2010 .

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.

D. Grassi, A. Migliorini, L. Montabone, S. Lebonnois, A. Cardesìn-Moinelo, G. Piccioni, P. Drossart, and L. V. Zasova. Thermal structure of Venusian nighttime mesosphere as observed by VIRTIS-Venus Express. Journal of Geophysical Research (Planets), 115:9007, 2010. [ bib | DOI | PDF version | ADS link ]

The mapping IR channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) on board the Venus Express spacecraft observes the CO2 band at 4.3 μm at a spectral resolution adequate to retrieve the atmospheric temperature profiles in the 65-96 km altitude range. Observations acquired in the period June 2006 to July 2008 were used to derive average temperature fields as a function of latitude, subsolar longitude (i.e., local time, LT), and pressure. Coverage presented here is limited to the nighttime because of the adverse effects of daytime non-LTE emission on the retrieval procedure and to southernmost latitudes because of the orientation of the Venus-Express orbit. Maps of air temperature variability are also presented as the standard deviation of the population included in each averaging bin. At the 100 mbar level (about 65 km above the reference surface), temperatures tend to decrease from the evening to the morning side despite a local maximum observed around 20-21LT. The cold collar is evident around 65S, with a minimum temperature at 3LT. Moving to higher altitudes, local time trends become less evident at 12.6 mbar (about 75 km) where the temperature monotonically increases from middle latitudes to the southern pole. Nonetheless, at this pressure level, two weaker local time temperature minima are observed at 23LT and 2LT equatorward of 60S. Local time trends in temperature reverse about 85 km, where the morning side is the warmer. The variability at the 100 mbar level is maximum around 80S and stronger toward the morning side. Moving to higher altitudes, the morning side always shows the stronger variability. Southward of 60S, standard deviation presents minimum values around 12.6 mbar for all the local times.

S. Lebonnois, F. Hourdin, V. Eymet, A. Crespin, R. Fournier, and F. Forget. Superrotation of Venus' atmosphere analyzed with a full general circulation model. Journal of Geophysical Research (Planets), 115:6006, 2010. [ bib | DOI | PDF version | ADS link ]

A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55-60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.