HDO and SO2thermal mapping on Venus

Abstract

We present the analysis of a four-year observational campaign 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. Data have been recorded in two spectral ranges around 1345 cm −1 (7.4 µm) and 530 cm −1 (19 µm) in order to probe the cloudtop at an altitude of about 64 km (SO 2 and HDO at 7.4 µm) and a few kilometers below (SO 2 at 19 µm). Observations took place during six runs between January 2012 and January 2016. The diameter of Venus ranged between 12 and 33 arcsec. Data were recorded with a spectral resolving power up to 80 000 and a spatial resolution of about 1 arcsec (at 7.4 µm) and 2.5 arcsec (at 19 µm). Mixing ratios were estimated from HDO/CO 2 and SO 2 /CO 2 line depth ratios, using weak neighboring transitions of comparable depths. The whole dataset demonstrates that the two molecules behave very differently to each other. The HDO maps are uniform over the disk. The disk-integrated H 2 O mixing ratio (estimated assuming a D/H of 200 VSMOW in the mesosphere of Venus) show moderate variations (by less than a factor of 2) over the four-year period. A value of 1.0−1.5 ppmv is obtained in most of the cases. The SO 2 maps, in contrast, show strong variations over the disk of Venus, by a factor as high as 5. Long-term variations of SO 2 show that the disk-integrated SO 2 mixing ratio also varies between 2012 and 2016 by a factor as high as ten, with a minimum value of 30 +/−5 ppbv on February 26, 2014 an a maximum value of 300 +/−50 ppbv on January 14, 2016. The SO 2 maps also show a strong short-term variability. It can be seen that the SO 2 maximum feature usually follows the four-day rotation of the clouds over a timescale of two hours, which corresponds to a rotation of 7.5 deg over the planetary disk, but its morphology also changes, which suggests that the lifetime of this structure is not more than a few hours.