B. Charnay, G. Tobie, S. Lebonnois, and R. D. Lorenz. Gravitational atmospheric tides as a probe of Titan's interior: Application to Dragonfly. Astronomy Astrophysics, 658:A108, February 2022. [ bib | DOI | arXiv | PDF version | ADS link ]

Context. Saturn's massive gravity is expected to causes a tide in Titan's atmosphere, producing a surface pressure variation through the orbit of Titan and tidal winds in the troposphere. The future Dragonfly mission could analyse this exotic meteorological phenomenon. Aims: We aim to analyse the effect of Saturn's tides on Titan's atmosphere and interior to determine how pressure measurements by Dragonfly could constrain Titan's interior. Methods: We model atmospheric tides with analytical calculations and with a 3D global climate model (the IPSL-Titan GCM), including the tidal response of the interior. Results: We predict that the Love numbers of Titan's interior should verify 1 + ℜ(k₂ - h₂) -0.5ex0.02-0.1 and ℑ(k₂ - h₂) < 0.04. The deformation of Titan's interior should therefore strongly weaken gravitational atmospheric tides, yielding a residual surface pressure amplitude of only -0.5ex5 Pa, with a phase shift of 5-20 h. Tidal winds are very weak, of the order of 3 10^-4 m s^-1 in the lower troposphere. Finally, constraints from Dragonfly data may permit the real and the imaginary parts of k₂ - h₂ to be estimated with a precision of 0.01-0.03. Conclusions: Measurements of pressure variations by Dragonfly over the whole mission could give valuable constraints on the thickness of Titan's ice shell, and, via geophysical models, its heat flux and the density of its internal ocean.

S. Rodriguez, S. Vinatier, D. Cordier, G. Tobie, R. K. Achterberg, C. M. Anderson, S. V. Badman, J. W. Barnes, E. L. Barth, B. Bézard, N. Carrasco, B. Charnay, R. N. Clark, P. Coll, T. Cornet, A. Coustenis, I. Couturier-Tamburelli, M. Dobrijevic, F. M. Flasar, R. de Kok, C. Freissinet, M. Galand, T. Gautier, W. D. Geppert, C. A. Griffith, M. S. Gudipati, L. Z. Hadid, A. G. Hayes, A. R. Hendrix, R. Jaumann, D. E. Jennings, A. Jolly, K. Kalousova, T. T. Koskinen, P. Lavvas, S. Lebonnois, J.-P. Lebreton, A. Le Gall, E. Lellouch, S. Le Mouélic, R. M. C. Lopes, J. M. Lora, R. D. Lorenz, A. Lucas, S. MacKenzie, M. J. Malaska, K. Mandt, M. Mastrogiuseppe, C. E. Newman, C. A. Nixon, J. Radebaugh, S. C. Rafkin, P. Rannou, E. M. Sciamma-O'Brien, J. M. Soderblom, A. Solomonidou, C. Sotin, K. Stephan, D. Strobel, C. Szopa, N. A. Teanby, E. P. Turtle, V. Vuitton, and R. A. West. Science goals and new mission concepts for future exploration of Titan's atmosphere, geology and habitability: titan POlar scout/orbitEr and in situ lake lander and DrONe explorer (POSEIDON). Experimental Astronomy, January 2022. [ bib | DOI | arXiv | PDF version | ADS link ]

In response to ESA's “Voyage 2050” announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn's largest moon Titan. Titan, a “world with two oceans”, is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan's remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low- eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a “heavy” drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan's northern latitudes with an orbiter and in situ element(s) would be highly complementary in terms of timing (with possible mission timing overlap), locations, and science goals with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan's equatorial regions, in the mid-2030s.

P. Rannou, M. Coutelier, E. Rivière, S. Lebonnois, M. Rey, and L. Maltagliati. Convection behind the Humidification of Titan's Stratosphere. Astrophysical Journal, 922(2):239, December 2021. [ bib | DOI | PDF version | ADS link ]

On Titan, methane is responsible for the complex prebiotic chemistry, the global haze, most of the cloud cover, and the rainfall that models the landscape. Its sources are located in liquid reservoirs at and below the surface, and its sink is the photodissociation at high altitude. Titan's present and past climates strongly depend on the connection between the surface sources and the atmosphere upper layers. Despite its importance, very little information is available on this topic. In this work, we reanalyze two solar occultations made by Cassini before the northern spring equinox. We find a layer rich in methane at 165 km and at 70S (mixing ratio 1.62% 0.1%) and a dryer background stratosphere (1.1%-1.2%). In the absence of local production, this reveals an intrusion of methane transported into the stratosphere by convective circulation. On the other hand, methane transport through the tropopause at a global scale appears quite inhibited. Leaking through the tropopause is an important bottleneck of Titan's methane cycle at all timescales. As such, it affects the long-term evolution of Titan's atmosphere and the exchange fluxes with the surface and subsurface reservoirs in a complex way. Global climate models accounting for cloud physics, thermodynamical feedbacks, and convection are needed to understand the methane cycle, and specifically the humidification of the stratosphere, at the present time, and its evolution under changing conditions at a geological timescale.

