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O2 emissions in the Venus nightglow
Icarus 217 (2012) 845–848
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Icarus journal homepage: www.elsevier.com/locate/icarus
O/O2 emissions in the Venus nightglow Tom G. Slanger a, Nancy J. Chanover b,⇑, Brian D. Sharpee a, Thomas A. Bida c a
Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA Astronomy Department, New Mexico State University, Las Cruces, NM 88003, USA c Lowell Observatory, Flagstaff, AZ 86001, USA b
a r t i c l e
i n f o
Article history: Available online 22 April 2011 Keywords: Venus, Atmosphere Atmospheres, Chemistry Aeronomy
a b s t r a c t The oxygen green line is one of the most characteristic features in the terrestrial visible nightglow; it can be seen in the Venus nightglow, but with much greater intensity variation than in the terrestrial case. Here we synthesize our current understanding of the green line in the Venus nightglow and discuss what might be expected observationally in the rising phase of solar cycle 24. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction In the sampling of small rocky planets in our Solar System, we find two kinds of atmospheres on the planets lying in or near the Sun’s habitable zone-one with a predominance of O2/N2 and two with CO2 atmospheres. It is likely that in other Solar Systems such atmospheres will be well represented, and therefore it is an important challenge to understand both the similarities and differences in the airglows of such environments. This is one of the contexts for studying the nightglow green line in the atmospheres of the terrestrial planets. Although the terrestrial nightglow has been studied for 150 years (Ångström, 1869), there are still numerous areas where our understanding is not yet adequate, and one of these is the source of the terrestrial green line in the mesosphere. The ultimate source is the energy of recombination of oxygen atoms produced during solar photodissociation of O2. The so-called Barth mechanism for O(1S) production (Barth and Hildebrandt, 1961) relates to the transfer of electronic energy from an excited O2 molecule to an O(3P) atom, which requires 4.17 eV. What is not fully understood is the nature of the O2 donor molecule. The recombination energy of O2 is 5.1 eV, so the O2 molecule in question has an electronic/vibrational energy between these limits. The O2 Herzberg states, O2(A, A0 , c), are the generally mentioned candidates for the relevant O2 , but there are also other choices. The first measurements and interpretation of the Venus visible nightglow were carried out with data from the Venera 9/10 orbiters (Krasnopolsky et al., 1977; Lawrence et al., 1977). Unexpectedly, strong emission was observed from transitions corresponding to the O2(c–X) Herzberg II system, which is almost indiscernible in ⇑ Corresponding author. Fax: +1 575 646 1602. E-mail address: [email protected] (N.J. Chanover). 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.03.031
the terrestrial atmosphere. This Venus atmospheric emission is 10 times more intense than the sum of terrestrial Herzberg emissions. Significantly, there was no evidence of green line emission on Venus, and therefore for 25 years it was believed that the nightglow green line is unique to a planet with an O2 atmosphere. Another O2 emission in common among the three atmospherebearing terrestrial planets is the O2(a–X) IR Atmospheric band transition at 1.27 lm. This is seen only from the a(v = 0) level because O2(a) has an extremely long lifetime, and its vibrational levels are quickly deactivated by collision (Slanger and Copeland, 2003). The Herzberg II emission is also seen only from the c(v = 0) level, probably for the same reason. In any case, the a–X transition is seen in all three planetary nightglows (Slanger et al., 2008). In this work, we summarize 10 years of ground-based observations of the green line in the Venus nightglow and discuss expectations for the strength of Venus’ green line in the rising phase of solar cycle 24. 2. Observations A new phase of Venus nightglow studies began when the green line was detected using the HIRES spectrograph on the Keck I telescope (Slanger et al., 2001). Based on the Venera results, there was no reason to believe that the green line would subsequently be seen. The terrestrial green line typically varies less than a factor of two from its nominal intensity of 150 R, thus it was surprising to find the green line at Venus with an intensity even larger than the terrestrial value when it was first detected on 20 November 1999. Fig. 1 shows the discovery spectrum as seen on the first night of its detection. The terrestrial and Venus lines in the figure are well separated as a consequence of using the Doppler shift associated with the velocity of the approaching or departing planet. With
