The role of organic haze in Titan's atmospheric chemistry: I. Laboratory investigation on heterogeneous reaction of atomic hydrogen with Titan tholin     

Yasuhito Sekine  Hiroshi Imanaka  Takafumi Matsui  Emma L.O. Bakes  Christopher P. McKay 《Icarus》2008,194(1):186-200

In Titan's atmosphere consisting of N₂ and CH₄, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N₂ and CH₄ gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×¹⁰⁻³×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.  相似文献   

Laboratory studies of methane and ethane adsorption and nucleation onto organic particles: Application to Titan's clouds     

Daniel B. Curtis  Courtney D. Hatch  Christa A. Hasenkopf  Owen B. Toon  Margaret A. Tolbert  Christopher P. McKay 《Icarus》2008,195(2):792-801

Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.  相似文献   

Elimination of natural organic matter by electrocoagulation using bipolar and monopolar arrangements of iron and aluminum electrodes     

M. Alimohammadi  M. Askari  M. H. Dehghani  A. Dalvand  R. Saeedi  K. Yetilmezsoy  B. Heibati  G. McKay 《International Journal of Environmental Science and Technology》2017,14(10):2125-2134

The aim of this research was to evaluate the efficiency of electrocoagulation (EC) for the removal of natural organic matter (NOM) by using iron (Fe) and aluminum (Al) electrodes. The effects of several operational parameters such as initial pH (3–10), time of electrolysis (5–30 min), initial concentration of organic matter (10–50 mg NOM/L), current density (0.25–1.25 mA/cm²), type of electrode material (n = 4, 2 sides × 11 cm × 10 cm, wall thickness = 2 mm, distance between each electrode = 5 mm), and type of connection of electrodes (bipolar and monopolar configurations) were explored for the removal of NOM from synthetic humic acid solution in a 2 L laboratory-scale EC cells (A _s/V = 0.110 cm^?1). The optimum conditions for the process were identified as pH = 3 and 7, electrolysis time = 20 and 10 min for Fe and Al electrodes, respectively. Using both electrodes at current density = 0.25 mA/cm² and initial concentration of organic matter = 50 mg/L, a NOM removal efficiency of almost 100% could be achieved in the bipolar mode. Based on the optimum conditions, specific reactor electrical energy consumptions were 14.90 kWh/kg Al (or 0.092 kWh/m³) and 2.88 kWh/kg Fe (or 0.11 kWh/m³). Specific electrode consumptions were obtained to be 0.0062 and 0.0382 kg/m³, and operating costs of the EC system were preliminary estimated at 0.057 and 0.119 $/m³ for Al and Fe electrodes, respectively.  相似文献   

Survival of endospores of Bacillus subtilis on spacecraft surfaces under simulated martian environments: implications for the forward contamination of Mars     

Schuerger AC  Mancinelli RL  Kern RG  Rothschild LJ  McKay CP 《Icarus》2003,165(2):253-276

