Hydrocarbon photochemistry in the upper atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.     

A study of chemical systems using signal flow graph theory: application to Neptune

Photochemistry of giant planets and their satellites is characterized by numerous reactions involving many chemical species. In the present paper, chemical systems are modeled by signal flow graphs. Such a technique evaluates the transmission of any input into the system (solar flux, electrons...) and gives access to the identification of the most important mechanisms in the chemical system. For a given chemical system, we first evaluate rate coefficients. Then, in order to obtain concentrations of each compound, we integrate the set of continuity equations by Gear's method. Gear's method is chosen rather than another classical method because it is recommended for a system of stiff equations due to the existence of greatly differing time constants. Finally, the technique of signal flow graphs is used. This method is applied to the production of hydrocarbons in the atmospheres of giant planets. In particular, the production of C2H6 in the atmosphere of Neptune from the photodissociation of CH4 is investigated. Different paths of dissociation of CH4 are possible from L alpha radiations. A chemical system containing 14 species and 30 reactions including these different paths of dissociation is integrated. The main mechanism of production of C2H6 is identified and evaluated for each model of dissociation. The importance of various reaction paths as a function of time is discussed.     

Laboratory simulations of Titan's atmosphere: organic gases and aerosols

Titan, the main satellite of Saturn, has been observed by remote sensing for many years, both from interplanetary probes (Pioneer and Voyager's flybys) and from the Earth. Its N2 atmosphere, containing a small fraction of CH4 (approximately 2%), with T approximately 90 K and P approximately 1.5 bar at the ground level, is irradiated by solar UV photons and deeply bombarded by energetic particles, i.e. Saturn mangetospheric electrons and protons, interplanetary electrons and cosmic rays. The resulting energy deposition, which takes place mainly below 1000 km, initiates chemical reactions which yield gaseous hydrocarbons and nitriles and, through polymerisation processes, solid aerosol particles which grow by coagulation and settle down to the ground. At the present time, photochemical models strongly require the results of specific laboratory studies. Chemical rate constants are not well known at low temperatures, charged-particle-induced reactions are difficult to model and laboratory simulations of atmospheric processes are therefore of great interest. Moreover, the synthesis of organic compounds which have not been detected to date provides valuable information for future observations. The origin and chemical composition of aerosols depend on the nature of chemical and energy sources. Their production from gaseous species may be monitored in laboratory chambers and their optical or microphysical properties compared to those deduced from the observations of Titan's atmosphere. The development of simulation chambers of Titan's extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titan's atmosphere by remote sensing and in situ analysis in the near future (Cassini-Huygens mission). It appears necessary, as a preliminary step to test on-board experiments in such chambers, and as a final step, when new space data have been acquired, to use them for more general scientific purposes.     

Constraints on the nucleus and dust properties from mid-infrared imaging of comet Hyakutake

We use Mie scattering theory to determine the expected thermal emission from dust grains in cometary comae and apply these results to mid-infrared images of comet Hyakutake (C/1996 B2) obtained preperihelion in 1996 March. Calculations were performed for dust grains in the size range from 0.1 to 10 micrometers for two different compositions: amorphous olivine (a silicate glass) and an organic residue mixture. The resulting emission efficiencies are complicated functions of wavelength and particle size and are significantly different for the two materials in question. The Hyakutake data set consists of three nights of high-resolution imaging (100-150 km pixel-1 at the comet) of the inner coma at 8.7, 11.7, 12.5, and 19.7 micrometers. Attempts to fit the observed colors (ratios of fluxes at different wavelengths) using a single grain composition failed. However, fits to the data were achieved for all three nights using a mixture of approximately 1 micrometer olivine grains and approximately 7 micrometers organic grains. The resulting olivine mass fraction was between 8% and 16% of the total dust mass-loss rate. We also estimate the radius of the nucleus to be r = 2.1 +/- 0.4 km.     