846
T.G. Slanger et al. / Icarus 217 (2012) 845–848
Fig. 1. Left: The terrestrial and Venus green lines as observed by HIRES/Keck I on 20 November 1999. Right: Comparison of VIRTIS spectrum (Garcı´a Muñoz et al., 2009) with DIATOM simulations of the four O2 band systems – Herzberg I, II, III, and Chamberlain (dotted). Also shown is the position of the (missing) 557.7 nm OI green line. All bands are from the v = 0 levels of the different states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a differential velocity of 12 km s1 or more and a spectrograph resolution of 40,000, spectra of high enough quality to resolve the terrestrial from the venusian line are routinely obtainable. This experiment was a potential breakthrough in our understanding of the Venus atmosphere, although the reason for the lack of detection by Venera 25 years earlier was not at all clear. Subsequent to the initial detection, we made regular observations of the Venus nightglow using two telescope/instrument combinations: HIRES on Keck I and the ARC Echelle Spectrograph (ARCES) on the Astrophysical Research Consortium’s 3.5-m telescope at Apache Point Observatory (APO). The HIRES slit was 0.861 7.000 , thus we extracted the Venus nightglow spectrum from multiple locations along the night side. We used ARCES with a slit of 1.6 3.200 , which we manually positioned at different locations on Venus’ night side. As discussed in Slanger et al. (2006), in measurements subsequent to November 1999, the green line has been much weaker in intensity in nearly a dozen observations made at Keck I and APO.
instrument on board ESA’s Venus Express spacecraft (Garcı´a Muñoz et al., 2009), which reproduces the visible emission seen by Venera very well. Fig. 1 shows the co-added spectrum, taken over a period of 450 h. Included in the figure are DIATOM simulations of the different O2 Herzberg band systems (Huestis et al., 1994). The position of the 557.7 nm green line is noted, conspicuous in its absence. The apparent presence of the Herzberg III system, most clearly at 472 nm, is noteworthy. This system has not been identified in the terrestrial atmosphere, and this difference may be related to the different collisional environment. 4. Quenching parameters A basic difference in the terrestrial atmosphere versus those of the two CO2 atmospheres relates to their absorption of solar radiation. Because absorption cross sections are much higher for O2 than CO2 in the far UV region – at 1200–1800 Å – to be absorbed by CO2 the light must penetrate to regions of higher gas density.
3. Discussion The key piece of information missing in our Venus green line studies is the emission altitude. In the terrestrial atmosphere there are two O(1S) sources – atom recombination in the mesosphere:
Oð3 PÞ þ Oð3 PÞ þ M ! O2 þ M;
ð1Þ
followed by
O2 þ Oð3 PÞ ! O2 þ Oð1 SÞ;
ð2Þ
and dissociative recombination in the ionosphere:
Oþ2 ðv Þ þ e ! Oð1 S;1 DÞ þ Oð3 PÞ:
ð3Þ
The former is typically the more intense in the quiescent nightglow, but the situation is reversed in aurora and the dayglow. It is clear that if we knew the emission altitude, then we would either have information on O-atom densities in the mesosphere, or on ion/electron densities in the ionosphere. To date there have been no green line observations other than our ground-based studies at Keck I and APO. The fact that the Akatsuki spacecraft did not go into a planned Venus orbit in December 2010 has prevented possible determination of an emission altitude for the green line. However, there has already been a study of the O2 Herzberg II system at Venus with the VEX/VIRTIS
Fig. 2. The radiating efficiency of O(1D) and O(1S) in the Venus atmosphere. For this calculation the [CO2] and temperature profiles of Krasnopolsky (2010) were used. The O(1S) + CO2 rate coefficient is 3.1 1011e(1330/T) cm3 s1 (Atkinson and Welge, 1972), and that for O(1D) + CO2 is 7.5 1011e(115/T) cm3 s1 (Sander et al., 2006). The radiative lifetime of O(1S) is 0.82 s, and that of O(1D) is 116 s (Froese Fischer and Tachiev, 2004).