S. Vinatier, C. Mathé, B. Bézard, J. Vatant d'Ollone, S. Lebonnois, C. Dauphin, F. M. Flasar, R. K. Achterberg, B. Seignovert, M. Sylvestre, N. A. Teanby, N. Gorius, A. Mamoutkine, E. Guandique, and D. E. Jennings. Temperature and chemical species distributions in the middle atmosphere observed during Titan's late northern spring to early summer. Astronomy Astrophysics, 641:A116, September 2020. [ bib | DOI | PDF version | ADS link ]

We present a study of the seasonal evolution of Titan's thermal field and distributions of haze, C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₈, C₄H₂, C₆H₆, HCN, and HC₃N from March 2015 (L_s = 66) to September 2017 (L_s = 93) (i.e., from the last third of northern spring to early summer). We analyzed thermal emission of Titan's atmosphere acquired by the Cassini Composite Infrared Spectrometer with limb and nadir geometry to retrieve the stratospheric and mesospheric temperature and mixing ratios pole-to-pole meridional cross sections from 5 mbar to 50 μbar (120-650 km). The southern stratopause varied in a complex way and showed a global temperature increase from 2015 to 2017 at high-southern latitudes. Stratospheric southern polar temperatures, which were observed to be as low as 120 K in early 2015 due to the polar night, showed a 30 K increase (at 0.5 mbar) from March 2015 to May 2017 due to adiabatic heating in the subsiding branch of the global overturning circulation. All photochemical compounds were enriched at the south pole by this subsidence. Polar cross sections of these enhanced species, which are good tracers of the global dynamics, highlighted changes in the structure of the southern polar vortex. These high enhancements combined with the unusually low temperatures (<120 K) of the deep stratosphere resulted in condensation at the south pole between 0.1 and 0.03 mbar (240-280 km) of HCN, HC₃N, C₆H₆ and possibly C₄H₂ in March 2015 (L_s = 66). These molecules were observed to condense deeper with increasing distance from the south pole. At high-northern latitudes, stratospheric enrichments remaining from the winter were observed below 300 km between 2015 and May 2017 (L_s = 90) for all chemical compounds and up to September 2017 (L_s = 93) for C₂H₂, C₂H₄, CH₃C₂H, C₃H₈, and C₄H₂. In September 2017, these local enhancements were less pronounced than earlier for C₂H₂, C₄H₂, CH₃C₂H, HC₃N, and HCN, and were no longer observed for C₂H₆ and C₆ H₆, which suggests a change in the northern polar dynamics near the summer solstice. These enhancements observed during the entire spring may be due to confinement of this enriched air by a small remaining winter circulation cell that persisted in the low stratosphere up to the northern summer solstice, according to predictions of the Institut Pierre Simon Laplace Titan Global Climate Model (IPSL Titan GCM). In the mesosphere we derived a depleted layer in C₂H₂, HCN, and C₂H₆ from the north pole to mid-southern latitudes, while C₄H₂, C₃H₄, C₂H₄, and HC₃N seem to have been enriched in the same region. In the deep stratosphere, all molecules except C₂H₄ were depleted due to their condensation sink located deeper than 5 mbar outside the southern polar vortex. HCN, C₄H₂, and CH₃C₂H volume mixing ratio cross section contours showed steep slopes near the mid-latitudes or close to the equator, which can be explained by upwelling air in this region. Upwelling is also supported by the cross section of the C₂H₄ (the only molecule not condensing among those studied here) volume mixing ratio observed in the northern hemisphere. We derived the zonal wind velocity up to mesospheric levels from the retrieved thermal field. We show that zonal winds were faster and more confined around the south pole in 2015 (L_s = 67-72) than later. In 2016, the polar zonal wind speed decreased while the fastest winds had migrated toward low- southern latitudes. data are only available at the CDS via anonymous ftp to <A href=“http:// cdsarc.u-strasbg.fr”>http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href=“http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/641/A116”>http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/641/A116</A>

T. Imamura, J. Mitchell, S. Lebonnois, Y. Kaspi, A. P. Showman, and O. Korablev. Superrotation in Planetary Atmospheres. Space Science Reviews, 216(5):87, July 2020. [ bib | DOI | PDF version | ADS link ]

Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit superrotation. Our understanding of superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus's and Titan's atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus's atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong superrotation. Recent gravity data suggests that these superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this superrotation. Moreover, atmospheric circulation models of tidally locked, strongly irradiated exoplanets have long predicted the existence of equatorial superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of superrotation on these objects.

C. Mathé, S. Vinatier, B. Bézard, S. Lebonnois, N. Gorius, D. E. Jennings, A. Mamoutkine, E. Guandique, and J. Vatant d'Ollone. Seasonal changes in the middle atmosphere of Titan from Cassini/CIRS observations: Temperature and trace species abundance profiles from 2004 to 2017. Icarus, 344:113547, July 2020. [ bib | DOI | arXiv | PDF version | ADS link ]

