2004 .

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.

T. Encrenaz, B. Bézard, T. K. Greathouse, M. J. Richter, J. H. Lacy, S. K. Atreya, A. S. Wong, S. Lebonnois, F. Lefèvre, and F. Forget. Hydrogen peroxide on Mars: evidence for spatial and seasonal variations. Icarus, 170:424-429, 2004. [ bib | DOI | PDF version | ADS link ]

Hydrogen peroxide (H 2O 2) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10 15-10 16 cm -2. However, a stringent upper limit of the H 2O 2 abundance on Mars (9×10 14 cm -2) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112deg). Here we report the detection of H 2O 2 on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring ( Ls=206deg) when the global water vapor abundance was 10 pr. μm. The spatial distribution of H 2O 2 shows a maximum in the morning around the sub-solar latitude. The mean H 2O 2 column density (6×10 15 cm -2) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 ( Ls=254deg), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).

F. Lefèvre, S. Lebonnois, F. Montmessin, and F. Forget. Three-dimensional modeling of ozone on Mars. Journal of Geophysical Research (Planets), 109:7004, 2004. [ bib | DOI | PDF version | ADS link ]

We present the first three-dimensional model simulations of ozone on Mars. The model couples a state-of-the-art gas-phase photochemical package to the general circulation model developed at Laboratoire de Météorologie Dynamique (LMD). The results do not contradict the classical picture of a global anticorrelation between the ozone (O3) and water vapor columns. However, the quantitative approach shows significant departures from this relationship, related to substantial orbital variations in the O3 vertical distribution. Over the period Ls = 180deg-330deg, low-latitude to midlatitude O3 is essentially confined below 20 km, has a weak diurnal cycle, and is largely modulated by topography. During the rest of the year (Ls = 330deg-180deg) the model predicts the formation of an O3 layer at 25-70 km altitude, characterized by nighttime densities about one order of magnitude larger than during the day. Throughout the year, high-latitude O3 peaks near the surface and reaches maximum integrated amounts (˜40 μm-atm) in the winter polar vortex, with considerable (30 to 50%) dynamically induced day-to-day variations. The most stringent comparison to date with O3 observational data reveals contrasted results. A good quantitative agreement is found in the postperihelion period (Ls = 290deg-10deg), but the model fails to reproduce O3 columns as large as those measured near aphelion (Ls = 61deg-67deg). Current uncertainties in absorption cross sections and gas-phase kinetics data do not seem to provide credible explanations to explain this discrepancy, which may suggest the existence of heterogeneous processes.