T.G. Slanger et al. / Icarus 217 (2012) 845–848
As a consequence, collisional quenching tends to be much more important at Venus and Mars than in the Earth’s atmosphere, although this is not necessarily a rule; the nature of the quencher is also important. However, for O(1S) and also O(1D), quenching at Venus/Mars will be more effective than terrestrial quenching because these species are generated at higher gas density. Fig. 2 shows the O(1S) and O(1D) radiating efficiencies at Venus, the principal quencher in both cases being CO2. The altitude of maximum O-atom recombination, which is synonymous with the emission peaks of the O2 Herzberg II and IR Atmospheric band systems, is 95 km (Garcı´a Muñoz et al., 2009), as indicated. At that altitude, the O(1S) efficiency is on the order of 0.02. On the other hand, the O(1D) efficiency is 2 108, and there is no chance of seeing mesospheric O(1D) radiation, as is also true in the terrestrial atmosphere. In any case, it has not been detected from any altitude.
847
5. Possible mechanisms The terrestrial and Venus atmospheres offer an interesting contrast in the Herzberg state recombination spectrum. In the terrestrial atmosphere, the observed O2(A, A0 ) bands are mainly from vibrationally excited levels (Broadfoot and Kendall, 1968), all having enough energy to make O(1S) by energy transfer:
O2 ðv Þ þ Oð3 PÞ ! Oð1 SÞ þ O2 ðXÞ:
ð4Þ
On the other hand, the O2(c, v = 0) seen in the Venus atmosphere, although much more intense than the terrestrial emission, does not have enough energy to make O(1S) by reaction (4). This suggests but does not prove that the O(1S) source does not lie in the mesosphere.
Fig. 3. Keck I/HIRES spectrum of Venus from July 22, 2010 showing the green line region. Top: This diagram shows the orientation of the HIRES slit with respect to Venus’ night side. Bottom: This plot represents the co-addition of three spectra, and shows the terrestrial green line and lack of Venus green line. The predicted location of the Venus green line, which was calculated from the relative Earth–Venus velocity at the time of the observation, is shown with a dashed vertical line. The best-fit terrestrial line position is 5577.323 Å (as compared with the NIST value of 5577.339 Å). The sunlight subtraction in this spectrum is imperfect, as shown by the Fraunhofer line near 5576.7 Å.
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T.G. Slanger et al. / Icarus 217 (2012) 845–848
A critical observation concerning any linkage between O2 Herzberg state emission and O(1S) is the fact that the Herzberg II emission is fairly stable between apparitions. From the Venera observations through Pioneer Venus (Bougher and Borucki, 1994), HIRES/ Keck I (Slanger et al., 2001), APO (Slanger et al., 2006), and VEX/VIRTIS (Garcı´a Muñoz et al., 2009), the intensity lies close to 4 kR, varying by less than a factor of two on either side. On the other hand, the O(1S) variations are much larger, with a peak of 150 R in 1999, to upper limits of 10 R at other periods, with no apparent correlation in intensity between O(1S) and Herzberg II emission. In the terrestrial atmosphere, the correlation between mesospheric Herzberg and green line intensities is quite persuasive (Stegman and Murtagh, 1991), as one might expect because both depend on O(3P) densities. That this is not the case at Venus suggests that the green line, when observed, is not coming from the mesosphere, but originates in the ionosphere via reaction (3), dissociative recombination, as is the case for the secondary terrestrial source (Shepherd et al., 1997). The peak Oþ 2 density occurs at 140 km (Schunk and Nagy, 2004), where Fig. 2 shows that the green line radiating efficiency is near unity. Although O(1D) and O(1S) production are of the same order of magnitude in reaction (3), Fig. 2 shows that the O(1D) radiative efficiency even at 130 km is still only on the order of 0.01, so the lack of its detection is not surprising. 6. Solar cycle effects If O2 Herzberg II emission at Venus is mesospheric and green line emission is ionospheric, this could explain the stability with time of the former and the variability of the latter. The solar radiation that dissociates CO2 to make O-atoms in the mesosphere is in the 1200–1700 Å wavelength range. In the ionosphere, COþ 2 must first be ionized at E > 13.8 eV, after which it reacts with O(3P),
