The characteristics of the O2 Herzberg II and Chamberlain bands observed with VIRTIS/Venus Express
Highlights
► Nightglow emissions properties of 0–ν″ progression are investigated. ► Three bands of the O2(A′3Δu–a1Δg) Chamberlain system are investigated. ► Simultaneous observations of VIRTIS/Venus Express in the visible and IR are compared. ► A one-dimensional model is applied to explain the peak altitude of the three systems.
Introduction
The oxygen emissions in the 300–700 nm spectral range, known as the Herzberg II bands, were observed for the first time in the Venera 9 and 10 data (Krasnopolsky et al., 1977). Successive laboratory experiments (Lawrence et al., 1977, Slanger, 1978) made this assignment. Moreover, a second weaker progression was recognized in the Venera 9 data, and assigned to the O2(A′3Δu–a1Δg) Chamberlain system (Slanger and Black, 1978).
The reported intensities for these two band systems were 2–3 kR (1R = 106 photon cm−2 s−1 (4π ster)−1) for the entire 0–ν″ progression in the Venera measurements and about 15 times less for the O2(A′3Δu–a1Δg) 0–ν″ progression (Krasnopolsky, 1983). Similar values for the Herzberg II band progression were reported by Bougher and Borucki (1994) from the Pioneer Venus Orbiter data.
From ground-based observations, a few rotational lines belonging to the (0–10) Herzberg II band were identified in observations with the Keck I and Apache Point Observatory telescopes by Slanger et al., 2001, Slanger et al., 2006, during an observation campaign to detect the O1S–1D green line of atomic oxygen.
More recently, a Herzberg II series of bands was observed using the visible channel of VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer), the imaging spectrometer on board the European mission to Venus, Venus Express. By averaging thousands of spectra acquired using the instrument in limb-mode observation, the 0–ν″ progression was clearly detected on the night side of the planet (García-Muñoz et al., 2009). A total limb intensity of 128.4 kR was reported by the authors for the summed intensity of the ν″ = 6–11 bands. Due to the weakness of these bands, the vertical profile was difficult to derive in the available data. However it was pointed out that the emission intensity was stronger at about 95 ± 1 km height, as shown in Figs. 4 and 5 in García-Muñoz et al. (2009).
The Herzberg II band system is not detected in the high resolution nightglow spectra of the Earth (Cosby et al., 2006), while emission lines of the c → b band system, which originates from the state, are detected (Slanger et al., 2003). By combining laboratory studies and airglow observations of the terrestrial planets, it was deduced that there exists a strict dependence between the Herzberg II/Herzberg I and the [M]/[O] ratios (Slanger and Copeland, 2003). In the case of Venus and Mars M is CO2, while for the Earth it is N2. According to the concentrations of CO2, N2 and O in the atmospheres of the three terrestrial planets at the nightglow altitudes, the [M]/[O] ratio explains the presence of an intense Herzberg II emission on Venus and its absence on the Earth (Greer and Murtagh, 1985, Slanger and Copeland, 2003).
Although the Herzberg I and Herzberg III systems are not ruled out in the Venus’ atmosphere, the expected intensities are very low. The Herzberg I system is expected to be about 20 times less intense than the Herzberg II in the Venus’ atmosphere, according to Krasnopolsky (1983). Considering the VIRTIS spectral sampling, it is hard to separate them from the Herzberg II bands. Hence it is not possible to unambiguously identify any feature belonging to the Herzberg I and III systems from VIRTIS data. In addition, at wavelengths shorter than 400 nm the stray light contribution is strong and the instrument sensitivity is not sufficient for weak band detection. As a consequence, the spectral region 300–400 nm must be treated with caution.
An extension of the previous work by García-Muñoz et al. (2009) concerning the O2 nightglow emissions in the Venus atmosphere is provided in the present paper by using VIRTIS/Venus Express data acquired during a recent observing campaign at limb. Details of selected data, and data processing are reported in Section 2. Using the new observing mode, a better coverage of the altitude region around 95 km, where the Herzberg II band emissions are known to occur, is obtained. It has also allowed derivation of the vertical profile for the detected bands. In addition, for limb data acquired in March 2007 and April 2008, it was possible to simultaneously investigate the nightglow O2 emissions in the visible and the IR Atmospheric band at 1.27 μm. Results are discussed in Section 3.
Section snippets
Observations with VIRTIS-M on Venus Express
The VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument on board Venus Express is an imaging spectrometer composed of two channels. One, called M, covers the range from 0.3 to 5.1 μm using 864 bins, called bands, and has imaging capabilities, while the second, called H, has a higher spectral resolution but no imaging capabilities. In this study, we describe results obtained with data acquired with the M-channel of VIRTIS. The M-channel can cover the visible spectral range from
Results
An estimate of the tangent altitude of the Herzberg II bands was reported by García-Muñoz et al. (2009), although a direct measure of individual limb profiles was quite difficult because of the low SNR. Owing to the refined limb observing technique with VIRTIS, described above, we were able to derive the tangent profile of the emissions.
In the present study, the (0–7), (0–8), (0–9), (0–10), and (0–11) Herzberg II bands are considered. The other bands, though unambiguously detected on the
One-dimensional simulations
The results reported here allow refining the modelling of the Venus nightside airglow. Based on the recent VIRTIS observations of the O2 nightglow emissions, both in the visible and infrared spectral regions, it is possible to constrain the production and collisional deactivation of excited O2 molecules. The model is based on a steady state equation for the population of the upper state of the transition (Gérard et al., 2013). The atomic oxygen and CO2 densities used in the model are taken from
Conclusions
We report the detection of eight bands of the O2 Herzberg II and three bands of the Chamberlain systems in the visible spectral range, observed by the VIRTIS spectrometer on board Venus Express.
The good sampling in the altitude range of about 90–120 km, achieved with the limb-tracking technique, allows investigation of the vertical profile of the detected bands. We found that the two systems peak at different altitudes. In particular, the Chamberlain bands peak about 4 km higher than the Herzberg
Acknowledgments
The authors thank ESA, ASI, CNES and all the national space agencies which support Venus-Express. AM is supported by ASI (Grant: ASI-INAF I/050/10/0). TGS supported by NASA Planetary Atmospheres Grant NNX08AO27G. Partial funding for this research was provided by the PRODEX program of the European Space Agency, managed in collaboration with the Belgian Federal Science Policy Office.
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