The Cassini/Composite InfraRed Spectrometer (CIRS) instrument has been observing the middle atmosphere of Titan over almost half a Saturnian year. We used the CIRS dataset processed through the up-to-date calibration pipeline to characterize seasonal changes of temperature and abundance profiles in the middle atmosphere of Titan, from mid-northern winter to early northern summer all around the satellite. We used limb spectra from 590 to 1500 cm-1 at 0.5-cm-1 spectral resolution, which allows us to probe different altitudes. We averaged the limb spectra recorded during each flyby on a fixed altitude grid to increase the signal-to-noise ratio. These thermal infrared data were analyzed by means of a radiative transfer code coupled with an inversion algorithm, in order to retrieve vertical temperature and abundance profiles. These profiles cover an altitude range of approximately 100 to 600 km, at 10- or 40-km vertical resolution (depending on the observation). Strong changes in temperature and composition occur in both polar regions where a vortex is in place during the winter. At this season, we observe a global enrichment in photochemical compounds in the mesosphere and stratosphere and a hot stratopause located around 0.01 mbar, both linked to downwelling in a pole-to-pole circulation cell. After the northern spring equinox, between December 2009 and April 2010, a stronger enhancement of photochemical compounds occurred at the north pole above the 0.01-mbar region, likely due to combined photochemical and dynamical effects. During the southern autumn in 2015, above the South pole, we also observed a strong enrichment in photochemical compounds that contributed to the cooling of the stratosphere above 0.2 mbar (∼300 km). Close to the northern spring equinox, in December 2009, the thermal profile at 74°N exhibits an oscillation that we interpret in terms of an inertia-gravity wave.

M. Sylvestre, N. A. Teanby, J. Vatant d'Ollone, S. Vinatier, B. Bézard, S. Lebonnois, and P. G. J. Irwin. Seasonal evolution of temperatures in Titan's lower stratosphere. Icarus, 344:113188, July 2020. [ bib | DOI | arXiv | PDF version | ADS link ]

The Cassini mission offered us the opportunity to monitor the seasonal evolution of Titan's atmosphere from 2004 to 2017, i.e. half a Titan year. The lower part of the stratosphere (pressures greater than 10 mbar) is a region of particular interest as there are few available temperature measurements, and because its thermal response to the seasonal and meridional insolation variations undergone by Titan remain poorly known. In this study, we measure temperatures in Titan's lower stratosphere between 6 mbar and 25 mbar using Cassini/CIRS spectra covering the whole duration of the mission (from 2004 to 2017) and the whole latitude range. We can thus characterize the meridional distribution of temperatures in Titan's lower stratosphere, and how it evolves from northern winter (2004) to summer solstice (2017). Our measurements show that Titan's lower stratosphere undergoes significant seasonal changes, especially at the South pole, where temperature decreases by 19 K at 15 mbar in 4 years.

Sebastien Lebonnois. Super-rotating the venusian atmosphere. Science, 368(6489):363-364, April 2020. [ bib | DOI | PDF version | ADS link ]

Among the intriguing mysteries that remain for planetary atmospheres, the phenomenon of super-rotation is still a teasing problem. An atmosphere in super-rotation rotates globally faster than the solid body of the planet. In our solar system, super-rotation is observed on Venus and the largest moon of Saturn, Titan (1). The challenge is to explain how angular momentum can accumulate in the atmosphere and what controls the atmospheric angular momentum budget. On page 405 of this issue, Horinouchi et al. (2) address this by analyzing observation data from the onboard cameras of the Venus-orbiting Akatsuki spacecraft.

J. M. Lora, T. Tokano, J. Vatant d'Ollone, S. Lebonnois, and R. D. Lorenz. A model intercomparison of Titan's climate and low-latitude environment. Icarus, 333:113-126, 2019. [ bib | DOI | PDF version | ADS link ]

Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan's atmosphere and climate, and several three-dimensional general circulation models (GCMs) now exist that simulate Titan with a high degree of fidelity. However, substantial uncertainties persist, and at the same time no dedicated intercomparisons have assessed the degree to which these models agree with each other or the observations. To address this gap, and motivated by the proposed Dragonfly Titan lander mission, we directly compare three Titan GCMs to each other and to in situ observations, and also provide multi-model expectations for the low-latitude environment during the early northern winter season. Globally, the models qualitatively agree in their representation of the atmospheric structure and circulation, though one model severely underestimates meridional temperature gradients and zonal winds. We find that, at low latitudes, simulated and observed atmospheric temperatures closely agree in all cases, while the measured winds above the boundary layer are only quantitatively matched by one model. Nevertheless, the models simulate similar near-surface winds, and all indicate these are weak. Likewise, temperatures and methane content at low latitudes are similar between models, with some differences that are largely attributable to modeling assumptions. All models predict environments that closely resemble that encountered by the Huygens probe, including little or no precipitation at low latitudes during northern winter. The most significant differences concern the methane cycle, though the models are least comparable in this area and substantial uncertainties remain. We suggest that, while the overall low-latitude environment on Titan at this season is now fairly well constrained, future in situ measurements and monitoring will transform our understanding of regional and temporal variability, atmosphere-surface coupling, Titan's methane cycle, and modeling thereof.