COþ2 þ Oð3 PÞ ! CO þ Oþ2 :
ð5Þ
The EUV ionizing radiation is very dependent on the phase of the solar cycle, being much lower in intensity at solar minimum than solar maximum (Marsh et al., 2007). On the other hand, in the 1200–1700 Å region, there is little difference in intensity. It follows that if the Herzberg II and green line sources are in the mesosphere and ionosphere, respectively, then the Herzberg II radiation will be relatively constant in intensity, while the green line intensity will be strongly dependent on the phase of the solar cycle. The most intense of our green line emissions occurred in November 1999, close to solar maximum. Most of the subsequent measurements were made in the declining phase of cycle 23 and in the long minimum between cycles 23 and 24. If our reasoning is correct, then we can expect that as cycle 24 heads towards a maximum, in mid-2013 (NOAA/SWPC), the green line intensity will again strengthen, although it seems that the peak in cycle 24 will be unusually low. A HIRES measurement of the visible Venus nightglow was recently made from Keck I on 22 July 2010. Fig. 3 shows that the green line is not discernible, and we assign a terrestrial/Venus ratio
of at least an order of magnitude. Thus, we reaffirm the idea that at solar minimum the green line is also minimal. Consistent with this reasoning is that the Venera observations of 1974 were also made at solar minimum. Acknowledgments This work was supported by Grant Number NNX08AO27G from NASA’s Planetary Astronomy Program. We thank the referees of this manuscript for their constructive comments. References Ångström, J.A., 1869. Spectrum des Nordlichts. Ann. Phys. 213, 161–163. Atkinson, R., Welge, K.H., 1972. Temperature dependence of O(1S) deactivation by CO2, O2, N2, and Ar. J. Chem. Phys. 57, 3689–3693. Barth, C.A., Hildebrandt, A.F., 1961. The 5577 Å airglow emission mechanism. J. Geophys. Res. 66, 985–986. Bougher, S.W., Borucki, W.J., 1994. Venus O2 visible and IR nightglow: Implications for lower thermosphere dynamics and chemistry. J. Geophys. Res. 99, 3759– 3776. Broadfoot, A.L., Kendall, K.R., 1968. The airglow spectrum, 3100–10,000 Å. J. Geophys. Res. 73, 426–428. Froese Fischer, C., Tachiev, G., 2004. Breit–Pauli energy levels, lifetimes, and transition probabilities for the beryllium-like to neon-like sequences. Atom. Data Nucl. Data Tables 87, 1–184. Garcı´a Muñoz, A., Mills, F.P., Slanger, T.G., Piccioni, G., Drossart, P., 2009. Visible and near-infrared nightglow of molecular oxygen in the atmosphere of Venus. J. Geophys. Res. 114 (E13), 12002–12014. Huestis, D.L., Copeland, R.A., Knutsen, T.G., Slanger, K., Jongma, R.T., Boogaarts, M.G.H., Meijer, G., 1994. Branch intensities and oscillator strengths for the Herzberg absorption systems in oxygen. Can. J. Phys. 72 (11–12), 1109– 1121. Krasnopolsky, V.A., 2010. Venus night airglow: Ground-based detection of OH, observations of O2 emissions, and photochemical model. Icarus 207, 17–27. Krasnopolsky, V.A., Krysko, A.A., Rogachev, V.N., Parshev, V.A., 1977. Spectroscopy of the night-sky luminescence of Venus from the interplanetary spacecraft Venera 9 and Venera 10. Cosmic Res. 14, 687–692. Lawrence, G.M., Barth, C.A., Argabright, V., 1977. Excitation of the Venus night airglow. Science 195, 573–574. Marsh, D.R., Garcia, R.R., Kinnison, D.E., Boville, B.A., Sassi, F., Solomon, S.C., Matthes, K., 