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Dr. Julianne I. Moses
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Photochemistry in Giant-Planet Atmospheres

Atmospheric photochemistry, or the chemistry that is initiated when an atmospheric molecule or atom absorbs a photon, can dramatically affect the composition of a planetary atmosphere (including the Earth's). A molecule that absorbs a photon can be excited to a higher energy level, making the molecule more reactive toward neighboring species. If the photon possesses sufficient energy, the molecule can be broken up (photodissociated) into fragments, and these fragments can react with themselves or with other atmospheric molecules to produce completely new, often disequilibrium, species.

Thermochemical equilibrium and atmospheric transport control the abundances of the “parent” molecules in a planetary atmosphere (e.g., Fegley and Lodders, 1994, Icarus 110, p. 117). On Jupiter, Saturn, Uranus, and Neptune, temperatures are so low that most equilibrium constituents condense deep in the atmosphere, and only the most volatile molecules survive to reach altitudes where they can interact with solar ultraviolet radiation. For instance, metals, rock-forming elements, and even H2O molecules are tied up in condensed phases so deep in giant-planet atmospheres that they seldom or never interact with ultraviolet photons. The interesting photochemistry on the outer planets is therefore centered around small amounts of volatile molecules that contain elements such as carbon, nitrogen, phosphorus, and sulfur.

Tropospheric Photochemistry

The troposphere is generally defined as the atmospheric region located below the temperature minimum in planetary atmospheres. Atmospheric convection is prevalent in tropospheres, and vertical and zonal motions tend to be rapid. Thermodynamic equilibrium controls the chemical abundances within the tropospheres of the giant planets, but photochemical and transport processes can also play a significant role. The thermodynamically stable form of nitrogen on the giant planets is ammonia (NH3). Although ammonia is tied up in condensed phases in the tropospheres (e.g., as NH4SH, NH3 ice, and/or a water solution cloud), its vapor pressure at the cloud tops of Jupiter and Saturn is sufficient enough that NH3 photochemistry is important. In the upper troposphere where ammonia is photolyzed (broken apart) by photons with wavelengths in the ~190–220 nm range, phosphine (PH3) is also present, and ammonia and phosphine photochemistry are linked (see Kaye and Strobel, 1984, Icarus 59, p. 314). The major photochemical products are expected to be hydrazine (N2H4), molecular nitrogen (N2), diphosphine (P2H4), aminophosphine (NH2PH2), and elemental phosphorus (e.g., P4), although direct evidence for any of these species is lacking. Deeper in the troposphere, hydrogen sulfide (H2S) may also be present. Although H2S is expected to be tied up in condensed phases such as NH4SH at deeper levels than ammonia, some longer wavelength ultraviolet radiation (e.g., wavelengths less than 260 nm) may survive scattering by tropospheric molecules and aerosols to dissociate H2S in the upper troposphere of Jupiter. Speculative details are provided in the review of Lewis and Prinn (1984, "Planets and Their Atmospheres: Origin and Evolution," Academic Press).

Stratospheric Photochemistry

The stratosphere is defined as the atmospheric region located above the temperature minimum in planetary atmospheres. Radiative processes dominate the transport of energy in stratospheres, and transport processes tend to be slow and of broad extent. Methane (CH4) is the most abundant equilibrium constituent (other than H2 and He) that can traverse the “cold trap” at the temperature minimum and be mixed into the upper atmospheres of the giant planets — other abundant species such as NH3, H2S, and H2O are tied up in condensed phases. Methane photolysis therefore dominates stratospheric photochemistry on Jupiter and the other outer planets. Our modern view of methane photochemistry on the giant planets was developed by Strobel (1969, J. Atmos. Sci. 26, p. 906), who recognized that long-lived disequilibrium hydrocarbons that are synthesized by methane photolysis in the upper atmosphere will slowly diffuse downward to deeper, hotter, and denser atmospheric regions where they will thermally decompose and react with H2 to reform methane. This recycling prevents the permanent conversion of methane into more complex hydrocarbons.

Many photochemically produced complex hydrocarbons have been identified on the giant planets (see the review of Moses et al. 2004, Chapter 7 in Jupiter: Planet, Satellites and Magnetosphere, edited by F. Bagenal, W. McKinnon, and T. Dowling, Cambridge University Press). These hydrocarbons are mainly observed at infrared and ultraviolet wavelengths. Photochemical products that have been observed include CH3, C2H2, C2H4, C2H6, C3H4, C4H2, and C6H6. Many of these products (particularly C2H2 and C2H6) help control stratospheric temperatures (e.g., Yelle et al. 2001, Icarus 152, p. 331). Although we have a good qualitative understanding of the production and loss of these hydrocarbons, quantitative details have yet to be fully resolved, in part due to a lack of good laboratory chemical-kinetics data at relevant temperatures and pressures.

My Research

My main interest with regard to this topic is in the development of theoretical models to help explain the observed compositions of giant-planet atmospheres. Exactly how are the observed hydrocarbons produced and destroyed? What are the main photochemical pathways? How do the different constituents vary with altitude, latitude, and longitude, and what are the relative roles of chemistry and atmospheric transport in explaining this variation? How important is auroral chemistry on Jupiter or the other giant planets? What are the polar haze layers composed of and how do they form? Why and how are the observed compositions of the giant planets different? How does the influx of external material affect stratospheric compositions? How did the comet Shoemaker-Levy 9 impacts affect the short - and long-term evolution of chemical species in Jupiter's stratosphere? Did Neptune experience a recent (~ 200 years ago) large cometary impact? What causes the cloud colors on Jupiter and Saturn? Are organo-nitrogen species formed in the troposphere of Jupiter? Is sulfur photochemistry important on Jupiter? Through photochemical modeling, I hope to be able to answer some of these questions.

Note:  Much of this material was cannibalized from Moses (2000), in From Giant Planets to Cool Stars (edited by C. A. Griffith and M. S. Marley), Astron. Soc. Pacific Conference Series, Vol. 212; Moses et al. (2004), in Jupiter: Planet, Satellites and Magnetosphere (edited by F. Bagenal, W. McKinnon, and T. Dowling), Cambridge University Press, and Moses et al. 2005, JGR 110, EO8001. Further references and information about giant-planet photochemistry can be found within these book chapters.

Last updated
April 4, 2008