Photochemistry of Saturn's Atmosphere
Identifieur interne : 001F57 ( Istex/Curation ); précédent : 001F56; suivant : 001F58Photochemistry of Saturn's Atmosphere
Auteurs : Julianne I. Moses ; Bruno Bézard [France] ; Emmanuel Lellouch [France] ; G. Randall Gladstone ; Helmut Feuchtgruber [Allemagne] ; Mark AllenSource :
- Icarus [ 0019-1035 ] ; 2000.
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Abstract
To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column densities of CH3, CH3C2H, and C4H2 above 10 mbar are 4+2−1.5×1013 cm−2, (1.1±0.3)×1015 cm−2, and (1.2±0.3)×1014 cm−2, respectively. The observed ISO emission features also indicate C2H2 mixing ratios of 1.2+0.9−0.6×10−6 at 0.3 mbar and (2.7±0.8)×10−7 at 1.4 mbar, and a C2H6 mixing ratio of (9±2.5)×10−6 at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2. The sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than ∼3×104 cm2 s−1 at pressures greater than 1 mbar in order to reproduce the C2H6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH3 radicals if the low-temperature chemistry of CH3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere—diacetylene, butane, and water condense between ∼1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.
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DOI: 10.1006/icar.1999.6270
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Julianne I. Moses<affiliation><mods:affiliation>Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas, 77058-1113, f1 moses@lpi3.jsc.nasa.govf1</mods:affiliation><wicri:noCountry code="subField">moses@lpi3.jsc.nasa.govf1</wicri:noCountry></affiliation><affiliation><mods:affiliation>Space Sciences Department, Southwest Research Institute, San Antonio, Texas, 78228-0510</mods:affiliation><wicri:noCountry code="subField">78228-0510</wicri:noCountry></affiliation><affiliation><mods:affiliation>Earth and Space Science Division, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California, 91109</mods:affiliation><wicri:noCountry code="subField">91109</wicri:noCountry></affiliation>Le document en format XML
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Moses</name><affiliation><mods:affiliation>Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas, 77058-1113, f1 moses@lpi3.jsc.nasa.govf1</mods:affiliation><wicri:noCountry code="subField">moses@lpi3.jsc.nasa.govf1</wicri:noCountry></affiliation></author><author><name sortKey="Bezard, Bruno" sort="Bezard, Bruno" uniqKey="Bezard B" first="Bruno" last="Bézard">Bruno Bézard</name><affiliation wicri:level="1"><mods:affiliation>DESPA, Observatoire de Paris, Meudon, 92195, France</mods:affiliation><country xml:lang="fr">France</country><wicri:regionArea>DESPA, Observatoire de Paris, Meudon, 92195</wicri:regionArea></affiliation></author><author><name sortKey="Lellouch, Emmanuel" sort="Lellouch, Emmanuel" uniqKey="Lellouch E" first="Emmanuel" last="Lellouch">Emmanuel Lellouch</name><affiliation wicri:level="1"><mods:affiliation>DESPA, Observatoire de Paris, Meudon, 92195, France</mods:affiliation><country xml:lang="fr">France</country><wicri:regionArea>DESPA, Observatoire de Paris, Meudon, 92195</wicri:regionArea></affiliation></author><author><name sortKey="Gladstone, G Randall" sort="Gladstone, G Randall" uniqKey="Gladstone G" first="G. 