HDO and SO2 thermal mapping on Venus III. Short-term and long-term variations between 2012 and 2016

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 mu m) and 530 cm(-1) (19 mu m) in order to probe the cloudtop at an altitude of about 64 km (SO2 and HDO at 7.4 mu m) and a few kilometers below (SO2 at 19 mu m). 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 mu m) and 2.5 arcsec (at 19 mu m). Mixing ratios were estimated from HDO/CO2 and SO2/CO2 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 H2O 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 SO2 maps, in contrast, show strong variations over the disk of Venus, by a factor as high as 5. Long-term variations of SO2 show that the disk-integrated SO2 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 SO2 maps also show a strong short-term variability. It can be seen that the SO2 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.