S. Lebonnois, D. Toublanc, F. Hourdin, and P. Rannou. Seasonal Variations of Titan's Atmospheric Composition. Icarus, 152:384-406, 2001. [ bib | DOI | 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.
E. Chassefière, F. Forget, F. Hourdin, F. Vial, H. Rème, C. Mazelle, D. Vignes, J.-A. Sauvaud, P.-L. Blelly, D. Toublanc, J.-J. Berthelier, J.-C. Cerisier, G. Chanteur, L. Duvet, M. Menvielle, J. Lilensten, O. Witasse, P. Touboul, E. Quèmerais, J.-L. Bertaux, G. Hulot, Y. Cohen, P. Lognonné, J. P. Barriot, G. Balmino, M. Blanc, P. Pinet, M. Parrot, J.-G. Trotignon, M. Moncuquet, J.-L. Bougeret, K. Issautier, E. Lellouch, N. Meyer, C. Sotin, O. Grasset, F. Barlier, C. Berger, P. Tarits, J. Dyment, D. Breuer, T. Spohn, M. Pätzold, K. Sperveslage, P. Gough, A. Buckley, K. Szego, S. Sasaki, S. Smrekar, D. Lyons, M. Acuna, J. Connerney, M. Purucker, R. Lin, J. Luhmann, D. Mitchell, F. Leblanc, R. Johnson, J. Clarke, A. Nagy, D. Young, S. Bougher, G. Keating, R. Haberle, B. Jakosky, R. Hodges, M. Parmentier, H. Waite, and D. Bass. Scientific objectives of the DYNAMO mission. Advances in Space Research, 27:1851-1860, 2001. [ bib | DOI | ADS link ]
DYNAMO is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/ CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for DYNAMO of 120-130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Additional data on the internal structure will be obtained by mapping the electric conductivity. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere and water from the planet.