Saturn's dynamic D ring     

Matthew M. Hedman  Joseph A. Burns  Mark R. Showalter  Philip D. Nicholson  Matthew S. Tiscareno  Bonnie J. Buratti  Roger Clark 《Icarus》2007,188(1):89-107

The Cassini spacecraft has provided the first clear images of the D ring since the Voyager missions. These observations show that the structure of the D ring has undergone significant changes over the last 25 years. The brightest of the three ringlets seen in the Voyager images (named D72), has transformed from a narrow, <40-km wide ringlet to a much broader and more diffuse 250-km wide feature. In addition, its center of light has shifted inwards by over 200 km relative to other features in the D ring. Cassini also finds that the locations of other narrow features in the D ring and the structure of the diffuse material in the D ring differ from those measured by Voyager. Furthermore, Cassini has detected additional ringlets and structures in the D ring that were not observed by Voyager. These include a sheet of material just interior to the inner edge of the C ring that is only observable at phase angles below about 60°. New photometric and spectroscopic data from the ISS (Imaging Science Subsystem) and VIMS (Visual and Infrared Mapping Spectrometer) instruments onboard Cassini show the D ring contains a variety of different particle populations with typical particle sizes ranging from 1 to 100 microns. High-resolution images reveal fine-scale structures in the D ring that appear to be variable in time and/or longitude. Particularly interesting is a remarkably regular, periodic structure with a wavelength of ∼30 km extending between orbital radii of 73,200 and 74,000 km. A similar structure was previously observed in 1995 during the occultation of the star GSC5249-01240, at which time it had a wavelength of ∼60 km. We interpret this structure as a periodic vertical corrugation in the D ring produced by differential nodal regression of an initially inclined ring. We speculate that this structure may have formed in response to an impact with a comet or meteoroid in early 1984.  相似文献   

The Ediacaran fossils of Charnwood Forest: Shining new light on a major biological revolution     

Charlotte G. Kenchington  Simon J. Harris  Philip B. Vixseboxse  Chris Pickup  Philip R. Wilby 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(3):264-277

Charnwood Forest (UK) hosts some of the oldest and best-preserved macrofossils known from the Ediacaran. It is the counterpoint to the more widely studied fossil sites of south-eastern Newfoundland (Canada), which include the recently-designated UNESCO World Heritage Site of Mistaken Point. Discoveries made in Charnwood Forest since 2008 have the potential to revolutionise our understanding of the evolution of complex macroscopic life and the subsequent development of ‘modern’ (i.e. Phanerozoic) ecosystems. The sites in Charnwood include the holotypes for several iconic Ediacaran taxa, and potentially both the oldest and youngest representatives of the deep-water Avalon Assemblage. These communities provide a unique opportunity to test models of community ecology, biological endemism and environmental sensitivity and adaptability in the Ediacaran. Here, we review the geology of Charnwood Forest and the palaeobiology of its biotas, and we summarise recent scientific advances in the context of our developing understanding of early macroscopic life. We review the application of Reflectance Transformation Imaging to these ancient communities, and signpost exciting new directions for research in Charnwood Forest, almost 170 years after the fossils were first brought to light.  相似文献   

Study of Carbon‐Bearing Particles in Ascending Geogas Flows in the Dongshengmiao Polymetallic Pyrite Deposit,Inner Mongolia,China          下载免费PDF全文

Guai Hu  Jianjin Cao  Philip K. Hopke  Robert Frantisek Holub 《Resource Geology》2015,65(1):13-26

