Dr. Julianne I. Moses
Recent Research

Saturn's Stratosphere: Photochemical Modeling

To investigate the details of hydrocarbon photochemistry on Saturn, Bruno Bézard, Emmanuel Lellouch, Randy Gladstone, Helmut Feuchtgruber, Mark Allen, and I 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. New constraints on CH₃, C₂H₂, C₂H₆, CH₃C₂H, and C₄H₂ are found through comparisons of model results with ISO emission spectra (Moses et al. 2000a, Icarus 143, p. 244). Upper limits for C₂H₄, CH₂CCH₂, C₃H₈, and C₆H₂ are also provided in the above paper, and the sensitivity of the 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. Diacetylene, butane, and water condense between ~1 and 300 mbar in our model and will dominate stratospheric haze formation at non-auroral latitudes. The formation and destruction mechanisms for CH₃C₂H and C₆H₆ at low temperatures, low pressures, and other conditions relevant to the Saturnian stratosphere need to be better understood.

Recent updates to the inferred thermal profile in Saturn's stratosphere by Lellouch et al. (2001, Astron. Astrophys. 370, p. 610) have caused us to re-evaluate the results regarding hydrocarbon abundances. Because the new preferred globally averaged temperature profile is colder in the stratospheric region of interest, the abundance results quoted in Moses et al. (2000a) need to be increased by factors of 1.2–1.4. The results of a new Saturn model that includes updated chemical parameters as well as an updated temperature profile, are shown in Figure 1 .

Aside from hydrogen, helium, and hydrocarbons, Saturn's stratosphere contains minor amounts of oxygen-bearing species such as H₂O and CO₂. The presence of these species in the stratosphere suggests that external material is being introduced into Saturn's upper atmosphere. Possible sources include direct ablation of interplanetary dust particles, an influx of material from rings or satellites, or a deposition of material following cometary impacts. Through back-of-the-envelope calculations regarding possible source strengths, we conclude that micrometeoroid ablation or ring-particle diffusion are the likely culprits for delivering the most oxygen-bearing material to Saturn over continuous time scales (Moses et al. 2000b, Icarus 145, p. 166). In the above paper, we also use photochemical models to investigate the influence of external material on atmospheric chemistry and compare the model results with ISO observations. We find that a globally averaged oxygen influx of (4 ± 2) × 10⁶ O atoms cm^–2 s^–1 is required to explain these observations. Models with a locally enhanced influx of H₂O operating over a small fraction of the projected disk do not provide as good a fit to the ISO H₂O observations. If the volatile oxygen compounds comprise one-third to one-half of the exogenic source by mass, then Saturn is currently being bombarded by (3 ± 2) × 10^–16 g cm^–2 s^–1 of extraplanetary material. To reproduce the observed CO₂/H₂O ratio in Saturn's stratosphere, some of the exogenic oxygen must arrive in the form of a carbon-oxygen bonded species such as CO or CO₂. Figure 2 (created by E. Lellouch) shows the ISO/SWS detection of at least 7 individual rotational lines of H₂O on Saturn.

Stratospheric constituents on Saturn vary with altitude, latitude, and season. Using a time-variable one-dimensional photochemistry/diffusion model, we have investigated this variation (see Moses and Greathouse, JGR 110, E09007). The model accounts for variations in ultraviolet flux due to orbital position, solar cycle variations, latitude and season, and ring-shadowing effects (see Figure 3). We find that hydrocarbon abundances at pressures less than ~0.01 mbar respond immediately to solar flux variations, but long vertical diffusion time scales in the 0.01-1 mbar region introduce phase lags in the response to insolation changes in the middle stratosphere. In the lower stratosphere at pressures greater than 1 mbar, vertical diffusion time scales are longer than a Saturnian year (29.5 Earth years), so that relatively long-lived hydrocarbons like (C₂H₂), C₂H₆, and C₃H₈ exhibit little seasonal variation in this region (see Movie 1). Species with short photochemical lifetimes (e.g., CH₃, C₂H₄, C₄H₂) continue to experience seasonal variations, even in the lower stratosphere. The model results are compared with observations from IRTF/TEXES (Greathouse et al. 2005, Icarus 177, 18–31) and Cassini CIRS (Howett et al. 2007, Icarus 190, 556-572) (Figure 4). Our models reproduce the observed meridional variation of (C₂H₂) reasonably well but significantly underpredict the high-latitude abundance of C₂H₆. We conclude that meridional transport is likely affecting the distribution of chemically long-lived C₂H₆, whereas shorter-lived (C₂H₂) is controlled by photochemistry and vertical diffusion. Given the different observed behavior of (C₂H₂) and C₂H₆, we constrain meridional transport time scales at 2 mbar on Saturn to be ~100-700 years, corresponding to meridional wind or diffusion speeds of ~0.4-2 cm/s. In the absence of other indicators of winds in Saturn's stratosphere, hydrocarbon observations may provide our best means of constraining stratospheric circulation.

Future Work

Preliminary 2-D photochemistry/transport models emphasize that the observed meridional behavior of (C₂H₂) and (C₂H₆) on both Saturn and Jupiter is puzzling. The acetylene (C₂H₂) mole fraction is observed to decrease with increasing latitude, whereas the ethane (C₂H₆) mole fraction is observed to slightly increase with increasing latitude. Pure 1-D photochemical models (no horizontal diffusion or advection) predict that the mole fractions of both species should decrease with increasing latitude. When advection or horizontal diffusion is considered, the increase in (C₂H₆) mole fraction with latitude can be reproduced under certain conditions (e.g., advection cells with downwelling at the poles), but under those conditions, the (C₂H₂) distribution mimics that of (C₂H₆) due to strong chemical coupling between the two species. The "decoupling" of the two species, as is suggested by their different meridional distributions, is puzzling and is a focus of current research.

Model Output

Moses and Greathouse (2005) concentrations as a function of altitude, latitude, and season

Note:  Much of this material was cannibalized from Moses et al. (2005, J. Geophys. Res. 110, E08001) and Moses and Greathouse (2005, J. Geophys. Res. 110, E09007). Further information about Saturn's stratosphere can be found within these papers and references therein.

Last updated
April 9, 2008