Society》1999,303(3):515-520

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Size and shape characteristics of drumlins,derived from a large sample,and associated scaling laws     

Chris D. Clark  Anna L.C. Hughes  Sarah L. Greenwood  Matteo Spagnolo  Felix S.L. Ng 《Quaternary Science Reviews》2009,28(7-8):677-692

Ice sheets flowing across a sedimentary bed usually produce a landscape of blister-like landforms streamlined in the direction of the ice flow and with each bump of the order of 10² to 10³ m in length and 10¹ m in relief. Such landforms, known as drumlins, have mystified investigators for over a hundred years. A satisfactory explanation for their formation, and thus an appreciation of their glaciological significance, has remained elusive. A recent advance has been in numerical modelling of the land-forming process. In anticipation of future modelling endeavours, this paper is motivated by the requirement for robust data on drumlin size and shape for model testing.From a systematic programme of drumlin mapping from digital elevation models and satellite images of Britain and Ireland, we used a geographic information system to compile a range of statistics on length L, width W, and elongation ratio E (where E = L/W) for a large sample. Mean L, is found to be 629 m (n = 58,983), mean W is 209 m and mean E is 2.9 (n = 37,043). Most drumlins are between 250 and 1000 metres in length; between 120 and 300 metres in width; and between 1.7 and 4.1 times as long as they are wide. Analysis of such data and plots of drumlin width against length reveals some new insights. All frequency distributions are unimodal from which we infer that the geomorphological label of ‘drumlin’ is fair in that this is a true single population of landforms, rather than an amalgam of different landform types. Drumlin size shows a clear minimum bound of around 100 m (horizontal). Maybe drumlins are generated at many scales and this is the minimum, or this value may be an indication of the fundamental scale of bump generation (‘proto-drumlins’) prior to them growing and elongating. A relationship between drumlin width and length is found (with r² = 0.48) and that is approximately W = 7 L ^1/2 when measured in metres. A surprising and sharply-defined line bounds the data cloud plotted in E–W–L space, and records a scale-dependent maximum elongation limit (approximated by E_max = L^1/3, when L measured in metres). For a given length, for some reason as yet unknown, drumlins do not exceed the elongation ratio defined by this scaling law. We also report and compare our statistics to an amalgamated sample (25,907 drumlins) of measures derived from around 50 published investigations. Any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling.  相似文献   

Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite     

Martin R. Lee  Caroline L. Smith  Sarah H. Gordon  Mark E. Hodson 《Meteoritics & planetary science》2006,41(8):1123-1138

Abstract— Laboratory dissolution experiments using the LL6 ordinary chondrite Bensour demonstrate that meteoritic minerals readily react with distilled water at low temperatures, liberating ions into solution and forming reaction products. Three experiments were performed, all for 68 days and at atmospheric fO₂ but using a range of water/rock ratios and different temperatures. Experiments 1 and 2 were batch experiments and undertaken at room temperature, whereas in experiment 3, condensed boiling water was dripped onto meteorite subsamples within a Soxhlet extractor. Solutions from experiment 1 were chemically analyzed at the end of the experiment, whereas aliquots were extracted from experiments 2 and 3 for analysis at regular intervals. In all three experiments, a very significant proportion of the Na, Cl, and K within the Bensour subsamples entered solution, demonstrating that chlorapatite and feldspar were especially susceptible to dissolution. Concentrations of Mg, Al, Si, Ca, and Fe in solution were strongly affected by the precipitation of reaction products and Mg and Ca may also have been removed by sorption. Calculations predict saturation of experimental solutions with respect to Al hydroxides, Fe oxides, and Fe (oxy)hydroxides, which would have frequently been accompanied by hydrous aluminosilicates. Some reaction products were identified and include silica, a Mg‐rich silicate, Fe oxides, and Fe (oxy)hydroxides. The implications of these results are that even very short periods of subaerial exposure of ordinary chondrites will lead to dissolution of primary minerals and crystallization of weathering products that are likely to include aluminosilicates and silicates, Mg‐Ca carbonates, and sulfates in addition to the ubiquitous Fe oxides and (oxy)hydroxides.  相似文献   

Comment on “Note on the Initial Value Problem for Coronal Loop Kink Waves” By P. S. Cally     

M. S. Ruderman  B. Roberts 《Solar physics》2006,237(1):119-121

In our comment we argue that the results obtained by Cally (2006) are based on a wrong solution of the initial value problem.  相似文献   

Chandra monitoring observations of the ultraluminous X-ray source NGC 5204 X-1     

T. P. Roberts  R. E. Kilgard  R. S. Warwick  M. R. Goad  M. J. Ward 《Monthly notices of the Royal Astronomical Society》2006,371(4):1877-1890

We report the results of a two-month campaign conducted with the Chandra X-ray observatory to monitor the ultraluminous X-ray source (ULX) NGC 5204 X-1. This was composed of a 50-ks observation, followed by ten 5-ks follow-ups spaced initially at ∼3, then at ∼10-d intervals. The ULX flux is seen to vary by factors ∼5 on time-scales of a few days, but no strong variability is seen on time-scales shorter than an hour. There is no evidence for a periodic signal in the X-ray data. An examination of the X-ray colour variations over the period of the campaign shows the ULX emission consistently becomes spectrally harder as its flux increases. The X-ray spectrum from the 50-ks observation can be fitted by a number of disparate spectral models, all of which describe a smooth continuum with, unusually for a ULX, a broad emission feature evident at 0.96 keV. The spectral variations, both within the 50-ks observation and over the course of the whole campaign, can then be explained solely by variations in the continuum component. In the context of an optically thick corona model (as found in other recent results for ULXs) the spectral variations can be explained by the heating of the corona as the luminosity of the ULX increases, consistent with the behaviour of at least one Galactic black hole system in the strongly Comptonized very high state. We find no new evidence supporting the presence of an intermediate-mass black hole in this ULX.  相似文献   

Holocene climate change in the eastern Mediterranean region: a comparison of stable isotope and pollen data from Lake Gölhisar,southwest Turkey     

Warren J. Eastwood  Melanie J. Leng  Neil Roberts  Basil Davis 《第四纪科学杂志》2007,22(4):327-341

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The Late Quaternary vegetational and climatic history of Easter Island     

J. R. Flenley  A. Sarah M. King  Joan Jackson  C. Chew  J. T. Teller  M. E. Prentice 《第四纪科学杂志》1991,6(2):85-115

Easter Island occupies an exceptionally isolated position in the south Pacific Ocean. It is entirely volcanic, and is famous for its giant statues. Late Quaternary sediments have been investigated in three craters: Rano Raraku, Rano Aroi and Rano Kao, giving a continuous record over the past 30 Ka. Pollen records indicate that the island was formerly forested. Palynological and sedimentological evidence suggests a Late Pleistocene climate slightly cooler and drier than the present. Deforestation by people occurred mainly between 1200 and 800 yr BP. This may have led to an ecological disaster and to the decline of the megalithic civilization. The depauperation of the present native flora owes more to human activity than to isolation.  相似文献   

Vertical motions in an intense magnetic flux tube     

Webb  A. R.  Roberts  B. 《Solar physics》1978,59(2):249-274

Solar Physics - The nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of anarbitrary temperature profile in...  相似文献