Dr.
Julianne I. Moses
Recent
Research
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
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