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METO 637

METO 637. Lesson 22. Jupiter. Jupiter. Jupiter and Saturn are known as the gas planets They do not have solid surfaces, their gaseous materials get denser with depth. What we ‘see’ is the top of the clouds at about I atmosphere pressure.

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METO 637

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  1. METO 637 Lesson 22

  2. Jupiter

  3. Jupiter • Jupiter and Saturn are known as the gas planets • They do not have solid surfaces, their gaseous materials get denser with depth. • What we ‘see’ is the top of the clouds at about I atmosphere pressure. • Jupiter probably has a core of rocky material amounting to about 15 Earth masses. • The main bulk of the planet is in the form of liquid metallic hydrogen. This implies a pressure of greater than 4 million bars. • Metallic hydrogen is an electrical conductor and the source of Jupiter’s magnetic field

  4. Jupiter • The outermost layer is composed of molecular hydrogen and helium. • The helium is liquid in the interior and gaseous in the outer layer. • Has high velocity winds which are confined to wide bands of latitude. Winds blow in opposite directions in adjacent bands. • Evidence is that the winds are driven by Jupiter’s internal heat sources, and not by the sun. • Vivid colors seen in the clouds are the result of chemical reactions within the clouds probably involving sulfur compounds.

  5. Saturn

  6. Saturn • Is the least dense of the planets – density of 0.7 is less than that for water. • Like Jupiter, Saturn is about 74% hydrogen, 25% helium, and trace amounts of water, methane, ammonia and ‘rocks’. • This composition is similar to the promordial Solar Nebula from which the solar system was formed. • As for Jupiter, Saturn’s interior consists of rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

  7. Saturn • The core of Saturn (and Jupiter) are hot (12,000 K). This high temperature is due to the slow gravitational compression of the planet (Kelvin-Helmholtz mechanism) • Jupier and Saturn have a rapid rotation - ~10 hours. This causes oblateness, although Saturn is affected the most (10%) • Saturn has prominent rings. These are quite thin – about one kilometer. • Ring particles seem to be mainly composed of water ice.

  8. Jupiter and Saturn • Atmospheric composition of Jupiter was investigated by several instruments on the Voyager spacecraft. The following instruments were flown: (1) IRIS – Infrared radiation (2) UVS – Ultraviolet Spectrometer (3) PPS – Photo-Polarimetry – aerosols (4) RSS – Radio Science – ions • As expected they found that the bulk of the atmosphere was composed of hydrogen and helium. • The fractional abundance of He is markedly smaller than that for the solar ratio (0.16) indicating gravitational separation from hydrogen within the interior of the planets.

  9. Thermal emission spectra from Jupiter – IRIS (Voyager 1)

  10. Jupiter and Saturn • Deep in the atmosphere thermal chemistry yields compounds which are mainly in thermo-chemical equilibrium. • Photochemistry can convert CH4 to heavier hydrocarbons and NH3 to N2H4 • Some of the chemical compounds formed are condensable. The temperature profile shows a distinct minimum at about 100 mb, which can act as a ‘cold trap’. • This limits the mixing ratios of condensable gases above the minimum. • On Jupiter NH3 is limited to a mixing ratio of about 10-7.

  11. Jupiter and Saturn • The condensates remain as aerosols. • On Jupiter dense water clouds form at ~270K, while near the 200K level H2S is thought to react with NH3 to form a cloud of solid NH4SH particles. • White crystals of ammonia precipitate out at ~154K, to produce the visible upper layer cloud. • Above the clouds photochemistry can take place.

  12. Jupiter and Saturn • The chemistry of the atmospheres of Jupiter and Saturn is greatly influenced by the reaction of other species with H and H2 • The atomic hydrogen is formed photochemically from the abundant molecular hydrogen. • Hydrides such as CH4, NH3 and PH3 also undergo photolysis to produce intermediate compounds such as CH2, CH, NH2 and PH2. These then participate in further reactions.

  13. Hydrogen • As noted before, molecular hydrogen is the dominant constituent. It dissociates at wavelengths less than 100nm in a dissociation continuum that begins at 84.5nm. • It also has an ionization continuum at 80.4 nm. Absorption in his continuum leads to the production of hydrogen atoms H2+ + H2→ H3+ + H H3+ + e → H2 + H (or 3H) • There is a net downward flow of atomic hydrogen from the ionosphere to lower altitudes. • Methane photolysis requires photons below 145nm, and ammonia requires photons below 160 nm.

  14. Synthesis of organic compounds • We noted before that Lyman alpha radiation from the sun is very intense. This can dissociate methane: CH4 + hν → CH3 + H → 1CH2 + H2 → 1,3CH2 + 2H → CH + H + H2 • The methylene radical (CH2) can then react to form observed products CH2 + H2 → CH3 + H CH3 + CH3 + M → C2H6 + M →

  15. Synthesis of organic compounds • Ethylene is formed by the reaction CH + CH4→ C2H4 + H2 • The ethylene is then photolyzed to acetylene C2H4 + hν → C2H2 + H2 • Acetylene is photochemically stable because its products C2H and C2 react with H2 to regenerate C2H2. • Higher hydrocarbons, even polymers, can be formed by reactions of C2H2 with other species, for example 1CH2 + C2H2 + M → CH3C2H (methylacetylene) • This product has also been observed in both atmospheres,

  16. Ammonia and Phosphine • The primary process is: NH3 + hν → NH2 + H • Followed by NH2 + NH2 + M → N2H4 + M (hydrazine) NH2 + H + M → NH3 + M • Analogous reactions are found for phosphine PH3 + hν → PH2 + H PH2 + PH2 + M → P2H4 + M PH2 + H + M → PH3 + M • P2H4 (a solid) is probably formed as a condensation product. • The concentrations of both ammonia and phosphine decrease rapidly above the tropopause.

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