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[1] We present observations of both the (0-0) and (0-1) bands at 1.27 and 1.58 mm of the O 2 (a 1 D g À X 3 S g À ) nightglow made with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument aboard Venus Express. The observations were conducted in both nadir and limb viewing modes, the latter constituting the first systematic investigation into the vertical distribution of the volume emission rate of the infrared oxygen nightglow in Venus' upper atmosphere. Limb measurements from 42 orbits covering the latitude range 7°S to 77°N are analyzed. The peak altitude of the volume emission rate occurs typically between 95 and 100 km, with a mean of 97.4 ± 2.5 km. The vertical profile is broader near the equator, with a full width at half maximum of 11 km, a factor 2 larger than at middle latitudes. A double peak is frequently observed, with the lower and upper peaks occurring near 96-98 km and 103-105 km, respectively. On average, the nightglow appears brightest in the vicinity of the antisolar point. This conclusion is consistent with past ground-based observations and nadir measurements by VIRTIS. We mapped the global mean O 2 nightglow intensity from VIRTIS data collected during 880 orbits. Patchy features of the nightglow intensity observed in nadir view are correlated with the thermal brightness at 4.23-4.28 mm. The observed positive correlation is consistent with downwelling (upwelling) of oxygen atoms accompanying compressional heating (expansion cooling) or with modulation by gravity waves. Finally, from simultaneous measurements of the 1.27 and 1.58 mm bands, we have estimated the ratio of the transition probabilities A 00 /A 01 to be 63 ± 8.

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratory's Table Mountain Facility ͑34.4°N, 117.7°W, elevation 2290 m͒. This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.

[1] The Herzberg II system of O 2 has been a known feature of Venus' nightglow since the Venera 9 and 10 orbiters detected its c(0)-X(v 00 ) progression more than 3 decades ago. We search for its emission at 400 nm-700 nm in spectra obtained with the VIRTIS instrument on Venus Express. Despite the weakness of the signal, integration over a few hours of limb observations of the planet's upper atmosphere reveals the unambiguous pattern of the progression. The selected data sample mainly the northern latitudes within a few hours of local midnight. The emission is ubiquitous on the nightside of Venus and can be discerned at tangent altitudes from 80 km to 110 km. The average emission vertical profiles of the c(0)-X(v 00 ) progression and the O 2 a(0)-X(0) band, the latter from simultaneous near-infrared spectra, are quite similar, with their respective peaks occurring within ±1 km of each other. We conclude that the net yield for production of the c(0) state is low, $1%-2% of the oxygen recombination rate, and that O( 3 P) and CO 2 are the two likely quenchers of the Herzberg II nightglow, although CO cannot be ruled out. We also derive a value of 2.45 Â 10 À16 cm 3 s À1 for the rate constant at which CO 2 collisionally quenches the c(0) state. Our VIRTIS spectra show hints of O 2 A 0 (0)-a(v 00 ) emission but no traces of the O ( 1 S-1 D) green line at 557.7 nm.

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