Th. Encrenaz 1, H. Feuchtgruber 2, E. Lellouch 1, Th. de Graauw 3, P. Morris 4, G. R. Davis 5, M. Burgdorf 6,
1 DESPA, Observatoire de Paris, F-92105 Meudon, France
2 Max-Planck Institut für Extraterrestrische Physik, D-85748 Garching, Germany
3 SRON, PO box 800, NL-9700 AV Groningen, The Netherlands
4 SRON, Sorbonnelaan, NL-3584 CA Utrecht, The Netherlands
5 Dept. of Physics, University of Saaskatchewan, Saskatoon, S7N 5E2, Canada
6 ISO Center, ESA, PO box 50727, Madrid 28080, Spain
Observations of Mars with ISO have been performed in July-August 1997, providing for the first time a complete spectral coverage from 2.3 to 180 microns. Grating spectra have been recorded with a resolving power of about 1500 below 45 microns (SWS) and about 200 above (LWS). In addition, Fabry-Perot scans of selected H2O lines have been recorded with SWS (R=30000) between 20 and 45 microns, and with LWS at longer wavelengths. A preliminary analysis of the SWS grating data below 20 microns can be found in de Graauw et al. (1997).
We present here an analysis of the SWS data between 20 and 45 microns, both with the grating and FP modes. These data have been compared to various synthetic spectra corresponding to different values of the surface temperature, the water mixing ratio at the surface, and the temperature profile. The grating spectra provide information about the brightness temperature (thus about the surface emissivity) in the range 20-45 microns, and about the total column density of water vapor. Calculations show that the grating spectral resolution is not sufficient to retrieve the H2O vertical distribution. In contrast, the Fabry-Perot H2O profiles clearly indicate a H2O water distribution confined in the lower atmosphere, within the first 10-12 kilometers above the surface. If the water vapor distribution is constrained by the saturation law, a good fit of the data is obtained with an atmospheric temperature at zero level of 228 K, as suggested by the Global Martian Climate Model of Forget et al (1998). Taking into account the surface temperature field derived from the GMC Model, we infer an emissivity of 0.83 at 40 microns.