[P-CG38_1PO1] Planetary atmosphere, ionosphere and magnetosphere

[PCG38-P01] Study of the Venus' upper haze

*Seiko TAKAGI¹, Mahieux ARNAUD², Wilquet VALERIE², Vandaele ANNCARINE², Naomoto IWAGAMI¹ (1.Graduate School of Science,the Univ. of Tokyo, 2.Belgian Institute for Space Aeronomy)

Keywords:Venus, upper haze, Venus Express, SOIR, cloud

Venus is completely shrouded by a thick cloud deck floating at 45–70 km. The major material of the cloud deck is thought to be H₂SO₄–H₂O droplets. The upper haze on Venus lies above the cloud layer surrounding the planet, ranging from the top of the cloud (〜70 km) up to as high as 90 km. The upper haze particles with an effective radius of 〜0.25 μm was suggested from Pioneer Venus Orbiter (PV) measurements. The particles were most likely composed of sulfuric acid in terms of refractive index 〜1.45. The haze vertical optical thickness in the polar region at 365 nm was found to be 0.8 above the main cloud of 1 μm particles by PV measurements. By comparison, the optical thickness of the haze above the main cloud at low latitudes was found to be 0.06 [Kawabata et al., 1980]. Knibbe et al. (1998) and Braak et al. (2002) observed a gradual decrease of the haze particle column density during the PV mission. Braak et al. (2002) reported a correlation between the decrease of SO₂ abundance [Esposito et al., 1988; Na et al., 1990] and that of the polar haze optical thickness. However, it is unclear how haze are produced and composition of haze.The upper layer detected (above the clouds) is characterized by a SO₂ mixing ratio increase with altitude from 85 to 105 km [Belyaev et al., 2012]. It shows a new source of SO₂ at high altitude. One possible source of SO₂ in the upper haze layer could be photo-dissociation of H₂SO₄ vapor resulting from evaporation of acid aerosol droplets. However, recent upper limit of H₂SO₄ from sub-mm ground-based observation makes this theory less likely [Sandor et al., 2012]. The cause of the phenomena given above is still controversial.The Solar Occultation at InfraRed (SOIR) on board Venus Express (ESA) is designed to measure the atmospheric transmission at high altitudes (70–220 km) in the IR (2.2–4.3 μm) with high resolution by solar occultation. The SOIR data obtained in 2006–2009 are analyzed to examine the upper haze at altitude above 90 km. Vertical and latitudinal distribution of haze extinction, optical thickness and mixing ratio are calculated in using SOIR data statistically. Extinctions and optical thickness at low latitude are two times thicker than those of high latitude. One of the notable results is that mixing ratios increase at altitude above 90 km at both high and low latitudes. It is speculated that sources of haze are transported upward from under altitude 90 km and haze is produced at high altitude. From comparison with the vertical distributions of SO and SO₂ mixing ratios reported by Belyaev et al. (2012), it is speculated about the correlation between sulfuric compound and haze.