2007. Modeling the whole atmosphere response to solar cycle changes in radiative and geomagnetic forcing. J. Geophys. Res. 112 (D11), 23306. Sander, S.P. et al., 2006. Chemical kinetics and photochemical data for use in stratospheric modeling, Evaluation Number 15. National Aeronautics and Space Administration, Jet Propulsion Laboratory. Schunk, R.W., Nagy, A.F., 2004. Ionospheres: Physics, Plasma Physics, and Chemistry. Cambridge University Press. Shepherd, G.G., Roble, R.G., McLandress, C., Ward, W.E., 1997. WINDII observations of the 558 nm emission in the lower thermosphere: The influence of dynamics on composition. Journal of Atmospheric and Solar-Terrestrial Physics 59, 655– 667. Slanger, T.G., Copeland, R.A., 2003. Energetic oxygen in the upper atmosphere and the laboratory. Chem. Rev. 103, 4731–4766. Slanger, T.G., Cosby, P.C., Huestis, D.L., Bida, T.A., 2001. Discovery of the atomic oxygen green line in the Venus night airglow. Science 291, 463– 465. Slanger, T.G., Huestis, D.L., Cosby, P.C., Chanover, N.J., Bida, T.A., 2006. The Venus nightglow: Ground-based observations and chemical mechanisms. Icarus 182, 1–9. Slanger, T.G., Cravens, T.E., Crovisier, J., Miller, S., Strobel, D.F., 2008. Photoemission phenomena in the Solar System. Space Sci. Rev. 139, 267–310. Stegman, J., Murtagh, D.P., 1991. The molecular oxygen band systems in the UV nightglow – Measured and modeled. Planet. Space. Sci. 39, 595–609.
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
O/O2 emissions in the Venus nightglow Tom G. Slanger a, Nancy J. Chanover b,⇑, Brian D. Sharpee a, Thomas A. Bida c a
Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA Astronomy Department, New Mexico State University, Las Cruces, NM 88003, USA c Lowell Observatory, Flagstaff, AZ 86001, USA b
a r t i c l e
i n f o
Article history: Available online 22 April 2011 Keywords: Venus, Atmosphere Atmospheres, Chemistry Aeronomy
a b s t r a c t The oxygen green line is one of the most characteristic features in the terrestrial visible nightglow; it can be seen in the Venus nightglow, but with much greater intensity variation than in the terrestrial case. Here we synthesize our current understanding of the green line in the Venus nightglow and discuss what might be expected observationally in the rising phase of solar cycle 24. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction In the sampling of small rocky planets in our Solar System, we find two kinds of atmospheres on the planets lying in or near the Sun’s habitable zone-one with a predominance of O2/N2 and two with CO2 atmospheres. It is likely that in other Solar Systems such atmospheres will be well represented, and therefore it is an important challenge to understand both the similarities and differences in the airglows of such environments. This is one of the contexts for studying the nightglow green line in the atmospheres of the terrestrial planets. Although the terrestrial nightglow has been studied for 150 years (Ångström, 1869), there are still numerous areas where our understanding is not yet adequate, and one of these is the source of the terrestrial green line in the mesosphere. The ultimate source is the energy of recombination of oxygen atoms produced during solar photodissociation of O2. The so-called Barth mechanism for O(1S) production (Barth and Hildebrandt, 1961) relates to the transfer of electronic energy from an excited O2 molecule to an O(3P) atom, which requires 4.17 eV. What is not fully understood is the nature of the O2 donor molecule. The recombination energy of O2 is 5.1 eV, so the O2 molecule in question has an electronic/vibrational energy between these limits. The O2 Herzberg states, O2(A, A0 , c), are the generally mentioned candidates for the relevant O2 , but there are also other choices. The first measurements and interpretation of the Venus visible nightglow were carried out with data from the Venera 9/10 orbiters (Krasnopolsky et al., 1977; Lawrence et al., 1977). Unexpectedly, strong emission was observed from transitions corresponding to the O2(c–X) Herzberg II system, which is almost indiscernible in ⇑ Corresponding author. Fax: +1 575 646 1602. E-mail address: [email protected] (N.J. Chanover). 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.03.031