Samples of ascending geogas flow particles were collected on to SiN grids directly in the Quaternary sediments overlying the Dongshengmiao polymetallic pyrite deposit, China. Corresponding soil samples were collected in the surface of Dongshengmiao district at the same time. After pretreatment, these SiN grid samples were analyzed by transmission electron microscopy. The tests focused on the characteristics of particles including size, shape, chemical composition, structure and association. The results show that there are numerous carbon‐bearing particles in particulate samples of the ascending geogas flow. The particles contain organic matter, carbonate or carbonate mixed with other minerals. These carbon‐bearing particles generally contain metallic elements like Fe/Zn/Au/Cu/Pb. However, all of the soil particles do not contain elemental C and only consist of common elements (O, Na, Mg, Al, Si, Ca, Ti) just like the composition of earth crust. Through a comparison between the particles from different sources, the carbon‐bearing particles were found to come only from the deep earth and carried useful information about concealed deposits as they pass through the deep‐seated orebodies. Given the influences of organic matter on mineralization, the carbon‐bearing particle may provide information on the deposit genesis. Combining the use of geogas particle for prospecting with characteristics of large depth, and the carbon‐bearing materials' close spatial and genetic relationship with orebodies, we propose a new prospecting method based on the characteristics of carbon‐bearing particles, including morphology, size, chemical component and ultra‐microstructure. This approach could be applied to the exploration of deposits deep in the earth and abundant in carbon‐bearing matter. This approach can provide efficient and effective deposit exploration.  相似文献   

Projections of the advance in the start of the growing season during the 21st century based on CMIP5 simulations     

XIA Jiangjiang  YAN Zhongwei  JIA Gensuo  ZENG Heqing  Philip Douglas JONES  ZHOU Wen  ZHANG Anzhi 《大气科学进展》2015,32(6):831-838

It is well-known that global warming due to anthropogenic atmospheric greenhouse effects advanced the start of the vegetation growing season(SOS) across the globe during the 20 th century. Projections of further changes in the SOS for the 21 st century under certain emissions scenarios(Representative Concentration Pathways, RCPs) are useful for improving understanding of the consequences of global warming. In this study, we first evaluate a linear relationship between the SOS(defined using the normalized difference vegetation index) and the April temperature for most land areas of the Northern Hemisphere for 1982–2008. Based on this relationship and the ensemble projection of April temperature under RCPs from the latest state-of-the-art global coupled climate models, we show the possible changes in the SOS for most of the land areas of the Northern Hemisphere during the 21 st century. By around 2040–59, the SOS will have advanced by-4.7 days under RCP2.6,-8.4 days under RCP4.5, and-10.1 days under RCP8.5, relative to 1985–2004. By 2080–99, it will have advanced by-4.3 days under RCP2.6,-11.3 days under RCP4.5, and-21.6 days under RCP8.5. The geographic pattern of SOS advance is considerably dependent on that of the temperature sensitivity of the SOS. The larger the temperature sensitivity,the larger the date-shift-rate of the SOS.  相似文献   

Fine-scale temporal recovery,reconstruction and evolution of a post-supereruption magmatic system     

Simon J. Barker  Colin J. N. Wilson  Aidan S. R. Allan  C. Ian Schipper 《Contributions to Mineralogy and Petrology》2015,170(1):1-40

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Community responses and adaptation strategies toward flood hazard in Jakarta,Indonesia     

Marfai  Muh Aris  Sekaranom  Andung Bayu  Ward  Philip 《Natural Hazards》2015,75(2):1127-1144

Natural Hazards - Jakarta, as the capital of the Republic of Indonesia, has long been known to be at risk from flood hazard. It is increasingly recognized that community responses and adaptation...  相似文献   

Use of the new official names for the Subsystems, Series and Stages of the Carboniferous System in international journals     

Philip H. Heckel 《Proceedings of the Geologists' Association. Geologists' Association》2006,117(4):393-396

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On the discovery of CO nighttime emissions on Titan by Cassini/VIMS: Derived stratospheric abundances and geological implications     

Kevin H. Baines  Pierre Drossart  Miguel A. Lopez-Valverde  Sushil K. Atreya  Christophe Sotin  Thomas W. Momary  Robert H. Brown  Bonnie J. Buratti  Roger N. Clark  Philip D. Nicholson   《Planetary and Space Science》2006,54(15):1552-1562