P. L. Read and S. Lebonnois. Superrotation on Venus, on Titan, and Elsewhere. Annual Review of Earth and Planetary Sciences, 46:175-202, 2018. [ bib | DOI | PDF version | ADS link ]

The superrotation of the atmospheres of Venus and Titan has puzzled dynamicists for many years and seems to put these planets in a very different dynamical regime from most other planets. In this review, we consider how to define superrotation objectively and explore the constraints that determine its occurrence. Atmospheric superrotation also occurs elsewhere in the Solar System and beyond, and we compare Venus and Titan with Earth and other planets for which wind estimates are available. The extreme superrotation on Venus and Titan poses some difficult challenges for numerical models of atmospheric circulation, much more difficult than for more rapidly rotating planets such as Earth or Mars. We consider mechanisms for generating and maintaining a superrotating state, all of which involve a global meridional overturning circulation. The role of nonaxisymmetric eddies is crucial, however, but the detailed mechanisms may differ between Venus, Titan, and other planets.

M. Sylvestre, N. A. Teanby, S. Vinatier, S. Lebonnois, and P. G. J. Irwin. Seasonal evolution of C₂N₂, C₃H₄, and C₄H₂ abundances in Titan's lower stratosphere. Astronomy Astrophysics, 609:A64, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]

<BR /> Aims: We study the seasonal evolution of Titan's lower stratosphere (around 15 mbar) in order to better understand the atmospheric dynamics and chemistry in this part of the atmosphere. <BR /> Methods: We analysed Cassini/CIRS far-IR observations from 2006 to 2016 in order to measure the seasonal variations of three photochemical by-products: C₄H₂, C₃H₄, and C₂N₂. <BR /> Results: We show that the abundances of these three gases have evolved significantly at northern and southern high latitudes since 2006. We measure a sudden and steep increase of the volume mixing ratios of C₄H₂, C₃H₄, and C₂N₂ at the south pole from 2012 to 2013, whereas the abundances of these gases remained approximately constant at the north pole over the same period. At northern mid-latitudes, C₂N₂ and C₄H₂ abundances decrease after 2012 while C₃H₄ abundances stay constant. The comparison of these volume mixing ratio variations with the predictions of photochemical and dynamical models provides constraints on the seasonal evolution of atmospheric circulation and chemical processes at play.

B. Charnay, E. Barth, S. Rafkin, C. Narteau, S. Lebonnois, S. Rodriguez, S. Courrech Du Pont, and A. Lucas. Methane storms as a driver of Titan's dune orientation. Nature Geoscience, 8:362-366, 2015. [ bib | DOI | arXiv | PDF version | ADS link ]

The equatorial regions of Saturn's moon Titan are covered by linear dunes that propagate eastwards. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth. This apparent contradiction has been attributed to Saturn's gravitational tides, large-scale topography and wind statistics, but none of these hypotheses fully explains the global eastward propagation of dunes in Titan's equatorial band. However, above altitudes of about 5 km, Titan's atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms-which develop at high altitudes during the equinox-on Titan's dune orientation, using mesoscale simulations of convective methane clouds with a GCM wind profile that includes super-rotation. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan's dunes, and implies an equatorial origin of dune sand.

S. Vinatier, B. Bézard, S. Lebonnois, N. A. Teanby, R. K. Achterberg, N. Gorius, A. Mamoutkine, E. Guandique, A. Jolly, D. E. Jennings, and F. M. Flasar. Seasonal variations in Titan's middle atmosphere during the northern spring derived from Cassini/CIRS observations. Icarus, 250:95-115, 2015. [ bib | DOI | PDF version | ADS link ]

We analyzed spectra acquired at the limb of Titan in the 2006-2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600-1500 cm^-1 spectral range at a resolution of 0.5 or 15.5 cm^-1 and probe the 150-500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm^-1 resolution allowed us to derive eight global maps of temperature, aerosols and C₂H₂, C₂H₆ and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm^-1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320-330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV.

B. Gans, Z. Peng, N. Carrasco, D. Gauyacq, S. Lebonnois, and P. Pernot. Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titan's atmospheric photochemistry. Icarus, 223:330-343, 2013. [ bib | DOI | PDF version | ADS link ]

A new wavelength-dependent model for CH₄ photolysis branching ratios is proposed, based on the values measured recently by Gans et al. (Gans, B. et al. [2011]. Phys. Chem. Chem. Phys. 13, 8140-8152). We quantify the impact of this representation on the predictions of a photochemical model of Titan's atmosphere, on their precision, and compare to earlier representations. Although the observed effects on the mole fraction of the species are small (never larger than 50%), it is possible to draw some recommendations for further studies: (i) the Ly-α branching ratios of Wang et al. (Wang, J.H. et al. [2000]. J. Chem. Phys. 113, 4146-4152) used in recent models overestimate the CH₂:CH₃ ratio, a factor to which a lot of species are sensitive; (ii) the description of out-of-Ly-α branching ratios by the ”100% CH₃” scenario has to be avoided, as it can bias significantly the mole fractions of some important species (C₃H₈); and (iii) complementary experimental data in the 130-140 nm range would be useful to constrain the models in the Ly-α deprived 500-700 km altitude range.