http://www.sciencedirect.com/science/article/pii/S167498711500095X

Pseudosection modelling is rapidly becoming an essential part of a petrologist's toolkit and often forms the basis of interpreting the tectonothermal evolution of a rock sample,outcrop,or geological region.Of the several factors that can affect the accuracy and precision of such calculated phase diagrams, "geological" uncertainty related to natural petrographic variation at the hand sample- and/or thin section-scale is rarely considered.Such uncertainty influences the sample's bulk composition,which is the primary control on its equilibrium phase relationships and thus the interpreted pressure-temperature(P-T) conditions of formation.Two case study examples-a garnet-cordierite granofels and a garnet-staurolite-kyanite schist-are used to compare the relative importance that geological uncertainty has on bulk compositions determined via(1) X-ray fluorescence(XRF) or(2) point counting techniques.We show that only minor mineralogical variation at the thin-section scale propagates through the phase equilibria modelling procedure and affects the absolute P-T conditions at which key assemblages are stable.Absolute displacements of equilibria can approach ±1 kbar for only a moderate degree of modal proportion uncertainty,thus being essentially similar to the magnitudes reported for analytical uncertainties in conventional thermobarometry.Bulk compositions determined from multiple thin sections of a heterogeneous garnet-staurolite-kyanite schist show a wide range in major-element oxides,owing to notable variation in mineral proportions.Pseudosections constructed for individual point count-derived bulks accurately reproduce this variability on a case-by-case basis,though averaged proportions do not correlate with those calculated at equivalent peak P-T conditions for a whole-rock XRF-derived bulk composition.The main discrepancies relate to varying proportions of matrix phases(primarily mica) relative to porphyroblasts(primarily staurolite and kyanite),indicating that point counting preserves small-scale petrographic features that are otherwise averaged out in XRF analysis of a larger sample.Careful consideration of the size of the equilibration volume,the constituents that comprise the effective bulk composition,and the best technique to employ for its determination based on rock type and petrographic character,offer the best chance to produce trustworthy data from pseudosection analysis.     

http://www.sciencedirect.com/science/article/pii/S1674987115001322

Zircon U-Pb geochronology has become a keystone tool across Earth science,arguably providing the gold standard in resolving deep geological time.The development of rapid in situ analysis of zircon（via laser ablation and secondary ionization mass spectrometry） has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable（e.g.detrital zircons as a sediment fingerprinting method）.The ease of acquisition,while bringing benefit to the Earth science community,has also led to diverse interpretations of geochronological data.In this work we seek to refocus U-Pb zircon geochronology toward best practice by providing a robust statistically coherent workflow.We discuss a range of data filtering approaches and their inherent limitations（e.g.discordance and the reduced chi-squared;MSWD）.We evaluate appropriate mechanisms to calculate the most geologically appropriate age from both ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios and demonstrate the cross over position when chronometric power swaps between these ratios.As our in situ analytical techniques become progressively more precise,appropriate statistical handing of U-Pb datasets will become increasingly pertinent.     

Comparison of Venusian Coronae with Giant Circumferential Dyke Swarms on Earth

正The surface of Venus features a large number(500)of quasi-circular tectono-magmatic features known as coronae.Coronae are characterized in part by their topographic characteristics,which can be diverse and     

Large Igneous Provinces,Their Giant Mafic Dyke Swarms,and Links to Metallogeny

正Giant mafic dyke swarms are a key component of the feeder system of Large Igneous Provinces(LIPs),large volume(0.1 Mkm~3;frequently above1 M km~3),mainly mafic(-ultramafic)magmatic events of intraplate affinity.     

Genesis and Distribution of Ultra-alkaline Magmatism within the East Antarctic Associated with the Kerguelen Plume Activity

正Alkaline magmatism is often associated with the initial or final stages of huge plume activity.The alkaline bodies are most often found within the boundary area of plume impact upon the continents.The initial stages of the     

Geochemistry and Emplacement of Post-collisional Shoshonitic Dyke Swarms,NW of Iran

正About 300 mafic dykes are intruded Eocene volcanic and pyroclastic rocks,north east of Meshkinshahr city in the NW of Iran.Most of dyke swarms display NNE-SSE trend.Their composition varies from basalt to tephrite and