The Martian Atmosphere as Observed by VIRTIS-M on Rosetta Spacecraft

The Rosetta spacecraft performed a flyby of Mars on its way to 67P/Churyumov-Gerasimenkoin February 25th, 2007. This event provided a unique opportunity to test the spacecraft and instrument status, and to acquire a scientifically highly valuable set of observations. In this paper we describe the measurements performed by the M-channel of Visual and Infrared Thermal Imaging Spectrometer (VIRTIS), and the first scientific results derived from their analysis. The VIRTIS-M instrument is a spectro-imager operating in the range 0.25-5 μm, with a spectral resolution of 10 nm in IR. The radiation emerging from the Martian atmosphere in this spectral range keeps record of a wide range of phenomena that can be effectively studied thanks to the accurate absolute radiometric calibration of the instrument. The range 3-5 μm is dominated by the thermal emission of the surface and the atmosphere. Namely, it is possible to exploit the variations of CO2 adsorbing coefficients in the 4.3 μm complex to constrain - from VIRTIS nighttime data - the air temperature profile in the lower atmosphere (5-30 km). The comparison of VIRTIS data with state-of-the-art Global Circulation Models shows as a general trend slight temperature excess accompanied by the presence of a wide equatorial regions with moderate deficiencies (~5K), possibly related to the occurrence of water ice clouds. Daytime data of the same spectral regions are dominated by CO2 non-LTE emission in the high atmosphere. A mapping of emission intensity confirms the strict dependence against sun zenith angle expected by theoretical models. Devoted limb observations also allow to retrieve, by a simple onion-peeling technique, the vertical profile of emission intensity that peaks around 105 km in southern tropical regions. Ozone content can be effectively monitored by the emission of O2 (a1Δg) at 1.27 μm, which is produced by dissociation of O3. A zero order correction for the intensity of incoming solar flux shows that polar regions are particularly rich in ozone, consistently with the expected anti correlation with water vapor content. Limb observations indicate how ozone emission in the South tropical region remains above the noise level at least up to 90 km. Nighttime oxygen emission was also observed above the North polar region. Aerosol scattering was observed in the 1-2.5 μm region above the night disk, pointing toward the occurrence of very high noctilucent clouds. Limb observation of aerosol species is quite complex to model, due to numerical difficulties in the treatment of anisotropic multiple scattering on spherical shells representation. Images at the highest spatial resolution available suggest however a meaningful aerosol occurrence up to 140 km above the surface.