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Project Progress

New spectroscopic observations of Venus were obtained with the IRIS2 imaging spectrograph on the 3.9m Anglo-Australian Telescope in two observing runs in 2007 June-July as part of an international coordinated program with the ESA Venus Express spacecraft. The observations are being analyzed in conjunction with earlier datasets from the same instrument in 2004 and 2005 and provide information on the composition, chemistry and circulation of the Venus atmosphere at a range of levels.

A particular focus has been to look at the water vapor content of the Venus lower atmosphere. Venus is extremely dry as compared with the Earth, but its high D/H ratio indicates that a substantial loss of water has occurred in the past. In-situ spacecraft measurements gave a wide range of values for the H₂O mixing ratio. Remote sensing observations using the near-IR nightside windows have given a more consistent picture pointing to a constant H₂O mixing ratio ~30 ppm below the clouds.

The IRIS2 observations in the near-IR windows and are being used to refine this value. In the course of this work we have found substantial discrepancies between different water vapor spectral line lists in the near-IR region. A comparison of eight H₂O line lists has shown that all line lists used for previous Venus studies are incomplete in hot line and/or contain systematic errors in line intensities. We are currently assessing the impact of these effects on past water vapor measurements, and analyzing our new data using the most complete modern lists.

The Venus IRIS2 observations have also been used to obtain temperature maps for the Venus mesosphere at an altitude of ~95 km by looking at the rotational line intensities in the 1.27 μm O₂ airglow band. The temperatures are typically 15-30 K higher than those expected from the standard Venus International Reference Atmosphere (VIRA). The observation of high temperatures at this altitude is consistent with the “warm layer” detected by the SPICAV instrument on Venus Express, and is believed to be the result of compressional heating of downwelling gas near the antisolar point.