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Astron. Astrophys. 333, 1092-1099 (1998)

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2. Observations

The observations were made using the Fourier Transform Spectrometer of the Canada-France-Hawaii 3.6 meter telescope located at the Mauna Kea Observatory (Hawaii). This instrument is a dual-input-dual-output interferometer, equipped with two InSb detectors operating between 0.9 and 5µm (Maillard & Michel, 1982). The filter (4100-4500 cm-1) was chosen in order to select the CO (2-0) vibrational band centered at 2.35µm.

The 1990 spectrum was recorded on May 31. The aperture was 8" so that the entire disk of the planet (6.8") could be observed. The solar longitude was LS =231[FORMULA] and the subearth point located at 24[FORMULA] S, with a local time around 3 p.m. The spectral resolution was 0.017 cm-1 and the Doppler shift was around 0.14 cm-1 (at 4280 cm-1 = central wavelength of the useful portion of the filter). The useful portion of the spectrum (i.e. the portion of the spectrum corresponding to the best filter transmission: 4240-4320 cm-1) exhibits 25 12 CO absorption lines, well separated from the corresponding telluric 12 CO lines. The spectrum however contains solar CO lines that could be mixed with martian CO lines. For this reason, a Moon spectrum was recorded at the same time and with the same spectral resolution. The Moon spectrum was corrected for Doppler shift with respect to the Mars spectrum, so that the solar CO lines be at the same wavelength position in the two spectra. The Mars spectrum was then divided by the rescaled Moon spectrum. This allowed us to remove the solar contribution to the martian CO lines, although this resulted in the telluric lines (water lines in particular) to appear twice (in emission and in absorption), as they were thus shifted in one spectrum with respect to the other. But this effect has no influence on our 12 CO martian lines.

On January 2, 1991, we recorded four spectra corresponding to four points on the disk of the planet (Fig. 1). The aperture was 2.5" while the planet was 13.5". This thus gives an average spatial resolution of about 1260 km at the centre of the disk. The spectral resolution was 0.049 cm-1 for the four spectra and the Doppler shift was about 0.18 cm-1 (at 4280 cm-1). The solar longitude was 358.5[FORMULA] and the subearth point was located at 13[FORMULA] S with a local time around 10.5 a.m. The geometry of the disk is shown on Fig. 1. The latitude, longitude and local time of the central point of the four positions are, respectevely: 22[FORMULA] N, 251 [FORMULA], 1.5 p.m. for position 1; 13 [FORMULA] S, 311.6[FORMULA], 10.5 a.m. for position 2; 37[FORMULA] S, 358.5[FORMULA], 7 a.m. for position 3 and 48[FORMULA] S, 275.7 [FORMULA] and 2 p.m. for position 4. No Moon spectrum could be recorded at the time the 1991 data were taken.

[FIGURE] Fig. 1. Geometry of Mars and the four positions of the aperture on the planet for the 1991 data.
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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998