Investigation at 1.27 μm, in the upper atmosphere of Venus, using VIRTIS/Venus Express A. Migliorini (1), G. Piccioni (1), J.-C. Gérard (2), M. Snels (3), S. Stefani (1), L. Zasova (4), P. Drossart (5) and the VIRTIS/Venus Express Team (1) IASF-INAF, Rome, Italy, (2) LPAP, Liege, Belgium, (3) ISAC/CNR, Rome, Italy, (4) IKI, Moscow, Russia, (5) LESIA, Obs. Of Paris, Paris, France. (Alessandra.Migliorini@iasf-roma.inaf.it / Fax: +39/0649934188) Abstract Venus Express gives the opportunity to study in great detail the O 2 nightglow in the IR spectral range, thanks to the extensive dataset acquired by VIRTIS, the Visible and Infrared Thermal Imaging Spectrometer on board the orbiter. The variability of the nightglow intensity has been debated in various papers [1, 2, 3, 4], and recently in [5]. One puzzle to solve is the unsatisfactory fit between data and synthetic spectrum at about 1.28 μm and thus we have further investigated the spectral properties of the emission. The spectral region around 1.27 μm is characterized by the presence of the bright (a−X)(0,0) O 2 band, the most intense nightglow emission observed on the night side of Venus. Another band, the (a−X)(1,1) O 2 band, is expected to occur at 1.28 μm, although it cannot be independently resolved from the (0-0) with VIRTIS, because of the relatively low spectral resolution. We find that the inclusion of this emission significantly improves the spectral fit around 1.27-1.28 μm. We also report the discovery of the presence of the (1-1) band and describe its vertical distribution. 1. Introduction Several emissions due to O 2 , OH and NO have been detected up to now in the Venus night side upper atmosphere by VIRTIS. In the infrared spectral range, the O 2 IR bands at 1.27 and 1.58 μm [5], the hydroxyl emissions [6, 7, 8] at 1.46 and 2.81 μm and the nitric oxide [9] at 1.22 μm have been observed for the first time, if we exclude the O 2 (0-0). In the visible spectral range, the oxygen emissions known as the Herzberg II system were also observed [10]. Atomic oxygen emissions were not detected yet in VIRTIS spectra, maybe because of the very low intensity. Variability in the spectral shape was observed in VIRTIS spectra around 1.27 μm. Spectra acquired at different altitudes, in the range 90-100 km, show a kind of “shoulder” at longer wavelengths than 1.27 μm. Any attempt to satisfactorily reproduce the O 2 emission with the (0,0) band available in the spectral range 1.2-1.32 μm has failed. From a photochemical point of view, other oxygen emissions are expected to be present in the planetary atmospheres, both in the visible and infrared spectral ranges. Emissions at 1.06 and 1.28 μm are predicted [11], though no observations have been reported so far, even in the case of the Earth. The energy level scheme of molecular O 2 consists of a triplet ground state, labeled X, and two excited states, a 1 Δ g and the b 1 Σ g singlet states. Vibrational- rotational transitions are associated to these two electronic transitions, giving rise to two band systems, one in the IR, called atmospheric infrared bands, and one in the visible spectral range, called the red bands. We propose a method to reproduce the O 2 emission around 1.27 μm, by comparing the VIRTIS spectra in the 1.2-1.3 μm spectral range, to a simulation taking into account both the (0,0) and the (1,1) O 2 bands . 2. Method We used PGOPHER software to generate line intensities of the O 2 molecule emission at 1.27 μm, due to the (0,0) transition. Lines are computed considering a rotational temperature of 185 K [8]. In addition, the O 2 emission due to the (1,1) transition was included in the simulation. Unfortunately, no spectroscopic information is available for this transition, and the rotational structure of the (1,1) band was assumed to be of the same as that of the (0,0) O 2 transition. Being the intensity of the (1,1) EPSC Abstracts Vol. 5, EPSC2010-243, 2010 European Planetary Science Congress 2010 c  Author(s) 2010