S. Lebonnois, C. Covey, A. Grossman, H. Parish, G. Schubert, R. Walterscheid, P. Lauritzen, and C. Jablonowski. Angular momentum budget in General Circulation Models of superrotating atmospheres: A critical diagnostic. Journal of Geophysical Research (Planets), 117:12004, 2012. [ bib | DOI | PDF version | ADS link ]

To help understand the large disparity in the results of circulation modeling for the atmospheres of Titan and Venus, where the whole atmosphere rotates faster than the surface (superrotation), the atmospheric angular momentum budget is detailed for two General Circulation Models (GCMs). The LMD GCM is tested for both Venus (with simplified and with more realistic physical forcings) and Titan (realistic physical forcings). The Community Atmosphere Model is tested for both Earth and Venus with simplified physical forcings. These analyses demonstrate that errors related to atmospheric angular momentum conservation are significant, especially for Venus when the physical forcings are simplified. Unphysical residuals that have to be balanced by surface friction and mountain torques therefore affect the overall circulation. The presence of topography increases exchanges of angular momentum between surface and atmosphere, reducing the impact of these numerical errors. The behavior of GCM dynamical cores with regard to angular momentum conservation under Venus conditions provides an explanation of why recent GCMs predict dissimilar results despite identical thermal forcing. The present study illustrates the need for careful and detailed analysis of the angular momentum budget for any GCM used to simulate superrotating atmospheres.

R. D. Lorenz, C. E. Newman, T. Tokano, J. L. Mitchell, B. Charnay, S. Lebonnois, and R. K. Achterberg. Formulation of a wind specification for Titan late polar summer exploration. Planetary and Space Science, 70:73-83, 2012. [ bib | DOI | PDF version | ADS link ]

Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration. The polar conditions have considerable seasonal variation and are distinct from the equatorial environment experienced by Huygens. Thus specific environmental models are required for these regions. This paper, informed by Cassini and groundbased observations and four independent Global Circulation Models (GCMs), summarizes northern summer polar conditions (specifically, regions north of 65degN, during the 2023-2024 period, or solar longitude L_s150^o-170deg) and presents a simple analytical formulation of expected, minimum and maximum winds as a function of altitude to aid spacecraft and instrument design for future exploration, with particular reference to the descent dispersions of the Titan Mare Explorer (TiME) mission concept presently under development. We also consider winds on the surface, noting that these (of relevance for impact conditions, for waves, and for wind-driven drift of a floating capsule) are weaker than those in the lowest cell in most GCMs: some previously-reported estimates of 'surface' wind speeds (actually at 90-500 m altitude) should be reduced by 20-35% to refer to the standard 10 m 'anemometer height' applicable for surface phenomena. A Weibull distribution with scale speed C=0.4 m/s and shape parameter k=2.0 embraces the GCM-predicted surface wind speeds.

S. Lebonnois, J. Burgalat, P. Rannou, and B. Charnay. Titan global climate model: A new 3-dimensional version of the IPSL Titan GCM. Icarus, 218:707-722, 2012. [ bib | DOI | PDF version | ADS link ]

We have developed a new 3-dimensional climate model for Titan's atmosphere, using the physics of the IPSL Titan 2-dimensional climate model with the current version of the LMDZ General Circulation Model dynamical core. Microphysics and photochemistry are still computed as zonal averages. This GCM covers altitudes from surface to 500 km altitude, with barotropic waves now being resolved and the diurnal cycle included. The boundary layer scheme has been changed, yielding a strong improvement in the tropospheric zonal wind profile modeled at Huygens descent position and season. The potential temperature profile is fairly consistent with Huygens observations in the lowest 10 km. The latitudinal profile of the near-surface temperature is close to observed values. The minimum of zonal wind observed by the Huygens probe just above the tropopause is also present in these simulations, and its origin is discussed by comparing solar heating and dynamical transport of energy. The stratospheric temperature and wind fields are consistent with our previous works. Compared to observations, the zonal wind peak is too weak (around 120 m/s) and too low (around 200 km). The temperature structures appear to be compressed in altitude, and depart strongly from observations in the upper stratosphere. These discrepancies are correlated, and most probably related to the altitude of the haze production. The model produces a detached haze layer located more than 150 km lower than observed by the Cassini instruments. This low production altitude is due to the current position of the GCM upper boundary. However, the temporal behaviour of the detached haze layer in the model may explain the seasonal differences observed between Cassini and Voyager 1. The waves present in the GCM are analyzed, together with their respective roles in the angular momentum budget. Though the role of the mean meridional circulation in momentum transport is similar to previous work, and the transport by barotropic waves is clearly seen in the stratosphere, a significant part of the transport at high latitudes is done all year long through low-frequency tropospheric waves that may be baroclinic waves.

D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Rannou, P. Lavvas, L. Q. Lobo, and A. G. M. Ferreira. Titan's lakes chemical composition: Sources of uncertainties and variability. Planetary and Space Science, 61:99-107, 2012. [ bib | DOI | arXiv | PDF version | ADS link ]

Between 2004 and 2007 the instruments of the Cassini spacecraft, orbiting within the Saturn system, discovered dark patches in the polar regions of Titan. These features are interpreted as hydrocarbon lakes and seas with ethane and methane identified as the main compounds. In this context, we have developed a lake-atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas present on Titan. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte Carlo simulations. Global circulation models (GCM) are also employed in order to investigate the possibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to 8500%) but the distributions of values are narrow. The relative standard deviations remain between 10% and 300% depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to 20%. Moreover, no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH₃CN, our work strongly underlines the need for experimental simulations and the improvement of Titan's atmospheric models.

