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  • 1
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    Photochemical modeling of Titan's atmosphere (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984), the most significant recent change in the reactions used is the updated methane dissociation scheme (Mordaunt et al. 1993). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990); using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992). We have run the model in both steady-state and diurnal modes, with 62 species involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.
    Keywords: INORGANIC AND PHYSICAL CHEMISTRY
    Type: Icarus (ISSN 0019-1035); 113; 1; p. 2-26
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  • 2
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    Environmental effects of an impact-generated dust cloud - Implications for the Cretaceous-Tertiary extinctions (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A model of the evolution and radiative effects of a debris cloud from a hypothesized impact event at the Cretaceous-Tertiary boundary suggests that the cloud could have reduced the amount of light at the earth's surface below that required for photosynthesis for several months and, for a somewhat shorter interval, even below that needed for many animals to see. For 6 months to 1 year, the surface would cool; the oceans could cool only a few degrees Celsius at most, but the continents might cool a maximum of 40 Kelvin. Extinctions in the ocean may have been caused primarily by the temporary cessation of photosynthesis, but those on land may have been primarily induced by a combination of lowered temperatures and reduced light.
    Keywords: GEOSCIENCES (GENERAL)
    Type: Science; 219; Jan. 21
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  • 3
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    Evolution of an impact-generated dust cloud and its effects on the atmosphere (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A simulation is carried out of the evolution of an optically thick dust cloud in the earth's atmosphere, and calculations are made of the effects that such a dust cloud would have on the amount of visible light reaching the surface and the temperature at the earth's surface. It is found that large quantities of dust remain in the atmosphere for periods of only three to six months. This duration is fixed by the physical processes of coagulation; these cause the rapid formation of micron-sized particles and sedimentation that quickly removes the particles from the atmosphere. The duration of the event is found to be nearly independent of the initial altitude, initial particle size, initial mass, atmospheric vertical diffusive mixing rate, and rainout rate. It depends to a slight extent on the particle density and the probability that colliding particles stick together to form a larger particle. In addition, the duration is limited by the rate at which the debris spreads from the initial impact site. A doubling code is used to calculate the visible radiative transfer in the dust clouds. It is found that light levels are too low for vision for one to six months and too low for photosynthesis for two months to one year.
    Keywords: GEOSCIENCES (GENERAL)
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  • 4
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    Radiocarbon dating of open systems with bomb effect (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The application of radiocarbon dating is extended to include systems that are slowly exchanging carbon with the atmosphere. Simple formulae are derived that relate the true age and the exchange rate of carbon to the apparent radiocarbon age. A radiocarbon age determination does not give a unique true age and exchange rate but determines a locus of values bounded by a minimum age and a minimum exchange rate. It is found that for radiocarbon ages as large as 10,000 years it is necessary to correct for the anthropogenic radiocarbon produced in the atmosphere by nuclear weapons testing. A one-term exponential approximation, with an e-folding time of 14.43 years, is used to model this effect and is shown to be accurate to within 3 percent for exchange time constants of 100 years and greater. The approach developed here is not specific to radiocarbon and can be applied to other radioisotopes in open systems.
    Keywords: INORGANIC AND PHYSICAL CHEMISTRY
    Type: Journal of Geophysical Research (ISSN 0148-0227); 91; 3836-384
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