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1998 .

(2 publications)

F. Billebaud, J. Rosenqvist, E. Lellouch, J.-P. Maillard, T. Encrenaz, and F. Hourdin. Observations of CO in the atmosphere of Mars in the (2-0) vibrational band at 2.35 microns. Astronomy Astrophysics, 333:1092-1099, May 1998. [ bib | ADS link ]

Following our high-resolution infrared observations of CO in the atmosphere of Mars in 1988 and 1989 at 4.7mu m (Billebaud et al., 1992), we recorded new spectra of CO: one covering the whole disk of the planet in 1990 and 4 spectra corresponding to 4 different locations on the planet in 1991. All these spectra were recorded in the (2-0) vibrational band at 2.35mu m. These data allow us to measure the CO abundance and to search for possible middle-scale spatial variations of this abundance in the case of the 1991 spectra. The CO mixing ratio derived from the 1990 data is in good agreement with the values we obtained in 1988 and 1989 (Billebaud et al., 1992), showing a great stability over a period of 3 years, with a value of the CO mixing ratio remaining in the range of 4.2-8.5 x 10(-4) . The results we obtained with the 1991 data also seem to comfort the stability of the CO mixing ratio, although the possible range is somewhat larger (5.5-11.5 x 10(-4) ). This common CO mixing ratio range for the four locations on the planet then tends to exclude the presence of any significant horizontal variations of the CO mixing ratio, even if, from our data, we cannot firmly rule them out.

F. Forget, F. Hourdin, and O. Talagrand. CO 2Snowfall on Mars: Simulation with a General Circulation Model. Icarus, 131:302-316, February 1998. [ bib | DOI | ADS link ]

Although CO2snowfall has never been directly observed on Mars, it has been suggested that such precipitation may explain the puzzling infrared measurements obtained by Mariner 9 and Viking during the polar night in each hemisphere. The radiative effect of the snow would strongly alter the radiative balance of the condensing polar caps and thus the CO2cycle and the global climate. We have simulated this phenomenon with a general circulation model (GCM). For that purpose, a new parameterization of CO2condensation in the atmosphere and on the ground has been developed, paying particular attention to mass and energy conservation and allowing for the possible sublimation of sedimenting CO2ice particles. Atmospheric condensation may result from radiative cooling on the one hand (especially when the atmosphere is dust laden) and from adiabatic cooling in upward motions on the other hand. This latter process can be very efficient locally. On this basis, we have modeled the effect of the CO2snowfall on the infrared emission by decreasing the local emissivities when atmospheric condensation was predicted by the model. This parameterization is based on physical considerations (radiative transfer through the CO2ice particles, snow metamorphism on the ground). Without tuning the model parameters, we have been able to accurately reproduce the general behavior of the features observed by Viking in the thermal infrared. These modeling results support the CO2snowfall scenario suggested from the observations. Overall, this new parameterization, used in combination with the digital terrain model topography and with allowance for a varying atmospheric dust content, allows the GCM to simulate the CO2condensation-sublimation cycle realistically. In particular, the seasonal variations of the surface pressure recorded by the Viking Landers can now be reproduced without artificially decreasing the condensation rate as was done in previous studies.

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