B. Charnay and S. Lebonnois. Two boundary layers in Titan's lower troposphere inferred from a climate model. Nature Geoscience, 5:106-109, 2012. [ bib | DOI | PDF version | ADS link ]

Saturn's moon Titan has a dense atmosphere, but its thermal structure is poorly known. Conflicting information has been gathered on the nature, extent and evolution of Titan's planetary boundary layer-the layer of the atmosphere that is influenced by the surface-from radio-occultation observations by the Voyager 1 spacecraft and the Cassini orbiter, measurements by the Huygens probe and by dune-spacing analyses. Specifically, initial analyses of the Huygens data suggested a boundary layer of 300m depth with no diurnal evolution, incompatible with alternative estimates of 2-3km (refs , , ). Here we use a three-dimensional general circulation model, albeit not explicitly simulating the methane cycle, to analyse the dynamics leading to the thermal profile of Titan's lowermost atmosphere. In our simulations, a convective boundary layer develops in the course of the day, rising to an altitude of 800m. In addition, a seasonal boundary of 2km depth is produced by the reversal of the Hadley cell at the equinox, with a dramatic impact on atmospheric circulation. We interpret fog that had been discovered at Titan's south pole earlier as boundary layer clouds. We conclude that Titan's troposphere is well structured, featuring two boundary layers that control wind patterns, dune spacing and cloud formation at low altitudes.

D. Cordier, O. Mousis, J. I. Lunine, S. Lebonnois, P. Lavvas, L. Q. Lobo, and A. G. M. Ferreira. About the Possible Role of Hydrocarbon Lakes in the Origin of Titan's Noble Gas Atmospheric Depletion. Astrophysical Journal, 721:L117-L120, 2010. [ bib | DOI | arXiv | PDF version | ADS link ]

An unexpected feature of Titan's atmosphere is the strong depletion in primordial noble gases revealed by the Gas Chromatograph Mass Spectrometer aboard the Huygens probe during its descent on 2005 January 14. Although several plausible explanations have already been formulated, no definitive response to this issue has yet been found. Here, we investigate the possible sequestration of these noble gases in the liquid contained in lakes and wet terrains on Titan and the consequences for their atmospheric abundances. Considering the atmosphere and the liquid existing on the soil as a whole system, we compute the abundance of each noble gas relative to nitrogen. To do so, we make the assumption of thermodynamic equilibrium between the liquid and the atmosphere, the abundances of the different constituents being determined via regular solution theory. We find that xenon's atmospheric depletion can be explained by its dissolution at ambient temperature in the liquid presumably present on Titan's soil. In the cases of argon and krypton, we find that the fractions incorporated in the liquid are negligible, implying that an alternative mechanism must be invoked to explain their atmospheric depletion.

A. Coustenis, S. K. Atreya, T. Balint, R. H. Brown, M. K. Dougherty, F. Ferri, M. Fulchignoni, D. Gautier, R. A. Gowen, C. A. Griffith, L. I. Gurvits, R. Jaumann, Y. Langevin, M. R. Leese, J. I. Lunine, C. P. McKay, X. Moussas, I. Müller-Wodarg, F. Neubauer, T. C. Owen, F. Raulin, E. C. Sittler, F. Sohl, C. Sotin, G. Tobie, T. Tokano, E. P. Turtle, J.-E. Wahlund, J. H. Waite, K. H. Baines, J. Blamont, A. J. Coates, I. Dandouras, T. Krimigis, E. Lellouch, R. D. Lorenz, A. Morse, C. C. Porco, M. Hirtzig, J. Saur, T. Spilker, J. C. Zarnecki, E. Choi, N. Achilleos, R. Amils, P. Annan, D. H. Atkinson, Y. Bénilan, C. Bertucci, B. Bézard, G. L. Bjoraker, M. Blanc, L. Boireau, J. Bouman, M. Cabane, M. T. Capria, E. Chassefière, P. Coll, M. Combes, J. F. Cooper, A. Coradini, F. Crary, T. Cravens, I. A. Daglis, E. de Angelis, C. de Bergh, I. de Pater, C. Dunford, G. Durry, O. Dutuit, D. Fairbrother, F. M. Flasar, A. D. Fortes, R. Frampton, M. Fujimoto, M. Galand, O. Grasset, M. Grott, T. Haltigin, A. Herique, F. Hersant, H. Hussmann, W. Ip, R. Johnson, E. Kallio, S. Kempf, M. Knapmeyer, W. Kofman, R. Koop, T. Kostiuk, N. Krupp, M. Küppers, H. Lammer, L.-M. Lara, P. Lavvas, S. Le Mouélic, S. Lebonnois, S. Ledvina, J. Li, T. A. Livengood, R. M. Lopes, J.-J. Lopez-Moreno, D. Luz, P. R. Mahaffy, U. Mall, J. Martinez-Frias, B. Marty, T. McCord, C. Menor Salvan, A. Milillo, D. G. Mitchell, R. Modolo, O. Mousis, M. Nakamura, C. D. Neish, C. A. Nixon, D. Nna Mvondo, G. Orton, M. Paetzold, J. Pitman, S. Pogrebenko, W. Pollard, O. Prieto-Ballesteros, P. Rannou, K. Reh, L. Richter, F. T. Robb, R. Rodrigo, S. Rodriguez, P. Romani, M. Ruiz Bermejo, E. T. Sarris, P. Schenk, B. Schmitt, N. Schmitz, D. Schulze-Makuch, K. Schwingenschuh, A. Selig, B. Sicardy, L. Soderblom, L. J. Spilker, D. Stam, A. Steele, K. Stephan, D. F. Strobel, K. Szego, C. Szopa, R. Thissen, M. G. Tomasko, D. Toublanc, H. Vali, I. Vardavas, V. Vuitton, R. A. West, R. Yelle, and E. F. Young. TandEM: Titan and Enceladus mission. Experimental Astronomy, 23:893-946, 2009. [ bib | DOI | PDF version | ADS link ]