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Moses</name><affiliation><mods:affiliation>Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas, 77058-1113, f1 moses@lpi3.jsc.nasa.govf1</mods:affiliation><wicri:noCountry code="subField">moses@lpi3.jsc.nasa.govf1</wicri:noCountry></affiliation></author><author><name sortKey="Bezard, Bruno" sort="Bezard, Bruno" uniqKey="Bezard B" first="Bruno" last="Bézard">Bruno Bézard</name><affiliation wicri:level="1"><mods:affiliation>DESPA, Observatoire de Paris, Meudon, 92195, France</mods:affiliation><country xml:lang="fr">France</country><wicri:regionArea>DESPA, Observatoire de Paris, Meudon, 92195</wicri:regionArea></affiliation></author><author><name sortKey="Lellouch, Emmanuel" sort="Lellouch, Emmanuel" uniqKey="Lellouch E" first="Emmanuel" last="Lellouch">Emmanuel Lellouch</name><affiliation wicri:level="1"><mods:affiliation>DESPA, Observatoire de Paris, Meudon, 92195, France</mods:affiliation><country xml:lang="fr">France</country><wicri:regionArea>DESPA, Observatoire de Paris, Meudon, 92195</wicri:regionArea></affiliation></author><author><name sortKey="Gladstone, G Randall" sort="Gladstone, G Randall" uniqKey="Gladstone G" first="G. 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Randall Gladstone</name><affiliation><mods:affiliation>Space Sciences Department, Southwest Research Institute, San Antonio, Texas, 78228-0510</mods:affiliation><wicri:noCountry code="subField">78228-0510</wicri:noCountry></affiliation></author><author><name sortKey="Feuchtgruber, Helmut" sort="Feuchtgruber, Helmut" uniqKey="Feuchtgruber H" first="Helmut" last="Feuchtgruber">Helmut Feuchtgruber</name><affiliation wicri:level="1"><mods:affiliation>Max-Planck Institut für Extraterrestrische Physik, Garching, 85740, Germany</mods:affiliation><country xml:lang="fr">Allemagne</country><wicri:regionArea>Max-Planck Institut für Extraterrestrische Physik, Garching, 85740</wicri:regionArea></affiliation></author><author><name sortKey="Allen, Mark" sort="Allen, Mark" uniqKey="Allen M" first="Mark" last="Allen">Mark Allen</name><affiliation><mods:affiliation>Earth and Space Science Division, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California, 91109</mods:affiliation><wicri:noCountry code="subField">91109</wicri:noCountry></affiliation></author></analytic><monogr></monogr><series><title level="j">Icarus</title><title level="j" type="abbrev">YICAR</title><idno type="ISSN">0019-1035</idno><imprint><publisher>ELSEVIER</publisher><date type="published" when="2000">2000</date><biblScope unit="volume">143</biblScope><biblScope unit="issue">2</biblScope><biblScope unit="page" from="244">244</biblScope><biblScope unit="page" to="298">298</biblScope></imprint><idno type="ISSN">0019-1035</idno></series><idno type="istex">79FC56D1DA0A9235A3D8895F07188E2A3528818B</idno><idno type="DOI">10.1006/icar.1999.6270</idno><idno type="PII">S0019-1035(99)96270-X</idno></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0019-1035</idno></seriesStmt></fileDesc><profileDesc><textClass><keywords scheme="KwdEn" xml:lang="en"><term>Saturn</term><term>atmosphere</term><term>atmospheres</term><term>composition</term><term>infrared observations</term><term>organic chemistry</term><term>photochemistry</term></keywords></textClass><langUsage><language ident="en">en</language></langUsage></profileDesc></teiHeader><front><div type="abstract" xml:lang="en">To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column densities of CH3, CH3C2H, and C4H2 above 10 mbar are 4+2−1.5×1013 cm−2, (1.1±0.3)×1015 cm−2, and (1.2±0.3)×1014 cm−2, respectively. The observed ISO emission features also indicate C2H2 mixing ratios of 1.2+0.9−0.6×10−6 at 0.3 mbar and (2.7±0.8)×10−7 at 1.4 mbar, and a C2H6 mixing ratio of (9±2.5)×10−6 at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2. The sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than ∼3×104 cm2 s−1 at pressures greater than 1 mbar in order to reproduce the C2H6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH3 radicals if the low-temperature chemistry of CH3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere—diacetylene, butane, and water condense between ∼1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.</div></front></TEI></record>Pour manipuler ce document sous Unix (Dilib)
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