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951.
952.
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
953.
954.
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
955.
The term Tertiary, subdivided into the Paleogene and Neogene, is traditionally used to represent the interval of geological time between the Cretaceous and Quaternary. In the 1990s, however, the Neogene and Paleogene were ratified by the International Union of Geological Sciences as periods/systems of the Cenozoic Era/Erathem, leaving the Tertiary officially undefined. The Tertiary nonetheless remains a formal term that has never been officially eliminated, and its widespread use today implies a long future. 相似文献
956.
Ray P. Norris Steven Tingay Chris Phillips Enno Middelberg Adam Deller Philip N. Appleton 《Monthly notices of the Royal Astronomical Society》2007,378(4):1434-1438
Infrared-Faint Radio Sources represent a new and unexpected class of object which is bright at radio wavelengths but unusually faint at infrared wavelengths. If, like most mJy radio sources, they were either conventional active or star-forming galaxies in the local Universe, we would expect them to be detectable at infrared wavelengths, and so their non-detection by the Spitzer Space Telescope is surprising. Here, we report the detection of one of these sources using very long baseline interferometry, from which we conclude that the sources are driven by active galactic nuclei. We suggest that these sources are either normal radio-loud quasars at high redshift or abnormally obscured radio galaxies. 相似文献
957.
958.
Uncertainty in floodplain delineation: expression of flood hazard and risk in a Gulf Coast watershed
Jason Christian Leonardo Duenas‐Osorio Aarin Teague Zheng Fang Philip Bedient 《水文研究》2013,27(19):2774-2784
This paper investigates the development of flood hazard and flood risk delineations that account for uncertainty as improvements to standard floodplain maps for coastal watersheds. Current regulatory floodplain maps for the Gulf Coastal United States present 1% flood hazards as polygon features developed using deterministic, steady‐state models that do not consider data uncertainty or natural variability of input parameters. Using the techniques presented here, a standard binary deterministic floodplain delineation is replaced with a flood inundation map showing the underlying flood hazard structure. Additionally, the hazard uncertainty is further transformed to show flood risk as a spatially distributed probable flood depth using concepts familiar to practicing engineers and software tools accepted and understood by regulators. A case study of the proposed hazard and risk assessment methodology is presented for a Gulf Coast watershed, which suggests that storm duration and stage boundary conditions are important variable parameters, whereas rainfall distribution, storm movement, and roughness coefficients contribute less variability. The floodplain with uncertainty for this coastal watershed showed the highest variability in the tidally influenced reaches and showed little variability in the inland riverine reaches. Additionally, comparison of flood hazard maps to flood risk maps shows that they are not directly correlated, as areas of high hazard do not always represent high risk. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
959.
Concentrations of chloride in excess of State of New Hampshire water‐quality standards (230 mg/l) have been measured in watersheds adjacent to an interstate highway (I‐93) in southern New Hampshire. A proposed widening plan for I‐93 has raised concerns over further increases in chloride. As part of this effort, road‐salt‐contaminated groundwater discharge was mapped with terrain electrical conductivity (EC) electromagnetic (EM) methods in the fall of 2006 to identify potential sources of chloride during base‐flow conditions to a small stream, Policy Brook. Three different EM meters were used to measure different depths below the streambed (ranging from 0 to 3 m). Results from the three meters showed similar patterns and identified several reaches where high EC groundwater may have been discharging. Based on the delineation of high (up to 350 mmhos/m) apparent terrain EC, seven‐streambed piezometers were installed to sample shallow groundwater. Locations with high specific conductance in shallow groundwater (up to 2630 mmhos/m) generally matched locations with high streambed (shallow subsurface) terrain EC. A regression equation was used to convert the terrain EC of the streambed to an equivalent chloride concentration in shallow groundwater unique for this site. Utilizing the regression equation and estimates of one‐dimensional Darcian flow through the streambed, a maximum potential groundwater chloride load was estimated at 188 Mg of chloride per year. Changes in chloride concentration in stream water during streamflow recessions showed a linear response that indicates the dominant process affecting chloride is advective flow of chloride‐enriched groundwater discharge. Published in 2010 by John Wiley & Sons, Ltd. 相似文献
960.