TandEM was proposed as an L-class (large) mission in response to ESAs Cosmic Vision 2015-2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini-Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini-Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through the atmosphere.

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.

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.

V. De La Haye, J. H. Waite, T. E. Cravens, A. F. Nagy, R. E. Johnson, S. Lebonnois, and I. P. Robertson. Titan's corona: The contribution of exothermic chemistry. Icarus, 191:236-250, 2007. [ bib | DOI | PDF version | ADS link ]

The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N ₂, CH ₄, H, H ₂, ³CH ₂, CH ₃, C ₂H ₄, C ₂H ₅, C ₂H ₆, N( ⁴S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N ₂, CH ₄, and H ₂. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N ₂ and CH ₄ INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be 8.3×10 Ns and 7.2×10 Cs.

M. Hirtzig, A. Coustenis, E. Gendron, P. Drossart, A. Negrão, M. Combes, O. Lai, P. Rannou, S. Lebonnois, and D. Luz. Monitoring atmospheric phenomena on Titan. Astronomy Astrophysics, 456:761-774, 2006. [ bib | DOI | PDF version | ADS link ]

For the past 8 years (1998-2005), we have used adaptive optics imaging (with VLT/NACO and CFHT/PUEO) to explore Titan's atmosphere, which is currently scrutinized in situ by the Cassini-Huygens mission. In the course of our work, we have found variations, such as as seasonal and diurnal effects, as well as temporary features in the southern polar region. The north-south asymmetry is shown to have changed since 2000 in the near-IR and to be currently organized in a brighter northern than southern pole. We study this evolution here. With our data, we also have new significant statistical evidence of diurnal effects in Titan's stratosphere, with a brighter (as much as 19%) morning limb appearing in our images in many cases, when the phase effect is expected on the evening side. The southern bright feature is probably a time-limited seasonal and/or meteorological phenomenon, revolving around the south pole (confined in its motion within the 80degS parallel) and located somewhere in the upper troposphere (18-40 km of altitude). Its behavior and possible nature are discussed here.

P. Rannou, F. Montmessin, F. Hourdin, and S. Lebonnois. The Latitudinal Distribution of Clouds on Titan. Science, 311:201-205, 2006. [ bib | DOI | PDF version | ADS link ]

Clouds have been observed recently on Titan, through the thick haze, using near-infrared spectroscopy and images near the south pole and in temperate regions near 40degS. Recent telescope and Cassini orbiter observations are now providing an insight into cloud climatology. To study clouds, we have developed a general circulation model of Titan that includes cloud microphysics. We identify and explain the formation of several types of ethane and methane clouds, including south polar clouds and sporadic clouds in temperate regions and especially at 40deg in the summer hemisphere. The locations, frequencies, and composition of these cloud types are essentially explained by the large-scale circulation.

S. Lebonnois. Benzene and aerosol production in Titan and Jupiter's atmospheres: a sensitivity study. Planetary and Space Science, 53:486-497, 2005. [ bib | DOI | PDF version | ADS link ]

Benzene has recently been observed in the atmosphere of Jupiter, Saturn and also Titan. This compound is required as a precursor for larger aromatic species (PAHs) that may be part of aerosol particles. Several photochemical models have tried to reproduce the observed quantities of benzene in the atmospheres of Jupiter (both low- and high-latitudes regions), Saturn and Titan. In this present work, we have conducted a sensitivity study of benzene and PAHs formation, using similar photochemical schemes both for Titan and Jupiter (low-latitudes conditions). Two different photochemical schemes are used, for which the modeled composition fairly agrees with observational constraints, both for Jupiter and Titan. Some disagreements are specific to each atmospheric case, which may point to needed improvements, especially in kinetic data involved in the corresponding chemical cycles. The observed benzene mole fraction in Titan's stratosphere is reproduced by the model, but in the case of Jupiter, low-latitudes benzene abundance is only 3% of the observed column density, which may indicate a possible influence of latitudinal transport, since abundance of benzene is much higher in auroral regions. Though, the photochemical scheme of C₆ compounds at temperature and pressure conditions of planetary atmospheres is still very uncertain. Several variations are therefore done on key reactions in benzene production. These variations show that benzene abundance is mainly sensitive to reactions that may affect the propargyl radical. The effect of aerosol production on hydrocarbons composition is also tested, as well as possible heterogenous recombination of atomic hydrogen in the case of Titan. PAHs are a major pathway for aerosol production in both models. The mass production profiles for aerosols are discussed for both Titan and Jupiter. Total production mass fluxes are roughly three times the one expected by observational constraints in both cases. Such comparative studies are useful to bring more constraints on photochemical models.