the terrestrial atmosphere. This Venus atmospheric emission is 10 times more intense than the sum of terrestrial Herzberg emissions. Significantly, there was no evidence of green line emission on Venus, and therefore for 25 years it was believed that the nightglow green line is unique to a planet with an O2 atmosphere. Another O2 emission in common among the three atmospherebearing terrestrial planets is the O2(a–X) IR Atmospheric band transition at 1.27 lm. This is seen only from the a(v = 0) level because O2(a) has an extremely long lifetime, and its vibrational levels are quickly deactivated by collision (Slanger and Copeland, 2003). The Herzberg II emission is also seen only from the c(v = 0) level, probably for the same reason. In any case, the a–X transition is seen in all three planetary nightglows (Slanger et al., 2008). In this work, we summarize 10 years of ground-based observations of the green line in the Venus nightglow and discuss expectations for the strength of Venus’ green line in the rising phase of solar cycle 24. 2. Observations A new phase of Venus nightglow studies began when the green line was detected using the HIRES spectrograph on the Keck I telescope (Slanger et al., 2001). Based on the Venera results, there was no reason to believe that the green line would subsequently be seen. The terrestrial green line typically varies less than a factor of two from its nominal intensity of 150 R, thus it was surprising to find the green line at Venus with an intensity even larger than the terrestrial value when it was first detected on 20 November 1999. Fig. 1 shows the discovery spectrum as seen on the first night of its detection. The terrestrial and Venus lines in the figure are well separated as a consequence of using the Doppler shift associated with the velocity of the approaching or departing planet. With
846
T.G. Slanger et al. / Icarus 217 (2012) 845–848
Fig. 1. Left: The terrestrial and Venus green lines as observed by HIRES/Keck I on 20 November 1999. Right: Comparison of VIRTIS spectrum (Garcı´a Muñoz et al., 2009) with DIATOM simulations of the four O2 band systems – Herzberg I, II, III, and Chamberlain (dotted). Also shown is the position of the (missing) 557.7 nm OI green line. All bands are from the v = 0 levels of the different states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a differential velocity of 12 km s1 or more and a spectrograph resolution of 40,000, spectra of high enough quality to resolve the terrestrial from the venusian line are routinely obtainable. This experiment was a potential breakthrough in our understanding of the Venus atmosphere, although the reason for the lack of detection by Venera 25 years earlier was not at all clear. Subsequent to the initial detection, we made regular observations of the Venus nightglow using two telescope/instrument combinations: HIRES on Keck I and the ARC Echelle Spectrograph (ARCES) on the Astrophysical Research Consortium’s 3.5-m telescope at Apache Point Observatory (APO). The HIRES slit was 0.861 7.000 , thus we extracted the Venus nightglow spectrum from multiple locations along the night side. We used ARCES with a slit of 1.6 3.200 , which we manually positioned at different locations on Venus’ night side. As discussed in Slanger et al. (2006), in measurements subsequent to November 1999, the green line has been much weaker in intensity in nearly a dozen observations made at Keck I and APO.