Experiments were conducted in a Mars simulation chamber (MSC) to characterize the survival of endospores of Bacillus subtilis under high UV irradiation and simulated martian conditions. The MSC was used to create Mars surface environments in which pressure (8.5 mb), temperature (-80, -40, -10, or +23 degrees C), gas composition (Earth-normal N2/O2 mix, pure N2, pure CO2, or a Mars gas mix), and UV-VIS-NIR fluence rates (200-1200 nm) were maintained within tight limits. The Mars gas mix was composed of CO2 (95.3%), N2 (2.7%), Ar (1.7%), O2 (0.2%), and water vapor (0.03%). Experiments were conducted to measure the effects of pressure, gas composition, and temperature alone or in combination with Mars-normal UV-VIS-NIR light environments. Endospores of B. subtilis, were deposited on aluminum coupons as monolayers in which the average density applied to coupons was 2.47 x 10(6) bacteria per sample. Populations of B. subtilis placed on aluminum coupons and subjected to an Earth-normal temperature (23 degrees C), pressure (1013 mb), and gas mix (normal N2/O2 ratio) but illuminated with a Mars-normal UV-VIS-NIR spectrum were reduced by over 99.9% after 30 sec exposure to Mars-normal UV fluence rates. However, it required at least 15 min of Mars-normal UV exposure to reduce bacterial populations on aluminum coupons to non-recoverable levels. These results were duplicated when bacteria were exposed to Mars-normal environments of temperature (-10 degrees C), pressure (8.5 mb), gas composition (pure CO2), and UV fluence rates. In other experiments, results indicated that the gas composition of the atmosphere and the temperature of the bacterial monolayers at the time of Mars UV exposure had no effects on the survival of bacterial endospores. But Mars-normal pressures (8.5 mb) were found to reduce survival by approximately 20-35% compared to Earth-normal pressures (1013 mb). The primary implications of these results are (a) that greater than 99.9% of bacterial populations on sun-exposed surfaces of spacecraft are likely to be inactivated within a few tens of seconds to a few minutes on the surface of Mars, and (b) that within a single Mars day under clear-sky conditions bacterial populations on sun-exposed surfaces of spacecraft will be sterilized. Furthermore, these results suggest that the high UV fluence rates on the martian surface can be an important resource in minimizing the forward contamination of Mars.  相似文献   

Possibilities for methanogenic life in liquid methane on the surface of Titan     

C.P. McKay  H.D. Smith 《Icarus》2005,178(1):274-276

Photochemically produced compounds on Titan, principally acetylene, ethane and organic solids, would release energy when consumed with atmospheric hydrogen, at levels of 334, 57, and 54 kJ mol⁻¹, respectively. On Earth methanogenic bacteria can survive on this energy level. Here we speculate on the possibility of widespread methanogenic life in liquid methane on Titan. Hydrogen may be the best molecule to show the affects of such life because it does not condense at the tropopause and has no sources or sinks in the troposphere. If life is consuming atmospheric hydrogen it will have a measurable effect on the hydrogen mixing ratio in the troposphere if the biological consumption is greater than . Life could develop strategies to overcome the low solubility of organics in liquid methane and use catalysts to accelerate biochemical reactions despite the low temperature. The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface.  相似文献   

On the mean particle size and water content of polar mesospheric clouds     

Gary E. Thomas  Christopher P. McKay 《Planetary and Space Science》1985,33(10):1209-1224

The ultraviolet spectrometer on board the Solar Mesosphere Explorer (SME) satellite has been measuring the scattering of ultraviolet sunlight from optically thin cloud layers in the upper boundary of the mesosphere (85 km) since the launch of the spacecraft in October 1981. These layers are present only at high latitudes during the summer season. During Summer 1983 an observing sequence was undertaken to measure the cloud radiance at two different scattering angles—one in the forward hemisphere at 50°, the other in the backward hemisphere at 130°. The data show a pronounced tendency for the brighter clouds to exhibit greater forward-scattering behavior, indicating that particle size may be the most important factor in determining the cloud brightness. We conclude that if the particles are monodisperse water ice aggregates, their radii do not exceed 70 nm. Estimates are provided for the water content and column particle number of the clouds, depending upon the unknown shape of the particle size distribution. Characterizing the distribution by two parameters, the spherical equivalent particle radius and the width of the distribution, the bulk cloud properties are shown to be dependent upon the limb radiance and the width parameter. For narrow widths the water ice content is less than that expected for the water vapor content at mesopause heights of a few parts per million. This confirms our earlier analysis using a smaller data set and assuming the cloud particles are monodisperse. However if the size dispersion is broad, the implied water ice exceeds the static atmospheric water supply. The calculated column number for the brightest clouds exceeds our estimates for the total supply of condensation nuclei. The alternative is that the brighter clouds are limited to a fairly narrow range of particle sizes from 40 to 60 nm. This conclusion is supported by theoretical time dependent calculations by Turco et al. (1982).  相似文献   