We present a quantitative analysis of CO thermal emissions discovered on the nightside of Titan by Baines et al. [2005. The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. Earth, Moon, and Planets, 96, 119–147]. in Cassini/VIMS spectral imagery. We identify these emission features as the P and R branches of the 1-0 vibrational band of carbon monoxide (CO) near 4.65 μm. For CH₃D, the prominent Q branch of the ν₂ fundamental band of CH₃D near 4.55 μm is apparent. CO₂ emissions from the strong v₃ vibrational band are virtually absent, indicating a CO₂ abundance several orders of magnitude less than CO, in agreement with previous investigations. Analysis of CO emission spectra obtained over a variety of altitudes on Titan's nightside limb indicates that the stratospheric abundance of CO is 32±15 ppm, and together with other recent determinations, suggests a vertical distribution of CO nearly constant at this value from the surface throughout the troposphere to at least the stratopause near 300 km altitude. The corresponding total atmospheric content of CO in Titan is 2.9±1.5×10¹⁴ kg. Given the long lifetime of CO in the oxygen-poor Titan atmosphere (0.5–1.0 Gyr), we find a mean CO atmospheric production rate of 6±3×10⁵ kg yr⁻¹. Given the lack of primordial heavy noble gases observed by Huygens [Niemann et al., 2005. The abundances of constituents of Titan's atmosphere from the GCMS on the Huygens probe. Nature, 438, 779–784], the primary source of atmospheric CO is likely surface emissions. The implied CO/CH₄ mixing ratio of near-surface material is 1.8±0.9×10⁻⁴, based on an average methane surface emission rate over the past 0.5 Gyr of 1.3×10⁻¹³ gm cm⁻² s⁻¹ as required to balance hydrocarbon haze production via methane photolysis [Wilson and Atreya, 2004. Current state of modeling the photochemistry of Titan's mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109, E06002 Doi:10.1029/2003JE002181]. This low CO/CH₄ ratio is much lower than expected for the sub-nebular formation region of Titan and supports the hypothesis [e.g., Atreya et al., 2005. Methane on Titan: photochemical-meteorological-hydrogeochemical cycle. Bull. Am. Astron. Soc. 37, 735] that the conversion of primordial CO and other carbon-bearing materials into CH₄-enriched clathrate-hydrates occurs within the deep interior of Titan via the release of hydrogen through the serpentinization process followed by Fischer–Tropsch catalysis. The time-averaged predicted emission rate of methane-rich surface materials is 0.02 km³ yr⁻¹, a value significantly lower than the rate of silicate lava production for the Earth and Venus, but nonetheless indicative of significant active geological processes reshaping the surface of Titan.  相似文献   

Is the dependence of spectral index on luminosity real in optically selected AGN samples?     

Su Min Tang  Shuang Nan Zhang  Philip F. Hopkins 《Monthly notices of the Royal Astronomical Society》2007,377(3):1113-1121

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Vertical structure of hyper-accreting disks and consequences for gamma-ray burst outflows     

Elena M. Rossi  Philip J. Armitage  Tiziana Di Matteo 《Astrophysics and Space Science》2007,311(1-3):185-190

We discuss the physics of the power source for gamma-ray bursts (GRBs). There is a great variety of stellar systems proposed as progenitors of long and short GRBs, but any current model for the engine ultimately involves the formation of a hyper-accreting disk around a newly-formed black hole of a few solar masses. The study of such disks can shed light onto the burst composition and energy content. We present preliminary results from disk vertical structure calculations. These include neutrino transport, a height-dependent determination of the nuclear composition, and a simplified treatment of turbulent mixing. We find that vertical mixing is rapid enough to largely erase compositional gradients, and as a consequence the upper layers of the disk reflect the neutron-rich composition of the midplane close to the black hole. We review the implications of this for the nuclear composition of outflows in GRBs. Our models suggest an increasing role for pairs in the upper regions of the disk, and we speculate that a pair-driven wind could be a significant source of cooling. Elena M. Rossi is a Chandra Fellow  相似文献