HDO and SO₂ thermal mapping on Venus

II. The SO₂ spatial distribution above and within the clouds

¹ LESIA, Observatoire de Paris, CNRS, UPMC, Univ. Denis Diderot, 92195 Meudon, France
e-mail: therese.encrenaz@obspm.fr
² SWRI, Div. 15, San Antonio, TX 78228, USA
³ Physics Department, University of California, Davis, CA 95616, USA
⁴ Department of Astronomy, University of Texas at Austin, TX 78712-1083, USA
⁵ Department of Atmospheric, Oceanic and Space Science, University of Michigan, Ann Arbor, MI 48109-2143, USA

Received: 11 July 2013
Accepted: 23 September 2013

Abstract

Sulfur dioxide and water vapor, two key species of Venus photochemistry, are known to exhibit significant spatial and temporal variations above the cloud top. In particular, ground-based thermal imaging spectroscopy at high spectral resolution, achieved on Venus in January 2012, has shown evidence for strong SO₂ variations on timescales shorter than a day. We have continued our observing campaign using the TEXES high-resolution imaging spectrometer at the NASA InfraRed Telescope Facility to map sulfur dioxide over the disk of Venus at two different wavelengths, 7 μm (already used in the previous study) and 19 μm. The 7 μm radiation probes the top of the H₂SO₄ cloud, while the 19 μm radiation probes a few kilometers below within the cloud. Observations took place on October 4 and 5, 2012. Both HDO and SO₂ lines are identified in our 7-μm spectra and SO₂ is also easily identified at 19 μm. The CO₂ lines at 7 and 19 μm are used to infer the thermal structure. An isothermal/inversion layer is present at high latitudes (above 60 N and S) in the polar collars, which was not detected in October 2012. The enhancement of the polar collar in October 2012 is probably due to the fact that the morning terminator is observed, while the January data probed the evening terminator. As observed in our previous run, the HDO map is relatively uniform over the disk of Venus, with a mean mixing ratio of about 1 ppm. In contrast, the SO₂ maps at 19 μm show intensity variations by a factor of about 2 over the disk within the cloud, less patchy than observed at the cloud top at 7 μm. In addition, the SO₂ maps seem to indicate significant temporal changes within an hour. There is evidence for a cutoff in the SO₂ vertical distribution above the cloud top, also previously observed by SPICAV/SOIR aboard Venus Express and predicted by photochemical models.