Photochemistry on Venus : a very fast evolution
1er septembre 2012
A team of astronomers of the Paris Observatory has just highlighted a very rapid variation of the sulphur dioxide (SO2) on the surface of Venus). Mapping of Venus was made in the infrared with the NASA telescope IRTF and with the spectro-imager TEXES in January 2012. It has highlighted of very strong variations of SO2 on 24 hours time scale, while there is no change in the map of water (H2O). The reason is probably related to the very short photochemical life of SO2 compared with that of H2O. This discovery will help a better understanding of the photochemistry and dynamics of the atmosphere of Venus, still poorly understood. The data are very complementary to those of Venus Express.
Planet Venus, permanently covered with a thick cloud deck of sulfuric acid, exhibits a complex atmospheric circulation and strong photochemical activity, with processes — like the sulfur cycle - which are not fully understood presently. Since 2006, the Venus Express spacecraft, in orbit around the planet, monitors its atmosphere, its circulation and photochemistry in order to bring answers to these questions. In parallel, ground-based observations of the Venus disk, at specific wavelengths characteristic of various atmospheric species, can bring complementary information.
- Figure 1 : Cartes de l’abondance de HDO au niveau des nuages, obtenues à partir du rapport de profondeur de deux raies voisines de HDO et CO2, de faible intensité. A gauche : 10 Janvier 2012 ; à droite : 12 janvier 2012. Avec l’hypothèse d’un rapport D/H égal à 200 fois la valeur terrestre (mesuré par Vénus Express), le rapport de mélange moyen de H2O est de 1.5 10-6, en accord avec les mesures de Vénus Express. Cliquer sur l’image pour l’agrandir
In January 2012, infrared observations of Venus were recorded with the TEXES imaging spectrometer (Texas Echelon cross-Echelle Spectrograph) at the NASA IRTF (InfraRed Telescope Facility) at Mauna Kea Observatory (Hawaii). The spectra recorded at 7.3-7.4 mm with a resolving power of 80000 probe the atmosphere of Venus at the level of the cloudtop. Maps of the water vapor abundance (measured through its proxy HDO) and the sulfur dioxyde SO2 have been measured during three successive nights. The abundances are measured from the line depth ratio of HDO and SO2 versus CO2, the major atmospheric component. H2O and SO2 are the two key elements involved in the condensation and saturation of the sulfuric acid present in the clouds. Surprisingly, the maps of the two components show spectacular differences. While the H2O map (Fig. 1), globally uniform, shows little variation on a timescale of 48 hours, the SO2 map (Fig. 2) shows local variations up to a factor 10 and very strong temporal variations on a timescale of 24 hours. These variations cannot be due to dynamical motions as they would imply very fast winds at the cloud level, not compatible with the wind speeds measured by Venus Express. More likely, the SO2 variations are due to the very short photochemical lifetime of sulfur dioxyde. Below the clouds, the atmosphere of Venus is strongly convective. Small SO2 convective cells, formed at the cloud level, could disappear in less than a day and be responsible for the observed maps.
- Figure 2 : Cartes de l’abondance de SO2 sur Vénus au niveau des nuages pendant trois nuits consécutives (10, 11 et 12 Janvier 2012), obtenues à partir du rapport de profondeur de deux raies voisines de SO2 et CO2, de faible intensité. Le rapport de mélange maximum de SO2 varie de 75 10-9 le 10 Janvier à 125 10-9 le 11 Janvier, avec une valeur intermédiaire le 12 Janvier. Ces valeurs sont compatibles avec les résultats de Vénus Express. Cliquer sur l’image pour l’agrandir
Reference HDO and SO2 thermal mapping on Venus : evidence for strong SO2 variability, T. Encrenaz, T.K. Greathouse, H. Roe, M. Richter, J. Lacy, B. Bézard, T. Fouchet, T. Widemann, Astron. Astrophys. 543, A153(2012) Contact Thérèse Encrenaz (Observatoire de Paris-LESIA, CNRS) Thierry Fouchet (Observatoire de Paris-LESIA, CNRS) Bruno Bézard (Observatoire de Paris-LESIA, CNRS) Thomas Widemann (Observatoire de Paris-LESIA, CNRS)
Contact Thérèse Encrenaz (Observatoire de Paris-LESIA, CNRS) Thierry Fouchet (Observatoire de Paris-LESIA, CNRS) Bruno Bézard (Observatoire de Paris-LESIA, CNRS) Thomas Widemann (Observatoire de Paris-LESIA, CNRS)
Dernière modification le 4 mars 2013