2003 .

D. Luz, F. Hourdin, P. Rannou, and S. Lebonnois. Latitudinal transport by barotropic waves in Titan's stratosphere.. II. Results from a coupled dynamics-microphysics-photochemistry GCM. Icarus, 166:343-358, 2003. [ bib | DOI | PDF version | ADS link ]

We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C 2H 2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions.

S. Lebonnois, F. Hourdin, P. Rannou, D. Luz, and D. Toublanc. Impact of the seasonal variations of composition on the temperature field of Titan's stratosphere. Icarus, 163:164-174, 2003. [ bib | DOI | PDF version | ADS link ]

We investigate the role of seasonal variations of Titan's stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C 2H 6), acetylene (C 2H 2), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around -20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small.

S. Lebonnois, E. L. O. Bakes, and C. P. McKay. Atomic and molecular hydrogen budget in Titan's atmosphere. Icarus, 161:474-485, 2003. [ bib | DOI | PDF version | ADS link ]

Using a one-dimensional model, we investigate the hydrogen budget and escape to space in Titan's atmosphere. Our goal is to study in detail the distributions and fluxes of atomic and molecular hydrogen in the model, while identifying sources of qualitative and quantitative uncertainties. Our study confirms that the escape of atomic and molecular hydrogen to space is limited by the diffusion through the homopause level. The H distribution and flux inside the atmosphere are very sensitive to the eddy diffusion coefficient used above altitude 600 km. We chose a high value of this coefficient 1 × 10 8 cm 2 s -1 and a homopause level around altitude 900 km. We find that H flows down significantly from the production region above 500 km to the region [300-500] km, where it recombines into H 2. Production of both H and H 2 also occurs in the stratosphere, mostly from photodissociation of acetylene. The only available observational data to be compared are the escape rate of H deduced from Pioneer 11 and IUE observations of the H torus 1-3 × 10 9 cm -2 s -1 and the latest retrieved value of the H 2 mole fraction in the stratosphere: (1.1 0.1) × 10 -3. Our results for both of these values are at least 50-100% higher, though the uncertainties within the chemical schemes and other aspects of the model are large. The chemical conversion from H to H 2 is essentially done through catalytic cycles using acetylene and diacetylene. We have studied the role of this diacetylene cycle, for which the associated reaction rates are poorly known. We find that it mostly affects C 4 species and benzene in the lower atmosphere, rather than the H profile and the hydrogen budget. We have introduced the heterogenous recombination of hydrogen on the surface of aerosol particles in the stratosphere, and this appears to be a significant process, comparable to the chemical processes. It has a major influence on the H distribution, and consequently on several other species, especially C 3H 4, C 4H 2 and C 6H 6. Therefore, this heterogenous process should be taken into account when trying to understand the stratospheric distribution of these hydrocarbons.

E. L. O. Bakes, S. Lebonnois, C. W. Bauschlicher, and C. P. McKay. The role of submicrometer aerosols and macromolecules in H 2 formation in the titan haze. Icarus, 161:468-473, 2003. [ bib | DOI | PDF version | ADS link ]

Previous studies of the photochemistry of small molecules in Titan's atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H 2. We describe the prompt reaction model for the formation of H 2 on aerosol surfaces and compare this with the catalytic formation of H 2 using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H 2 with a peak formation rate of 70 cm -3 s -1 at 420 km. We also conclude that catalytic H 2 formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of 0.1 cm -3 s -1 at 210 km) than microphysical aerosols.