HDO and SO₂ thermal mapping on Venus: evidence for strong SO₂ variability

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 Lowell Observatory, Flagstaff, AZ 86001, USA
⁴ Physics Department, University of California, Davis CA 95616, USA
⁵ Department of Astronomy, University of Texas at Austin, TX 78712-1083, USA

Received: 16 April 2012
Accepted: 13 June 2012

Abstract

We have been using the TEXES high-resolution imaging spectrometer at the NASA Infrared Telescope Facility to map sulfur dioxide and deuterated water over the disk of Venus. Observations took place on January 10–12, 2012. The diameter of Venus was 13 arcsec, with an illumination factor of 80%. Data were recorded in the 1344–1370 cm^-1 range (around 7.35 μm) with a spectral resolving power of 80 000 and a spatial resolution of about 1.5 arcsec. In this spectral range, the emission of Venus comes from above the cloud top (z = 60–80 km). Four HDO lines and tens of SO₂ lines have been identified in our spectra. Mixing ratios have been estimated from HDO/CO₂ and SO₂/CO₂ line depth ratios, using weak neighboring transitions of comparable depths. The HDO maps, recorded on Jan. 10 and Jan. 12, are globally uniform with no significant variation between the two dates. A slight enhancement of the HDO mixing ratio toward the limb might be interpreted as a possible increase of the D/H ratio with height above the cloud level. The mean H₂O mixing ratio is found to be 1.5 +/−0.75 ppm, assuming a D/H ratio of 0.0312 (i.e. 200 times the terrestrial value) over the cloud deck. The SO₂ maps, recorded each night from Jan. 10 to Jan. 12, show strong variations over the disk of Venus, by a factor as high as 5 to 10. In addition, the position of the maximum SO₂ mixing ratio strongly varies on a timescale of 24 h. The maximum SO₂ mixing ratio ranges between 75 +/−25 ppb and 125 +/−50 ppb between Jan. 10 and Jan. 12. The high variability of sulfur dioxide is probably a consequence of its very short photochemical lifetime.