P. Rannou, S. Lebonnois, F. Hourdin, and D. Luz. Titan atmosphere database. Advances in Space Research, 36:2194-2198, 2005. [ bib | DOI | PDF version | ADS link ]

We have developed in the last decade a two-dimensional version of the Titan global circulation model LMDZ. This model accounts for multiple coupling occuring on Titan between dynamics, haze, chemistry and radiative transfer. It was successful at explaining many observed features related to atmosphere state (wind, temperature), haze structure and chemical species distributions, recently, an important step in our knowledge about Titan has been done with Cassini and Huygens visits to Titan. In this context, we want to make the results of our model available for the scientific community which is involved in the study of Titan. Such a tool should be useful to give a global frame (spatial and time behaviour of physical quantities) for interpreting ground based telescope observations.

F. Hourdin, S. Lebonnois, D. Luz, and P. Rannou. Titan's stratospheric composition driven by condensation and dynamics. Journal of Geophysical Research (Planets), 109:12005, 2004. [ bib | DOI | PDF version | ADS link ]

Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation (the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.

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.

S. Lebonnois, E. L. O. Bakes, and C. P. McKay. Transition from Gaseous Compounds to Aerosols in Titan's Atmosphere. Icarus, 159:505-517, 2002. [ bib | DOI | PDF version | ADS link ]

We investigate the chemical transition of simple molecules like C ₂H ₂ and HCN into aerosol particles in the context of Titan's atmosphere. Experiments that synthesize analogs (tholins) for these aerosols can help illuminate and constrain these polymerization mechanisms. Using information available from these experiments, we suggest chemical pathways that can link simple molecules to macromolecules, which will be the precursors to aerosol particles: polymers of acetylene and cyanoacetylene, polycyclic aromatics, polymers of HCN and other nitriles, and polyynes. Although our goal here is not to build a detailed kinetic model for this transition, we propose parameterizations to estimate the production rates of these macromolecules, their C/N and C/H ratios, and the loss of parent molecules (C ₂H ₂, HCN, HC ₃N and other nitriles, and C ₆H ₆) from the gas phase to the haze. We use a one-dimensional photochemical model of Titan's atmosphere to estimate the formation rate of precursor macromolecules. We find a production zone slightly lower than 200 km altitude with a total production rate of 4×10 ^-14 g cm ^-2 s ^-1 and a C/N4. These results are compared with experimental data, and to microphysical model requirements. The Cassini/Huygens mission will bring a detailed picture of the haze distribution and properties, which will be a great challenge for our understanding of these chemical processes.

S. Lebonnois, D. Toublanc, F. Hourdin, and P. Rannou. Seasonal Variations of Titan's Atmospheric Composition. Icarus, 152:384-406, 2001. [ bib | DOI | PDF version | ADS link ]

In order to investigate seasonal variations of the composition of Titan's low stratosphere, we developed a two-dimensional (latitude-altitude) photochemical and transport model. Large-scale advection, hidden in the vertical eddy diffusion for one-dimensional models, is accounted for explicitly. Atmospheric dynamics is prescribed using results of independent numerical simulations of the atmospheric general circulation. Both the mean meridional transport and latitudinal mixing by transient planetary waves are taken into account. Chemistry is based on 284 reactions involving 40 hydrocarbons and nitriles. Photodissociation rates are based on a three-dimensional description of the ultraviolet flux. For most species, the model fits well the latitudinal variations observed by Voyager I giving for the first time a full and self-consistent interpretation of these observations. In particular, the enrichment of the high northern latitudes is attributed to subsidence during the winter preceeding the Voyager encounter. Discrepancies are observed for C ₂H ₄, HC ₃N, and C ₂N ₂ and are attributed to problems in the chemical scheme. Sensitivity to dynamical parameters is investigated. The vertical eddy diffusion coefficient keeps an important role for the upper atmosphere. The wind strength and horizontal eddy diffusion strongly control the latitudinal behavior of the composition in the low stratosphere, while mean concentrations appear to be essentially controlled by chemistry.

S. Lebonnois and D. Toublanc. Actinic fluxes in Titan's atmosphere, from one to three dimensions: Application to high-latitude composition. Journal of Geophysical Research, 104:22025-22034, 1999. [ bib | DOI | PDF version | ADS link ]

We present a study on diurnally and annually averaged values of the actinic fluxes used in one-dimensional (1-D) photochemical models, as well as a 3-D radiative transfer model, based on Monte Carlo calculations with application to the atmosphere of Titan. This study shows that the commonly used value =30deg for the mean incident angle at the equator in photochemical models of Titan is not the best choice, though changing the value has no dramatic effects on photochemistry. The results of the 3-D code give direct access to the photolysis rates at any point in the atmosphere. The necessity of 3-D values in a deep atmosphere such as Titan's is demonstrated particularly for high-latitude winter conditions. These 3-D photolysis rates are used to model the latitudinal variations of the chemical composition of Titan's atmosphere in a 1-D photochemical model adapted to different latitudes. This study shows that these kinds of simple photochemical models cannot reproduce the observed latitudinal behavior and that we need to develop real 2-D photochemical models of Titan's atmosphere.