instrument on board ESA’s Venus Express spacecraft (Garcı´a Muñoz et al., 2009), which reproduces the visible emission seen by Venera very well. Fig. 1 shows the co-added spectrum, taken over a period of 450 h. Included in the figure are DIATOM simulations of the different O2 Herzberg band systems (Huestis et al., 1994). The position of the 557.7 nm green line is noted, conspicuous in its absence. The apparent presence of the Herzberg III system, most clearly at 472 nm, is noteworthy. This system has not been identified in the terrestrial atmosphere, and this difference may be related to the different collisional environment. 4. Quenching parameters A basic difference in the terrestrial atmosphere versus those of the two CO2 atmospheres relates to their absorption of solar radiation. Because absorption cross sections are much higher for O2 than CO2 in the far UV region – at 1200–1800 Å – to be absorbed by CO2 the light must penetrate to regions of higher gas density.
3. Discussion The key piece of information missing in our Venus green line studies is the emission altitude. In the terrestrial atmosphere there are two O(1S) sources – atom recombination in the mesosphere:
Oð3 PÞ þ Oð3 PÞ þ M ! O2 þ M;
ð1Þ
followed by
O2 þ Oð3 PÞ ! O2 þ Oð1 SÞ;
ð2Þ
and dissociative recombination in the ionosphere:
Oþ2 ðv Þ þ e ! Oð1 S;1 DÞ þ Oð3 PÞ:
ð3Þ
The former is typically the more intense in the quiescent nightglow, but the situation is reversed in aurora and the dayglow. It is clear that if we knew the emission altitude, then we would either have information on O-atom densities in the mesosphere, or on ion/electron densities in the ionosphere. To date there have been no green line observations other than our ground-based studies at Keck I and APO. The fact that the Akatsuki spacecraft did not go into a planned Venus orbit in December 2010 has prevented possible determination of an emission altitude for the green line. However, there has already been a study of the O2 Herzberg II system at Venus with the VEX/VIRTIS
Fig. 2. The radiating efficiency of O(1D) and O(1S) in the Venus atmosphere. For this calculation the [CO2] and temperature profiles of Krasnopolsky (2010) were used. The O(1S) + CO2 rate coefficient is 3.1 1011e(1330/T) cm3 s1 (Atkinson and Welge, 1972), and that for O(1D) + CO2 is 7.5 1011e(115/T) cm3 s1 (Sander et al., 2006). The radiative lifetime of O(1S) is 0.82 s, and that of O(1D) is 116 s (Froese Fischer and Tachiev, 2004).
T.G. Slanger et al. / Icarus 217 (2012) 845–848
As a consequence, collisional quenching tends to be much more important at Venus and Mars than in the Earth’s atmosphere, although this is not necessarily a rule; the nature of the quencher is also important. However, for O(1S) and also O(1D), quenching at Venus/Mars will be more effective than terrestrial quenching because these species are generated at higher gas density. Fig. 2 shows the O(1S) and O(1D) radiating efficiencies at Venus, the principal quencher in both cases being CO2. The altitude of maximum O-atom recombination, which is synonymous with the emission peaks of the O2 Herzberg II and IR Atmospheric band systems, is 95 km (Garcı´a Muñoz et al., 2009), as indicated. At that altitude, the O(1S) efficiency is on the order of 0.02. On the other hand, the O(1D) efficiency is 2 108, and there is no chance of seeing mesospheric O(1D) radiation, as is also true in the terrestrial atmosphere. In any case, it has not been detected from any altitude.
847
5. Possible mechanisms The terrestrial and Venus atmospheres offer an interesting contrast in the Herzberg state recombination spectrum. In the terrestrial atmosphere, the observed O2(A, A0 ) bands are mainly from vibrationally excited levels (Broadfoot and Kendall, 1968), all having enough energy to make O(1S) by energy transfer:
O2 ðv Þ þ Oð3 PÞ ! Oð1 SÞ þ O2 ðXÞ:
ð4Þ
On the other hand, the O2(c, v = 0) seen in the Venus atmosphere, although much more intense than the terrestrial emission, does not have enough energy to make O(1S) by reaction (4). This suggests but does not prove that the O(1S) source does not lie in the mesosphere.