Photoelectric Charging of Submicron Aerosols and Macromolecules in the Titan Haze     

E.L.O. BakesChristopher P. McKay  Charles W. Bauschlicher Jr. 《Icarus》2002,157(2):464-475

We quantify the charge states of submicrometer aerosols and aromatic macromolecules in Titan's organic haze. The aerosol charge is balanced between the recombination of positive ions with the aerosol plus the ejection of electrons from the aerosol via the UV-driven photoelectric effect and the recombination of electrons with the aerosol. During the day, the dominant charge state for submicro-meter aerosols is positive. Macromolecules composed of fewer than 32 carbon atoms with low electron affinities (<1.0 eV) are neutral, while the rest are mainly neutral and negatively charged with a small fraction (∼10%) becoming positively charged at higher (≥300 km) altitudes. At night, Titan's aerosol population becomes uniformly neutral and negatively charged. The time taken for a nighttime aerosol to change from being negatively charged to its most probable daytime positive charge is on the order of a few seconds for the largest submicrometer aerosols, while macromolecules tend to persist in an anionic charge state for one to several Earth days. Charging strongly influences aerosol agglomeration via Coulomb attraction and may account for the seasonal variations in the albedo of the Titan haze at midrange (∼200-250 km) altitudes. Enhanced agglomeration may also efficiently produce a source of condensation nuclei for the daily rainout of methane. In addition, the difference in aerosol charge between Titan's day and night (or summer and winter) phases will produce dramatically different chemistries which must be accounted for in future photochemical models. Finally, if there are PAH-like macromolecules in the Titan haze, Cassini Huygens should be able to observe these charge differences, with neutral macromolecules emitting strongly at 3.3 and 11.2 μm, cationic macromolecules emitting between 6.2 and 8.6 μm, and anionic macromolecules emitting in both infrared spectral regions.  相似文献   

EXPERIENCES WITH ANALYTICAL PLOTTERS     

W. M. McKay  I. J. Dowman  J. E. Farrow 《The Photogrammetric Record》1985,11(66):729-743

Three authors, representing commercial, educational and governmental photogrammetric organisations, report on their initial uses of analytical plotters.  相似文献   

Weathering of surface clasts as an indicator of the relative age of glacial deposits on Jameson Land, East Greenland     

MARIANNE LlLLIESKÖLD  BJÖRN SUNDQVIST 《Boreas: An International Journal of Quaternary Research》1994,23(4):473-478

The relative age of glacial deposits on Jameson Land was investigated by weathering-based techniques. Two related methods, measurement of rind thickness and of Schmidt hammer rebound values. were added to a dating programme of luminescence-, amino acid- and ¹⁴C-analyses. Rind thicknesses differ significantly between areas which represent three glacial phascs, spanning a time interval from late Saale to Late Weichselian. Rebound values are more susceptible to external factors than the rind values but may, if these external factors are known, serve as a complement to the interpretation of rind development.  相似文献   

Tridymitic jasperoid deposits in N. W. Queensland     

W. J. McKay  T. Finlow-Bates 《Mineralium Deposita》1977,12(1):96-104

A number of small, fault-controlled, siliceous, copper deposits in the Precambrian of North-West Queensland contain most of their silica as tridymitic jasperoid. The tridymite has pseudomorphically replaced the dynamic metamorphic assemblages that resulted from earlier fault movements. It is proposed that seismic pumping (Sibson et al., 1975) forced copper and silica-charged hydrothermal solutions along major faults. At elevated pressures quartz-copper lodes were emplaced but in lower pressure zones on feather faults at the end of major fault zones or in small fault off-shoots tridymitic jasperoid deposits formed. As tridymite is unstable at quite low burial pressure the preservation of the tridymitic jasperoid desposits reveals the limited amplitude of uplift and erosion in north-west Queensland since the deposits formed in the middle Proterozoic.  相似文献