Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1409m&link_type=abstract
American Astronomical Society, DPS meeting #28, #14.09; Bulletin of the American Astronomical Society, Vol. 28, p.1118
Astronomy and Astrophysics
Astronomy
Scientific paper
Existing atmospheric chemistry models predict that O_2 should be a minor, but detectable, constituent of the Venus Mesosphere (60-100 km) [e.g., Sze and McElroy 1975; Winick and Stewart 1980; Krasnopol(') sky and Parshev 1983]. The predicted average mixing ratios above the cloud tops ( ~ 65 km) are between ~ 1.5 and 3.5x10(-6) [Yung and DeMore 1982]. Evidence for ground-state O_2 at mesospheric levels has been provided by observations of intense airglow in the near-infrared O_2(a(1Delta_g ) ) bands (near 1.269mu m) [Connes et al. 1979; Crisp et al. 1996]. Observed hemispherically-averaged airglow intensities exceeding 1 MR (10(12) photons cm(-2) s(-1) (4pi sr)(-1) ) indicate ground-state O_2 production rates exceeding 10(12) molecules s(-1) , but the airglow observations do not constrain the equilibrium column abundance. In spite of these observational and modelling results, the absorption of sunlight by ground-state O_2 has not yet been detected above the Venus cloud tops. The current upper limit on the O_2 column abundance corresponds to a column averaged mixing ratio near 3x10(-7) [Trauger and Lunine 1983]. Atmospheric modelers have speculated that O_2 mixing ratios in the early 1980's may have been reduced by chemical reactions associated with the high SO_2 abundances detected above the cloud tops in the late 1970's and early 1980's. This suggests that the O_2 abundance should be higher now because the SO_2 amounts above the clouds have decreased by almost a factor of 10 since 1979 [Esposito et al. 1988; Na et al. 1994; Na and Esposito 1995]. On 23-24 January 1995, we used the Anglo-Australian Telescope's Ultra-High-Resolution Facility [Diego et al. 1995] to make new observations of O_2 above the Venus cloud tops on the Venus day side. We observed the 763.6325 and 763.2165 nm O_2 lines at a spectral resolving power, lambda /Delta lambda , = 600,000 with a square, 1.5 arc-second field of view. These observations reveal no evidence for O_2 above the Venus cloud tops within our detection limits (Delta intensity/intensity = 0.005), corresponding to ~ 1x10(-6) (1 ppm). These results suggest that the enhanced SO_2 concentrations seen in the early 1980's may not explain the anomalously low O_2 mixing ratios observed during that period.
Crisp Dave
Mills Franklin P.
Ryan Sean G.
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