Fig. 3. Keck I/HIRES spectrum of Venus from July 22, 2010 showing the green line region. Top: This diagram shows the orientation of the HIRES slit with respect to Venus’ night side. Bottom: This plot represents the co-addition of three spectra, and shows the terrestrial green line and lack of Venus green line. The predicted location of the Venus green line, which was calculated from the relative Earth–Venus velocity at the time of the observation, is shown with a dashed vertical line. The best-fit terrestrial line position is 5577.323 Å (as compared with the NIST value of 5577.339 Å). The sunlight subtraction in this spectrum is imperfect, as shown by the Fraunhofer line near 5576.7 Å.
848
T.G. Slanger et al. / Icarus 217 (2012) 845–848
A critical observation concerning any linkage between O2 Herzberg state emission and O(1S) is the fact that the Herzberg II emission is fairly stable between apparitions. From the Venera observations through Pioneer Venus (Bougher and Borucki, 1994), HIRES/ Keck I (Slanger et al., 2001), APO (Slanger et al., 2006), and VEX/VIRTIS (Garcı´a Muñoz et al., 2009), the intensity lies close to 4 kR, varying by less than a factor of two on either side. On the other hand, the O(1S) variations are much larger, with a peak of 150 R in 1999, to upper limits of 10 R at other periods, with no apparent correlation in intensity between O(1S) and Herzberg II emission. In the terrestrial atmosphere, the correlation between mesospheric Herzberg and green line intensities is quite persuasive (Stegman and Murtagh, 1991), as one might expect because both depend on O(3P) densities. That this is not the case at Venus suggests that the green line, when observed, is not coming from the mesosphere, but originates in the ionosphere via reaction (3), dissociative recombination, as is the case for the secondary terrestrial source (Shepherd et al., 1997). The peak Oþ 2 density occurs at 140 km (Schunk and Nagy, 2004), where Fig. 2 shows that the green line radiating efficiency is near unity. Although O(1D) and O(1S) production are of the same order of magnitude in reaction (3), Fig. 2 shows that the O(1D) radiative efficiency even at 130 km is still only on the order of 0.01, so the lack of its detection is not surprising. 6. Solar cycle effects If O2 Herzberg II emission at Venus is mesospheric and green line emission is ionospheric, this could explain the stability with time of the former and the variability of the latter. The solar radiation that dissociates CO2 to make O-atoms in the mesosphere is in the 1200–1700 Å wavelength range. In the ionosphere, COþ 2 must first be ionized at E > 13.8 eV, after which it reacts with O(3P),
COþ2 þ Oð3 PÞ ! CO þ Oþ2 :
ð5Þ
The EUV ionizing radiation is very dependent on the phase of the solar cycle, being much lower in intensity at solar minimum than solar maximum (Marsh et al., 2007). On the other hand, in the 1200–1700 Å region, there is little difference in intensity. It follows that if the Herzberg II and green line sources are in the mesosphere and ionosphere, respectively, then the Herzberg II radiation will be relatively constant in intensity, while the green line intensity will be strongly dependent on the phase of the solar cycle. The most intense of our green line emissions occurred in November 1999, close to solar maximum. Most of the subsequent measurements were made in the declining phase of cycle 23 and in the long minimum between cycles 23 and 24. If our reasoning is correct, then we can expect that as cycle 24 heads towards a maximum, in mid-2013 (NOAA/SWPC), the green line intensity will again strengthen, although it seems that the peak in cycle 24 will be unusually low. A HIRES measurement of the visible Venus nightglow was recently made from Keck I on 22 July 2010. Fig. 3 shows that the green line is not discernible, and we assign a terrestrial/Venus ratio
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