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Issue A&A
Volume 483, Number 3, June I 2008
Page(s) L29 - L33
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200809761
Published online 01 April 2008



A&A 483, L29-L33 (2008)
DOI: 10.1051/0004-6361:200809761

Letter

First detection of hydroxyl in the atmosphere of Venus

G. Piccioni1, P. Drossart2, L. Zasova3, A. Migliorini1, J.-C. Gérard4, F. P. Mills5, 6, A. Shakun3, A. García Muñoz5, N. Ignatiev3, D. Grassi7, V. Cottini1, F. W. Taylor8, S. Erard2, and the VIRTIS-Venus Express Technical Team

1  INAF - IASF (Istituto di Astrofisica Spaziale e Fisica Cosmica), via del fosso del Cavaliere 100, 00133 Rome, Italy
    e-mail: Giuseppe.piccioni@iasf-roma.inaf.it
2  LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
3  Space Research Institute of Russian Academy of Sciences (IKI), Profsojuznaja 84/32, 117997 Moscow, Russia
4  LPAP, Université de Liège, 17 allée du 6 août, B5c, 4000 Liège, Belgium
5  Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia
6  The Fenner School of Environment and Society, Australian National University, Canberra, ACT 0200, Australia
7  INAF-IFSI (Istituto di Fisica dello Spazio Interplanetario), via del fosso del cavaliere 100, 00133 Rome, Italy
8  Atmospheric, Oceanic and Planetary Physics, Oxford University, Oxford OX1 3PU, UK

(Received 11 March 2008 / Accepted 26 March 2008)

Abstract
Context. Airglow emissions, such as previously observed from NO and O2(a-X) (0-0) on Venus, provide insight into the chemical and dynamical processes that control the composition and energy balance in the upper atmospheres of planets. The OH airglow emission has been observed previously only in the Earth's atmosphere where it has been used to infer atomic oxygen abundances. The O2(a-X) (0-1) airglow emission also has only been observed in the Earth's atmosphere, and neither laboratory nor theoretical studies have reached a consensus on its transition probability.
Aims. We report measurements of night-side airglow emission in the atmosphere of Venus in the OH (2-0), OH (1-0), O2(a-X) (0-1), and O2(a-X) (0-0) bands. This is the first detection of the first three of these airglow emissions on another planet. These observations provide the most direct observational constraints to date on H, OH, and O3, key species in the chemistry of Venus' upper atmosphere.
Methods. Airglow emission detected at wavelengths of 1.40-1.49 and 2.6-3.14 $\mu$m in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft is attributed to the OH (2-0) and (1-0) transitions, respectively, and compared to calculations from a photochemical model. Simultaneous limb observations of airglow emission in the O2(a-X) (0-0) and (0-1) bands at 1.27 and 1.58 $\mu$m, respectively, were used to derive the ratio of the transition probabilities for these bands.
Results. The integrated emission rates for the OH (2-0) and (1-0) bands were measured to be $100 \pm 40$ and $880\pm90$ kR respectively, both peaking at an altitude of $96 \pm 2$ km near midnight local time for the considered orbit. The measured ratio of the O2(a-X) (0-0) and (0-1) bands is $78 \pm 8$.
Conclusions. Photochemical model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth. The observed ratio of the intensities of the O2(a-X) (0-0) and (0-1) bands implies the ratio of their transition probabilities is $63\pm6$.


Key words: planets and satellites: individual: Venus -- infrared: solar system -- techniques: spectroscopic -- astrochemistry -- molecular processes -- radiation mechanisms: non-thermal



© ESO 2008


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