2008 .

A. Crespin, S. Lebonnois, S. Vinatier, B. Bézard, A. Coustenis, N. A. Teanby, R. K. Achterberg, P. Rannou, and F. Hourdin. Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model. Icarus, 197:556-571, 2008. [ bib | DOI | PDF version | ADS link ]

The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-2198]. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 m s -1 at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C 2H 2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15deg S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80deg N), some compounds (C 4H 2, C 3H 4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50deg S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model. This restriction should also be kept in mind as a possible source of other discrepancies.

V. De La Haye, J. H. Waite, T. E. Cravens, I. P. Robertson, and S. Lebonnois. Coupled ion and neutral rotating model of Titan's upper atmosphere. Icarus, 197:110-136, 2008. [ bib | DOI | PDF version | ADS link ]

A one-dimensional composition model of Titan's upper atmosphere is constructed, coupling 36 neutral species and 47 ions. Energy inputs from the Sun and from Saturn's magnetosphere and updated temperature and eddy coefficient parameters are taken into account. A rotating technique at constant latitude and varying local-time is proposed to account for the diurnal variation of solar inputs. The contributions of photodissocation, neutral chemistry, ion-neutral chemistry, and electron recombination to neutral production are presented as a function of altitude and local time. Local time-dependent mixing ratio and density profiles are presented in the context of the T and T Cassini data and are compared in detail to previous models. An independent and simplified ion and neutral scheme (19-species) is also proposed for future 3D-purposes. The model results demonstrate that a complete understanding of the chemistry of Titan's upper atmosphere requires an understanding of the coupled ion and neutral chemistry. In particular, the ionospheric chemistry makes significant contributions to production rates of several important neutral species.

F. Lefèvre, J.-L. Bertaux, R. T. Clancy, T. Encrenaz, K. Fast, F. Forget, S. Lebonnois, F. Montmessin, and S. Perrier. Heterogeneous chemistry in the atmosphere of Mars. Nature, 454:971-975, 2008. [ bib | DOI | PDF version | ADS link ]

Hydrogen radicals are produced in the martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth's stratosphere, and may significantly perturb the chemistry of the Earth's upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.

A. Sánchez-Lavega, R. Hueso, G. Piccioni, P. Drossart, J. Peralta, S. Pérez-Hoyos, C. F. Wilson, F. W. Taylor, K. H. Baines, D. Luz, S. Erard, and S. Lebonnois. Variable winds on Venus mapped in three dimensions. Geophysical Research Letters, 35:13204, 2008. [ bib | DOI | PDF version | ADS link ]

We present zonal and meridional wind measurements at three altitude levels within the cloud layers of Venus from cloud tracking using images taken with the VIRTIS instrument on board Venus Express. At low latitudes, zonal winds in the Southern hemisphere are nearly constant with latitude with westward velocities of 105 ms-1 at cloud-tops (altitude ˜ 66 km) and 60-70 ms-1 at the cloud-base (altitude ˜ 47 km). At high latitudes, zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear (values lower than 15 ms-1), indicating the possibility of a vertically coherent vortex structure. Meridional winds at the cloud-tops are poleward with peak speed of 10 ms-1 at 55deg S but below the cloud tops and averaged over the South hemisphere are found to be smaller than 5 ms-1. We also report the detection at subpolar latitudes of wind variability due to the solar tide.

T. Encrenaz, T. K. Greathouse, M. J. Richter, B. Bézard, T. Fouchet, F. Lefèvre, F. Montmessin, F. Forget, S. Lebonnois, and S. K. Atreya. Simultaneous mapping of H 2O and H 2O 2 on Mars from infrared high-resolution imaging spectroscopy. Icarus, 195:547-556, 2008. [ bib | DOI | PDF version | ADS link ]

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude L was 332deg (end of southern summer). Data have been obtained at 1235-1243 cm -1, with a spectral resolution of 0.016 cm -1 ( R=8×10). The mean water vapor mixing ratio in the region [0deg-55deg S; 345deg-45deg W], at the evening limb, is 15050 ppm (corresponding to a column density of 8.32.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H 2O 2 abundance is 1510 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H 2O 2 abundance remains to be understood and modeled.

Y. Sekine, S. Lebonnois, H. Imanaka, T. Matsui, E. L. O. Bakes, C. P. McKay, B. N. Khare, and S. Sugita. The role of organic haze in Titan's atmospheric chemistry. II. Effect of heterogeneous reaction to the hydrogen budget and chemical composition of the atmosphere. Icarus, 194:201-211, 2008. [ bib | DOI | PDF version | ADS link ]

One of the key components controlling the chemical composition and climatology of Titan's atmosphere is the removal of reactive atomic hydrogen from the atmosphere. A proposed process of the removal of atomic hydrogen is the heterogeneous reaction with organic aerosol. In this study, we investigate the effect of heterogeneous reactions in Titan's atmospheric chemistry using new measurements of the heterogeneous reaction rate [Sekine, Y., Imanaka, H., Matsui, T., Khare, B.N., Bakes, E.L.O., McKay, C.P., Sugita, S., 2008. Icarus 194, 186-200] in a one-dimensional photochemical model. Our results indicate that 60-75% of the atomic hydrogen in the stratosphere and mesosphere are consumed by the heterogeneous reactions. This result implies that the heterogeneous reactions on the aerosol surface may predominantly remove atomic hydrogen in Titan's stratosphere and mesosphere. The results of our calculation also indicate that a low concentration of atomic hydrogen enhances the concentrations of unsaturated complex organics, such as C 4H 2 and phenyl radical, by more than two orders in magnitude around 400 km in altitude. Such an increase in unsaturated species may induce efficient haze production in Titan's mesosphere and upper stratosphere. These results imply a positive feedback mechanism in haze production in Titan's atmosphere. The increase in haze production would affect the chemical composition of the atmosphere, which might induce further haze production. Such a positive feedback could tend to dampen the loss and supply cycles of CH 4 due to an episodic CH 4 release into Titan's atmosphere.